[aprssig] Indoor/outdoor digital thermometers - and Geothermal

Robert Bruninga bruninga at usna.edu
Wed Feb 18 09:42:27 CST 2015

Geothermal balancing...

>> All I needed was a pair of digital readouts with a wired thermister in
a convenient package.

> Check out Parts Express www.parts-express.com Search for 'temperature
display'  (Rick Green, N8BJX)

Here is the one I got at Lowes.  Opened the back, took out the thermistor
conveniently right there in your face, and remoted it with about 3' of
wire so now I have a pair of remote digital thermometers for $8 each.


The value was immediate.  By now KNOWING the input and output temperatures
of the ground water in my geothermal heatpummp and the flow rate, I was
able to adjust the flow rate DOWN until I found a maximum BTU transfer
rate.  The contractor had simply installed the hardware, waved their arms
and said it was done (two years ago)...  But I was suspicious...

It was moving 21 gallons per minute (two water pumps in parallel) with a 4
degree temperature drop or 21*8.3*4 = 697 BTU/min
I then turned off one pump, 16 GPM and the delta T went up to 16*8.3*6 =
800 BTU/min
Closed the valve some to 13 GPM and delta T went up to 13*8.3*8 = 863

I had insisted on full instrumentation on the geothermal heatpump and they
had given me standard temperature/pressure combo meters, but the scale was
mechanical and went from 30F to 250F and so trying to see the small
delt-T's was meaningless.

By the way, my geothermal when first installed was getting 50F ground
water (start of season).  Now (2 yrs later) it is down to 45F (mid season)
of pulling heat out of the ground.  Unfortunately, my system is HEATING
only, so unlike most gepothermal systems that then put all the heat back
into the ground during the summer cooling season, mine has a slower
recovery.  Again, the contractor said the temp would not change, but
again, they really don't do the detail engineering behind their systems
and just install them like they always do... so I had them install 130%
more geothermal wells than their design called for, since I anticipated
the slow heat loss in a heating-only system.

I guess, I need to start using the geothermal for cooling if for no other
reason than to better balance my earth heatisink!  The reason this is not
simple, is because my system drives my hot-water-radiatiors and I have no
air ducts for the cooling application.  Though I have a plan for
installing them in the attic for summer cooling the 2nd floor and let
trickle-down-economics* cool the rest of the house (* the only place where
the term makes any sense!).


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