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Review my calcs for heat loss from pipes, w/ + w/o a recirculating system?

michaelbluejay | Posted in Energy Efficiency and Durability on

I’d like to request a check of my math for heat loss from hot water supply lines, with and without a recirculating system.

(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.

(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 BuildItSolar , 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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  1. GBA Editor
    Martin Holladay | | #1

    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. If you possibly can, you want to re-route your 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.

  2. Jon_R | | #2

    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).

  3. michaelbluejay | | #3

    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?

    1. michaelbluejay | | #4

      Correction: Martin said to run the pipes through the building's "thermal envelope", which somehow I misread as its "conditioned space", probably b/c I'm conditioned (no pun intended) (okay, maybe a little) to thinking about how it's important to run ducts through conditioned space rather than an unconditioned attic. I see now that the "thermal envelope" probably includes the unconditioned attic, so I plan to run the pipes there.

      In Austin, the attic temp. is above the ground temp. 99% of the time, plus I'd insulate the pipes and then they'd be buried under a ton of cellulose. The trick will be trying to get all the joints in the walls and not the attic, to protect against leaks in the attic.

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