plumbingwaterwater-heater
Once I saw on an episode of This Old House that they were putting in hot water return lines: pipes that recirculate idle hot water back to the heater so you get hot water right away, instead of wasting time and water.
My wife and I are looking to getting our first home and I think this might be a beneficial investment to make at some point. I also am skeptical of the cost.
Is it worth it to install them? Or is it a matter of convenience?
Without natural gas or propane, your only source of heat is electricity. Electric tankless water heaters would probably require a new circuit to be run. Here's a table of typical power and wire requirements:
http://www.rheem.com/documents/rte-13-tankless-electric-spec-sheet
The smallest unit they recommend for a shower is the RTE13 which takes 13 KW or 54A@240V. This requires #6 wire. If your incoming temp is 40F and desired out temp is 110F, you need 70F of temp rise and the RTE13 will provide 1.27 GPM.
Many people think they want tankless because the mfg tell them tankless is more efficient. There is some truth to this for gas fired water heaters but in the case of electric water heaters, the difference in efficiency is very small.
I suspect you have a set-up like this?
You are concerned that when the water gets pumped back into the "water heater" it could possibly pollute your main water line?
The only thing I could suggest is putting a check valve on the "cold" water supply so that it can only supply fresh water. That is helpful if the pumps output pressure is higher than your supply pressure (This could happen on someday with low supply pressure or if you do not have main water pressure regulator)
In this situation the return is a dedicated port on the "water heater/boiler" and there should already be a check valve/pressure regulator/non return valve just before the water heater. So the rest of the house will run just on the cold water supply and never get any recirculation contamination.
Best Answer
After doing a bit of research on what exactly hot water return lines are I found this page which goes into a lot of detail about how they work and their benefits and drawbacks.
The big drawback I see is that you will need to make sure that all your hot water lines are well lagged to minimise the inevitable heat loss that will occur with the hot water sitting in the pipes. Otherwise you will be heating water for it to go to waste when you aren't using it.
So you will need to add up all the costs:
and decide whether it's a fair price to pay for instant hot water.
Another alternative would be to change the pipework so you don't have any long runs or even install secondary heaters for parts of the house.