[Closed] Help! Shower cycling hot and cold

Obvious question: is it fitted the right way around?

Does turning on either the hot or the cold tap while the shower is on make any difference?

Thanks for the reply

Yes, pretty certain it is the right way round.

Before the cartridge was changed it was at first cycling every few minutes. Then got progressively worse so that it ended up just staying cold. Since the cartridge was changed it is back to cycling every few minutes.

Can you run another hot tap while the shower is on, even just a little? If that goes cold at the same time as the shower, it looks like a boiler problem; if the water there is still hot while the shower is cold it points to a shower problem.

It does sound like my combi boiler which was diagnosed as probably a clogged heat exchanger, though.

... I should have said. Up until a month or so it was working fine.

I and wondering if it a flow issue causing the boiler to cycle on and off.

Had a similar thing a few years ago with a Combi and realised it was becuase we hadn't installed the pressure reducing valves in the pipework. The boiler will only supply based on demand, and if the cartridge keeps asking, it will get a surge and then the cartrdige will try and back off again, so boiler backs off and then you repeat..

I know you said it did work, but we found that with flow changes in the mains, the probelm started fairly randomly. With a system boiler (boiler + tank) you have a system blaance valve that taakes the demand issue away!

edit: siomething like that anyway!!

You could just convince yourself it is a spa shower and its good for endorphins, then embrace the experience. You would probably pay a fortune for a shower that does that by design.

It is for a rental property. I am pretty embarrassed for the inconvenience it has caused the tenant (there is no bath). I have given her some money off this months rent to compensate. However, I imagine her patience is wearing pretty thin.

I'll try to go round there this evening to investigate.

Can you run another hot tap while the shower is on, even just a little? If that goes cold at the same time as the shower, it looks like a boiler problem; if the water there is still hot while the shower is cold it points to a shower problem.

Thanks, I'll check that.

Had a similar thing a few years ago with a Combi and realised it was becuase we hadn’t installed the pressure reducing valves in the pipework. The boiler will only supply based on demand, and if the cartridge keeps asking, it will get a surge and then the cartrdige will try and back off again, so boiler backs off and then you repeat..

I guess that is almost the inverse of the above? In which case it should work better at a low temperature rather a high temperature. Thanks

You could just convince yourself it is a spa shower and its good for endorphins, then embrace the experience. You would probably pay a fortune for a shower that does that by design.

🙂 I imagine if a try to convince the tenant of that. I will get a bucket of ice cold water on me.

... I asked the plumber to check whether it was flow issue when I found out we would have to wait for the cartridge. He seemed decidedly nonplussed, so I let it drop. He is now doubting the tenants word that there is still an issue. I think I am going to need a new plumber.

The plumber was convinced it was the cartridge in the shower so ordered another (at £200 plus £100 labour)

You could have bought a new shower for that much and have enough left for a pint and a pie or 5.

Are there any isolating valves feeding the shower which you could temporarily adjust to see if it is a pressure balancing issue?

Also, did the plumber check the shower inlets for scale build-up?

I think I am going to need a new plumber.

This was caused by a clogged heat exchanger when it happened to us.

Plumber told me it was full of "sand". To which my response was its limescale, because, a) how would sand get into the water supply without a leak, b) if it was somehow getting to the supply it would be coming out of the cold tap, and c) the water here is harder than Chuck Norris.

By this point they had managed to flush said "sand" out of the heat exchanger and into the hot water system, blocking every screen in every tap rendering the plumbing inoperable.

So yeah, second opinion might not be the worst.

It does sound like the boiler is sensing a flow problem and starting/stopping.

The secondary heat exchanger problems mentioned above are common.
You can usually take these out fairly easily yourself and clean them internally with brick acid (Screwfix).

Plumber told me it was full of “sand”. To which my response was its limescale, because, a) how would sand get into the water supply without a leak, b) if it was somehow getting to the supply it would be coming out of the cold tap

Quite easily really, we had similar at our old house.
There was a repair done on the supply a few streets away during the day when nobody was home, debris in the pipework came out of whatever tap was opened first. In our case this was the kitchen sink cold tap after running for a few seconds, but could just as easily been the boiler if a hot tap was used first.

Are there any isolating valves feeding the shower which you could temporarily adjust to see if it is a pressure balancing issue?

I am not sure, I'll check but I don't anything obvious.

Also, did the plumber check the shower inlets for scale build-up?

No but I will, thanks.

You could have bought a new shower for that much and have enough left for a pint and a pie or 5.

That was my thought but it is a concealed showed which would have meant some tile smashing

The secondary heat exchanger problems mentioned above are common.
You can usually take these out fairly easily yourself and clean them internally with brick acid (Screwfix).

I will ask the plumber but I expect the same reaction as the doctor gives when you say you have looked the symptoms up on the internet. I think professional pride may get in the way of solving the problem.

We had something similar with our shower a while back. There's a filter where the water goes in, which had got completely clarted up with crunchy crystals - I think something to do with being in a hard water area. Cleaned this out and it was good as new, and has been for a few years. Might be worth a look.

We had something similar with our shower a while back. There’s a filter where the water goes in, which had got completely clarted up with crunchy crystals – I think something to do with being in a hard water area. Cleaned this out and it was good as new, and has been for a few years. Might be worth a look

Good point, thanks. There maybe a filter/restrictor in the shower head. I'll check that. Being a concealed shower, I am not sure the inlet to the shower unit can be accessed in a non-destructive way. I'll check though.

Mine was doing this a few months ago. Clogged up heat exchanger, as others have suggested. Whole system including pipework was less than a year old. Suspect insufficient / no inhibitor went in originally! The Ideal engineer replaced the heat exchanger under warranty, which was rather decent of him considering! Has worked perfectly ever since.

Forgot to mention, it only had an issue with the shower and the the bath tap, all the others were too small to 'call' for enough flow to be a problem.

Mine was doing this a few months ago. Clogged up heat exchanger, as others have suggested. Whole system including pipework was less than a year old

Thanks. Looking a likely candidate.

Don't forget to consider that the incoming water temperature to the house is quite a bit higher now than in the winter, so there's much less work for the boiler to do to satisfy the demand, especially if there's significant hysteresis in the system.

If you have a reasonably modern boiler I would also consider derating it slightly if it has the facility to do so.

I've only just looked at this thread because the title made me think it was a new Peloton class.

I’ve only just looked at this thread because the title made me think it was a new Peloton class.

Brilliant!

So the plumber came back this morning and had a look at the shower, he changed the flow on the boiler and the temperatures.
He said that the flow of the hot is too high so the boiler is cutting the temperature and that's why it has been running cold.
He seemed confident that it was fixed and ran the shower for a while...

...the tenant has just got back to me. They havn't changed anything with the temperature on the shower or the boiler and it is still cycling hot and cold.

The plumber suggesed running the tap low at the same time which she did and it stays hot.

I am not entirely convinced that it is the only permanent solution. Particular as I have paid £300 for a new cartridge that wasn't needed. Any further thoughts appreciated?

New plumber?

We had a similar issue a few weeks back, but it was mainly the bath cycling.

After multiple parts and a heat exchanger, it was diagnosed as a blocked silicone pipe within the boiler. But this did take multiple attempts to find the problem.

Yeah you need a new plumber, the one you have clearly doesn't know what he is doing.

We have a pressure balancing valve on our shower and I keep the incoming water pressure turned down below maximum.

Without both or either of these, our shower used to cycle hot/cold with the old boiler.

Quick solution - what about a shower head with much more flow? Same as having a tap on.

It's a multipoint water heater not a combi boiler so no secondary heat exchanger. If It's a chaffoteaux britany It's probably the diaphragm that's worn or starting to split so you'd be better getting a gas engineer rather than a plumber to look at your water heater next. If the cartridge has been changed and you still have no hot water I doubt its the shower that's faulty.

Thanks all, really appreciated and useful advice.
I am going to try out with the shower head removed - just the hose. Assuming that is okay will then purchase a rainflow head. Fingers crossed that will sort it. I'll report back.

-... to confirm, the combination of fitting a rain-flow shower head and reducing the outlet temperature from the boiler (and therefore increasing the flow) did the trick.

Thanks all