this post was submitted on 04 Nov 2025
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Ah well in that case, definitely.
Have you considered a recirculating pump?
That would have been nice, but our plumbing doesn't allow it. We would have to put in return pipes and that means tearing up the walls. Hard no, sadly.
Grundfos makes a valve that bridges hot and cold and automatically opens when the hot side is cool. It means your cold isn’t always cold and if your hot side isn’t potable it’s a problem, but might be a possibility. I use one in my system, because with on demand hot water I was running gallons down the drain waiting for hot water. https://www.amazon.co.uk/595926-Valve-grundfos-Recirculating-Bypass/dp/B0DN1D9P5B
I don't fully understand how this works. The pump and valve are used to circulate hot water though the hot pipe and back through the cold pipe.
But... doesn't that mean you never have cold water? Because the cold pipe is by definition also hot now?
It seems as if this just shifts the problem from "wasting hot" to "wasting cold" to get the water you want.
Yeah it’s confusing.
When the hot side of the valve is cold, the valve opens. When it’s hot, it closes. The pump pushes the cold water on the hot side through until warm water reaches the valve, then it closes, and the pump can just keep going without moving any water. As the water at the valve cools down again, the valve opens again.
With an aerator on the tap it’s not much more than a second of warm water so the wasted water isn’t really much volume at all - 100mL vs several litres. I also have a separate drinking water tap which is really my only cold water application so a bit of water at 30° instead of 18° doesn’t bother me.
Thank you Flax, this was a very useful explanation! I will look into it and figure whether I can install it in a suitable place in my bathroom.