Seeking advice on issue with hot water recirculation system

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djwbd92

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My home has a hot water recirculation pump installed that doesn’t use a dedicated return line (The Grundfos Comfort System). This setup uses a thermostatic bypass valve at the far end of the loop in the 2nd floor master bathroom, so that the cooled water in the hot water line is pushed back to the water heater when the pump is running. Once the water in the hot water line gets hot, the bypass valve closes to prevent the hot water from continuing to flow into the cold water line. I’ve had this pump installed for 6-7 years, and during much of that time it’s worked very well. At worst the cold water might get slightly warm once the pump had been running for a while.

Unfortunately, sometime in the last 1+ years, something has changed such that the cold water actually becomes uncomfortably, if not dangerously, hot if the pump has been running for 10 or more minutes. I have replaced the thermostatic valve, and tested that the new valve is properly shutting off once the water in the hot water line gets hot, so I’m fairly certain that is not the problem. Could the hot water be bleeding into the cold water somewhere else in our plumbing system, like in one of the shower cartridges or shower valves?

I had a plumber come by to look at this, and despite my telling him that the bypass valve was new and showing him how the water being pushed up the hot water line stopped once it reached temperature, he insisted that the bypass valve was the problem and told me we should get a dedicated return line installed, to the tune of $3800, since that is the “right” way to do it. He insisted they could install the return line without punching holes in my walls by installing it from the crawlspace, which I’m not even sure if that makes sense.

I’d greatly appreciate any thoughts on other explanations for why the current recirculation setup is causing our cold water to become so hot. Also, I realize that a dedicated return line would work better in that it would avoid causing any temperature changes to the cold water, but is it really possible to install a dedicated return line in an existing house without opening up the walls and generally making a mess?
 

Tuttles Revenge

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Without seeing the layout of the house and the relationship between your water system and the water heater its impossible to give an informed opinion. It is possible if the water lines are all exposed in the crawlspace and the water heater is in a location where it is accessible from the crawlspace too.

To test whether a faucet or a shower valve is bleeding hot into the cold you can feel the angle stops or the shower valves to see if they feel warm to the touch. Best to do a test when nobody has used any hot water and if you can turn the pump to run manually to simulate the problem. Only the angle stop that is connected to the bypass should get warm since it is receiving hot water by design.

Some people like to shut off all the cold angle stops under each sink to prevent any crossover. Process of elimination.

I know when we've installed Touch or Touchless faucets they can cause bleed over because they have a solenoid valve to stop and start flow, but the faucet cartridge itself is in the On position.
 

jadnashua

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In the last year, have you changed anything else in the plumbing?

The most frequent culprits of cross-0ver problems are single-handle valves like your shower, kitchen, or vanity ones. On some designs, when the cartridges are starting to fail, they can provide an unintended cross-over.

Some people put a Y on their washing machine lines to get warm instead of all hot or cold...that can cause this as well. Or, if you have a shutoff on your showerhead, and leave the main valve on.

You might have an issue with some tempering valves on your water heater.

I do not know if that design utilizes a check valve IN the pump assembly, but that can also be an issue.
 

Jeff H Young

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valid answers above. I'm not the biggest fan djbwd of your circ system but they work fairly well. also its not 4000 dollars and guess what 4000 dollars might not make a great circ system either if the hot main splits off in 2 different directions a dedicated return line might not be that swell either.
thermostatic valve or shower valve likely cause
 

djwbd92

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In the last year, have you changed anything else in the plumbing?

The most frequent culprits of cross-0ver problems are single-handle valves like your shower, kitchen, or vanity ones. On some designs, when the cartridges are starting to fail, they can provide an unintended cross-over.

Some people put a Y on their washing machine lines to get warm instead of all hot or cold...that can cause this as well. Or, if you have a shutoff on your showerhead, and leave the main valve on.

You might have an issue with some tempering valves on your water heater.

I do not know if that design utilizes a check valve IN the pump assembly, but that can also be an issue.


In the last year we've had the water heater and kitchen faucet replaced, but this problem predated both of those. There is no Y on the washing machine lines. I'll have to run some more tests.
 

djwbd92

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Circling back for any future readers of this thread. The root cause looks to have been a bad mixing valve in one of our showers. Now that it has been replaced, the problem appears to have disappeared (fingers crossed).
 
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