Hot water recirculating pump

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Jim C.

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Portland, OR 97219, USA
I'm putting in a hot water recirculating pump with a dedicated line. The plan is to control it with Smart Recirculation Control's model 32. The unit mounts between the water heater and the pump and cycles the pump by detecting a change in the water temperature, which is achieved by simply opening the valve then closing it then reopening it.
The water heater (conventional electric tank type) is about 55 pipe-feet from the farthest valve which includes about ten feet of vertical.
My plumber suggests installing a Grundfos Comfort 10-16 A PM BU/LC pump rated at 2 gpm. The one I've found is a Grundfos UP 15-10 SU7P/TLC rated at 6 gpm. From what I gather, that 55' of pipe holds about 1-1/4 gallons of water so my plumber's pump would take about 35 seconds for the water to make it to the far end while mine would take about 12 seconds. Why would I go with the 2-GPM-rated one and not the 6-gpm one? Or is there more to it than simply GPM?
Second, both of our pumps have more bells and whistles than I need. All I need is an appropriate pump, preferably one with either a stainless steel or bronze body, so any suggestions you might have would be appreciated.
Thanks in advance.


Retired Defense Industry Engineer xxx
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New England
FWIW, the Copper Institute's design guidelines say to not design a hot water system for more than a 5 fps flow rate...on a 1/2" pipe, that's equivalent to 4gpm. While you could put a valve in the line on a 6 gpm pump, it is also going to use more power.

IMHO, the better solution is to put the system on a timer so that you'll get hot water there when you need it, but you don't need a big pump to do that. The one on mine only draws like 9W, and a HP=7xx something watts, so it's quite small, quiet and works. Mine runs maybe a couple of minutes the first time it turns on with the timer, then maybe 45-seconds about 4-5 x per hour. Make sure to insulate your lines or you'll be wasting more water.

Another reason to have it run and keep the lines hot during the period of use...say it moves 2-3 gallons into the tank before you achieve full hot after the lines have cooled off, that's 2-3 gallons less hot water you have on tap to use because you've put that into the bottom of the WH in the process. If it runs and maintains the outlet temp, you have the full capacity of the WH to use at any time.
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