Passive recirculation loop? Help solve a mystery.

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Taki

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New (to me) home. Two water heaters in line in the basement (photo attached), master bathroom on upper floor in the opposite corner of a long house. Takes 3-4 minutes for hot water to get there...
Looking at the heaters, I see what seems to be a loop of a smaller size pipe coming into the drain outlet of the first (cold side) heater. I'm attaching the picture. There is also a patch made with PEX, which made me think maybe something like a valve or a pump was there and got removed.

Could that be a passive recirculation loop? Naturally, I opened the valve and waited overnight. I didn't take any time or temperature measurements, but it seemed like hot water got there faster BUT it wasn't as hot as it should be! Thinking about it while standing in the lukewarm shower, I figured it's pulling cold water from the bottom of the first heater in parallel to the hot water from the second heater. Unacceptable! Went downstairs wrapped in a towel, shut the valve off, back to the shower - the water is hot again!

So, am I correct in the assumption that it is a passive recirculation loop put in by the previous owners (and eventually abandoned for some reason)? And if so, how do I solve the cold water mixing issue? I was thinking about a check valve of some sort, but would the regular spring loaded valves open by the thermosiphon pressure? Is there a swing valve of some sort that could be used? How do I make the system work?

I hate wasting so much water and energy waiting for the hot water to show up.

Thanks for the advice.

wh1.png
wh2.png
 

Terry

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To use it as a recirc line, it needs a check valve. A spring check won't work, and I'm not sure about a swing check.
Most of the time, we put a pump with spring check on those.
Having a clip on aqua stat is nice.

grunfos_recirc_chart.jpg
 
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John Gayewski

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Someone removed the check valve a spring check won't work. Add a pump. You really don't want stagnant lukewarm water sitting in your pipes growing organisms.
 

Taki

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Someone removed the check valve a spring check won't work. Add a pump. You really don't want stagnant lukewarm water sitting in your pipes growing organisms.
Would something like this installed horizontally work? It should offer minimal resistance to the hydrosiphonic flow I think. I am just curious how these pump-less systems work - I would probably go with a pump and a temperature sensor.

Also, John, with a working check valve and no backflow, wouldn't all the water from the loop go back only through the heater and be fully heated? Seems like that would elimknate any health risks. Just trying to wrap my head around it.
 

Master Plumber Mark

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keep in mind that their is always a reason that they took out that pump and installed that ball valve in the line.... perhaps they started to have pin holes in that return line and decided to just kill it off completely.... we have seen copper return lines get paper thin after less than 10 years and pin holes begin to appear

you could probably install a swing check valve in that horizontal line with the arrow(flow) pointing towards the heater and see if it will stop the water from backing up that line but still allows enough flow in the right direction to still move the water....

or just leave it off and alone and live happily ever after....
 

Taki

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keep in mind that their is always a reason that they took out that pump and installed that ball valve in the line.... perhaps they started to have pin holes in that return line and decided to just kill it off completely.... we have seen copper return lines get paper thin after less than 10 years and pin holes begin to appear

you could probably install a swing check valve in that horizontal line with the arrow(flow) pointing towards the heater and see if it will stop the water from backing up that line but still allows enough flow in the right direction to still move the water....

or just leave it off and alone and live happily ever after....
Yeah good point. I was wondering why it got cancelled, but the line is under pressure, so probably not because of leaks. Too bad houses don’t have logbooks documenting what kind of stuff went down before your time.
I will try a swing valve. Can’t live happily ever after while wasting 5 min of water every morning waiting for the hot water to arrive. I bet it’ll be even longer when winter comes.
 

Reach4

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I remember somebody posting a suggestion to use a swing check valve, but tilting the valve so that the flapper was just barely open. But during hot water use, there would be enough pressure drop in the return pipe to swing the flapper closed.
 

John Gayewski

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Would something like this installed horizontally work? It should offer minimal resistance to the hydrosiphonic flow I think. I am just curious how these pump-less systems work - I would probably go with a pump and a temperature sensor.

Also, John, with a working check valve and no backflow, wouldn't all the water from the loop go back only through the heater and be fully heated? Seems like that would elimknate any health risks. Just trying to wrap my head around it.
The water goes through the loop but without a pump it can be so slow that the end of the line is built up with bacteria. Since it takes 160f to kill this bacteria instantly it flows from the pipe into the water and out of you faucet being that your not running water hot enough to kill it.

The purpose of the pump isn't to sanitize the water, it's to keep the pipe hot enough to be inhospitable to legionella. A pumpless system isn't legal anymore for this reason. This is also why a professionally designed system doesn't have a hot button or any other non thermostatic means of controlling the temp.
 

Fitter30

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Also when heaters are in series the first heater get the majority of lime out of the water and you don't get the total gallon capacity from the two tanks. 2 Tanks piped reverse return get a total cap and the lime is more equal.
 

Taki

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I remember somebody posting a suggestion to use a swing check valve, but tilting the valve so that the flapper was just barely open. But during hot water use, there would be enough pressure drop in the return pipe to swing the flapper closed.
Nice memory! I found the thread from 2016 here, and will try both approaches: either 45 deg angle on the swing valve (as suggested by nomad297, or perhaps moving the loop to the second heater without a check valve, since the water pulling through the loop would be already hot, coming from the first heater. The first approach seems to be preferable.

Man, both plumbing and the internet, being a series of tubes (Al Gore), can be equally complicated...
 
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