Determining Rate of Heat Transfer in Hydronic Systems

A friend of mine recently asked me how I made all the heat transfer calculations on the new FOM heating system. He’s also in the HVAC business but does forced air and geothermal mostly, and has not dealt much with hydronic heating systems. We’ve both seen a number of “hack jobs” where outdoor wood boilers have been lashed up to existing forced air or new radiant systems. I asked one homeowner who was having some problems with his new radiant system to show me the heat loss and heat flow calculations for the job. The ensuing blank stare told me this was not going to be a fun visit. With all the computer programs out there these days that simply plug and chug and give you numbers close enough to work with, there’s no excuse for not doing things properly. But I digress, we are supposed to be talking about heat transfer!

Heat transfer calculations in hydronic systems are dead easy, so there’s no excuse for not running the numbers. In the USA we still use BTU for our units for heat. Recall that 1 BTU is equal to the amount of heat that causes a temperature rise of 1oF of 1 pound of water. Let’s say we have something like a geothermal system that uses a ground loop. A fluid, usually water or water mixed with glycol to prevent freezing, is circulated through the ground loop outside and a heat exchanger inside, either transferring heat from the refrigerant circuit (summer cooling) or transferring heat from the ground to the refrigerant circuit (winter heating).

  1. A geothermal system is operating at steady state in cooling mode. If the flow rate in the ground loop is equal to 10 gallons per minute, the temperature of the water entering the loop from the heat exchanger is 110oF and the water returning from the ground loop is 90oF, what is the amount of heat, in BTU per hour, that is being conducted into the ground?

Answer:

heat transfer(BTU) = ΔT * Flow rate(gpm) * 8.3 * 60, where ΔT is the temperature drop around the ground loop in degrees F from input to output.

We simply multiply ΔT times the flow rate times weight of 1 gallon of water times 60 minutes. We have to multiply by 60 to rationalize our units since flow is in gpm but we want to know the number of BTU’s per hour. Since 8.3 * 60 = 498 it’s common to write the heat transfer equation simply as ΔT times flow times 500, to make it easier to do ‘rule of thumb’ calculations in one’s head. So the answer to our question is then:

20 * 10 * 500 = 100,000 BTU per hour.

So remember, it’s simply delta T times flow times 500. Not so hard, is it?

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