Hydronics GPM Calculation and More…

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


The podcast on delta T for A/C the other day got me thinking about the formula I learned in school about calculating the GPM of a hydronic system. 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. Then, multiply your rated input 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 0.85 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 0.85 (your nozzle size), and you get 119,000 BTU/hr input.

The input would be 119,000 x 0.80 efficiency = 95,200.

500 – a constant that stands for a pound of water times 60 minutes – 8.33 x 60 = 499.8 (As you can see, we fudged the number a bit.)

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

To find a system’s gallons per hour:

BTU/ (500 x TD)
100,000/(500 x 20)
100,000/10,000= 10 GPH

That's 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. Using the data from the last calculation, you solve for the 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. For example, there could be mud in the pipe insulating it.

Just something for the wet-heads.

—Brian M.

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One response to “Hydronics GPM Calculation and More…”

  1. Brian,

    A few adjustments that could be made to this: 1) Oil pump pressure can affect the combustion input rate. I think 135 psi is standard bit it’s important for a tech to know if conclusions are to be drawn. 2) I think here you meant “Output” is 119kbtu input rate x 80% combustion efficiency =92.5kbtu. Perhaps specify that output is the useful heat (mostly) transferring into the water. 3) 500 is the weight of 1 gallon of water x 60 minutes. Maybe defining the relationship of what a BTU is could help in completing the thought 4) Why not carry the example through by using the output calculated from the oil boiler instead of switching numbers mid article? Probably would make the process more clear. One might wonder where 100,000 btu number came from when calculating the gpm flow 5) The net output into the water can be used to get overall system flow but testing at an individual room or baseboard only works if all the flow goes through that room. That’s probably an important distinction to make.

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