# Month: March 2018

## Calculating Target Delta T w / Manufactures Data

This tip was a COMMENT on the sensible heat ratio tip left by Jim Bergmann. As usual Jim makes a great point, once you get the “sensible” capacity for a piece of equipment at a set of conditions you can easily calculate a true target Delta T.

Another interesting thing you can do with this information is to determine the approximate target temperature split under any load condition. There are some additional footnotes on that chart likely saying the return air conditions are at 80 degrees at each of the respective wet-bulb temperatures.

To do so, find the sensible capacity at any set of conditions, for example at 95 degrees outdoor air and 1400 CFM, the sensible capacity is:

At 72 wb 25,010 BTUH

At 67 wb 31,730 BTUH

At 63 wb 37,360 BTUH

At 57 wb 37,930 BTUH

Using the sensible heat formula, BTUH = 1.08 x CFM x Delta T

Delta T = BTUH /(1.08 x CFM)

So…..

Delta T = 25, 010/(1.08 x 1400)

or 16.6°

Delta T = 31,370/(1.08 x 1400)

or 20.74°

Delta T = 37,360/(1.08 x 1400)

or 24.70°

Delta T = 37,930/(1.08 x 1400)

or 25.08°

So you can see also that the target temperature split has a lot also to do with the return air and outdoor air conditions and it has a lot of variation

— Jim Bergmann w/ MeasureQuick

## Hydronics GPM Calculation and more…

This tech tip was written by a friend of HVAC School, Brian Mahoney HVAC instructor at Western Suffolk BOCES/Wilson Tech. Thanks Brian!

The podcast on delta T for A/C the other day got me to thinking about the formula I learned in school about calculating the GPM of a hydronic system using a handy formula. We will be using the following values:

Td – temp difference of your supply vs return

Net boiler output(btu) use the boiler plate rating or get fancy and do an efficiency test and multiply your rated input multiplied by your efficiency rating. On an oil system, the unit could be down-fired.

It may be rated for 1 gallon per hour (140,000 BTU per hour input, but it may be firing with a .85 gallon per hour nozzle. So you have to do the math:
1 gallon of #2 fuel oil contains about 140,000 BTUs. Multiply that by .85 (your nozzle size) and you get 119,000 btu/hr input. Input would be 119,000 x .80 efficiency = 95,200.

500 – a constant which stands for a pound of water times 60 minutes – 8.33 x 60 = 499.8 (we fudge a bit.)

This is the weight of water at 60 degrees. You could look up the weight at the temp you are working with and multiply by sixty but it wouldn’t be far off.

To find a system’s gallon per hour:
BTU/ (500 x TD)
100,000/(500 x 20)
100,000 / 10,000= 10 GPH

Nice, but is there anything else you can do with this? How about a room that’s not warm enough. Is your baseboard supplying enough heat? You could look up the specs for that product, maybe. But what if it has dirty fins or mud in the pipe that is affecting temperature transfer. How would you know?

By using your Testo temp clamps on either end of the baseboard you find your temperature difference and using the data from the last calculation you solve for net BTU output of the baseboard

Btu = GPH x 500 x td
10 x 500 x 2 = 10,000 btu/hr

Now you know what you are getting. So you can check the specs of that baseboard and see if it’s giving you its rated output. If it is you don’t have enough baseboard or you have a problem with the room; thermal bypass for instance.

If it’s not performing as rated and the fins are clean you have an internal problem such as mud in the pipe insulating it.

— Brian M.

## Measuring Air For Techs (Podcast)

In this discussion with Bill Spohn from TruTechtools.com we cover the practical steps and tools for YOU to start measuring airflow today… if not sooner.

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE.

## Short #2 – Delta T (Podcast)

Delta T (Evaporator air temperature split) what it is, what it means and how to avoid some common pitfalls.

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE.

## Refrigerant Charging Basics For Air Conditioning & Heat Pump Systems

Before I start on this one… At HVAC School we focus on a wide range of topics, many of them are very basic. My experience as a trainer for over 16 years has taught me that no matter what I assume others SHOULD know, it doesn’t change that fact that they often do not. This write up is very basic but you may find that some of the content will be useful for you to give apprentices or junior techs or it may give you a new idea of how to explain it to them… or maybe not. Either way, I feel an obligation to cover even the most basic concepts in the trade to help ensure that nothing gets missed.

Thanks for understanding.

Before reading this you need to understand some of the terms surrounding air conditioning charging and diagnosis, specifically the term saturation

Next, you need to know something of the basic refrigerant circuit, I suggest that you understand these words and concepts before you ever dive into attempting to charge an air conditioner.  Many who start here may ask “what should my pressures be”, this is NOT how you charge a system so if you are reading this to try and find that answer just be aware, it isn’t that simple.

In order to set a proper charge on an A/C system, you must first know the type of metering device.  The piston / fixed orifice type system primarily uses the superheat method and the TXV / EEV primarily uses the subcooling method.  When setting a charge, it is always preferable to set the charge in cool mode.  Whether you set the charge in heat or cool mode, you should always follow the manufacturers recommended charging specifications.  In this section, we will discuss manufacturer recommended charging and some indicators that you have set a proper charge in heat mode.

But first, There are some things that Trump these guidelines and should make you stop and do more diagnosis

A properly running A/C system with indoor and outdoor temperatures above 68 degrees will have a suction saturation above 32 degrees (freezing), don’t leave a system with a below 32˚ saturation suction without doing more diagnosis even if the superheat/subcool looks correct.

If you see a liquid line pressure that is more than 30 degrees saturation above outdoor temperature (like a 440 psi liquid pressure on an R410a system on a 90 degree day), do not proceed until you have further addressed the possible causes of high head, regardless of what the superheat or subcool might be reading.

Always purge your hoses to prevent introducing air into the system and never mix gauges when using low loss fittings and different types of refrigerants.

Charge in the liquid phase (tank upside down) and add the refrigerant slowly and carefully to ensure you do not flood/slug the compressor with liquid refrigerant. You can do this by watching your manifold sight glass or using a special liquid preventing adapter such as the Imperial 535-C Kwik Charge.

These precautions will prevent causing system damage.

Note: This is only a basic guide for charging. There are innumerable conditions that can alter refrigerant pressures, superheat, subcool and saturation that are not related to the refrigerant charge. This is not intended to cover the complete diagnosis of the refrigerant circuit.

### Superheat Charging

To charge a system using superheat, you will need to monitor the actual temperature of the low-pressure suction line, the saturation temperature of the low side suction gauge and the indoor and outdoor temperatures entering the unit(s).

Most if not all manufacturers have a charging chart available with their respective units.  With the information you have gathered on indoor and outdoor temperatures, you can calculate the recommended superheat or in a pinch, you can use a superheat calculator such as the Trane superheat calculator or a free app like our superheat calculator or even better the MeasureQuick app. A good calculator will require that you determine the wet bulb temperature in the return air stream.  Without a sling or digital psychrometer or hygrometer, you will not be able to determine wet bulb temperature.

Once you know the target superheat you can adjust the system charge to hit it. Let’s say, the recommended superheat was 18 degrees, you would add/remove refrigerant to the system until the actual temperature of the suction line was 18 degrees above the indicated saturation temperature from your low-pressure gauge. Adding charge will decrease the superheat and recovering refrigerant will increase the superheat.

### Subcool Charging

To charge a system using subcool, you will need to monitor the actual temperature of the liquid line and indicated saturation temperature on the high-pressure gauge.  Information on the entering temperatures is not necessary to charge the unit by the subcooling method.

Most manufacturers have recommended subcool charging information with the units if it is designed for a TXV (TEV).  If for some reason, there is no information with the unit, or if it has worn off, you can set a typical residential air conditioner charge to 10 to 12 degrees of subcooling which is a relatively safe range to use.

Let’s say for example the manufacturers recommended subcool is 14, you would add enough refrigerant to the system so the actual temperature of the liquid line was 14 degrees less than the saturation temperature, as indicated on the high-pressure gauge for that particular refrigerant. Adding more refrigerant will increase the subcool reading and recovering refrigerant will decrease the subcool reading.

### Approach Method

Lennox factory information asks that we charge by the approach method on TXV systems. I suggest charging to at least a 6˚ subcool before even attempting to calculate the approach method.

The approach method is a calculation based on the relationship of liquid line temperature to outdoor temperature.  To calculate approach, subtract outdoor ambient from actual liquid line temperature.  The outdoor temperature used to calculate approach should always be taken in the shade and away from the hot condenser discharge air. To increase the approach differential you would remove refrigerant to decrease it you would add refrigerant.

Some Lennox heat pump systems come with a subcool chart next to the approach chart. This subcool chart is for < 65˚.  This means the subcool chart is only valid when the outdoor temperature is below 65˚.  Follow the instructions on the unit carefully when charging in subcooling in <65˚ temperatures.  The method requires that you block sections of the coil to achieve higher head pressures before setting by subcooling.

### Heat Mode Charging for Heat Pumps

In most, if not all, cases you will charge a unit in heat mode according to the manufacturer’s recommendations.  In those cases where no information is available, there are other indicators that you may use to set a proper charge in heat mode.

First, make sure you switch your hoses so the suction gauge is reading off of the “common suction” port that taps in between the compressor and reversing valve. You may put your high side gauge on either the discharge or liquid (on most systems) depending on what you are checking.

Before doing any heat mode charging use common sense, if installing a new system the best bet is to calculate line distance and weigh in any additional charge before moving on to the detailed testing phase.

The first one is the 100˚ over ambient discharge temperature rule.  The general rule to this is that a properly charged unit will have a discharge line temperature of 100˚ above the outdoor ambient temperature.  If the discharge line is too hot. you would add refrigerant which would lower the discharge temperature.  Alternately, if the discharge line were too cool, you would remove refrigerant to raise the discharge temperature.  This rule is to be used only as an indicator and, in some instances, may not be accurate given some other factors such as dirty coils, excessive superheated refrigerant entering the compressor, etc.

Another common rule of thumb is suction pressure will be close to the outdoor temperature in an R-22 system, this is totally a fluke and has no scientific basis other than it just generally tends to work out that way. this means that on a properly functioning R22 system if it is running in heat mode and its 40 degrees outside the suction pressure tends to be around 40psig. This guideline obviously doesn’t work on an R-410A system or any other refrigerant.

A more applicable guideline is 20˚- 25˚ suction saturation below outdoor ambient temperature. This means if it is 50˚ outside the suction saturation temperature would generally be between 25˚and 30˚on a functioning system.

Remember that in heat mode the colder it gets outside, the lower the suction pressure and the hotter it gets inside, the higher the head pressure.  Since the roles of the coil are reversed in heat mode, if you notice an abnormally high head pressure it may be due to a dirty air filter or evaporator coil.  A dirty condenser coil would cause the suction pressure to drop below normal and also cause superheat problems.

Once heat mode a charge is set, whether by manufacturer specification or an alternative method, you can still verify the subcool and superheat on the unit in some cases.  Do not confuse the superheat or subcool methods recommended by the manufacturer though when running in heat mode.  These are only used for setting the charge in cooling mode and not in heat. Look for heat mode specific or low ambient guidelines.

Finally and most importantly is ALWAYS TEST EVERYTHING. Airflow, Delta T, Superheat, Subcool, Suction Pressure, Head pressure, Amps, Incoming voltage, Filter etc…

Read manufacturers specs, understand the units the units you are working on, only then will guidelines and rules of thumb help instead of hinder you.

— Bryan

## Do You Need to Be Recovering Into a Vacuum?

This topic came up because I was testing out the new MR45 digital recovery machine and that machine goes off by itself when it hits a 20″ Hg vacuum. This is a cool feature but it is good to know when that level of vacuum is overkill and when it’s not enough according to EPA requirements.

Why would you need to recover into a vacuum you might ask? Well, so long as you are above a PERFECT VACUUM (and you always do) there are still molecules of refrigerant in a system even at 0 pisg (14.7 PSIA at sea level). In low pressure systems like centrifugal  chillers the entire system charge can often be in a vacuum when the system is off, this means that recovery on these systems means you START below 0 PSIG and go down from there.

First off let’s pretty much assume that none of you are using recovery machines OLDER than 1993 so really only look at the right side of the chart above.

If you are working on an air conditioning system with UNDER 200 lbs you are safe taking your recovery to 0 or atmospheric pressure. If the system you are working on has OVER 200 lbs of refrigerant or if you are working on a medium pressure or low pressure system you will need to pull the system into a vacuum.

The EPA does make an exception if the system has a know leak and pulling into a vacuum will result in contamination of the recovered refrigerant. Here is an excert from the EPA final rule summary from 1995 (still in force)

Also let me clarify that 25mm hg absolute is another way of saying 25 torr or 25,000 microns, it’s just a finer scale and it goes from 760 torr (760,000 microns) down as the vacuum gets deeper whereas inches of mercury (“hg) goes up as the vacuum gets deeper.

— Bryan

## Beating the Ego, Ignorance and Insecurity Monster w/ Andrew Greaves (Podcast)

In this simultaneously heavy and lighthearted discussion, Bryan Orr and Andrew Greaves discuss ego, Dunning Kreuger, insecurity and apprenticeship in the trades. AK HVAC on YouTube – https://www.youtube.com/user/akgreaves

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE.

## Short #1 – Refrigerant Circuit Basics (Podcast)

In this short episode we review the basics of the refrigerant circuit. Compressor, Condenser, Metering Device and Evaporator Find out more at HVACRSchool.com.

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE.

## Checking Evaporators on Furnaces

In Florida, there are not many gas furnaces, At least not as many as up North. Sometimes we can look like real dummies compared to techs who work on them everyday.

One thing to know about 80% gas furnaces with cased evaporator coils is that you can often check the evaporator coil by removing the high limit and running an inspection camera up through the opening.

You may also be able to use a mirror and flashlight but you usually won’t see much due to the heat exchanger being in the way. Otherwise, you are stuck removing the entire blower assembly… and that’s no fun at all.

Another practice is benchmarking the static pressure drop across a new coil when it is dry and wet when installed or during the first service call. You can then easily watch coil loading over time without the need to look at the coil visually.

— Bryan

The receiver is also often called a “liquid receiver” and you will see it on everything from small self-contained refrigeration units to very large commercial and industrial systems.

Many new techs who are used to residential air conditioning confuse receivers with accumulators. While an accumulator is located in the suction line before the compressor and prevents liquid from entering the compressor a receiver is located in the liquid line after the condenser and stores liquid refrigerant.

The liquid receiver stores refrigerant when the system is operating at less than maximum heat load and is generally designed so that receiver can hold all of the system charges and still be no more than 80% full. This allows you to pump down the entire system charge into the receiver without danger of creating hydrostatic pressure (very high pressures resulting from full liquid expansion) in the receiver.

The multi-position service valve at the outlet of the receiver is called a “king valve” and can be used for refrigerant circuit access as well as fully front seated (turned clockwise) for pump down.

Because a receiver has both liquid and vapor present inside many techs argue that the refrigerant cannot be “subcooled” in the receiver. The truth is that while the refrigerant that interacts between the liquid and vapor at the top of the receiver is at saturation the refrigerant below the liquid line can be and usually will / should be subcooled.

— Bryan

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