Robert Laws wrote:

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

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.

sm_jamieson wrote:
Robert Laws wrote:


...To solve this one needs a thermostatic shower mixer that controls the
cold supply, not the hot.....

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


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.

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

Robert Laws wrote:
sm_jamieson wrote:
Robert Laws wrote:


...To solve this one needs a thermostatic shower mixer that controls the
cold supply, not the hot.....

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


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.

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

I'd just fit a straight forward mixer valve with a pressure balancing
valve fitted to the pipe work - http://www.plumbworld.co.uk/344-0000

A