How do you calculate the amount of heat that is lost through un-insulated hot water pipes?

Question

Ultimately, I would like to calculate the amount of money wasted by not isulating the pipes. I know the diameter and length of the pipes, the initial temperature of the water in the pipes and the ambient temperature of the space through which the pipes pass.

Answer 1

I remember a rule of thumb that said the heat loss was a milliwatt per square centimetre per Celsius degree of temperature difference. Wonder how it compares with 2 BTU per square foot per Fahrenheit degree?

If you don't insulate the pipes, and the heat escapes into the house, you will need either less central heating (good) or more air conditioning (bad). It depends where you live.

Answer 2

I like all the equations and stuff, but I think that I have a simpler way to measure the heat loss.

Run your hot water at two different flow rates and measure the temperature at the faucet. You will have to measure the flow rate by timing the filling of a known amount - say a 1 gallon container, then measure the temperature of the water coming out. Now run the water at a different flow rate, measure the flow rate and temperature. From the temperature difference and flow rate difference you can calculate the approximate heat loss in your pipes to that faucet as follows:

Let F1 and F2 be the two flow rates in milliliters/second ( you do the conversion) and the two temperatures T1 and T2 in degrees centigrade. Assume that the Total Heat loss is a constant H in watts. Not really true but close enough.

Also let S be the Source Temperature which is assumed to be the same in both cases.

So now you end up with two equations ( 4.186 converts from calories to joules so we can get joules/second or watts)

( S - C1) * F1 * 4.186 = H and
( S - C2) * F2 * 4.186 = H

Solve these two equations for H and you will have an approximation for the amount of heat lost in your pipes.

Answer 3

I don't think so -- the foam should provide the insulation barrier you need -- I'm just wondering how you managed to jam the insulation in between the pipe and the joist if they were up against each other?Another solution would have been to simply slip some aluminum tape (it's like flashing) in between with the sticky side of the tape on the wood -- that way the pipe would be against the aluminum to make a heat barrier. Still, I can't see where a hot water pipe would ever get hot enough against the wood to cause a fire -- typical hot water that runs in residential buildings is simply not hot enough to cause a fire...

Answer 4

If you're a homeowner or business owner, just insulate the pipes! The price of fuel isn't going to get any lower, you'll protect the pipes from freezing and your building from the possibility of water damage from frozen pipes, and you'll improve the resale value of your building.

If you're an engineering student, you need to approach this by looking for the proper heat transfer coefficient between the pipe and its surroundings. Since you didn't mention airspeed around the pipe, it will be a free convection heat transfer coefficient h, which you'll get from a correlation relating the Nusselt number (Nu = h*L/k, a dimensionless form of the heat transfer coefficient) to the Grashof number (the dimensionless number for intensity of free convection, too complicated to put here) Also, you didn't mention whether the pipe was horizontal or vertical, as the correlations for these are different.

Once you work out the heat transfer coefficient h, you can calculate the initial heat loss per unit length of pipe Q/L = ha(Tp_initial - Tambient) where a is the pipe surface area per unit length, Tp_initial is the initial temperature of the pipe, and Tambient is the temperature of the surroundings. Use this worst case estimate of heat loss per unit length, along with the known length of pipe, and the known flowrate of water through the pipe, in an energy balance calculation to estimate the the worst-case final temperature of the water leaving the uninsulated section of the pipe. This calculation will tell you whether you can use the initial per-unit-length heat loss for the entire pipe (if Tpipe stays reasonably constant end to end) or whether you'll have to account for the variation in pipe temperature along the length of the pipe. (if Tpipe drops more than, umm, 10-20% of the way to Tambient)

Besides your "Heat Transfer", "Transport Phenomena" or "Thermal and Fluid Science" text, a good place to look for heat transfer correlations and how to use them is Perry's Handbook of Chemical Engineering. You can find the thermal properties of air there too (needed for that Grashof number).

I hope that's enough to get you started.

Answer 5

A good rule of thumb for heat loss from piping is about 2 BTU/ degree F per sqft of pipe.

Calculate surface area of pipe x 2 x temperature difference between water in pipe and ambient air. Not a great way to do it but it will get you close enough for what you're doing.

Heat loss from insulated pipes is a little tougher.

Answer 6

Consideing the heat loss is due to conduction and through the walls of pipe to the surroudings.

Q = - k A (T2-T1) where,

A = 3.142*ø of pipe *Length of pipe,

k = Thermal conductivity of pipe material in W/m² K

T2 = Temp of fluid inside pipe &

T1 = Ambient temperature

Answer 7

confusing issue. look in google. this can assist!