DIY Doctor

Expansion vessel for mixer shower

Postby Barry Bunsen » Sat Apr 26, 2008 2:06 pm

The instructions for a Triton thermostatic shower fitted with a combi boiler state an expansion vessel must first be fitted between the stop tap and the combi itself UNLESS an expansion vessel is already be present in the combi. This would have to be in addition to the larger central heating expansion vessel in the combi.
Could someone explain if combi boilers usually have two expansion vessels and also is it normal to fit one when installing a thermostatic shower.
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Simply Build It

Postby htg engineer » Sat Apr 26, 2008 4:07 pm

The manufacturers instructions for most showers state that the main cold water supplying the boiler should be fitted with an expansion vessel/pressure accumulator - if your shower breaks down the manufacturers warranty will be void if not fitted in accordance to their instructions.

The purpose is to allow for expansion of the water so that it doesn't damage components in the shower or the boiler.

Now there's different theories depending on who you ask.

Same say no need for one.

Some people say, you only need the expansion vessel if the cold feed to the boiler has a non-return valve fitted which would prevent expansion back down the cold supply.

Some say the float operated valve on a WC will do the same job as long there's no non-return valves where the hot water could become trapped between the shower and combi.

Maybe other people on here have heard different or have other theories.

I have never seen a mixer shower where a expansion vessel has been fitted to the mains cold water supply, and I have never known anyone have any problems with their shower or boiler because there was not one fitted.

For the price though starting around £15 - you know the manufacturers warranty is valid, and the system is safe ????

I was intending to fit one in my house will only take a few minutes - but have been using the shower daily now for 2 months. I will get round to it eventually.

Hope this helps
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Postby Barry Bunsen » Sat Apr 26, 2008 6:30 pm

Thanks for your time. You have been most helpful.
Just for interest does anyone out there fit expansion vessels to thermostatic showers or has anyone had problems because they did not fit one?
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Postby rosebery » Sun Apr 27, 2008 8:14 am

"anyone had problems because they did not fit one?"

No. HE's post summarises the situation very nicely so nothing more to add.

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Postby Skids » Sun Apr 27, 2008 10:03 pm

Hi Barry Bunsen

Yes, there is an expansion vessel on the ‘sealed’ CH system, which is there to accommodates the 4% of the system volume when hot (e.g. if the system holds 100ltr when cold, there will be 104ltr when its hot), there is also an DHW expansion valve/vessel which takes the this expanded volume.

Think of your electric shower ~ when you switch it off, it keeps running for a bit until the water is running ‘cold’ this ensures that when it turns off, there is no hot water in the pipe, which would cause damage, either to the pipe or at the fitting at the either end.

In the combi/condensing boiler, when you turn ‘off’ the hot tap the boiler senses that the flow has stopped and the heat is cut off, which also stops the flow of water, however there is still hot water in the hot pipe system to the tap? As hot water has more volume that cold water its got to be accommodated somewhere.

If the boiler is fed from a CWSC in the loft the expanded water will/should be accommodated in the ‘cold fed pipe’ to the boiler. If the boiler is mains fed there will/should be a double check valve fitted which will not allow the water to expand back down the mains cold supply.

Always follow instructions, also check out the instructions for the boiler, if you need more info contact the boiler manufacturer direct, as although in principle the same (they heat water) all boilers are different, as in it may not have two EV’s.


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Postby Barry Bunsen » Mon Apr 28, 2008 9:29 am

Thanks for your time Skids. Very useful
I suppose the combi is always under this pressure with any hot water tap - thermostatic shower or otherwise.
One thing that puzzles me. I reckon the hot water in the pipe between the combi and the tap or shower has already expanded. It wouldnt continue to expand as it is now cooling slightly. The small volume of cold water just hitting the combi heater as the tap is turned off might expand but the existing hot water in the pipes would be contracting.
Perhaps I'm wrong. Someone else must have thought this through and decided otherwise. Not worth losing sleep over 15-20 pounds is it?
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