Shower Mixer - Hot Pressure to High

[dogcatfish]

Good morning all. I've just replaced a set of separate bath taps with a cheap shower mixer as we're about to gut and replace the other room with the only shower. Been running a few tests and it's proving very hard to get a comfortable temperature balance, which I gather is normally down to having poor pressure from the hot tank.

We've got a combi boiler do the whole hot system is at mains pressure, and the cold for sinks and showers is from the tank. We've always had really good hot water, and other than humming quite loudly, the shower mixer has always been brilliant. However, I've found that on the two tap mixer it's fine in bath mode, but on shower mode as soon as the cold is turned on the pressure vanishes and all the hot water crossflows into the cold system (hot water at cold sink taps!).

While a check valve on the bath feeds would solve the crossflow problem, do you reckon it would allowthe temperature to be comfortably balanced? it's only a temporary fit so I didn't go all out with a thermostaic tap, and I'm concerned that fitting any type of prv would deny the benefit of having a combi boiler for good hot supply... I'm also this might become a problem when it comes to replacing the original shower valves which have worked well.

 

[Charnwood]

The hot and cold are both from the mains, so ought to be roughly the same pressure.
The flow rate of the hot should be a bit less than the cold, so you seem to have an odd problem.

A check valve is a must in the current circumstances.

 

[dogcatfish]

Mark
Thanks for the advice, installing a check valve was going to be my first thought to stop thebackflow problem. However, the cold is on tank pressure and very weak while the hot is at mains (the opposite to the usual shower problems). However, I don't think this'll solve any temperature/pressure problems. I'm wondering why the main shower copes fine, but the only way to get this one stable is to open the cold fully then crack the hot open a tiny amount - ok temperature but pressure very low...

 

[Charnwood]

There's too big a differential between the tank fed cold and mains hot. Most mixers can cope with a dofferential. Yours can't cope with what it's getting.

Could you get a cold mains feed to the shower?