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.

On 2007-04-10, Frank Cusack wrote:

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.

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

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.

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

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

On Tue, 10 Apr 2007 16:10:21 GMT Wayne Whitney wrote:
On 2007-04-10, Frank Cusack wrote:

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.

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

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.

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.

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