New heat pump water heater and thinking of adding hot water loop. Pump or thermosyphon?

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Deathtofishy

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Good Morning,

I'm going to be replacing my hot water heater in the next couple weeks with a hybrid water heater (rheem performance platinum) and also am planing on doing a bathroom remodel this summer, which is my furthest water fixture from the heater. I was looking at considering a hot water loop because currently its roughly 45-60 seconds (depending on season) to get full hot water at the shower.

My hot water heater is in the basement, and the furthest fixture is on the second story of the house. In researching it seems like I have an option for either a recirculating pump or I can make a thermosyphon loop to the bottom of my hot water heater.

The thermosyphon seems simple enough and sounds attractive to eliminate the need for a pump.

Only negative I've seen is some possiblity of bacterial grown in pipe water that is left stagnant? I do have a garage sink and basement laundry that are not run very often, but every other line would be ran daily.

Any issues with using pex on this system? I'm planning on running 3/4" pex to the furthest fixture (having all other fixtures in the house tee off this fixture) and then having the return be 1/2" to the hot water heater.

The Rheem's cold water supply actually connects at the bottom of the tank, so I'm not sure if I should tee into this or the drain, or if it matters?

Are there are any other downsides or considerations with a system like this?
 

John Gayewski

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Thermosyphon and gravity systems aren't legal. But I do think a pump has been known to kill the effeciancy of a hybrid heater. Make sure whatever you do you insulate the piping with a minimum of 1"thick insulation. Generally 1/2" is considered too small for a recirc line these days
 

Deathtofishy

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Interesting. I had no idea they weren't legal. I can't find that written anywhere when doing research on this.

I just assumed they weren't common (locally) because Its rare to have have a hot water heater in a basement (they stopped building them in my city in the 1960's), or crawlspace where I am located.

However it looks like there are some concerns on effeciency/short cycling with a pump. Rheem has even released a technical service bulletin about this which I attached for any future searches on this subject. Ironically they also show a hot water return on their brochure when using multiple hybrid tanks in a commercial application.

It seems one needs to make sure there isn't more than a 15 degree drop in temp from the heater output to input from loop with a pump. It may also limit you on what modes you can run, specifically the energy saver mode.

With all this in mind I may skip the reciculating system idea all together? I suppose I could run the return line and A/B it with it valved off and see what energy use difference I'm getting within the Rheem app. Worse case I abandon the return line and I'm out some 1/2" pex and a little time. I'm replacing all the 70+ year old lines to the bathroom anyways and most of the run will be in the basement asides from the one wall up to the bathroom.
 

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