What should I expect from this Heat Pump?

[Fathompin]

I'm experimenting with a geothermal heat pump system as a DIY project. I believe my unit may have suffered compressor efficiency issues. I am wondering just what efficiency I should expect. I’m new to this forum; it showed up in a Google search and seemed appropriate. The story follows if you are interested:

I bought a new-old-stock, 9 year old, R22, 3.5 ton geothermal heat pump, for my 1.8K sq ft home in SW Ohio. I hope to run open loop in my fairly remote neighborhood with excellent ground water supply. I set it up without checking R22 temps / pressures as the manual indicated that was not necessary to just set up the system. I was getting about 107 degrees (max) out of my VENTS and running 8 gpm, 57 degree water through the water coax (very happy camper at this point!).

However, one day the room air vent temperature abruptly dropped 10 degrees after only about a couple of hours of light usage over a couple weeks in early autumn. At that point I figured something went wrong and I’d better check out the R22 pressures and temperatures. Now I kick myself for not doing this thorough testing at the very start so that I had some starting parameters to compare to; part of the reason I didn’t do this was I had to go to my dad’s house since he had the test equipment. I’m a novice learning.

The installation manual had recommended values that I was just barely hitting (highest values for suction and lowest values for discharge pressures). I added refrigerant just in case there was a leak, then recovered it when I saw I could not make much difference to lower the suction or raise the discharge pressures. I gathered at that point the compressor was not working as well as before. I am unsure why this may have occurred even after calling the manufacturer service tech; he did not offer much of an opinion after discussing the examples below, saying the 30 rise in air temperature I am still getting was within spec (this is a capillary system, not TXV which was his first thought). I am thus posting here, wondering about the difference that occurred.

For example, the manual reads typical is 60-70 suction pressure, 10-18 superheat, 205-230 discharge pressure, 1-5 subcooling, 12-14 water temperature drop, 23-29 air temp rise.

I removed refrigerant (after adding) to get it down from above 70 to 69 psi suction, 7 superheat, 200 psi discharge, 10 subcooling, but this raised the temperature of the compressor discharge tube to around 200 degrees (I consider this the hairy edge for compressor operation so I had to stop my efforts to reduce the discharge pressure at 69 psi). My goal was to drop the suction pressure to get more superheat as my water temperature is 57 degrees, thus 10 degrees water temperature drop (47 degree) and 10 superheat (37 degrees) means I’d like to be at the lower side of 60 psi (35 degree refrigerant saturation temperature) instead of ~70 psi (42 degree saturation) where I am now.
So let’s say my unit was working that first couple of hours when I first set it up at midpoint recommended values, 65 psi suction, 225 psi discharge, then my superheat would be about perfect and I could drop my flow to get about 10 degree water temp decrease, my discharge saturation temperature would be 110 degrees and with 3 degrees subcooling I estimate my vent air temperature would be about 107 degrees, and this appears to be right where I started. Yet factory tech says 97 degree at air vent is all I can expect and something wasn't right initially, but how can that be? I can't help but think my compressor is now less efficient than that first few weeks, though its still somewhat effective, and I kind of want that initial 107 degrees back, as it sure heated things up faster.

[superlen]

I would start by verifying the basics. Make sure you *still* have 8gpm of 57deg water flowing in, and measure the temp leaving as well so you can calculate total heat removed from water.


Len

[Fathompin]

Originally Posted by superlen

I would start by verifying the basics. Make sure you *still* have 8gpm of 57deg water flowing in, and measure the temp leaving as well so you can calculate total heat removed from water.


Len

Yes, I check this often. I have since dropped the flow to a little less than 6 gpm and I am now seeing a seven degree water temperature drop. Using that lower flow and ground water temp at 57 degree I am operating on the edge with only around 5 superheat, this is something I don't like, but the extra 2 gpm flow did little to buy me more superheat and only made my heat/water extraction much worse (just a few degrees water temperature drop), so using higher ground water flow seemed like a bad trade off considering nothing I do for water flow changes the 30 degree delta air flow exchange, it only changes my superheat.

[jeff5may]

I don't know what model of water you are using, but normal water will vary much less than the numbers you posted in your intro. R22 has a somewhat low heat capacity, much lower than that of water. Since you are running an air handler on the indoor side, this can be your gauge of heat transfer. It is a temperature drop vs. flow rate vs. temperature split function that can be optimized PDQ.

If you have a stable supply of groundwater that doesn't wander in temperature, your cap-tube based heat pump will pretty much run the same way all the time. However, if the groundwater wanders up and down in temperature (even a few degrees), your system balance (and "hot air output") point will be utterly dependent on this seemingly minor change. It may not be trivial that your heat output decreased as the outdoor (and maybe groundwater) temperature plummeted from fall to winter. Reducing the flow rate will amplify this change.

Mind you, I'm taking potshots here. Not enough information to make an accurate assessment by any means.