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Ground source heat pump loop ground temperature

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Posted by Astro on December 28, 2004, 2:15 pm
 
A few weeks ago, I had a GSHP system installed. Along with the loops, I  
installed temperature sensors so that I could monitor the temperature of  
the ground at the loops. In this way, I figured that I could monitor  
system efficiency during the course of the seasons. This is a four ton,  
direct expansion system with four, 100ft vertical loops. The ground here  
is solid granite for hundreds of feet down and around. Little to no water  
movement.

The measurements I'm making are as follows -

Run the system normally for some period, i.e. a couple of days of  
operation. While the system is running, I'm monitoring the vent, room, and  
outside temperatures throughout the house. I'm also monitoring the  
temperature of the refrigerant lines to/from the compressor unit and have  
measured the current draw at the breaker panel.

Next, I turn the system off for a period, typically the better portion of  
a day. During this period, I monitor the loop temperature sensors to see  
ground temperature recovery.

Finally, I have a monitor buried at roughly the same depth (these are all  
vertical loops) so as to provide a ground control temperature.

The results I'm getting are illuminating (and disappointing!)

Control ground temperature started at 51F on 12/8/04. It is now 49.9F on  
12/28/04.
Total system run-time during this period has been 150 hours.

The ground around the loops is freezing within <24 hours of system  
operation. Along with the loop cooling goes the vent temperatures. When  
the system is fresh (i.e. loop temperature recovered to the mid 40's), the  
best vent temperature is around 93F. (My ductwork is tight and well  
insulated). Maximum vent temperature is within 2F of the temperature  
measured right at the air handler.

Ground loop recovery is getting progressively worse as the usage  
increases. I assume this is because the ground around the loops is getting  
colder. Currently, it takes about 6 hours for the loops to "unfreeze" from  
about 30F to 32+F. Then, the loops rise at 1.1F/hour until they reach 38F.  
Then they slowly rise a couple degrees above that. Measurements just a  
week ago showed loops rising up to a about 42F before leveling off to  
about 45F. My suspicion is that within a couple more weeks of use the  
loops won't even rise above freezing.

My computed BTU output for the system runs from 32000BTUs/hr to  
42000BTUs/hr. This system is rated at 4 tons, and I was told that it  
typically puts out closer to 5 tons at the start of the season and it  
drops to about 40000BTU by the end of the season as the ground cools.

The system was "professionally installed". The installer came highly  
recommended by the manufacturer.

On the initial install, the loops were backfilled incorrectly resulting in  
the tubing making minimal contact with the surrounding ground. These  
measurments are from after I corrected this problem.

The system was supposed to have six loops, 100ft but the drill rig broke  
during installation. However, the manufacturer states that four, 100ft  
loops should be sufficient.

The loops were supposed to be installed at least 10ft apart but they are  
6ft apart.

After doing many hours of research, I feel that there are several issues  
here.

First, the thermal diffusivity of the ground is such that much more ground  
loop is needed to balance the thermal load being placed on it. I have  
typically read that about 250ft/ton is required for conventional gshp  
systems and under good conditions, this can drop to 200ft/ton for a direct  
expansion sysetm. so from the getgo, I'm at least 50% of the desired loop  
size.

Second, the loops are too close together. At 6ft apart, the BTU capacity  
of the surrounding ground is seriously compromised. Most of what I've read  
suggests that 225 sq. ft/loop is required for long term ground temperature  
stability. This is running at 16% of this.
The manufacturer stated that even 8ft loop separation is within spec.

Ok, now besides telling me that I've been ripped off, I'm interested in  
other peoples experiences with GSHPs. Has anybody besides me taken the  
time to actually quantify the performance of their GSHP system? I've read  
everything on the standard GSHP sites and none of what I've seen actually  
shows what expected ground loop temperature is in the immediate vicinity  
of the loops. I've read about the overall ground temperature of the loop  
field, but haven't encountered any literature discussing what I'm  
measuring.

I've also seen measurements of water loop systems, where the water  
temperature to/from the ground loop is measured. This would be a good  
indication of the loop field temperature. Anybody have numbers from their  
systems?


I am now investigating remediation for the system. I am considering two  
options:
1) Install 6 additional vertical loops, spaced 15ft apart from one  
another. this would give the required 225 sq. ft/loop and would increase  
the system to 250ft/ton.

2) go to a horizontal loop field since drilling here is so damned hard and  
expensive.

Any suggestions on an effective, long term remediation would be greatly  
appreciated. Spare me the snide remarks about installing a gas burner and  
throwing away the GSHP. I actually have a working baseboard hot water  
system that this system is supposed to replace. Now I'm alternating  
between the systems to save oil.


Posted by Tim Keating on December 28, 2004, 2:53 pm
 


snip..


A couple of items to think about..

   1. You may have to wait a season to get full benefit..
  During the summer the GSHP will build up the stored heat, which you
can then tap in the winter.

   2. If this is a closed loop system.. Try adding some tap water to
each well casing.  Water is excellent conductor of heat.

   3. Consider adding some solar hot water panels to your setup to
boost well temps in winter.   Probably a lot cheaper than drilling
additional wells.   Exact configuration depends on your site..   A
self draining system which dumps hot water into the well casings has
the advantage of simplicity.      

Posted by daestrom on December 28, 2004, 4:53 pm
 

Sorry to hear of your troubles, I have been looking forward to installing a
GSHP here in update NY.  But your experience is giving me second thoughts.
In my area, there is a lot less granite, and a lot more ground water, so
maybe your experience is unique to your situation?  Anyway, I'm planning a
horizontal loop system.

I'm curious, how are you measuring the ground loop temperatures when the
system is off?  Are these temperatures near the surface or deep within the
'wells'?  With 100 ft deep wells, I would have expected the bottom to be
much warmer than the surface.

daestrom



Posted by Astro on December 29, 2004, 1:21 am
 (snipped out original long posting)


My guess is that in an area with plenty of water movement, the problem  
will be greatly reduced as water in motion is great at carrying heat. My  
bores are pure granite. I live on a "mountain" which is a local diabase  
intrusion. I have two wells on the property. The backup well which the  
previous owner drilled in 1990 is 950 ft. deep and has a recovery of  
1.25gpm!

As for the measurements - I embedded temperature sensors in epoxy then  
lowered these 50ft into the boreholes along with the loops.

I believe that you have to go considerably deeper to start seeing warming.  
I have read that the geothermal gradient is on the order of 75F/mile. When  
the loops were first installed, the baseline measurements showed a  
temperature of 51F at all loops.

After all the research I've done now, I would suggest that anybody  
considering a geothermal system pay for a thermal diffusivity analysis to  
see what the thermal gradient will be around the loops during actual use.  
For example, I would like to see the thermal gradient based on a 100%  
runtime of a 50k BTU load. Can the heat from the surrounding strata come  
in as fast as it's being pumped out? How fast would it come in? Where's  
the break point - i.e., if you run the system X% of the time, it will be  
pumping out the same amount of heat as can diffuse back in. I think that  
is the magic number for any loop configuration. Without this analysis, I  
wouldn't trust any GSHP installation.


As for Tim's comments:


That will certainly help some. However the balance here is definitely  
weighted towards heating degree days, so my guess is that, if I'm lucky,  
I'll get really good Air Conditioning performance early in the season,  
then as the ground warms up, it will warm back to somewhat above the  
baseline temperature. The problem I see is that the current loop field is  
so small that in a matter of a couple weeks of use, the heating system  
will draw out all the useful heat. But time will tell. Thanks for the  
thought.


Another good idea. Actually, since they botched the backfill on  
installation, I did that myself using hot water (we have a direct hot  
water hose connection), so the bores got filled with 120+ water. In  
practice, the heat in the water was sucked out PDQ by the ground thermal  
mass.


Funny you should mention that one. I was running an analysis on that  
option as well and may yet go that route. The problem, and please correct  
me if I'm wrong, my references show about 25,000BTU/day per panel is  
pretty good. So if I put 8 panels on my roof, then I'd be pumping in, at  
best, about 200,000 BTUs/day into the system. Now, when the system is  
running to spec, it should be pumping out about 50,000 BTUs/hr. Some of  
this will come directly from the compressor power, and the rest from the  
ground. So I was figuring on some 35,000BTU/hr being pumped out. During my  
test runs, I found the system to be running between 16+ hours/day at 15F.  
So I'd still be pumping out 16*35000 = 560,000BTU/day which is  
360,000BTU/day more than I'd get from the solar panels.

I still think solar could be a good addition as that would really decrease  
the thermal load on the wells.

One other option - if I install solar, then would it not be more efficient  
to just add radiant heating wherever I can to my house? Then I'd be  
pumping the heat right to where it's being used. as such, all those BTUs  
would go into the house and reduce the load on the ground thermal mass.

Lots of thoughts. Please keep em coming.

Thanks!

Posted by Sylvan Butler on January 6, 2005, 11:02 pm
 
Interesting idea.  Is that a fairly regular test or analysis?  Eg, for
well drilling they can test flow/recovery to know if it is sufficient.
I wonder if they could do something similar for a GSHP...

A neighbor has a GSHP and really likes it.  However his is open loop,
and the outflow maintains a nice pond (1/2acre?).


The fact that the ground sucked the heat out of the water so quickly is
a good sign for summer heat storage.  After all, that heat did not just
disappear, it was dispersed via conduction.


Probably.

One interesting point with solar though, is that lower operating
temperature is more efficient/effective at harvesting and transporting
that solar energy.  For radiant heating, you probably want water at
least 100F at the panel output.  However to dump heat into the ground
you could probably get significant benefit at only 80F or even colder
output temperatures.

In the summer you could easily get temperatures high enough to provide
all your domestic hot water needs.  That may even be a better design
starting point -- add solar water heating sufficient to provide all
domestic needs, and if it works out well, add capacity for space
heating.  Lower initial investment for a trial.

sdb
--
Wanted:  Omnibook 800 & accessories, cheap, working or not
sdbuse1 on mailhost bigfoot.com

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