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

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 in message
oups.com...

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.

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

Jim Alexander wrote:

"Robert Laws" wrote in message
oups.com...

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.

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

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.

"Jim Alexander" wrote in message
news

"Robert Laws" wrote in message
oups.com...

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.

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

Fitted a Mira mixer to my combi system 10 years ago never had a problem

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!!!!

Tony

"Robert Laws" wrote in message
oups.com...

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?

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

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

On 10 Jan 2007 01:36:51 -0800, "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

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.

In article .com,
"Robert Laws" writes:
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?

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.

--
Andrew Gabriel

Andrew Gabriel wrote:
In article .com,
"Robert Laws" writes:


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?

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.

--
Andrew Gabriel


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

"Robert Laws" wrote in message
oups.com...

Andrew Gabriel wrote:
In article .com,
"Robert Laws" writes:


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?

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.

--
Andrew Gabriel


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'.

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

Jim A

Jim Alexander wrote:



Have a look at the bar mixer available from Lidls next Thursday for £19..99

Jim A

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

thanks, Robert

"Robert Laws" wrote in message
oups.com...

Jim Alexander wrote:



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

Jim A

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

http://www.lidl.co.uk/uk/home.nsf/pa...ower_Mixer.ar6