Month: March 2018

Here is another great explanation from Michael Housh from Housh Home Energy in Ohio.Thanks Michael!

I’m going to layout and compare the Sensible Heat Rate equations for both the air-side and water-side of HVAC, to help draw similarities and dive deeper into the science behind these equations.  This is the beginning of a series to try and help us all gain a deeper knowledge of where these equations come from. The more we learn about the two, the more similarities can be drawn between them. This will hopefully allow a technician to be more comfortable when faced with different systems in the field.  I should also note that while the equations can be complex, they are a great reference for those who would like to build them into spreadsheets (or other formats).


Sensible Heat Rate Equations:



Q = 1.08 * CFM * TQ = 500 * GPM * T
Q = sensible heat transferred (Btu/hr)Q = sensible heat transferred (Btu/hr)
CFM = quantity of air (ft3/min)GPM = quantity of water (gallons/min)
T= dry bulb temperature difference (°F)T= dry bulb temperature difference (°F)


The only thing I will say about the Delta-T side is that it is the measurement of dry-bulb temperature, this is something I think all technicians know and have a decent grasp on.


Like most things in our industry these are “rules of thumb” equations, however, both derive from the same lower level equation, which is as follows:


Q = M * C * T



Q = sensible heat transferred (Btu/hr)

M = mass of the fluid (lb/ft3)

C = specific heat of the fluid (Btu/lb)

T= dry bulb temperature difference (°F)


I’ve often heard Bryan say that air-conditioning is about moving pounds of refrigerant.   We move pounds of refrigerant to create air-conditioning, and we move pounds of air to deliver that air-conditioning to the space.  As you may have gathered from the above equation the Sensible Heat Rate is derived from moving pounds of a substance (in our case air or water).


I’m not going to dive into the details of the above equation at this time, but wanted to share where both of these equations stem from. What I’d like to breakdown in this article is a deeper understanding of where the 1.08 (air) and 500 (water) constants come from.   Both CFM and GPM are actually what provides the “pounds” of the fluid (air is a fluid), based on density and specific heat.


Density is defined as its mass per unit of volume (or weight per unit of volume), and specific heat is the rate at which an object will give off or absorb thermal energy.  Both density and specific heat are moving targets, but in the “rule of thumb” below are the values that are used.


Density (lb/ft3)Specific Heat (Btu/lb)
Air @ 70°F & at sea-level (14.7 psia)0.0750.24
Water @ 60°F62.371.0


I’m going to solve for the water-side first.  We have to take into account that our measurement for water is Gallons Per Minute, so for anyone who doesn’t know, there are approximately 7.48 gallons in 1 cubic foot.  Using the density from the table above we can solve for the weight of one gallon of water 62.37 / 7.48 = 8.34 lb @ 60°F.  Since our end result of the Sensible Heat Rate equation is Btu/hour we have to convert our GPM -> GPH (gallons per hour).  So, our 500 is a simplification of the following values:


Constant = 8.34 (lbs/gal) * 60 (min) * 1 (specific heat) = 500


When we provide the GPM in the Sensible Heat Rate equation for water, we have already accounted for its density (mass), specific heat, and converted to gallons per hour.


Next, let’s look at the air-side.  Here we have to account (just like in the water-side), that our measurement is in Cubic Feet per Minute, and since we are solving for Btu/hour we will have to convert CFM – > CFH (cubic feet per hour), we also have to use the density to account for the mass of air that we are moving, and the specific heat.  So, our 1.08 is a simplification of the following values:


Constant = .075 (density [ lbs/ft3]) * 60 (min) * 0.24 (specific heat) = 1.08


So, just like the water side, when we provide CFM to the Sensible Heat Rate equation, we have already accounted for its density (mass), specific heat, and converted to cubic feet per hour.


I hope I haven’t utterly confused you on such a technical topic, but stay tuned for more in the series to help bring the Sensible Heat Rate equation (and the air / water side) closer together.

— Michael Housh

This tip is written by 19 year service tech Frank Mashione. Thanks Frank.

Here is a tech tip from the field that had stumped me. Lucky I knew the right person to call to get back on track.

This business has complained about heat calls for quit a while. I have heard other techs mention it in passing. I ran into our installation crew there about a month ago on a different job. Yesterday was my turn to take a crack at it.

They had called a different company day before and they said it was working fine. When I arrived checked thermostat set at 73 and 65 in store, so I checked another thermostat set at 73 and 70 in the store.

I Got up on the roof found both units locked out on high limit, which is 4 blinks for these Trane gas packs. I found it odd that the units were locked out on high temperature but the indoor fan was not running or inducer. I cycled power and the unit restarted. Next I checked gas pressure, it was on the high end of manufacturers specs so I adjusted it back just a bit.

In the back of my mind, I thought I had it. I checked static pressure found it at 0.6″ wc which isn’t too bad especially for the area I work in which I regularly see static off the charts.

The temperature rise was 55° which was acceptable but the return temperature was reading high. I put my wireless air probe in the location of the high limit. After some run time, the temperature in the high limit was approaching the trip point of the limit. This made it clear that the limit was doing its job by tripping.

The unit would run about ten minutes then trip on limit. After four trips it would lock out for an hour.

This got me looking again at the return temperature and I realized that it was much higher than the indoor temp.

The cause of the problem was the supply was installed too close to the return making return temperature high tripping out high limit. After nineteen years in the business still finding new things to learn.

Frank Mashione service technician

We’ve all been new at one time or another so there is no need to get all judgy about some of the mistakes new techs make just because they are inexperienced.


These are some very preventable mistakes that occur due to simple oversight and carelessness that need to happen 0% of the time.

Caps and Seals

Leaving caps off is never OK. While it’s true that Schrader valves and back seating service valves “should” seal completely and shouldn’t be left leaking it is always possible that a little leakage can happen. Besides, keeping bugs and dirt out of the ports is reason enough to keep the caps on.

Bill Johnson (co-author of RACT) made a really good point on a recent podcast. When a system is apparently low (which you can verify through non-invasive temperature tests) you shouldn’t just pull off the caps and attach the gauges. First, look for oil at the ports and leak check them to eliminate port leaks as a possible cause. Once you remove the caps and attach your manifold you won’t be able to know if the ports were a leak point or not.

Every time I remove caps I look inside them to make sure they are in place unless it is a flare hex cap that doesn’t require a seal.

It’s a good practice to keep all caps and screws together and in the same place on every call. This helps to ensure they don’t get accidentally knocked into the dirt, lost or forgotten.  Put those caps back on, finger tight for caps with seals and snugged up with a wrench for hex flare caps (Trane residential units for example).

Leaving Disconnects Out / Off

Obviously, nobody TRIES to forget the disconnect but it still happens all the time and it’s almost always because the tech gets in a hurry or distracted and usually both, and it can be eliminated easily by some best practices.

Most often the disconnect is left off or out during maintenance or during very simple repairs. This is because the tech will often run test the equipment, then perform the maintenance or minor repair and leave without run testing again. This order of test first then clean / repair isn’t my favorite for several reasons will silly mistakes being one of them.

I advocate for performing the comprehensive run test at the very end of a repair or maintenance meaning you are observing the system running right before you leave with the last action being resetting the thermostat or controls back to the desired setpoint. When you run test last you don’t forget silly things that prevent the system from running.

Always do a final walk of the job before leaving and check disconnects, setpoints, cleanup and check for tools.

Making Poor Electrical Connections 

I see it all the time. Capacitors tested and the spade connections left loose, contactor lugs not properly torqued, stranded wires with some of the strands cut off to make the wire fit, crimp connections on solid wire…. the list goes on and on. Here are the top mistakes to avoid.

  • When forcing on a female spade (on a capacitor for example) it should be very snug. If it is loose at all, pull it off and pinch down the spade sides a bit to ensure it’s a snug fit
  • When making a crimp connection only do so on a stranded wire and use an appropriately sized connector. Position the jaws so that the indent crimp is made on the side of the connector OPPOSITE the split in the barrel. Even better is to use a crimper specifically designed for insulated terminals that compresses the entire barrel.
  • Never cut strands of wire to make a conductor fit under a lug. Use the proper connection (termination) type for the conductor.
  • Never leave exposed wire, strip back insulation only to the length required to make the connection and no more.
  • Don’t leave connections under tension. Use straps and zip ties to keep tension away from connections so that they aren’t left under a pulling/disconnecting force.
  • Make appropriate connections for the job, never leave connections open to the environment unless they are rated for it.

When making any electrical connection always pull the connection to make sure it is a snug fit before walking away.

Failing to See the Obvious 

So much is made of good workmanship (how things look) and diagnosis (figuring out what’s wrong) and rightfully so. However, for a new tech, nobody expects you to do the best looking work out there or to diagnosis the super difficult situation. You are expected to use common sense and spot things that are out of the ordinary or that can lead to issues. Here is a quick list of things to look out for that you can see with little to no experience.

  • Look for refrigerant oil stains, often oil stains or residue can lead you straight a refrigerant leak.
  • Use a mirror and a flashlight and look for dirty evaporator coils and blower wheels. You may make a diagnosis but if you leave the system with a dirty coil or a blower wheel you still look silly.
  • Check the air filter and let the customer know you checked it. A home or business owner may not know much about HVAC but they know what an air filter is and reporting the condition back helps give them confidence.
  • Watch for rub outs on copper lines, feeder tubes, external equalizers and sensing bulbs and wires. You can often find or prevent a problem just by looking for areas of contact between tubes and/or wires.
  • Inspect control wiring for cuts or UV damage outside. If the weedwhacker doesn’t get the wire often the sun will.
  • Look for past workmanship that may be done incorrectly. Just because that fan motor or capacitor is new doesn’t mean it is the right size and wired properly. Always double check your own work as well as work done by others.
  • Before making a repair double check the previous diagnosis and check that the part you have is actually the correct part. There is NOTHING worse than removing a compressor t find out the one you have isn’t the correct one. ALWAYS double check the diagnosis and the part.

There are many other things that could be added to the list, but for a new tech if you do the following you will be on the road to success even if you are green.

  • Read product manuals and never stop learning
  • Listen carefully to senior techs and ask lots of questions
  • Help other techs when they are in a pinch
  • Smile and treat customers with respect
  • Compete with yourself to do each job better than the last
  • Walk  every job before you leave to make sure everything is buttoned up (Screws, caps, disconnects)
  • Ask every customer is you have done everything to their satisfaction and if there is anything you can improve.
  • Do all the little things with exceptional detail. Cleaning drains, washing condensers etc… always do it with a level of detail that exceeds your peers and you will build a reputation for excellence.

If you do these things your co-workers, customers, and managers will generally overlook the mistakes you make just because you are green.

— Bryan

Michael Housh from Housh Home Energy in Ohio wrote this tip to help techs determine the air side charge on a pressure tank. Thanks, Michael!

Determining the air-side charge of an expansion tank in a hydronic heating system is a relatively easy task.  A properly sized and charged tank is designed to keep the system pressure about 5.0 psi lower than the pressure relief while the system is at maximum operating temperature.


The proper air-side charge is equal to the static pressure of the fluid at the inlet of the tank plus an additional 5.0 psi allowance for the pressure in the top of the system.  The air-side of the tank should be checked and adjusted before adding water to the system, if the tank is already installed and the system has pressure in it, the pressure should be drained at the tank to 0 psi before testing the pressure on the tank.


The formula for calculating the air-side pressure is relatively easy and directly related to the highest point in the system from the inlet of the expansion tank.


Pa = H * (Dc / 144) + 5


Pa = air-side pressure in the expansion tank (psi)

H = height from the inlet of the tank to the highest point in the system (ft)

Dc = density of water at its coldest state / typically filling (lb/ft3)


The above graph shows us the relationship between density of water and temperature between 50°F – 250°F.


A lot of the “rule of thumb” equations for hydronic systems are based on the density of water @ 60°F is 62.37, so we could simplify the above equation into a rule of thumb equation by first solving for the density (Dc).


Dc = 62.37 / 144 =0.433


Substituting ‘Dc’ into the original equation would give us a slightly less complicated equation that can be used as a rule of thumb to solve for the air-side pressure.


Pa = H * 0.433 + 5


Below is a graph that shows us this rule of thumb equation and the required air-side pressure based on the height of the system piping.

— Michel H.

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)


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

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.

Just something for the wet-heads.

— Brian M.

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