Review my calcs for heat loss from pipes, w/ + w/o a recirculating system?
我想request a check of my math for heat loss from hot water supply lines, with and without a recirculating system.
ASSUMPTIONS:
(1) 75’ of run, pipes under slab, 68°F ground temperature, 120°F heater temp (∆52°F).
(2) Two draws per day of the 75’ run (daughter showers in the morning and son in the evening), no other hot water use.
The pipes should lose all the heat between draws according to calcs not shown here.
CONSTANTS:
(1) 27 BTU/hr./ft. of 1/2” pipe for ∆52°F (Engineering Toolbox)
(2) 8.33 BTU/∆°F/g (Rheem)
(3) 0.76 g per 75’ of pipe (HandyMath)
(A)For a constantly-heated pipe, yearly heat loss should be 75’ x 27 BTU/hr x 24h/d x 365.25 d/y = 18M BTU (rounded).
This jibes with a writer at BuildItSolarhttp://www.builditsolar.com/Projects/Conservation/Recirc/RecircEnergy.htm, who said his system used an extra 200g of propane per year. At 91k BTU/g, that’s 200g x 91k BTU/g = 18M BTU (rounded).
(B)With no recirc. system, loss should be 75’ of pipe x 2 draws x 0.76 g/75’ of pipe x 8.33 BTU/∆°F/g x ∆52°F x 24h/d x 365.25 d/y = 6M BTU (rounded).
CONCLUSION:Recirculating system uses horrific amounts of energy, but not the whole amount of BTU loss calculated, because even without the system there’s still significant heat loss from the pipes.
Is this correct? Thanks!
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Replies
Michael,
Jon made two important observations.
I won't comment on your calculations, since Jon's observations point to how tricky and dynamic such calculations will prove to be. I'll just provide some advice.
1.如果你可以,你想要你的改道hot water lines to a new location inside your home's thermal envelope.
2. Once this is done, you want to insulate your hot water lines.
3. You may want to read this article, which has relevant advice:Hot Water Circulation Loops.
Soon the surrounding ground heats up and the heat loss drops. And in the Winter, much of the heat isn't lost, it goes into the house.
Even so, I'd use an intermittent, not continuously circulating system.
24h/d should be removed from B).
Thank you very much to you both. One point of running the numbers would inform whether and how to make a change to my hot water supply. For now I'll assume my calcs in the ballpark, after fixing the 24/d error that Jon found.
It's not practical, affordable, or aesthetically okay to run hot water lines inside the building envelope. So I'm leaning towards running them through a well-insulated attic in Austin (climate zone 2A), which has an average annual temperature of 69°F. My calculations for that show that the pipes should retain their heat well:
* R value = (h x sf x ∆°) / BTU, or BTU x R = h x sf x ∆°, or BTU = (h x sf x ∆°F) / R.
* 1' of 1/2" pipe is 0.13sf of surface area.
* There's 0.01G in 1 ft. of pipe. At 8.33 BTU/∆°F/g & an Austin avg. temp. of 69°F, heat in pipe subject to loss is 8.33x51x0.01=4.25 BTU.
* With R30 insulation (above & below), loss is (1 x 0.13sf x 51°) / R-30 = 0.22 BTU/hr.
* So it would take 4.25 BTU ÷ 0.22 BTU/hr = 19 hours to lose all its heat.
* That compares to the loss of 27 BTU/hr for uninsulated pipe, 27÷0.22=123x faster. 4.25 BTU ÷ 27 BTU/hr = 9 minutes.
Actual loss should be even less since the attic will be warmer than outside air, but I couldn't find any figures for average attic temperatures.
Your thoughts?