As a geologist I learned many years ago that groundwater is 50-55F in the NE (approximately). In fact I have a well and yesterday I pumped water and checked; it was 53F (air temp has been in the teens and lower recently). EWT is 35F on my thermostat. If you view most videos online the GW temperature is usually said to be 55F so when folks buy a system they expect that to be the temp of the EWT. NOT SO! Installers should make sure customers know that there is an incomplete transfer of heat, and you can't expect the EWT to be the same as GW; even if that's what manufactures say it is. Now my question: what is a reasonable difference between GW and EWT? I know there are many factors, i.e., GPM pumped, borehole depth, loop length, etc. but what is needed to get an efficient system? Is a 15 degree difference between GW and EWT to be expected?

"Is a 15 degree difference between GW and EWT to be expected?" A loop that is designed to be both cost efficient to install and run will swing above and below "GW" temps 30° +. It is too bad that your installer didn't educate you on this. Most ground loops will get down around 30° under heavy prolonged loads. This is nothing to be alarmed about. Heat Pumps are still very efficient at EWT's in the 30's.

This is assuming you have a closed loop system

Along with the heat transfer efficiency aspect, the ground water temp and the temp of the ground surrounding a closed loop have very little to do with each other

this is among the geo wives tales or i should say poorly expressed points. The "undisturbed ground temperature may be 55ish, but once a loop is there, it is no longer "undisturbed" is it?
As Dewayne points out, in the cost V benefit conversation on closed loops, allowing the temperature to drop in deep winter makes the most economic sense.

RE open loop, EWTs do drop in deep winter but not as much as closed loop systems.

Thanks for the education. I'm still learning about my relatively new GT system. I have a 3 ton system, closed loop, DSH, 2,000 SF in main part of house. My master suite (not in this calculation) has a standard Air HP, which is very good. I have 3 bore holes, each about 140-150 deep, grouted with bentonite. I appreciate all the knowledge on this forum, and am learning more about these systems (I wish I knew this before installation). With these really cold temps in SE PA, close to zero at night, I wondered about the EWT and LWT, and if the bore holes were sufficient, with EWT around 35F. How does a contractor know the loop is doing what is expected, and whether it's the correct length and efficiency? What compressor temps are expected with a CM TEV 30 system (mine is 160F when in stage 2)? What does LT1 and LT2 tell you? Where can NEWBIE's learn about all these data and systems? Thanks for your help. Hank

Posted By dr_rock on 21 Feb 2015 10:05 AM
How does a contractor know the loop is doing what is expected,
and whether it's the correct length and efficiency?

He measures pressure drop and temperature drop
across the heat pump's internal heat exchanger.

- the pressure drop measurement is converted to
flow rate (GPM), based on tables supplied by the
heat pump manufacturer.

- GPM x (EWT - LWT) x some_constant = BTU/hr

If you EWT is 35° this time of year after the colder than normal winter, I would say that your loop is doing just fine. Some loops are down in the upper 20's

There is another forum where you can learn as well : www.geoexchange.org

Below is an open well system in Buffalo, NY.

As you can see, EWT is a constant 51F.

http://welserver.com/WEL0762/



A closed loop extracts the heat out of a given field.

http://welserver.com/WEL0601/

Sixth graph from the top is the EWT over the whole season. It is around 30.4F right now.

For that reason, Geo heat pumps are rated at 32F EWT for closed loops, and 50F for open loops to account for those differences. The 30F EWT is kind of a standard design most installers shoot for, since it is the best compromise between efficiency and upfront costs. Keep also in mind that 30F is at the end of the heating season, whereas average annual EWT is usually above 40F.

No need to have discussions about that with clients, unless they ask. Most customers don't care too much about the technicalities. They want to know how much it will cost them, how much operational savings they have, and if it will make their home comfy (although the last question I have not heard for a while). The loop dropping down to 30F is accounted for in the operational costs.

Very nice informative charts.

I particularly like Annual Entering Water Temp (LoopTemp) for 601 - shows how the loop should be designed.

And I like HP Stage (Stage) for 601 - shows how the staging is supposed to work.

Nice job!

Best regards,

Bill