Review my calcs for heat loss from pipes, w/ + w/o a recirculating system?
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 (âˆ†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.
(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 http://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/âˆ†°F/g x âˆ†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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