Here is a strange one. Started with a running toilet (just a small drip so didn't bother fixing) then noticed the water in the tank was warm... almost hot. Very strange.

Then I turned off the hot water to and from the HW tank.
Water still flows freely through the hot water taps in all the house, although it quickly turned cold.

I turned on the hot again and noticed that if you turn on the cold water tap close to the shower (with the Foremost mixer) the water runs quite hot for a second or two then cold.
Sooooo. I figure the foremost valve must be flowing between H and C. Take it apart. Looks OK but when you turn it on just a bit, warm water flows, on full(H) very hot flows.

Any suggestions??

What happens when you shut off the angle stops to the "formost mixer valve"?
And what's a foremost mixer valve anyhow? A lavatory valve? Sounds like a cross connection caused by a faulty cartridge. Regards, Tom

Thanks for getting back to me. There are no shut off valves to the shower mixer, and I'm going to have to tear a wall out to get at the valve to replace it... which is why I want to be really sure that is the problem. There is one other shower downstream from the problem but I suspect the Foremost because the valve gets warm when the cold tap is running in the sink nearby (Foremost is the name of the manufacturer of the valve - although they don't seem to make them any more.) Everything else I have been able to test by turning off the stop valves, except the one other shower. (The separate tub is on separate H and C taps.)
Anyway I'm going to take apart the valve again and see if I can see anything amiss.. . Then out comes the wall!
I'll let readers know what happens. I'm starting on it now.
This is a great site and thanks for your concerns.


Sure enough. I took apart the black plastic part of the valve and there is a pressure compensator with a torn rubber gasket between the H and C. That's the problem! Think of all the hot water I have been wasting over the years!
Now if I can fing a new cartridge!

Couldn't find the cartridge. The company doesn't make them anymore! Tore the wall out and installed a brand name mixer. No more hot water in the toilet!
Thanks everyone! Great site!

And we thank you for the update. Glad you got fixed up. Tom

Identical problems as above and yes it is the pressure compensator membrane. It's apparently made in Taiwan but they never answer emails. Foremost discontinued the design so it appears one is SOL.

Does anyone else know about these pressure compensator membranes and where one could get a replacement?

Otherwise I face the same problem as the water is flowing across the H to C side or reverse whenever there is flow.