tankless water heater and voids

[Frank Cusack]

I have a tankless water heater. It takes 3 seconds for it to detect a flow, and 3 seconds to heat up. So, assuming cold water in the (output) pipes, it takes however long it takes for all the existing water to run through the pipes (call this T1) PLUS 6 seconds for hot water to come out of the tap. ok so far.

What happens though is that after getting hot water and then shutting off the tap, there is now a 6 second cold water "void" in the pipes. so hot water comes out for T1 seconds, then 6 seconds of cold water, then hot again.

This is annoying if, e.g., you run the shower, then use hot water at the sink. Initially it's hot, but then it goes cold.

Any ideas on how to fix this? I'd thought of having a recirc pump and/or a small storage tank, but I'm not sure that either one will address the problem adequately if at all.

[Wayne Whitney]

Yes, this is commonly called the "cold water sandwich".

Well, I have recently put a 2.5 gallon electric tank water heater just after my tankless gas water heater. The effect is to spread out the temperature change, e.g. instead of getting cold water for 6 seconds, you get slightly less hot water for 30-60 seconds. Plus the heater in the tank helps a little, as it turns on for most of that period.

A more complicated solution would be to use a house-wide tempering valve. E.g. set your water heaters at 140 F, and set your tempering valve at 130 F. Then when the output temperature of the system dips a little, the tempering valve should compensate.

Cheers, Wayne

[hallerb]

tankless sure seem to require a lot of work arounds and adding tank type at fixtures costs more for energy, and creates standby losses.

[Frank Cusack]

Thanks, I think I'll try the first approach. Sounds easy to do.

-frank