Grundfos comfort UP10-16 installation - why drain for return?

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Mikes1098

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Our hot water system has a Grundfos comfort UP10-16 recirc pump installed -- with a dedicated return line. The return is plumbed into the cold line going into the hot water heater - which seems ok to me.

However, when the sprinklers go on and the water pressure drops, it seems to get into a vapor lock somehow on the pressure side. The pressure side of the pump (returning to the hot water heater) gets insanely hot, like there is steam in the line... while the suction side is cold. So clearly, no flow. Once I purge the line, everything works fine again.

It seems a bunch of people say to plumb the return into the drain.... including this YouTube video by Grundfos ( at 1:15):


But an equal amount of people are passionately saying that is wrong :)..... I too originally felt this was only done because it was easier and not correct. But the YouTube video made me question why they would do that on the new install from the manufacturer.

Does anyone know why installing the return through the drain port would be the recommended configuration? I did notice the pump only has a max of 4 ft of head.... this is very close to the distance to the top of my tank..... not sure if that is why.....

Thanks!
 
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breplum

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Return going in to cold line on WH is fine, as long as there is a check valve.
Once there is a check valve, there also must then be a expansion tank.
 

Terry

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I've run water heater recircs both ways, to the incoming cold and to the drain. Always with a check valve on the cold supply and on the 1/2" hot recirc. I guess that's two check valve if you're counting.
I like to plumb to the drain best. It is nice to have some options though.
 

Mikes1098

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Interesting....There is a check valve in the 1/2" recirc line..... but no check valve on the supply line. And no expansion tank. I could see that slowing the loss of pressure when the sprinklers go on. I do know the cold supply line gets warm when that happens..... I wonder if that would fix my problem.

Thanks!!
 

John Gayewski

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Text book piping is pump toward the heater and the check valve. Pipe the recirc into the drain. There are other ways but this keeps your system open so no need for other components. The water will only circulate in the loop because it's pulling and pushing at the same time, which keeps the cold unaffected. This is the way its taught (in school) and drawn by professional designers. But different situations call for different piping.

Most people do what they have either seen before or done before and worked well which is fine.
 

Mikes1098

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Fitter30.... here is a picture.... I figure that is better than a diagram :) The only thing outside of the picture is a 2nd ball valve in the recirc pipe to isolate the pump and a spring loaded check valve (again in the recirc line).

Actually looking a little closer.... I think there is a check valve in the supply line. Wonder if it is stuck open?

Screenshot 2022-08-19 211524.png


If I understand Breplum and Terry - they would add check valve in the cold water supply line and an expansion tank. The items in orange.

Screenshot 2022-08-19 213636.png
 
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Mikes1098

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Text book piping is pump toward the heater and the check valve. Pipe the recirc into the drain. There are other ways but this keeps your system open so no need for other components. The water will only circulate in the loop because it's pulling and pushing at the same time, which keeps the cold unaffected. This is the way its taught (in school) and drawn by professional designers. But different situations call for different piping.

Most people do what they have either seen before or done before and worked well which is fine.
It does feel like a "cleaner" loop going into the drain..... to your point, there would be no influence from the supply and it would be an isolated loop.
 

Fitter30

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All water heaters on the cold side have a dip tube that directs the cold to the bottom of the tank. Going through the drain valve location does the same.
 

Fitter30

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W 3.9 feet of head = 1.7lb everything has to 100% If theres 50' of pipe and 10 fitting your over the pump capabilities especially with a swing check.
 

John Gayewski

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All water heaters on the cold side have a dip tube that directs the cold to the bottom of the tank. Going through the drain valve location does the same.
This is like saying walking across the street is the same whether you do it in Manhatten at 12 noon or crossing the street in Mayberry. They are both crossing the street.
 
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