My plumber friends and I (a handyman) can't seem to solve this vexing problem. The house is an ordinary single story, maybe 20 feet from the closest fixture and 75 feet from the WH to the furthest fixture. When the Grundfos pump is running, we get hot water within 30 seconds. When we started working on this project, if the pump was not running, no fixtures ever got hot water. The pump was plumbed into the cold water line, so we switched it to return to the bottom of the water heater, and with a check valve. When the pump is on, all works the same as before, fine. And now when the pump is off, we *eventually* get hot water along the system, but we're talking like 5+ minutes to even the close fixtures. There's no way there is that much cold water between the WH and the first fixture. When the hot water arrives, it's warm and only slowly turns to true hot. To me, it seems like cold is mixing in. Everyone disagrees with me, but how else could the water be so slow and then only warm for awhile?

Thanks for any ideas.

You are drawing cold water back into the system through a spot that either needs a check, or has one that isn't working. turn the pump on and let everything get hot; while the pipes are hot, stop the pump and then turn on the hot water at a faucet. Somewhere, a pipe will go from hot to warm/cold very quickly.

Pay my airfare and I will diagnose it, otherwise you do not tell us enough about the piping system to give an answer.

Excellent idea, KC. Will try that late today or tomorrow. I think all but one or two faucets are separate hot/cold handles, so it should be easy. Thanks, I will report back.



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Haven't been able to test yet, but have noted cold line in to WH is warm, despite having a dedicated return line for the pumped hot recirculation loop. Normal?



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The cold water influent pipe to a water heater typically is warm due to convection from the tank of warm water it is connected to



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OK, I finally got to test this. Got every fixture hot, then turned off pump. I used a bbq digital thermapen so I could be precise! All were within a degree or two whether or not the pump was running. All tested to about 130F, except a shower that hit only 122F (it has an anti-scald, with a dial that lists temperature on it).

I also tried turning off the pump and the cold supply going into the water heater. All faucets ran dry of hot water pretty quickly, except the shower above, which I would expect since it intentionally bleeds cold into the hot for anti-scald, I assume.

Hmmmmph. I don't get it. How is it possible that water takes literally forever to get hot without the pump, yet once hot, you can switch off the pump and it stays hot?

Anti-scald devices don't put hot water into the cold line, they limit the hot water input coming into the mixing chamber. I suspect that closest to the shower you have a crossover valve that is bad. Turn the supply to the crossover valve off. It should be under the lavatory if it was installed like normal. Then test the hot water again with the pump off. It should get hot water as normal to the rest of the fixtures if the cold isn't bleeding across into the hot water line.

I checked all the lavs, and there are no crossover valves. The nearest lav is a two-handle type of faucet, nothing unusual about how the supply lines meet at the valve. I think it's a Newport Brass.

More info on the shower that's 10F cooler than the others... it is next to a tub that has huge hot pressure (the hot pressure in the shower is very low, but the cold is fine). It's also furthest out from the WH. It's also a Newport Brass.

Here's where I am confused. Let's assume there is cold crossing over somewhere, at that shower. Is that small flow (approx 10% of normal flow of cold coming through at full hot setting, when I tested with the cold supply off) enough to prevent hot water from ever recirculating around to any fixture without the Grundfos pump running?

Ok. Complete brain fart. I didn't think things through properly. Grundfos uses a thermally regulated crossover valve to keep hot water at the taps. If the pump isn't on, the hot water that would normally heat up and close the valve off never gets to the valve's thermally operated element, allowing the cold water to continuously have a direct path into the hot water. While the pump is off, the valves will remain in an open position at all times unless the valve were heat up sufficiently to actuate. There is quite possibly nothing wrong with your system. If you don't want to run the pump, then you will need to remove the entire system or the erratic temperatures will continue.

I suspected the pump was a problem early on, thinking maybe it had a bad valve, and checked with Grundfos. This particular pump doesn't have any valving, just pumps all the time (model UP 15-18 SU). And it is plumbed into the hot return, and goes into the bottom of the WH. Some of their other pumps have that thermo valve (I think they call it the Comfort Pump).

As a crutch fix here, we can install an aquastat to shut off the pump when the return line temp exceeds 115F. And then a timer to limit the hours, regardless of temp. But it sure seems a huge waste of propane to heat all that water in copper.

It still bugs me that I can't figure out why the water would never get hot without the pump, even so close to the WH. And why the most distant shower has so little hot pressure.

The thermal valves are supposed to be at the furthest fixture, not at the pump. The grundfos system isn't a good choice for a propane tank, but really no worse than any other. If your water lines are insulated, you can reduce energy costs, but putting an aqua stat on your system won't alleviate the problem. YOU WILL HAVE CROSSOVER CONSTANTLY with the pump off due to thermally regulated crossovers.

I think I understand, You're saying there should be a temp-sensitive valve somewhere at the end. But why, since the pump is not regulated at all by temp (there are no wires, nothing, attached to the pump)?

I just don't know where that valve could possibly be. We've looked everywhere. I'm unimpressed by a lot of the craftsmanship in this '07 remodel, so it could simply be del one wrong or put in someplace stupid like a wall, and I would not be shocked.

The valves are designed to be placed under a lavatory sink or kitchen sink and connected to the supply lines there. They are designed to attach to flexible water supply lines so it is unlikely that they are behind a wall.
The grundfos system adds pressure to the hot water side constantly. Normally during a cross connection, the hot and cold pressure would be equal, so water doesn't move much until a tap is opened. When that happens, roughly equal amounts of water from the hot and cold water lines will come through both the hot and cold taps when used. Let us say that the incoming water is 60 degrees and the heated water is 140 degrees. when the hot or cold is used, the water coming out will be roughly 100 degrees (disregarding heat losses and gains throughout the rest of the system). If there was a leak in the system somewhere such as a running toilet, it will generally fill with hot water if it is close enough to the cross connection as thermal expansion from the hot water tank will add pressure to the hot side of the line and force more hot into the cold pipes until a tap near the cross connection is opened again and restores a rough equilibrium. The Grundfos system adds that constant pressure to the hot water side, and causes the same effect; however, they do this with a thermally operated valve that has a paraffin component that swells with heat and will block the pipe and prevent further cross connection from occurring.
Let's say that the paraffin expands sufficiently at about 120 degrees. The water is sitting in the pipes, and heat transfer and losses are occurring in a normal fashion. After say 20 minutes, the water at the Grundfos valve has dropped to 118 degrees. The paraffin starts shrinking and allows a small amount of cross connection. The pump at the hot water tank provides additional hot water pressure so 118 degree water seeps into the 60ish degree water at the tap, warming it slightly. The valve returns to 120 degrees and closes, and the cold water line is full of luke warm water. The hot water sits at the valve waiting to either be used or ready to pass into the cold water. Now the cold water pushes its way back toward a lower pressure area. This is toward the entrance of the house, but it doesn't make it there. As hot water is removed from the hot water tank, it creates a reduced pressure, causing the water to make a loop backwards through the cold water piping til it reaches the inlet of the hot water tank.
Now if you have a dedicated return line, the Grundfos valve is actually detrimental. A regular recirculation system has a dedicated loop for the hot water that doesn't involve the cold supply to the rest of the house. A Grundfos valve will simply cause a problem. Since the hot water has its own loop, it never crosses to the cold piping and therefore never closes the valve. This means that every time you run water, you will have the aforementioned 100 degree water indefinitely as you get half hot and half cold with every twist of the faucet. A conventional recirc system REQUIRES that you remove the Grundfos cross connection, although the pump is still fine to use.
I hope this clears up the confusion.



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I checked again, for sure no valve like that at any fixture. All plumb straight to the fixture from the copper supply, except the faucet at the far end of the house, where the faucet has both hot and cold going into what looks like a mixing chamber.

Just curious, why would a valve like the picture be installed? The pump is independent of any valve. Grundfos suggests the aquastat at the return line before the pump to shut off the pump if water is hot enough. That at least reduced the time the pump is running, but doesn't solve the mystery.

quote: except the faucet at the far end of the house, where the faucet has both hot and cold going into what looks like a mixing chamber

Shut off the stops at that faucet and see if the problem goes away while the pump is off. I can't think of any easier way to describe the Grundfos comfort system to you. Maybe someone else can explain it so that you can understand.

I actually didn't see the text you wrote below the picture until now. Whoops, sorry about that. I get it now, thanks for typing out that detail.

We have a dedicated return line, and no valve. Just the Grundfos pump at the WH. When I referred to the Comfort system, I was saying that is not what we have.



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To recap in case anyone is interested...

- fixtures will never get hot water no matter how long they run
- turn on pump and water turns hot quickly
- turn off pump and water stays hot
- with cold supply off, one shower at the end of the house has a trickle of cold through it when turned to full hot, and that shower has very poor hot pressure (dramatically less than its full cold pressure)

You have a cross connection somewhere at a single handle faucet in that case. Or you installed the check valve backwards on the return line that you just replumbed…

Thanks for sticking with me here. Hopefully the attached pic of valve shows up (looks plumbed correctly -- pic is upside down... we do have gravity here in CA). Tomorrow when the water is all cooled down in the lines I will close off all single handle faucets and see if I can get hot without the pump on.






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OK, I closed off all the single handle faucets (2 in kitchen, laundry, a bath). Nothing I can do to shut off the two single handle showers.

It took 12 gallons at the tub before the water got hot, and the tub is approx the furthest away from the WH, maybe 125' if you took a very circuitous path. Water hit 130F.

I opened up the supplies to all again, and then went to the single-handle laundry, which is approx the closest to the WH. It took about 2 more gallons to get hot, and it only reached 122F. I waited 30 mins, checked that laundry again and it took at least a gallon to get hot again, and only reached 120F. Turned on the pump, and the temp rose to 125F.

I've attached a pic of the layout and temp results.

I suspect that your check valve isn't working properly. It may be stuck open. When all the stops are closed it only leaves the 2 showers and the check valve. It may be a shower cartridge that is bad, but the check is the easiest to eliminate from the equation. When the pump is off and you turn water on in a faucet, does it get very hot between the check and the pump?

I'll need to let it cool off before I test that (pump has been running for a few hours). I'll report back.

Tested that -- no, the copper pipe is hot between the check valve and the water heater (the return line going to the bottom of the WH). It is *not* hot between the check valve and the pump. Just a little warm, huge difference in temp from the pipe into the WH.

That was after the pump was off and you ran water till it got hot in one of the fixtures somewhere?
At this point the only place that you can be crossing hot and cold is at one of the showers. I would replace the cartridge in the one that is problematic first. It may be a broken O-ring or something causing the reduced flow and letting water cross into the hot line. Or replace the cartridge in both of them. One trip to the supply house that way.

That was after it had cooled down, and then pump off, run a fixture. I didn't wait for the water to turn hot, just 5 mins or so (3-4 gallons of water).

Since the crossover test (closed cold supply, hot water faucet on) was negative, no flow, is it even possible that crossover could be causing it to take 13 gallons until the distant fixtures go hot? Wouldn't they at least get warm after a couple gallons?

initially you are mixing cold water with cold water, then warm with cold, then hot with cold. depending on the amount of flow, yes, it is possible.

Going to check and replace that cartridge today.

I would think that if it was mixing, the temperature would slowly be rising. It doesn't. It goes from cold for maybe 12 of those gallons, and then the last gallon goes to warm and hot very quickly. Thus why I'm skeptical, especially since we can't find any sign of crossover (other than the ridiculous 13 gallon flow!).

Dip tube fine, shower valve ok (partially clogged, which was killing the water pressure). Looked under the house. The WH outbound supply line down the house is 1", and the return is 3/4". Lines are relatively straight, so I can't imagine that there's 13 gallons of water between the WH and most distant fixture. Looks like at most 150' of line (probably more like 120'), which would be ~6 gallons. The only thing I can think of is that when the pump is off, and we turn on a hot fixture, the water comes from both the return and supply line, doubling the time it takes for hot to get there. The closest fixture is very close, and still takes a few minutes (no measure of volume taken yet), and that could explain that problem as well.

Hmmm.

That would mean a cross connection or faulty check valve.