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Andrew Smith
Andrew Smith, Plumber
Category: Plumbing
Satisfied Customers: 3767
Experience:  City and Guilds qualified plumber
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Last year I had a new shower mixer fitted (replacement

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Hi. Last year I had a new shower mixer fitted (replacement for something similar) and I had a power pump installed to take advantage of the large shower head.
Unfortunately the cold water supply seems to be creeping into the hot system and eventually the overflow from the tank in the attic kicks off. At one point we could turn the shower mixer to the cold setting and that seemed to work, but not of late.
Any ideas if it's the shower mixer or the power pump please?
Thanks. Karl
Even if the pump is boosting the hot and cold water, the two pressures never actually mix until the shower valve.If you fit non return valves to the shower or pipes that feed the shower then it will cure this problem
Customer: replied 2 years ago.
The shower mixer - a Phoenix traditional HFVP001 - already has check/non-return valves fitted in each of the two inlets, so why would I need to introduce more of these before this unit?
The hot one must have failed for the cold water to force its way back into the hot system and fill the tank.Unless you have another mixer in the house?
Customer: replied 2 years ago.
No more mixers in the house and both check valves were replaced by a plumber on Friday.
If there are isolation valves before the shower then turn the cold one off and see if this cures the tank overflowing.Turn off at night and check in the morning before you turn back on again, mark the water level with a crayon if you need to.
Customer: replied 2 years ago.
Morning. Firstly, I'm an IT director so please forgive my ignorance and lack of plumbing jargon, and note I'm applying logic only and def not any knowledge/expertise.
Here's some further info which might help:
When the issue started a year back only two things changed - the shower mixer swapped out, and a 'monsoon' pump installed to give us sufficient pressure for the new larger shower head. Oh, and subsequently a new kitchen sink mixer tap in lieu of single taps.
I don't have a cold shut off for the shower supply (and the pipes are within a tiled stud wall, so not easy to get to) but I did reduce the cold water pressure to 2 bar and overnight no overflow. So, that's good news and the only downside is the cold pressure is a tad low, particularly in the kitchen. This is with the shower mixer in the cold position. Significance of this is that I've reduced the pressure before now and with the mixer in the (mostly) hot position (I.e. Just leaving it how we use it each day), but that led to an overflow after about 5 hours. Not as fierce as before but nevertheless overflowing.
When I shut off the hot, from the inlet and outlet pipes on the monsoon pump, then no overflow as expected (and no hot water). I don't have a separate shut off valve.
I have a gas heated water tank in the airing cupboard and the supply tank is in the loft. Not entirely easy to get to but I have my ladder rigged while I'm monitoring.
When the plumber returned and swapped out the check valves it appeared that they had become a little squashed and distorted (twice now). On the Phoenix HFVP001 they are in the elbow joint which bolts on to the inlet pipes, and on that side of the elbow if you know what I mean. Therefore, I was wondering if they were being damaged because the plumber had too much inlet pipe inside the elbow joint. He tells me he leaves 25m as standard for fitting showers. On the second occasion he questioned whether these should be on that side of the elbow joint and fitted the replacements check valves on the other side.
Would having a monsoon pump fitted, where there's virtually no pressure until it kicks in make any difference? I.e. Having lots of cold pressure on one side of the shower mixer and virtually no hot on the other?
If the shower mixer check valves were installed correctly could it be possible that these are too weak for our system/situation?
Many thanks for your patience.
Sometimes too much pressure differential can cause a problem even if you have non-return valves fitted.Can you fit a full size non-return valve into the copper pipework after the shower pump?It proves this is the fact as when the shower is on cold only there is no mixing of hot and cold, therefore it cannot force its way up thge hot pipe.
Customer: replied 2 years ago.
Hi. It does find its way up that pipe eventually, even when on cold. Our plumber did fit a non return valve (large heavy duty looking affair) after the monsoon pump, on the outlet pipe. However, the effect was that the pump wouldn't detect the drop in pressure/demand for water so it wouldn't kick in. The plumber then removed the inside of that valve to get our system going again.
Any other ideas Andy?
The only other way to cure this is to run a new cold supply from the tank to supply the shower so it is a balanced supply.To save another tank connection, you cold tee into the cold feed that feeds the cylinder but just after the tank, not down by the cylinder.
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Customer: replied 2 years ago.

I gave this some thought, after bringing in a heating and plumbing company for another view (who recommended I spend over £800 reverting the system to what it was before installation of the monsoon pump and new shower mixer), and decided to go with my own 'fix'. I will have a local plumber install a concealed cold shut off valve (chrome lever to match the other fittings) just under the shower mixer.

That means we simply turn this on and off when we use the shower. No more overflow.