Hot Water Mixing in my cold water line - recirculation pump and mixer

holograham · July 31

My hot water setup is 2 tanks with a mixing valve (previous owner put this in to basically have unlimited hot water) plus a recirculation pump to have continuous hot water on the 3rd floor.

Issue I noticed is when I use the garden hose - the water oscillates from cold to hot to cold again. I also have noticed this occasionally in other sinks/tubs in the house.

When I feel the cold water line leading into the mixing valve it is hot so assume there is some action that is "leaking" hot water back up the cold water line and gradually back to where the rest of the cold lines branch to the rest of the house.

When I turn off the recirculation pump - the issue stops.

Is the fix as simple as a backflow preventer at the mixing valve cold water line?

Jamie Hall · July 31

Even simper than that. There should be a simple check valve on the recirculation line where it returns to the feed to the hot water tank(s). There may even be one — which isn't working any more.

holograham · July 31

https://forum.heatinghelp.com/discussion/comment/1807087#Comment_1807087

Can you explain that a little more? Why on the recirculation line? Wouldnt that just push hot water into the dedicated hot water loop? How would it cross over to the cold line?

holograham · July 31

Heres a closer picture of the pump and where the recirculation line enters the tank

Jamie Hall · July 31

Well it depends some on how the whole thing is plumbed. The check valve (or check valves) may have to be somewhere else. The idea is to make sure that cold water cannot get into the recirculation line. The recirculation line must go into the cold water feed to the hot water tanks. Honestly I'd really have to see a diagram on the system to give you better advice.

holograham · July 31

I didnt commision this so reverse engineering what was done. I am the owner and not a professional plumber.

The way I understand it is that there is a dedicated recirculation line - so that doesnt cross over. If you look at the top middle you'll see a guage. That is where there is a cold water mixing value that takes hot water from the 2 hot water tanks and mixes in cold (you'll see a large T and then another T that splits cold water back down to the tanks.

I suspect that the pressure differential (either from the recirculation pump or when the cold water is used it "pulls" some hot water off the mixing valve back up through the cold lines. Could be wrong. The other theory is there is some crossover in the walls I cant see but I dont really even fully understand crossover like that.

hot_rod · July 31

check vslves, thermal expansion tanks, and a specific way to pipe 3 way mix valves with recirc

It depends on the brand snd model if the valve

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_11_NA_Domestic%20water%20heating_0.pdf

IMG_8228.png

Larry Weingarten · August 1

Hi, I think I see a spring check under the upper red handled drain valve, below the pump. It may or may not work. I'd suggest running water as you have been, but feel the lines around that valve and see if they cool down during use. If they cool a while after you start using hot water, it is likely that the now cold water in the bottom of the tank is being pulled backwards through the recirc line, causing the problem. It's a common problem.

Yours, Larry

holograham · August 2

https://forum.heatinghelp.com/discussion/comment/1807180#Comment_1807180

Hmmm not quite following this - how would the cold water coming back through the recirc line cause a cold water line (like my hose spigot) to alternate hot/cold water?

My theory was that the cold water line going into my mixer valve was getting backflow (I can feel the line get hot). When I run cold water - there would be some dueling with my cold water line from the main and the warm water backflow - perhaps with slugs of hot water mixing with slugs of cold water hence why I would get alternating hot and cold (though predominately cold). That said I am still understanding how recirc lines work.

My mixer brand is a Caleffi so it appears there needs to be a check valve on my cold water line going into the mixer valve. That would prevent hot water backflow into the cold line.

Larry Weingarten · August 2

Hi, There may be multiple problems, but maybe best to start by simply feeling the lines around the mixing valve under different operating conditions. I'd also get a couple of yards away and feel the cold supply to the mixer. This would tell you if that is really where the hot is coming from. Sounds like you're on the right track.👍️

Yours, Larry

holograham · August 3

https://forum.heatinghelp.com/discussion/comment/1807333#Comment_1807333

I feel hot stemming from the cold line off the mixer that travels back to the cold line branches to the house. I assume that is the problem - would a simple check valve work to stop that?

hot_rod · August 3

Caleffi mix valves are available with or without checks in the H &C port, do you know which model you have? The checks fit into the tailpieces where the press connections are made.

The manual that came with that valveshowsthecorrect piping when used with a recirc pump.
As shown in this attachment, both checks need to be in place.

IMG_0767.png

Is there a thermal expansion tank on the system? Are you on a well tank system?

Does the hot water temperature rise above the valve setting? First draw in the morning, for example. If so that is common with thermostatic mix valves when warm recirculated water enters the cold port of the mix valve.

Two ways to deal with that issue, pipe a bypass valve as shown and explained above. Or use a mix valve that has a ASSE 1070 listing. That valve will have scald protection which closes down the valve if cold side is lost. Which also prevents temperature creep

If you add a check, a spring type that is low lead NSF listed.

holograham · August 28

https://forum.heatinghelp.com/discussion/comment/1807361#Comment_1807361

I do not know what model I have - I cant see any markings to indicate on the valve itself.

There is no thermal expansion tank on the hot water tanks that I can see. Not on. a well tank system - municipal water.

Hot water temp I think does rise above the valve setting - unsure about first thing in the morning though.

I believe the simple thing missing is the check valve on the cold water supply into the mixer. The thing that doesnt make sense from that diagram is the recirculation return is piped to the drainage port of the hot water heater not back via the cold supply into the tank. Does that change things? Not quite understanding the bypass.

Lyle {pheloa} Carter · August 28

If you can not determine which mixing valve you have. Someone will have to remove the mixing valve and look at the inlets or tailpieces to determine if there is an integral check valve and if it is working. Once that's determined, if they are not present, they can be ordered or the valve can be replaced with a valve that does have checks. As far as the creeping temperatures are concerned that may be caused by lack of control on the recirculating pump. There should be some type of control on the pump to turn it on or off, either based on return temperature or time. Piping the return into the water heater drain is not most manufacturers usual method but I have seen it work without problems.

Teemok · August 28

It looks like that pump is uncontrolled and that can be a problem for a mixing valve with the recirc piped wrong. If you can remove the mixing valve it would simplify things a lot. Meaning, if the tank storage volume is enough for you when the tank temp. is set at normal usage of 120-127F. To the Legionnaire fear mongers: I already know.

The twin tank set up wants symmetry in piping and it looks like it's close as far as the in/Cold and out/Hot piping. That should be confirmed and corrected if not. The existing recirculation line is not symmetrical at all. Here's my suggestion, mixing valve or not. Preferably not. The recirc. line should tie in to the shared cold line before the existing branch to the mixing valve cold port. This will mean recir. water is returned equally to the tanks and has the option to go directly back to the mix valve cold port should the mixing valve close down the hot port due to it being hot. A check valve is needed on the cold pipe before where the recirc will T in and the recirc. line needs a check valve too. The existing one is fine if it's still functional. This must be proven or just replace it as good measure. The pump should be at the the very least thermostatically controlled with an aquastat so the pump doesn't create an overheated loop.(when the tank is kept hotter than the loop and a mix valve is used) I like a timer for master on and off times, an adjustable recycle timer tuned to the building with an aquastat. Caleffi has excellent info here: https://idronics.caleffi.com/sites/default/files/magazine/file/idronics_21_na.pdf

Teemok · August 28

Edit: Some of the below is relevant for hot in the cold. Problems are most of the time the other way around and I was giving advise for that. My mistake. A bad check is prime suspect. The above advice still stands.

Determining where the cold water is getting into the hot line is a logic problem. As Larry said above, feeling pipes during a hot water draw can eliminate/ identify if it's a mixing valve or recirc. check valve issue. If no faults are found with the near tank stuff, I'd look at shower valves or any single handle valve as a crossover point. Are the tank temperatures set Identical? With the recirc piped to just one tank, I would guess they have different outlet temperatures sometimes. I don't think any mixing valve likes fluctuating temps. on the hot port. I doubt that's the primary reason for cold pulses though.

holograham · August 30

Thank you all for the responses.

To clarify - I dont have cold pulses on hot water. Its only hot pulses when cold water on. Easiest way I checked this was with my garden hose. This also makes the crossover issue less likly (I think) since the garden hose cold water line I can trace directly from the cold water line in my basement to the spigot - there doesnt appear to be any opportunity for crossover along this path.

The recirc pump has no timer or sensor - it was on continuously. When I unplug it the problem stops - however I dont get instant hot water anymore and my 3rd floor bathroom takes 3+ mins to get hot water.

I will have to ask my plumber what mixer valve he installed. The setup is twin hot water tanks + a mixing valve as the previous owner wanted essentially unlimited hot water. This does work and I dont really want to change it. I have family in the house who like their baths/long hot showers.

I have felt the lines and its clear to me that warm water is traversing back up the cold water line (I assume this action happens because no or broken check valve and when cold water is run there is an occasional slug of hot water that is "pushed from recirc"/"pulled from the cold water tap flow" and it mixes with the cold water line.

hot_rod · August 30

The correct piping depends on what type of valve you have an ASSE 1070 or ASSE 1017.

https://idronics.caleffi.com/magazine/11-domestic-water-heating

This issue shows the various pipings.

Yes there needs to be a working check and a thermal expansion tank in the piping if you check the incoming cold.

Screenshot 2024-08-30 at 10.34.33 AM.png

holograham · August 31

Thanks will need to figure out what valve I have.

Hot Water Mixing in my cold water line - recirculation pump and mixer

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