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[madbrain]

Sorry, they are all gone now, including the refurbished units.  The plug-in XTBR will remain available for now.

OK.

You have a complete loop without a one-way valve.  So without the pump running, you draw water from both the hot and cold return.  That causes the quick drop in temperature as the hot and cold mix at the faucet.  If you don't want to run the pump whenever hot water is being drawn, you need a one-way valve in the return to block the cold water from mixing with the hot at the faucet.

Thanks. I can't really tell where the valve should be. But I'll certainly tell a plumber. If that turns out to be the issue for WH#1, where do I Paypal you to thank you ?

[madbrain]

Looking at those photos I think Jeff is right.  I also think the recirc pump is plumbed in wrong but the photos don't reveal what's behind the exhaust stack.  I don't see what's below which is what I'd expect with the recirc line being pumped to the cold water input rather than to the hot:

Here is a video that hopefully shows all the relevant connections.
https://drive.google.com/file/d/1qA7g1c8-Uj8HDyKJuiD5SBP18vLp3dhe/view?usp=sharing

[brobin]

That video is helpful, thanks.  If you turn off the pump and turn off the valve on the line in the foreground do you still get hot water out of the taps?  I ask this to verify that the line in the foreground is indeed the recirc line.

[madbrain]

That video is helpful, thanks.  If you turn off the pump and turn off the valve on the line in the foreground do you still get hot water out of the taps?  I ask this to verify that the line in the foreground is indeed the recirc line.

Which valve do you mean ? The blue flat valve on the right side ? The red circular valve on the left ? Or something else ?

There is another copper piece on the pipe just underneath that red valve. You can see it near the very end of the video. It goes to a pipe that's plumbed to the pump on the right side. I'm not sure it's a valve, or how to operate it.

Yes, I'm quite useless with plumbing unfortunately. Never had to deal with something like this in 25 years as a homeowner. Only my second home, though.

[brobin]

The flat blue one.

[madbrain]

The flat blue one.

Thanks. I just turned off the pump first, but left the valve open.

Tested at the bathtub in the downstairs bathroom. Open the hot water faucet (separate handle). Got hot water around 115F after 30 seconds. at t = 4 minutes, it had dropped to 98F, which I consider too cold. Problem reproduced. I closed the faucet. I then closed the blue valve and went back to the bathttub. Opened hot water faucet again. Temperature slowly crept back up. It got to 135F at t = 6.5 minutes . I left it open for a while. At t = 12 minutes, it had dropped to 131.5F, which I believe is normal since cold water is mixing in to the water heater while drawing from it.

I then checked the sink in that same bathroom, using the separate hot water handle. I got a very very slow stream of warm water. Not a big enough flow to wash one's hands. It seems like there is almost no hot water pressure. My guess is the blue valve being closed is causing a problem there. I didn't get hot water above about 100F, but it was hard to measure as the stream was so small.

I also checked the shower in the same bathroom. Everything seemed normal there as far as both pressure and hot water temperature. Didn't get hot water temperature above about 115F. I assume the shower valve has an anti-scalding feature.

I went to check all the other other faucets and shower valves that are also served by this same water heater.
Bar downstairs sink, single-handle : got about 120F. Pressure was normal.
Main kitchen sink upstairs, single-handle : about 115F. Pressure normal.
Island kitchen sink upstairs, single-handle : about 115F. Pressure normal.
Half bathroom sink upstairs, separate handle : about 115F. Pressure normal.
Laundry room upstairs sink, single handle : about 115F. Pressure normal.
Guest bathroom #1 upstairs sink, separate handle : no water on either hot or cold sides. Both valves were open.
Guest bathroom #1 upstairs,  shower valve : about 115F. Pressure seemed normal.
Guest bathroom #2 upstairs sink, separate handle : 115F. Pressure seemed low.
Guest bathroom #2 upstairs,  shower valve : about 115F. Pressure seemed low.
We seldom use the 2 guest bathroom upstairs, so it's hard for me to be more accurate about the pressure.
It was about t = 45 minutes when I finished testing everything.

So, closing the blue valve with the circulation pump off seems to have resolved the issue for the bathtub and shower in the downstairs bathroom. Hot temperature is no longer massively dropping quickly at either of them. However, it is causing pressure issue with the sink hot water faucet in the same bathroom. And it seems to be causing issues with the two upstairs guest bathrooms as well. The reason I wrote "seems" for my observations is that guest bathroom #2 hasn't been used in years, probably 7 years, either the faucet or shower. Guest bathroom #1 was last used around Xmas time, and no issue was reported by the guest at either the sink or shower, but the circulation pump was on during that time, and of course the blue valve was left open.

I think in the course of the experiment above, more than 50 gallons of water were drawn from the 50 gallon water heater, so it was in the process of regenerating, and this may explain the lower temperature readings of 115F upstairs, but I'm not certain if that's the only reasons. Some of the single-handle faucets at the bar, kitchen and laundry might have anti-scalding features. The shower valves probably all do. I'll wait for the water heater to stop regenerating. It's know = 1 hr 10 minutes and the water heater has fully regenerated - I just went to look and it was silent, so I'll take a few more temperature readings without emptying it this time.

115F would be hot enough for any shower or bath, so the above results look good, as far as temperature goes, but of course, not for the pressure issues. 115F might not be hot enough for laundry in the hot cycle, but the washer will heat the water up, using more electricity, though, and taking longer.

Edit: so, I repeated it after WH had regenerated. Got the same results for pressure.

But I was able to get temperature above 130F at all sinks and showers that had pressure. I even hit 141F in one case. I guess none of these fixtures must have anti-scald. A bit surprising. None of the shower valves were replaced by me. But the 2 single-handle faucets in the kitchen and the single-faucet faucet in the bar were, and not with low-end stuff. I guess it's less important to have anti-scald for kitchen faucets than shower valves since that feature is not in my kitchen faucets.

I'm going to turn the blue valve and pump back on now to see if the pressure issues are resolved. Edit 2:

Downstairs bathroom sink : got a very small stream of hot water.

Pressure definitely improved with the pump on / valve open for this case.

Guest bathroom #1 upstairs sink, separate handle : no water on either hot or cold sides. Both valves were open.

No difference with pump on and valve open. So, must be a faucet gone bad.

Guest bathroom #2 upstairs sink. Pressure seemed low.

No difference in pressure with pump on and valve open.

Guest bathroom #2 upstairs,  shower valve : pressure seemed low.

I believe the pressure was higher in this case with the pump on/valve open, but didn't do a scientific measurement to be able to conclude one way or the other. It was enough pressure to take a shower in either case.

[brobin]

Just to be sure which is which, close the round red handled valve near the ceiling and turn off the pump.  You shouldn't get any hot water.