Calculating BTU Transfer Using The Water Formula

I thought it might be cool to write about how BTU transfer is calculated and share a little shortcut. There is a quick back of the envelope formula used for calculating the BTU/h values for heating water. The “water formula” says:

Q (Energy) = 500 x  f  x DeltaT (in F)

Q = rate of heat transfer (Btu/hr.)

f= flow rate (gallons per minute or gpm)

DeltaT = temperature change (degrees F)

500 = the “fluid factor” this is based on water as the heat transfer fluid. The fluid factor is obtained by using the weight of a gallon of water (8.33 lbs.) multiplied by the specific heat of the water (1.0) multiplied by 60 (minutes). This comes out to 499.8 when using water.

Simply put, if you know a beginning temperature, an ending temperature, and a flow rate, you can calculate the heat transfer in BTUs.

As mentioned, water has a specific heat of 1.0. But a 30% or 50% aqueous glycol solution has a different value. Of course if you are using glycol you must know the specific heat for the brand of glycol you are using, at the temperature range used, and the % of the mixture. For example, a 30% glycol-water mix would have the specific heat of around .90. All propylene glycol (PG) brands will have similar values; here you can see the chart of specific heat of a cool corn-based glycol. Anyway, in the temperature range used in most domestic water heating applications (100-200F) for 30%  glycol use 450 instead of 500 in the water formula.

Here’s an example, using 100% water for simplicity. Lets say we know the flow rate is 5 GPM and the entering temperature of a system (solar, boiler, heat recovery, whatever) is 120 and the leaving temperature is 125. So, 5 degrees are added at 5 GPM. The BTU of this transfer is 5 x 5 x 500 or 12,500 BTU per hour.

If a 30% glycol solution was used, de-rate it by multiplying using 450 and you would get 11.250 BTU/h.

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