Hot water recirculation pump causing very hot water at taps

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William Fisher

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The hot water recirculation line is a dedicated line and used a Laing SMT 303BSW pump and is plumbed to the bottom of the hot water tank.

I found out that the recirculation was not working because the pump was dead. I replaced it with a Laing LHB08100081 Low Flow Circulating Pump. It is supposed to the replacement for the older pump. I set the pump at 3 and started it.

Well, the issue is that the water at the faucets is now over 150 degrees. It is almost like the recirculation pump is defeating the balancing valve on the top of the tank.

The hot water heater is a Bradford White Combination Water Heating unit.

https://www.bradfordwhite.com/profe...efender-combi1-ttw-power-vent-models-c-sw2-tw

By the way, the thermostatic mixing valve on the water heater seems to be working well. When the recirculate pump is forcing hot water through it from the tank (my theory) it shuts the water flow down to almost nothing to try to stay under 140 degrees.

I have seen you write about recirculation pumps. I hope you have an idea or two..or three.

Thanks.
 
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William Fisher

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Try turning the temperature down on your water heater.
That would work. However, the water heater is a call a combi water heater. It has to run at 160 to 180 degrees to heat a coil the goes through the water heater. That coil contains water that is used to heat the water used for the in floor heating. There is thermostatic mixing valve on top of the water heater that mixes hot and cold to reduce the water temperature to 100 to 140 degrees.

https://www.dsgsupply.com/Product/Bradford-White-CDW2TW75T10BN-475
 

Fitter30

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That new pump number is the replacement for the old. Are you sure the mixing valve isn't sticking open? Have a thermometer on the supply line after mixing valve? Normally radiant floor heat doesn't need to run anywhere near 180* most run under 100*. The higher the water temp the most minerals precipitate out of the water.
 

James Henry

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I would think you could readjust the mixing valve and that wouldn't effect the heating end of it if the domestic hot water is on a dedicated line.
 

William Fisher

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The odd part is that if I shut off the recirculation pump I get 120 hot water or whatever I want. And I can use the mixing valve on top of the hot water tank up and down and the water heater is at 165 degrees. As soon as I turn on the recirculation pump everything goes out the window. Somehow there is very hot water getting into the hot water lines. I checked the direction of the pump and it looks right.

The floor water runs at about 100 degree. The issue is the efficiency of the hot water heater. There are two separate circuits. The floor water circulates through a sealed copper coil inside the hot water heater. You need to run the hot water tank at 165 to get the water to maintain 100 degrees as it goes through the hot water tank.
 
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Fitter30

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Looking at the timer dial pump discharge is on the right. With pump off run hot water after getting hot water turn water off with hands on the piping either side turn pump on which side gets hot first?
 
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Jadnashua

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It can depend on the design of your tempering valve. Because you are not actually consuming any water while recirculating, there's no need for additional water to flow into the tank while recirculating, so there can be no additional water coming in to temper it when it goes back out - you get whatever the tank has in it. Some tempering valves have a special port to compensate for that, and essentially use the return water as the tempering water for what's in the tank. I think you may need to change your tempering valve or, turn the pump off once the incoming water is warm enough, or, shut the circulation down with a thermostatically controlled one. Many of those shut circulation down once the water reaches about 105-degrees.

Look at it a different way...when actually consuming some hot water, the tempering valve adds some cold, inlet water to it to drop the ultimate temperature. WHen recirculating, there's no incoming cold, so you need to either provide it with a tempering valve plumbed accordingly, or stop the circulation entirely once the water is your desired temperature.
 

William Fisher

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Even though not a pro I think your answer makes alot of sense. :) It really is a good way to look at it. Kind of the old mass balance equation. The hot water heater need to push out as much as comes in so no way for it to add cold. The tempering valve has no choice but to pass the water being pushed in so no cold can be added. Thanks. :)
 

Jeff H Young

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Jadnashua I think figured this out how can water enter the pipe if none is being used
 

wwhitney

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Nice find. So with a tempering valve, you have to put some of the pump output into the tank, and some into the cold water line upstream of the tempering valve. That way the suction side can pull through the tempering valve from both the tank and the cold water line.

The article only seems to address dealing with the steady state condition of a constantly running recirculator, though. For example, if 120F water leaving the tempering valve cools off to 115F at the return near the tank at the pump's flow rate, and the tank is at 150F, you want to put 1/7 of the return into the tank and 6/7 of the return into the cold water line. That gives you an outgoing temperature of 1/7 * 150 + 6/7 * 115 = 120F. You set up some some throttling valves to be close to that, and then presumably the tempering valve does the rest to provide the correct ratio.

But if you don't want be constantly heating your house with the return loop, and prefer an on-demand system, then sometimes the return loop will have cooled to ambient, say 70F. At which point you want to put 5/8 of the return into the tank, and 3/8 into the cold water line. Because 5/8 * 150 + 3/8 * 70 = 120F.

So that brings up the question, if you set the throttling valves to be roughly balanced (50/50), does the tempering valve have the range to handle both cases? I guess it must. [Which also raises the question of why you need throttling valves at all, just pipe the output of the pump to a tee that goes to both the tank and the cold water line, with the appropriate check valves shown.]

Cheers, Wayne
 

Reach4

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It's not clear to me that you need throttling valves, since the pump is variable rate.

Also, I think you can just turn the pump off, and it should still work, don't you think?
 

Whatnot

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Do any thermostatic mixing valves work backwards? If they did then you could just put it after the pump. Push water in the the mix port, connect the hot port to the bottom of the tank and the cold tee into the tank outlet.
 
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