Cost of operating a gravity recirc loop?

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md9918

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I have about a 40 foot run of 3/4" pipe between my hot water heater and just about every fixture in the house, which, when combined with today's low-flow fixtures, means a full minute wait for hot water. I am intrigued by the idea of a gravity fed recirc loop-- nearly instant hot water for $100 in parts and a Saturday afternoon, all powered by the laws of physics! (And it will force me to finally get around to flushing the HWH)

I'm curious though-- those of you who have installed one, how has it affected your utility costs? It seems like it might be noticeably more expensive to keep the pipes hot 24/7.

Another option I'd considered is 1/2" pex home runs for the hot water for the two bathrooms and kitchen-- by reducing the diameter of the pipe from 3/4" copper to 1/2" pex, I'd be reducing the amount of cold water to be purged by about 40%, so hot water 40% faster. At least with this option, I'm not heating the pipes constantly. Is there any downside to this option?
 

John Gayewski

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I install them in my houses. I love it but don't notice the cost difference. The only problem is... its not legal so mine are set up to easily install a pump if for some reason someone decides to enforce the law. Insulate the piping and there's not much cost at all. It's could be added up for someone with the motivation.
 

jadnashua

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1/2" pex may not be great to a bathroom group unless you have at least two home runs. THen, you might get hot at your shower, but the sink is still cold. If you ever try to fill a tub, especially if it's a larger, soaker one, pex isn't your friend in 1/2".

Unless you can make sure you don't have any dips or uphill portions to your gravity loop return, it may not work, and you need to be very careful about the check valve you select, as that can be enough resistance to stop the flow.

A pump will cost more to buy, but if you've insulated the hot pipe runs and the return, considering you could put it on a timer or a time-of-use control (less convenient), it doesn't need to cost much to run. You don't need a huge pump (mine is only about 9W when running), and if it's got an aquastat, doesn't need to run all that often (mine runs maybe 45-seconds about every 10-minutes). My total run from the WH to the furthest distance is about 40'.

Another advantage to recirculating the hot is that your total hot water volume is greater as that in the tank AND the lines is all useable as hot rather than dumping a few gallons down the drain and cooling off the WH tank with the replacement, cold water thus extending the amount you have. Depending on where you are, water is or probably will be, an escalating cost and scarcity item, so saving it is good for everyone.

If I go away for a week or longer, I shut the recirculation off. On one trip, I was away multiple weeks...I did not see any significant change in my gas bill.
 

md9918

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Thanks guys. I'll give it a try with gravity first, if no dice, I'll look into adding a pump.
 

Mr tee

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$100 in parts sounds optimistic, as does retrofitting a gravity system.
 
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