Mixer shower and combi boiler problems

Sorry for the boring thread but STW usually has a bit of helpful advice with stuff like this! Finally got a plumber in to fit a mains shower after almost 2 years of putting up with a temperamental/weak electric shower. We have a glow worm swiftflow combination boiler so no hot water tank etc. The mixer is a thermostatic one, can’t remember the brand off hand. Anyway, to start with the shower is perfect but then goes cold after a minute or so then back to hot, so on and so forth. I think it’s something to do with the thermo tap cutting off hot water to maintain temp which then turns the boiler off so it goes cold again til the tap decides it needs more hot.

My girlfriend said it all worked fine the other day when she used it whilst the washing machine was running so maybe a problem with the hot and cold pressures not matching? Would I be better off with a standard/not thermostatic mixer?

Got the plumber coming round again on weds but any suggestions to help in the meantime would be great 🙂

I had something similar when I plumbed in out mixer shower. Similar set up to you. I’d not opened the isolation/cut of thingy for the hot tap feed fully. Found once fully open has been fine.

I’m not a plumber.

Starts hot, then goes cold for a minute or less?
Sounds like the boiler cycling too much – it senses ‘over-temperature’ for the output, then turns off until the water is at an acceptable temp., then turns on again. Trouble is, you feel it at the tap, as there is a lag while it warms up again.
Could be a number of things.
Not enough hot flowing through the shower – try it with the bath taps to see if the same happens.
If it doesnt, then it will be a fault with the shower plumbing somewhere. It could be a lack of flow at the head – try it again with the head off. Possible a shower mixing problem – water pressures should be pretty much the same for H+C on a combi, so doubtful to be that.
If you have the same h+C cycling on the bath tap, then suspect a fault with the boiler. At best, it’ll be a sensor, at worst the heat exchanger will be limescaled/gunged up and not allowing a good flow through it.
Try it with the bath tap, then the shower head off, and reply with your findings.

My shower was similar.

I had to turn up the boiler temperature before a shower and reduce the water flow at the shower head so water would stay in the boiler longer for the heat exchange.

I guess the pressure was too high at the shower and too low at the boiler.

Have you contacted the people who installed it? They will have loads of experience and I joined a plumbers forum lol

OH’s house had similar recently.
Plumber had fitted a pressure regulator factory set for 3 bar.
Why he chose not to read the specs which stated 3bar with the furthest tap open fully.
It was showing under one when I checked which caused the symptoms you describe and a flow rate less than a dehydrated alcoholic with kidney stones!
Definitely worth checking first as its just a case of turning a screw

I went through 3 thermostatic shower controls in 4 years. Would work fine first fitted then become erratic.Finally fitted a manual control one and have had no problems since.

It was showing under one when I checked which caused the symptoms you describe and a flow rate less than a dehydrated alcoholic with kidney stones!
Definitely worth checking first as its just a case of turning a screw

What exactly am I looking for and where? I doubt I can get much access as it’s quite tight, plumber had to take the toilet out to get to the taps but I can get the side of the bath off and have a quick look.

Had a similar issue recently, and it turned out to be a clogged heat exchanger on the boiler. Back pressure switched off the heat, shower went cold, pressure then reduced, went warm, etc., etc.

Now piping hot.

The plumber is coming back on Wednesday so I will get him to check the boiler over as well 🙂

Was just wondering if I could take out the thermostatic cartridge and run it as a standard mixer tap or would that not work?

Clogged heat exchanger for us too when we had this problem.

Was just wondering if I could take out the thermostatic cartridge and run it as a standard mixer tap or would that not work?

No.

With a combi, you definitely want a thermostatic valve. Your plumber needs to check what’s happening at other hot water outlets. It’s pretty simple to determine if the problem is with the shower valve or the boiler.
Either way, it doesn’t sound like something you have the skills to resolve. So leave it to the plumber. He’s coming on Wednesday.

Low flow rate swirching off boiler .
There is sometimes a hall effect sensor in the output main. WHen you turn on a tap the hall efect switches on the boiler . Your thermostatic valve opens up when switched on to full flow HOT .

THen as the hot water reaches th mixer bar the thermostatic cartrrdge balance the flow by letting in more cold and reducing the hot side.

The combi boiler hall effect sensor now slows right down , so the boiler thinks you have turned down or switched off a tap, and turns off the flames.

The water goes cold, but as it starts to cool your thermostatic mixer bar turns off the cold and speeds up the hot, triggering hall effect sensor and firing up the boiler.

To see if I am correct ( ish ) and you have a shower over bath ,, then have a shower with the hot tap running slowly , somewhere about 4pint wee rate and see if it runs . If you dont have shower over bath then , if you can , switch your hot water temp right down , to keep the flow rate high , and he hall effect ensor spinning .

PS I am not a plumber or heating engineer.

We had a swiftflow 100 in the last house, if you didn’t have the flow through the shower sufficiently it would do that, it’s an overheat protection to stop you being scalded with too little flow. If your shower head has one of those flow conditioner filters they can restrict the flow enough to do it.

PS I am not a plumber or heating engineer

That’s why he should wait to see one on Wednesday.
There whole load of variables that could be the cause.
Most commonly, the item used to regulate the temperature of the domestic hot water coming out of your combination boiler is called a thermistor. A plumber/heating engineer will know that and it might be one of the things he tests to try and diagnose the fault.

Yeah just gonna wait rather than try anything myself then. Was just hoping there might be a quick fix!