thermostatic shower mixer that controls only the cold

[Robert Laws]

I know this has bee ndiscussed recently, but I don't think this particular question has been answered.

When a direct heating combi boiler is used there can be a conflict between a thermostatic shower mixer and the boiler's own temperature control. Ideally, to get the most powerful shower, one would like the boiler runnign aty full tilt, with the water flow adjusted so that the temperature is nice.

With a thermostatic mixer that controls the hot flow things can go unstable: Shower is too hot; thermostat reduces the flow of hot; hot water then gets hotter and shower output gets hotter; thermostat reduces the hot flow further; boiler cuts out as its outlet temp is too high; shower goes cold; hermo opens up the hot supply; shower gets too hot ....

To solve this one needs a thermostatic shower mixer that controls the cold supply, not the hot and ther woul dbe a limiter in the hot supply so that 'full on' gives a temperature a bit above a comfortable shower tempertaure.

Are there any thermostatic mixers that control only the cold flow?

Specifically, for my application, is there one that would mount on the bath as a conventioanl bath/shower mixer with a spout for the bath and a hose connection for the shower?

thanks

Robert

[sm_jamieson]

If you have a combi that tries to maintain a constant temp output then this cycling will not be a problem. But in my previous house this did happen to some extent. The combi only had 2 settings at 30% and max. Note: you could still have the problem with a cold control thermostatic mixer depending on plumbing. If the mixer turned up the cold, this could rob the combi input, thus less water goes through it and it gets hotter etc. Not as likely but just about possible. As far as what the mixer controls, this information does not seem forthcoming. You would have to examine the internals to see what it does. Simon.

[Robert Laws]

That's a good point about the drawing of cold also affecting the supply of water to the boiler.

I'll have to check my boiler to see whether it does any kind of continuous adjustment of teh flames or whether is just oges on/off.

it might be better for me to not have a thermostatic mixer at all: Simply use the hot tap flow rate to control the temperature, assuming I can get it cold enough that way, but then the shower temp would fluctuate depening on othehr taps i n the hosue being used. but at least it would not go into unstable oscillations from icy to boiling!

R

[Jim Alexander]

Cannot answer your specific question but I have previously installed a thermostatic bar type mixer (Screwfix or other Italian unbranded) on a Combi installation and it has performed well. Since the temperature of the combi delivery lags any imposed change in flow I would anyway have expected a decent thermostatic mixer to cope.

Jim A

[sm_jamieson]

Just to add due to relevancy, my current bathroom refit, I have a modulating combi, a pressure balancing/equalising valve and then a triton tyne thermostatic bar mixer. Since thermo valves have some delay in adjusting temp when another tap is turned on, I thought the balancing valve would help. I have not yet had a proper shower with this setup (still tiling !) but have tested the shower with hair washes etc. I was a bit worried that the 3 things in this equation would fight at some point, so its a bit of an experiment, but has seemed OK so far. Worst case, out comes the balancing valve ... Cheers, Simon.

[TMC]

However IIRC there were 2 types of mixer temperature balanced and pressure balanced. One being for normal systems and one for Combi systems. I think I used the pressure balanced one

Google found Plumbworld which says that eiother type can be used ofr a combi system!!!!

[Andy R]

Aqualisa make 2 different versions of an apparently identical bath/shower mixer. One is for combi's, one is not. Maybe this is the issue it resolves.

Rgds

Andy R

[auctions]

I'd just fit a straight forward mixer valve with a pressure balancing valve fitted to the pipe work -

[Pete C]

Hi,

Sounds like the cold supply is inadequate, and theres not enough to satisfy the cold input to the boiler and to the mixer.

Try opening a cold tap on the same pipe to the shower mixer and see what happens to the flow when a hot tap is opened and the boiler fires. Might be worth measuring the flow with a bucket before and after.

If there's plenty of flow maybe the shower inlet strainers are blocked or it's scaled up.

cheers, Pete.

[Andrew Gabriel]

You need a thermostatic mixer designed for Multipoint and Combi use. They are more complicated than just controlling the cold, but they know they can't adjust the output temperature by just adjusting the hot flow. They are also very fast acting (unlike the wax pellet bar type), but are more expensive as a result. They are also designed for high pressure cold feed a hot feed which can rapidly change from low to high pressure and vice versa whilst in use.

[Robert Laws]

Thank you all for your helpful comments. it is clear that I need to obtain a mixer designed to cope with this situation and not just buy 'any old thing'.

many thanks,

Robert

[Jim Alexander]

Have a look at the bar mixer available from Lidls next Thursday for £19.99

Jim A

[Robert Laws]

Please can you clarify? google cannot find anything to match this.

thanks, Robert

[Jim Alexander]

Please can you clarify? google cannot find anything to match this.