Tag: Manual J

This is a piece about oversized air conditioners.

Though the symptoms and consequences of oversized heating equipment are similar to those of air conditioners, you’ll notice that the focus throughout the article will be on the cooling side. Specifically, from the perspective of climate zone 1 (hot and humid).
I’m gonna skip right through the lecturing about proper equipment sizing, selection, and duct design. There are trained professionals for that and I’m not one of them. Instead, we are going to riff from the perspective of a system that has already been installed and is doing damage.
We are gonna go over some of the symptoms, their characteristics and why making improvements to oversized HVAC it’s a slippery slope.

So, what does an oversized system looks like?
Like any other one, you’ve worked on. Except, these systems:

• Can’t keep the occupants comfortable throughout various rooms in the house.
• Comfort complaints are intensified at night.
• It short cycles periodically, but it specifically does so when it’s less than 94° outside and still feels warm inside. Even when the thermostat it’s showing 67° as the room temperature.
• The relative humidity is consistently high (over 55%) or, at best, goes through big swings throughout the day. These swings will normally track with the operation cycles.
• Light films of condensation might be visible on supply vents.
• Duct work sweating.
• Excessive noise from vents. Returns, supplies or both.
• The temperature feels (noticed I said feels, not reads) significantly warmer around the perimeter areas of the space (larger exposure to exterior walls) than on the interior ones (hallways and such).

If you pull up to a service call and any meaningful combination of these symptoms are the reason you’re there, put the gauges back in the truck. There is no need to worry about subcooling or superheat. I promise.

But why? What’s so wrong with oversized equipment anyways?

Run time is the obvious place to start. The lack thereof that is.
Oversized equipment will naturally result in larger and colder air volume being moved throughout the space. Invariably, the wall control will reach its setpoint faster and the system will cycle off before it had the chance to do its job.

What is its job exactly?

Let’s start with the mean radiant temperature. The linked article explains it very well but in short, human comfort has as much to do with the temperature of the surfaces around us as the one displayed by the thermostat.

Our body temperature is normally 98 degrees, our skin is closer to 94. So, if we were to stand by a wall with a surface temperature of 75 degrees our bodies will cool off by radiating heat to it at a more comfortable rate than if we were to stand by a wall at 85 degrees. And the same goes for couches, beds, kitchen counters, etc.

An AC system must run long enough to keep a cooler and consistent temperature on all the surfaces of a home. If the outdoor temperature it’s in the ’90s and yet, the system runs for only 10 to 15 minutes each cycle, this won’t be enough to keep the mean radiant temperature of the surfaces in your home under control. Therefore, you’ll be uncomfortable despite the thermostat reaching, and “maintaining” an indoor temperature in the 60’s. This phenomenon is worsened at night when the outdoor temperature drops and the AC runs even less.

Apparatus dewpoint (ADP) is next. ADP is the effective surface temperature of the cooling coil. Or as we call it, coil temperature. I will use these 3 terms interchangeably.

While a system is off, the evaporator coil will be at a temperature close to that of the return air path and its surrounding surfaces. This temperature is much higher than that of when the system is running. Once the system cycles on, the return air temperature will dictate the evaporator saturation temperature based on the DTD and it will reach the ADP.

But just because the refrigerant entering the evaporator is at 40 degrees doesn’t mean that all of the coil will immediately drop to this temperature. This process takes time. The cold refrigerant has to make several passes before it can first, absorb the heat from all of the evaporator’s body mass, for it then, come down to the design ADP.

If we are having average run cycles in the 10 to 15 minutes range, this won’t be enough to ensure that the whole evaporator surface reaches its design operating temperature, and dehumidifies the air before the system cycles off. Therefore, the dehumidification capacity of the system will be consistently and greatly compromised, resulting in poor relative humidity control in the space.

This phenomenon is seriously worsened when dealing with high-efficiency systems. To increase SEER ratings manufacturers have found ways to drop the compression ratio and therefore power consumption. To achieve this, they have increased the suction saturation temperature through the use of larger coils. So, not only does the evaporator starts out warmer, but now it has more surface to bring down to temperature. The shorter run times of oversized systems will accentuate the otherwise negligent consequences of having a larger and warmer cooling coil surface temperature.

Did you just say SEER?! At no other time, an AC system is more efficient than when is not running, right? Because is not using any energy. So, wouldn’t it make sense to provide the consumer with a system that cycles off more often then? Nope. To begin with, the upfront costs of having larger equipment installed are normally more than that one of smaller capacity.

Also, and more importantly, the single, highest point of energy consumption for an AC system is when it turns on. Once a system cycles on and off more times than necessary throughout the day, the presumed savings of not having it run for a given amount of time go out the door.
And to top it all off, the clients are ticked off! Not only did their electric bill not go down much if any, but now they are also uncomfortable.

So how can we fix it?

Well, to fix it we would have to replace the system with one of the appropriate capacity. But that’s probably not gonna happen right away is it? Not until the consumer has enough pain to motivate the expense anyways.

Before we invariably end up talking about extending runtime and/or lowering airflow I want to make a quick stop on static pressure.
When there is an oversized system connected to existing, older ductwork. As soon as you start diagnosing the issue, you’ll run into a high external static pressure reading. At this point, a light bulb will go off in your head “it’s the ductwork”!

You’ll carry on to quote duct improvement solutions that will drop the TESP, maybe even throw some return air path upgrades. Let’s say the customer agrees, and once the work is done you perform a complimentary (or not) test and balance and ultimately confirmed that the TESP is now within acceptable levels.

“I’m going to be a hero” you may say to yourself. Well, if you did in fact improved the duct system to a point where the equipment is now moving more air than before, then the problem just got worse.

I get that it’s a controversial stance but, next time you realize you are in front of one of these situations ask yourself:
More, colder air. Do I really want to make this oversized system run better?

About extending run time

If the envelope doesn’t change, then the alternative left would be extending runtime. There is a number of ways to achieve this:

• Strategically place remote temperature sensors on the warmest areas of the house that report to the thermostat and therefore will trick the system into running more. The thermostat may feature dehumidification specific algorithms.

• Purposely de-balance the airflow distribution throughout the house, so there is more air hitting the exterior surfaces and as little as possible on the interior areas where the wall control may be located – the ceiling on this strategy is pretty low in my experience.

• And all of the above plus reducing the airflow to its minimum possible setting to run a colder coil temperature and run a lower SHR. Therefore, the dry bulb temperature as sensed by the wall control won’t drop as fast…maybe.

Doesn’t sound that bad, does it? Except these will also result in colder supply air temperatures. This is the leading cause of sweating ducts and vents in these scenarios, but that’s not the worse part.

This will directly result in localized, colder surfaces throughout the envelope as well. Condensation on vents and ductwork you can notice fairly early, before they become a problem. But what about the condensation you can’t see? The one that had been forming on building materials for a while and wasn’t a problem until now that a coconut tree sprung out of one the walls. A “moisture” remediator gets called next and what follows it’s an unfortunate tale of lawsuits and bad reviews.

I am not saying that improvements to ductwork and runtime shouldn’t be made to an oversized system but…
Have you ever heard of the bull in a china shop metaphor?

The china shop is the house and the oversized HVAC is the bull.

Genry Garcia
Comfort Dynamics, Inc.


Every year when outdoor temperatures rise there is a rash of news stories and articles about air conditioning. We had an early heat wave this year and lot of people have come out and referred to the idea of a rule of thumb of what temperature you can achieve indoors based on the outdoor temperature, most commonly used is the “20° rule”. Here is a link to an article like this.

There is no such thing as a universal 20° rule, it is simply the difference between the indoor and outdoor design conditions and it varies based on location and design

There are times where 20° is the design difference between indoor and outdoor temperatures, specifically when the design outdoor temperature is 95° and indoor is 75° for cooling. Before we go any further let’s specify EXACTLY what we are talking about

This is all about designing an air conditioning system and has nothing to do with DIAGNOSING it. When a tech goes to a home their job is to diagnose and test the HVAC system not to quote rules of thumb.


When a contractor designs an air conditioning system they have to size it for the space being cooled (I’m just going to focus on cooling here). The size of the unit needs to be based on a design indoor and outdoor temperature and humidity.

The indoor temperature design for homes is fixed by ACCA at 75° and the designer can choose 45%, 50% 0r 55% indoor design humidity.

The outdoor design temperature comes from temperature data specific to the location and is based on a temperature that will only be (statistically) exceeded 1% or 0.4% of the time in that location.

We do not design air conditioning systems for the hottest possible day with the lowest possible indoor temperature the customer may want because that would result in over-sizing for 99.6% of the year and over-sizing isn’t a good thing for many reasons including –

  • Higher initial cost
  • Larger Ducts Required
  • Poor humidity control
  • Short Cycling resulting in less efficiency and shorter system life
  • Generally lower rated efficiency on larger capacity systems

Take a look at the chart above, you can see from a glance that Florida has outdoor design conditions of 90° to 95° depending on the city and which column you use for design. Because the indoor design conditions stay fixed at 75° regardless the design difference on a peak design day vary from 20° down to as low as 15°.

Nevada (for example) is completely different –

You can see Reno is much like Florida in terms of dry bub temperature but Las Vegas is 106° – 108° and still a 75° indoor temperature design so a 31° to 33° difference must be designed for.

ACCA manual J does allow some oversizing to find a proper system match, from 15% greater than the load for straight cooling and 25% greater on heat pump systems (where the heating load is greater than the cooling load) but that isn’t a lot of wiggle room.

It’s also important to remember that system performance also changes based on outdoor and indoor temperatures and we must select out equipment capacity based on the specific design conditions rather than AHRI conditions which are 80° indoor and 95° outdoor temperatures.

So here are some facts to get straight-

  • There is no universal design rule of thumb 
  • This 20° rule has no relationship to the old 20° delta T rule (which is also a bad rule of thumb) 
  • Oversizing isn’t a good idea ESPECIALLY in humid climates
  • The fact that you can get your home to certain temperature in one part of the county has nothing to do with another location
  • How cold you can get your house on a hot day is just a representation of the capacity of your system in comparison to the load, it’s not something to be proud of.
  • Did I mention that over-sizing isn’t a good thing? Just because you can get your house to 66° on a 140° degree day in Nome, Alaska doesn’t mean you have a better air conditioner. It means someone put in a unit that is much too large. 

So when a contractor emails their clients before a heat wave (Like I did recently) or when a LOCAL news channel runs a story and quotes something like, “You can expect your home to maintain only 20° lower inside than the outdoor temperature” withhold judgement for a minute and consider. That 20° rule may be a good guideline for their market and they may be telling it to consumers to reduce nuisance service calls on a rare 100° day in a place like Savannah where the design temperature is 93°.

As HVAC professionals we understand that some designs will do better than others and with modern multi-stage / variable speed systems we can get away with a little more over-sizing than we used to. We also (should) know that we size systems based on heat gain and loss and not based on square footage and that oversizing a system because the customer wants it like a “meat locker” has unintended consequences.

Now there will always be the techs who use silly rules of thumb rather than proper diagnosis procedures. If a customer calls you to look at their A/C don’t just walk up to the thermostat and say something like “It’s 80° in here and 100° outside so it’s doing good”. You need to properly test the equipment and I would suggest doing actual capacity calculations using in duct psychrometers and MeasureQuick in addition to everything else if it seems like the system just isn’t keeping up.

Check the charge, check ducts and insulation, do a good job of making sure everything is as it should be for the customer…. but sometimes, on unusually hot days, a properly designed and installed system may bot maintain 75° inside.

So these are my takeaways –

  • Size systems properly not based on rules of thumb
  • Still use proper system diagnosis and commissioning. Don’t use a rule of thumbs as an excuse not to do a proper diagnosis.  
  • It’s OK to use a rule of thumb specific to your market to communicate with customers so long as it’s based on real design conditions 
  • Don’t oversize systems especially in humid climates
  • Don’t be a jerk to people on the internet 

So in Orlando… you can expect your A/C to maintain 75° on a 95° day in most cases and if the temperature rises above that it may not keep up.

— Bryan

If you want to know more about the ACCA design process take a look at this quick sheet with design instructions


I get questions all the time about performing “load calculations” and “rules of thumb” as well as how to do it properly. This article isn’t about load calculation but the only good answer is to find a quality ACCA approved Manual J software and get used to using it.

You may have heard from others in the field that Manual J tends to “undersize” the equipment. If you are an engineer or designer you may be shocked at how “oversized” most equipment is when compared to Manual J. Like most things, the truth can be found somewhere in between and here are the reasons for this.

System selection is just as important as Manual J because if you don’t match the proper equipment to the BTU load or if you fail to consider the factors you can easily end up with a system that is undersized or that does not deal with the humidity load of the space.

Here are some common factors that contractors & designers fail to properly factor into their Manual J

Duct leakage rate 

We can guess, but the only way to really KNOW the rate of leakage is to do a duct leakage test like the one shown below by Corbett Lunsford.

Building Envelope Leakage Rate

The rate of leakage in and out of a structure is one of the most overlooked aspects of a load calculation. Once again, we can guess based on the age and construction type but the true leakage rate can vary wildly. The only true way to test leakage rate is by measuring it with a blower door.


Even insulation is often a guess in areas where you cannot access walls or portions of the attic. It takes a combination of experience and thermal imaging or other R-value measuring tools to truly calculate heat loss / gain through insulation.


This one is very challenging to calculate. You may have two homes with nearly identical construction, orientation, and layout, one with significant shade from trees and another without. This shade can represent a significant decrease in radiant heat transfer to the walls, windows and roof and will vary seasonally based on the season and various times of day. Shade is something you can factor in using common sense. While I wouldn’t suggest “under sizing” just because of shade, you can be sure that a well-shaded structure will have lower radiant gains which will have an impact in all seasons.


For every cubic foot of air  you move out of a building you are also moving one cubic foot of air into the building, either through a designed path, through cracks and gaps, or when a door or window opens. One way or another when you move air out you are also moving it in. It is always better to move that air into the building through a designed path where the air can be controlled, measured and likely treated (ERV, HRV, Dehumidification) instead of through cracks and gaps that can be in any number of undesirable places. In addition to this, there are new standards being enforced surrounding ASHRAE 62.1 & 62.2 that mandate mechanical fresh air be brought into all structures. This fresh air needs to be considered as to heat gains and losses both sensible and latent.

Because of all these factors and industry pressures I have found that design professionals have a tendency to underestimate heat gains and losses (on existing, untested structures) while contractors and field personnel tend to oversize equipment based on “experience” or “rules of thumb” and usually a combination of both. This happens because it is much more common for a contractor to get a complaint of a system “not keeping up” than humidity, or power consumption issues because the thermostat displays the temperature in big bold numbers while humidity and power are a bit more abstract.

Design professionals are under pressure not to oversize equipment by the industry (and rightfully so) but may not be fully aware of all of the “as built” conditions that exist.

But for sake of argument, lets say the heating season losses and cooling season gains have been perfectly calculated

We now bump up against some of the most common areas of misunderstanding by contractors and field staff which is properly matching the equipment to the load. here are some of the biggest mistakes.

  • Failing to cover both sensible and latent (humidity) loads in the cooling season
  • Using nominal tonnage of equipment to estimate capacity (no a 3-ton cannot be counted on to produce 36,000 BTU)
  • Looking at AHRI ratings to find capacity instead of at the manufacturer performance data
  • Oversizing / Undersizing either the heating or cooling side by considering one and not the other

In order to properly select equipment, it is recommended that you use ACCA manual S to ensure that you don’t miss any of the steps. ACCA has a great quick guide on system selection you can read HERE

Here are some tests you can apply to your residential design to help double check that your selection matches the design.

  • The sensible cooling capacity of the system you choose should not be more than 15% greater than the sensible heat gain of the space
  • The latent cooling capacity of the system must be equal to or greater than the latent load of the space
  • In heat pump applications with greater heating loads than cooling loads (common) the cooling system must not be more than 25% than the sensible cooling gain
  • The heat Pump + electric aux heat capacity should not exceed the heat loss by more than 15%
  • In the case of fuel-burning appliances choose the next size greater than the heat loss of the space

All of these need to take into account the manufactures specifications matching the load calculation conditions to the specifications of your equipment at the same conditions. While a furnace may produce the same heat output no matter the conditions, air conditioning and heat pump equipment output will change depending on indoor and outdoor conditions.

The final step is configuring the equipment to the proper air flow levels so that the sensible/latent capacity will match the design. If the system was designed for 400 CFM per ton then ensuring that the equipment is set to output that airflow is critical.

— Bryan

Every piece of air conditioning equipment is capable of moving a certain amount of heat BTUs (British Thermal Units) at set conditions. In most cases during the cooling mode, a portion of those BTUs will go toward changing the temperature of the air and a part will go towards changing vapor water in the air into water that collects on the evaporator and then drains out.

The BTUs that go towards changing the TEMPERATURE of the air are called SENSIBLE and the ones that go toward removing water from the air are called LATENT. The percentage of the capacity that goes toward sensible cooling at a given set of conditions for a given piece of equipment or space is called SENSIBLE HEAT RATIO (SHR). So a system that has an SHR 0f 0.70 and 30,000 Total BTUs of capacity at a set of conditions would produce 21,000 BTUs of sensible cooling and 9,000 BTUs of latent removal because 30,000 x 0.7 = 21,000 and the rest 30,000 x 0.3 = 9,000.

Higher SHR (closer to 1.0) = More change in temperature and less humidity removed

Lower SHR = less change in temperature and more humidity removed

In the HVAC industry, there is a set of standard conditions used to compare one piece of equipment to another. When a system has an SHR rating listed it would often be at AHRI conditions unless the specs state otherwise.

When doing a load calculation a good designer will calculate and consider the internal and external latent and sensible loads and match up with equipment accordingly based not only on one set of design conditions but on the range of seasonal and occupant conditions that the structure is likely to experience based on the use, design and climate. By following ACCA (Manual J & S) and ASHRAE (62.2 & 62.1 for example) standards a designer will have guidelines to follow and this includes matching the space SHR to a piece of equipment that will make a good match at similar conditions. It does often need some digging into manufactures specs to interpret this data for the equipment.

In the example above from a Lennox unit, you can see that the SHR is listed and highly variable based on outdoor temperature, air flow setting as well as indoor wet bulb and dry bulb temperatures. In this example, you would need to multiply the total capacity x SHR to calculate the actual sensible and latent capacity.

This example from Carrier has no SHR listed, instead, it lists the specific sensible and total capacities. You can easily calculate the SHR by dividing the sensible capacity by the total capacity and the latent is simply the sensible subtracted from the total.

The cool thing is that this understanding can help both designers and commissioning technicians to match equipment properly and even make further adjustments using airflow to get a near perfect match which leads to lower power consumption, less short cycling and better humidity control.

— Bryan

How an AC System got to be Oversized (Maybe)

But My Old Unit Worked Fine?!

Most of us have heard this at some point. This complaint comes typically from a particularly unhappy customer after the installation of a brand new AC system. Throughout this article we’re going to explore the possible root causes of this situation, but first some ground rules:

  • We are going to navigate this issue from the perspective and comfort needs of climate zone 1…in other words it’s hot and humid!
  • Also, the intention is to describe the impact that progressive accumulation of a set of factors can have on the comfort (or lack thereof) of people within a building envelope over time, which means that, while a hypothetical story, it is based on actual events plus some reasonable assumptions.

So, going back to our complaint.

The complaint comes from whom we’re going to call homeowners C. The C’s just bought their first house about seven years ago. The home was ready to move in, but they did have to replace the old dark shingle roof shortly after. The new roof is much more cosmetically appealing with the light tile they used instead. It is a 1980 production home, and with the converted garage it adds up to about 1,450 square feet. Is not much, but it’s home, and they’re proud of it. So much so that when the old AC system started having some problems a few months back, they decided it was time for a new one.

Through the reference of friends/family, they got your number and requested a free estimate. You showed up on time, wore your booties, petted their dog and had the right answers for all their questions. “You had me at 0% interest!” said the husband while the wife asks “What are all these returns you said we need?”. She only asked because you’ve pointed out how all three bedrooms and the converted garage have only the undercut on the doors as return air paths.

The existing unit is a 14-year-old, 4-ton split system. “Do you guys feel this system keeps the house comfortable?” you asked. “Sure,” they said, “but we can probably use a bigger one. It gets warm in here every time we have a party”.  But you know better than that, you convince them to stay with the 4 ton, but as an act of good faith, you’ll run a new refrigeration line set. It’s only 50 feet, but the existing suction line is 3/4,” and you know that the manufacturer recommends a 7/8” for better system capacity and performance. The liquid line will, of course, be 3/8”.

So, the system gets replaced including the new lines and the returns are added for all four spaces. That’s when the fun starts! For months you’ve had to go back every 2 or 3 weeks, all to address the same chief complaint. They feel uncomfortable no matter how low they set the temperature. You’ve tried replacing and relocating the thermostat. You’ve also been in touch with the manufacturer a few times and confirmed that the subcooling and superheat are within range. The TESP is high, but that’s “OK” because this new air handler has a constant CFM motor (2.3 or 3.0) and according to the fan performance table the unit is indeed moving enough air.

“What did I do wrong??” you say to yourself. Nothing, but the C’s are very upset! Right now, that 0% interest doesn’t feel as sexy as it did back when you were petting their dog. What’s worse, they are blaming you for it! They are convinced that all their predicaments started when the new system was installed, but little do they know that it was many years in the making.

From the top

It’s 1980 and the house is brand new. It’s a slab on grade, 1,200 square feet initially with 9’ ceilings, 3/2 with an adjacent garage. A dark shingle roof housing the ductwork in a ventilated attic and the back of the house faces West. The back yard can be admired from inside the house since there are some seriously large glass sliding doors. A load calc gets done on the house, and an engineer estimates that a 3-ton system will work (perhaps a smaller one would have worked too). The indoor unit it’s installed in a closet with a louvered door inside the conditioned space.

Enter homeowners A. The A’s are a mild-mannered semi-retired couple and some peace it’s all they’re after at this stage in their lives. They like the new house and add a back porch where they can hang out and BBQ occasionally. But the now porch covered back of the house that faces West gets too hot in the afternoon. “Let’s plant a couple of trees dear,” she says while the husband changes the subject with the hope that she’ll forget about it. The trees get planted and the perspective of enjoying some shade in the afternoon looks promising.

Fast forward 15 years and among other things it looks like it’s time for a new AC system. Mrs. A has been thinking about converting the garage into a room so her husband can watch sports and drink beer undisturbed. She hasn’t however decided on the wallpaper yet, but it’s happening. I addition, as the system has aged, it has decreased performance and can’t seem to keep up on the hot summer days which can only mean one thing…they need a bigger unit!

A 3 ½ ton system gets installed. Same ductwork but a new 8” supply duct gets added to the garage. “That’s 200 CFM for you, Mr. A,” said the installing contractor as he got into his truck and drove off. The garage finally gets converted into a room. The garage door gets removed, and a block wall goes up instead. A Brand new TV, minibar and a nice comfortable chair but no dedicated return air path to go with that new supply vent. Just the door undercut. The conditioned space is now roughly 1450 square feet (don’t know the aspect ratio of the garage; let’s use round numbers).

As the turn of the century approaches, traffic has been getting hectic, and there were a couple of near hits last hurricane season. The A’s have had it and are ready to sell the house and move on! By now those young trees that Mr. A reluctantly planted are towering over the back of the house providing cool shade.

The cosmetic inclined home remodel

Cue homeowners B. Now the B’s are in it to win it, they have some capital to burn and show up sledgehammer in hand. It’s time for some serious remodeling.

They want the house to look modern but also want it to be safer. As such they rip out all the windows and replace them with low SHGC, low E, impact glass windows. As for the sliding glass doors, they replace them with smaller, wood framed double doors and a similar type of glass as the windows. Now they don’t have to board up the house three times every hurricane season, they thought.

For the interior, among other things they get a new kitchen and add a 100 CFM exhaust hood. To give the house the ultimate modern look, they replace all the existing lighting with ceiling recessed lights. About two dozen of those evil things throughout the house! No LED bulbs, they are the vented cans type so they can dissipate the heat up into the attic by convection.

When all the construction work is done, they move in and crank down the air conditioner. But something’s wrong…”We made all these improvements to the house, new, more efficient windows and doors and the thermostat won’t go below 76?! It must be time for a new AC unit.”

As they’re walking through the local big chain store of home goods one day, they see a kiosk where they are promised a better life by way of installing a new “Comfort System” for their home. “Just what we need!” they said. The B’s secure a conveniently prompt, free visit from the ‘Comfort Advisor/Specialist/Commission Based Former Car Salesman.’ This so-called specialist shows up at their house, and after listening to their problem, he concludes that their unit must be too small.

“You need a 4 ton for this house I am sure. Not only that, because today is Wednesday we are going to give you a complementary UV Light kit that is going to clean the air in your house” LOL!

The system gets replaced with a 4 ton. The owners still have some comfort issues, but it does feel more refreshing in the house, and so, they move on with their lives.

Fast forward a decade or so, and now they no longer like it there, one of them got a job offer out of town, they don’t like their local Sedano’s supermarket, traffic got worse – I don’t know, you pick the reason.

The C’s buy the house from the B’s and here you are, with a brand new, perfectly operational 4 ton AC system and a very unhappy customer.

Where did it go wrong?

By now you probably know where I’m going with this, but let’s try to break it down anyway:

1 – How the block load decreased:

Beware of the shade. The West and south facing sides are the two with the most heat gain incidence. By building a porch over the Westside and adding trees, a good chunk of the block load got chopped off.

In addition to that, the fenestration factor of the load was profoundly affected by upgrading the windows and doors. The more energy efficient windows and doors installed have also decreased the amount of heat that the AC system was supposed to handle.

Lastly, there is a new roof. As compared to shingles (especially dark colored ones), roof tiles will reflect some heat as opposed to absorbing it. This keeps the roof surface cooler and therefore, also the attic. A cooler attic will result in a cooler ceiling and of course, less heat gain. I highly recommend this excellent research study by the FSEC on the subject.

2 – How the cooling load increased:

As the AC system kept getting bigger, the lack of return consistently worked to offset the decrease of the block load.

The table below can be found on page 28 of ACCA’s Manual D.

You can also read this article by Allison Bailes quick before you keep going that way it’ll make more sense.

Remember when in anticipation of the garage being converted into a conditioned space, how the system was oversized by the ½ ton? How come that didn’t cause any trouble?

Because we lost about 100 CFM of return air from the former garage and replaced it with unconditioned infiltration air coming into the envelope through every crack and crevice. If a supply vent brings in 160-180 CFM of cold air and all you have as return is a door undercut of 1” on a 36” door, on an isolated room you’ll only get 80 CFM back. Because this is not a tight envelope, the balance will likely come from the attic through unsealed access doors and ancillary spaces. Therefore, this tonnage increased was pretty much a wash and an extra expense on the utility bill.

But remember how the car salesman/comfort specialist sold homeowners B a 4 ton? And they upgraded all those windows and doors…how come that didn’t cause any trouble?

Let’s start with the consequences of connecting a 4-ton air handler to the existing ductwork designed for a 3 ton.

The old 4-ton unit had a PSC evaporator motor. Connecting the larger air handler to the smaller air distribution system will cause the TESP to go up, but it’s also going to move more air than the previous, smaller unit. It will likely not move enough air for the system to deliver its 4-ton nominal capacity, but it will move more air none the less.

If the duct configuration remains unaltered, this, in turn, will proportionally increase the airflow out of each vent. Meaning that if you had 100 CFM coming out of a given supply vent and the total system airflow is increased by 20%, then there’d be 120 CFM approximately being delivered by that same vent.

But what does that mean? Remember how we can only get about 80 CFM worth of return air? Now there is more airflow going into each room with a door closed, but we are getting less a percentage of it back. If for example there were four bedrooms with the doors closed, and the total combined airflow for these rooms was 700 CFM then we are getting 320 CFM of it back or 46%. If with the bigger unit the total airflow went up by 20% then there are 840 CFM going in but still, only 320 CFM coming back which is now 38%. Consequently, the pressure in the central open area is even lower and there is a higher pressure differential driver bringing unconditioned air into the building envelope. If there isn’t a low resistance path for the return air to travel from the isolated rooms to the core/main return area, then upsizing the system will result in more infiltration, not necessarily more cooling of the space.

I get that these doors may not always be closed, but when they are this is going to happen. These rooms not being isolated all the time will intermittently mask this condition and make it much harder to diagnose.

To add insult to injury, the reverse stack effect has kicked in.  Remember what else the modern homeowners B added? High hat can lights! And these aren’t the LED sealed can ones available nowadays. No, these are the evil ones, full of little openings. Not only is the pressure differential between the attic and the conditioned space higher, but now there is a highway for the reverse stack effect of bringing nasty, humid attic air into the space.

Lastly, there is the occasional use of the kitchen exhaust hood at 100 CFM without makeup air and the more than likely small percentage of duct leakage to factor in. At this point, it wouldn’t be unthinkable to have at least 5 – 10% duct leakage. Duct leakage has the same space depressurization effect as the lack of return mentioned above (for different reasons). If you want to look at some hard numbers on its consequences, check this article by Neil Comparetto.

But wait for a second! Isn’t this an air conditioner?! If the air conditioner is bigger, then it should be able to handle more significant loads.

Correct, higher loads of design conditions at 80 °F DB and 67 °F WB, not of infiltration air at 75 °F Dew Point! Imagine that you were trying to thread the needle and every time you miss the hole somebody kept handing you a thicker thread.

The capacity of the system went up

So now is the present day and it’s your turn to screw up. Of course, you meant well and wanted to do the best possible job but, you were doomed right out of the gate. If you see a 4-ton system and the consumers tell you that it has been keeping the house comfortable, then what do you replace it with? A newer, shinier and more efficient 4-ton system…with a constant CFM ECM (not the same as a continuous torque) evaporator motor…and a larger diameter suction line…and added low resistance return air paths to all the isolated rooms.

The new ECM motor will deliver the design airflow against up to 1” WC for most manufacturers. This means that though at an energy penalty, now the system is moving as much air as it was designed to do, so the system capacity improved over the old unit.

With the larger diameter suction line, you got about 2.5% of total capacity back over the old system with the 3/4 line. About 1,000 BTU/H, not much but the system capacity did go up.

Adding returns to all the infamous isolated rooms put it over the top. Now the pressure differential between these rooms and the core return area is nearly if not zero. Because of this, the system is no longer pulling in as much infiltration air as it was before. All the volume that is being delivered by the supply vents is now smoothly making it back to the air handler in the form of cool, crisp return air. And this is where it gets hairy.

Because the block load of the house was decreased by the new roof, new windows and doors and the shading of the trees, and now, because there is less infiltration of unconditioned air, there is more air moving across the evaporator and the total system capacity went up, there is less of a load for the AC system to handle which makes it…(drum roll)… oversized!

Sure the ceiling recessed lights are still there and the reverse stack effect is still doing its thing but, in light of the new circumstances, the occupants will have double down on those to have a chance at the system, not short cyclin – as much.

By now you, the installing contractor have figured it out. You take a big gulp, brace yourself for impact and walk up to poor homeowners C. After an elaborate but oversimplified explanation of the situation, you gather the courage and confess to them: “Mam, this unit is oversized” …. pause for effect… “But My Old Unit Worked Fine?!”


— Genry Garcia

This article is written by HVAC contractor and Building Science Whiz, Michael Housh. Thanks Michael!

For a while, I’ve fallen into this camp where I feel Manual J overshoots heating loads. I would like to first off say, that Manual J is only as good as the information you give it, but we often run into this problem where we have to make educated guesses in the field. Most of us will likely assume insulation values in the wall assemblies (based on age, house type, etc.), windows are often an educated guess (they all suck anyway), and the huge infiltration rate (without a blower door number you’re guessing).

Through some conversations with friends, I was inspired to look at my utility bills and see what knowledge I could gain, and I figured I could take you along for the ride. In my experience utility companies have a fairly easy way to look at your usage rates for given periods of time. I pulled up my gas usage for 2018 and start slicing and dicing.

The first thing I did was determine what it costs to heat my domestic hot water by averaging the usage during the summer months, which came out to 15.2 CCF (CCF = volume of 100 cu. Ft. of natural gas). I was then able to adjust the data to a usable format, which is the Therm.

1 Therm = 100,000 BTU’s. The CCF measurement actually equals a Therm if the average heat content of gas is 1,000 BTU in your area, however national averages are typically higher than that. For where I am in Ohio our average heat content of gas is 1,070 BTU’s per CCF, so I was able to calculate the number of Therm’s used for a given month.

For the sake of brevity, I did a quick generic block load (as I normally would) on just the first floor of my house (which is @ 2400 sq. ft.), I also have some basement area that’s finished, but haven’t put it into the model yet. I will spare you on the exercise, but the concept is important to understand, that Manual J is rather linear based on outdoor temperature (a good exercise on your own is to generate a load, manipulate the design temperature to several different values and plot them on a graph).

If looking at just usage vs. Manual J there’s a huge difference, however, my design temperature is 5° and 2018 was a pretty mild winter, so I looked at the historical weather data for 2018 and added them to my spreadsheet. I also did some calculations (gracefully giving my boiler 70% AFUE), to show the approximate output.

My home was primarily built in the 1950s with an addition that was likely in the 1980s. For a home of this nature (and especially without a blower door number), I default to “loose” for an infiltration rate. I then took my worst case month (January), I manipulated the design temperature to 31° in my model, which was the average for that month, and began to adjust the infiltration rate up to “average” and came out with a pretty similar result to my actual usage.

Once satisfied that the model was pretty close to usage, I set the design temperature back to 5°.

This is a 10-15% difference from the original load/model. Now, all of this stuff is just a little thought experiment I decided to let play out, I hope to expand on these ideas further in the future and hopefully spark some more thoughts on this. DO NOT go out and start doing this, but DO start challenging the norm and come to a deeper understanding for yourself.

— Michael

Fenestration is a fancy architectural term that means “openings in the outside of the building”. You will see this word a lot when you read ACCA manual J or when you are doing a manual J load calculation.

Fenestration loads include heat losses and gain through windows, doors, skylights etc… and can make up a significant portion of the overall load as well as being the biggest variable load throughout the day as the sun moves across the horizon.

There are two big things to watch for when entering fenestration loads

  1. Look for tags on doors and windows that say NFRC (National Fenestration Rating Council) certified. If they have this mark it means that the entire door or window was rated including the glass, frame etc… over the entire rough opening rather than the glass only. If it is not NFRC certified then you are better off using ACCA manual J tables.
  2. Use the full rough opening of the doors or windows you are entering into manual J rather than just the part you can see. Many doors and windows will be pretty standard or at least consistent across the building so once you get one rough opening and U-factor you will often be able to use it over again.

There you have it… Try to use the word “fenestration” next time you play scrabble with your grandma for extra awesome points.

— Bryan

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