Need help to find out what is causing occasional cold showers

[Mikesm]

Hi. We have 4 yr old house with a 80 gal HTP Phoenix gas hot water (the 160,000 BTU unit) and a backup HTP electric hot water heater (with the valves closed, so it's not part of this issue). The heater is set to run at 145 degrees, and it's output is connected to a Watts LFN170-M3 Commercial water tempering valve, to try and keep the hot water temps in the house at 120 degrees or so and extend the 80 gallon hot water supply. The house also has a hot water recirculation system, using a Bell and Gossett NBF-22U pump (that is controlled by an automation system, so it be programmed to turn on and off at certain times or conditions). The mechanical room that contains this equipment is on one side of a large 2 story house with basement, so it's a long way to the far side of the loop.

Here are some pics of the setup:

In the last few months, we have experienced occasional cold showers, which should not be happening. They did not happen before the last 6 months. I am an engineer, and have temp sensors wired into various parts of the piping, including the gas heater input, gas heater output and tempering valve output. I also have a water flow sensor, so I can tell how much water is being used. Unfortunately I only keep about 3 months of data around, so I can't compare this to when the house was built.

Here is a chart of what the temp sensors showed this morning, and a cold shower was experienced at the opposite end of the house at approx 9 AM, and also at around 2:30 PM at a different shower in the house:

At 9 AM, you can see the hot water output temp crash, and the mixing valve (the Watts tempering valve) output being even colder. At 2:30 PM, the mixing valve output also crashes, but the hot water output of the stays above 120 degrees. Now, this doesn't make much sense, because only about 20 gallons or so of water is being consumed in that shower (I think it was the only hot water consumed at that time) at 9 AM. At 2:30, multiple showers were going on, and a lot more hot water was being used, but the hot water output didn't crash, but the mixing valve temp dropped significantly.

The HTP should not be running out of hot water for just 20 gallons of use!

Now, the recirc valve is switched on between 6 AM and 11 PM. If it is shut off, the hot water in parts of the house turn cold. Now, you will notice that in the plumbing, there don't seem to have any heat traps. In factor, the output of the recirc pump comes in near the input of the tempering valve, and the valve was hot all the time. The specs for the valve say the water should be between 40-80 degrees F, so I had a new plumber come in an look at this, and he knew nothing about "heat traps", but he installed a backflow preventer valve just after the recirc pump line output into the cold. This should prevent hot water from going to the input of the tempering valve. However the part of the pipe past the backflow preventer still seems to get still seems to get hot from time to time.

Does anyone know what could be causing this problem?

Thanks!
Mike

 

[Breplum]

Double check that you don't have any cross connection from anywhere like shower valves. The Kohler K304 shower valve is notorious for causing cross connection within the valve. (Also seen with KWC shower faucets) To check, just turn cold water supply at the WH off, and open any hot fixture and let it depressurize. Water flow should stop completely. Flow would indicate an issue.
The check valve was worth a try.
Beyond those things, I'd have to deduce that the mixing valve is the culprit, though I've never run into a bad mixing valve in 49 years (doesn't mean anything).

 

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[Jeff H Young]

As soon as we say something never happened the onle exception pops up LOL . pressure ballancing or temp ballancing valves tend to be culprits as mentioned

 

[Mikesm]

thanks for the replies everyone. i should point out that one of the symptoms is also not cold water from cold water faucets. This was caused by backflow into the cold line from the tank I think by thermal siphoning, but the recirc line plumbed where it is doesn't help.

Is it common that the recirc pump needs to run continuously? I'm happy to try and replacing the valve - it's only about 5 yrs old, but it did have hot water on the cold side for quite awhile, so maybe that damaged it.

 

[Breplum]

As to continual running of recirc pump: A good recirc system has an aquastat, timer or on demand pushbutton or motion sensor and should be insulated with the thickest insulation available. There are lots of options.
So, not just a dumb pump plugged in to an outlet.

 

[Fitter30]

Mixing valve how long has it been installed? Your running 145° pulling out more minerals valve needs to disassemble soaked in vinegar cleaned and reassemble. Can buy new guts swap them out. Hopefully not having to cut the valve out and add unions so the valve can be seviced.

 

[WorthFlorida]

My opinion is he recirculating check valve (at the furthest faucet) is staying open due to mineral deposits, as Fitter3 suggest.

 

[Mikesm]

Interesting. The hot water loop should be running at 120 or less - that's the whole point of the tempering valve, so at the very far end of the loop it should not be that hot. We have a separate hot water return, but I didn't know there was a check valve installed at the far end somewhere. I can definitely see how the tempering valve might have mineral build up in it, but I don't think it lends itself to disassembly: https://s3.amazonaws.com/s3.supplyhouse.com/product_files/0559125-install.pdf Should this be replced then?

I can program the pump behavior pretty easily - it is shut off at night and when the home is in vacation mode. It's easy to to integrate the burger alarm's motion sensors to it too - that's a good idea, but we need hot water in the kitchen faucets too, but just the bathrooms. How long would it normally take to push hot water through a loop? Turning it on too late may result in a slug of cold water getting dispensed.

thx!
Milo

 

[Breplum]

Most properly sized recirc pumps (meaning very low hp...note: often they are incorrectly sized too high) ought to take up to 70 seconds but distance is of course part of the equation. When we do motion detectors for recirc. they are at the approach or in the room.
The check valve for recirc is always at the water heater and never at the end or hidden.
Without an aquastat, the piping for circ loop should always be hot. And if insulated, even the return will be plenty hot.

 

[Master Plumber Mark]

Its probably a bad mixing valve.... crank it all the way to its hottest setting and then back it off
and do it a few times and see what happens

it probably needs new repair parts or replacement

plumbing is a mystery sometimes..... I would start there

 

[Fitter30]

To bad who ever installed the mix valve didn't use unions. They make rebuild kits or for a few dollars more a new valve to swap out and time to service the old one.

 

[Tuttles Revenge]

In 2019 I had installed a similar style tank with tankless built in.. with a recirculation pump. The return water port was aimed directly at the temperature sensor which caused hot water to return and tell the sensor not to turn the heater on. Was a HUGE PITA to diagnose and solve.

 

[Master Plumber Mark]

To bad who ever installed the mix valve didn't use unions. They make rebuild kits or for a few dollars more a new valve to swap out and time to service the old one.

That one will be a bitch to change out but its probably the logical place to start

 

[John Gayewski]

I can't quite follow the piping enough to logic it out, but that check valve should be on the recirc line isolating it from the hot and cold water lines. To me it looks like it could be wrong. To me it looks like there's nothing stopping the cold water from entering the pump backwards when the pressure drops during a shower.