Hello, I'm new on here and am looking for some advice. I had a closed loop geothermal system installed a year and a half ago with a 40 gallon desuperheater buffer tank that is plumbed into our 50 gallon LP hot water heater. Since the day it was installed whenever the geothermal system is running our domestic hot water will increase in temperature up to 30+ degrees at the various faucets/tub-showers in the house. When the geothermal system is not running, the domestic hot water is where it should be based on where I have our hot water heater set. I have taken temps in both scenarios with the domestic running anywhere between 110-120 degrees when the geo unit is off and upwards of 150 degrees when it is running. My question is: why is the hot water temperature increasing upwards of 30 degrees when the geothermal unit is running? My geothermal contractor believes I have a stuck mixing valve somewhere in the plumbing of the house. I'm confused by that theory. However, I am lacking in knowledge of plumbing and could use any help available to get to the bottom of this. Main concern: I don't want one of my young children to burn themselves if they turn the hot water on while the geo unit is running. Thanks!

My system has a dip switch that allows you to select between 125 or 150 max desuperheater output temp. Maybe yours has the same? The higher setting comes with a recommendation to use an anti-scald valve in your plumbing. If you post your equipment make and model, you may get a more detailed response.

Hi Stickman, Thank you for the reply. My system is a Geocomfort GEO SEER Compass Vertical Two-Stage Model GXT048C11MM1C. The 40 gallon buffer tank is a Bradford White that feeds into a 50 gallon Bradford White that is LP heated. Where would the dip switch be located? Thank you! James

Most houses don't have a mixing valve. But if the above approach doesn't work, you should add one to make sure that the hot water temp is never too high at the faucets. Edit: Hmm, exactly as the manual recommends. Also, no indication that the DSH temp is adjustable.

Here's a link to your installation manual. http://residential.geocomfort.com/products/literature/20D082-01NN.pdf
You'll want to read over pages 22-27. Be very careful poking around inside the unit...

Thanks for the information. Troubleshooting tonight over the phone with a plumber, I did find that hot water is back feeding into the cold water line that feeds the buffer tank. He is coming over later this week to trouble shoot that concern-most likely a bad cartridge or blending valve in the tub or faucet. However, he doesn't think that has anything to do with why the water temperatures are increasing 30+ degrees when the geo unit is running. I have a message in with the geo contractor regarding the dip switch. It's a waiting game now :) Thanks again for the replies. Keep sending info if you have it and I'll post once the issue has been solved :)

Not sure how a bad faucet would allow hot water to siphon back to the cold line. Are you sure the buffer and primary tanks are plumbed as per the installation manual? The geo contractor blaming a stuck valve "somewhere in the house" sounds a little suspect to me, but I'm a long ways away from there. Also, see bold type at bottom of page 24 that states NOT to use the desuperheater with a gas water heater - not sure if that includes LP as well. DISCLAIMER - I am but a homeowner...

Can you take some photos of your set up plumbing and post them here? If your house hot water is only drawn from the LP tank, there's no reason why the geo should cause such a hearing gain when running. Even if the buffer tank was 150, when that hot water is drawn into the LP tank, it would mix with the 110-120, and cool down somewhat, I would think.
Also, where in the USA are you?

If you have a tempering valve on your LP hot water tank it is possible the check valve on the hot water inlet is stuck open. There are other ways hot can cross connect to cold ie, hot water recirc, mixing valve and desuperheater piping etc. All have the potential to cross connect if not piped or checked properly. Toss a pump into the mix and anything can happen.

No connecting to a gas water heater. Interesting! I'll be running that by my Geo contractor! Thanks for that heads-up.

I'll try and post some pics later today.

Located in south central Wisconsin.

Are you sure you have the hot output of the buffer tank going into the cold input of the gas H.W. tank. Somewhere your geo hot output is getting directly into your home hot system. That is the purpose of a buffer tank to stop this problem. Water source cold goes directly to the cold on the buffer tank, hot on buffer tank goes to cold on domestic hot water tank , then hot on domestic tank goes throughout home. With this setup properly you do not usually need a mixing valve. Most mixing valves are used on a hot water boiler where temps are very high.

I cannot get my pics small enough to fit the 100kb max :( . My brother has the same geo system in his home. I checked his plumbing set up and it seems to be the same. I'm wondering if there isn't an issue with the thermostat or stuck valve on the domestic heater (as someone mentioned earlier in the thread). I'll have the plumber trouble shoot both tomorrow.

The desuperheater actually never shuts off, the only thing which turns off is the circulation pump pushing water through the heat exchanger on the exhaust pipe from the compressor.
You might have some thermal siphoning there, putting a check valve in the DSH line will solve that problem.

I have some updates and some improvements in the situation. Had a plumber over on Thursday afternoon and we did some troubleshooting. We had hot water returning to the cold line from our house addition. Plumber replaced the guts of the valve in the tub in the house addition and switching the check valve from the line running from buffer tank to domestic heater tank to the cold water in line to the buffer tank. That took care of the hot water coming through to the cold water line. I did some troubleshooting with the domestic hot water heater. I turned it off over night and drained all of the hot water in the morning. I then turned it on and set it to 100 degrees. Had 100 degree water at the faucets. Set it to 110 degrees and had 122 degree water at the faucets. Set it to 120 degrees and had 130 degree water at the faucets. Set it to 130 degrees and had 142 degree water at the faucets. This all was done while the geo unit was not running. Next step is to replace the thermostat on the domestic hot water heater. For now, I think we've isolated the two main problems. 1) the bad valve on the tub was allowing hot water to circulate while the geo unit was running. 2) the domestic hot water heater's thermostat is messed up. Once the thermostat is replaced I'll have to monitor water temps while geo unit is off and when it is running. Buffer tank is plumbed in correctly. My brother has the same set up and his is plumbed into a gas water heater-even though the manual says not to do that. He's never had any issues. Thanks for all the feedback on this issues. I appreciate it. I'll post again once the situation is completely resolved.