Posted By tinoue on 25 Aug 2010 07:54 AM
I think many people forget that there's multiple factors,
and heat transfer rate is just one. Ground heat capacity
is also very important.
'Zactly!
If my calculations are correct, the BTU transfer rate across
the walls of SDR-11 HDPE loop pipe works out to about:
7.1 BTU/hr-LinearFootOfPipe-°F (independent of diameter)
At 40°F EWT, my 3-ton WF Envision would like to extract
17k or 24k BTU/hr (stage1 or stage2, respectively) from
my 1000 feet of loop. That's 17 to 24 BTU per foot of pipe,
for an average delta-T across the pipe wall of 2.3°F in
stage1 or 3.4°F in stage2.
Thus, switching to (an equal length of) copper would have
raised my EWTs by 2.3°F to 3.4°F -- and improved COPs
by 0.1 to 0.2 (i.e., from the mid-4's to ...uh, the mid-4's).
Bottom line: Pipe conductivity is a gigantic non-issue
in my system. Using copper in place of HDPE would
have saved me a few pennies per month.
...ymmv,
Looby