Combi boiler - shower going hot/cold/hot

[theplanner24]

Morning all - so I know I need to call the plumber but I like to have done some research first!

I have a baxi potterton combi boiler in the loft - 10 years old. Had various parts replaced a week or so ago due to other issues

Shower on first floor - thermostatic bar shower - runs hot and cold - watching the boiler it fires up but then the 🔥 light goes off temp decreases and then fires up again a minute or so later causing the shower to constantly cycle through hot and cold

It doesn't affect any other taps in the house.

Shower is around 2 years old. I do recall the old shower if you didn't have it on just the right flow setting would go cold so I think maybe the issue has always been there

Parts replaced last week included diverter valve and heat exchanger

So what options does that leave me to try? I cannot afford for a new boiler.

I've heard @pigletjohn can be helpful on this forum so hoping for some insight!

Hopeful bump 😅

Hello

Pretty easy to diagnose and fix this yourself. And relatively cheap in all instances.

First, let's prove what the issue is...

The hot water flow going through the boiler and off to the shower is not remaining high enough for the hot water demand flow switch to be continuously triggered. So the boiler goes off and the shower gets cold. The cold water causes the thermostatic valve to open up the hot side a bit more and so the hot flow through the boiler increases and triggers the switch. The boiler fires in and there is hot water going to the shower. So the thermostatic valve throttles the hot water, which reduces the hot flow, which untriggers the switch and the boiler goes off. The cycle continues.

Please confirm this by turning the shower on and witnessing the cold/hot cycling and then gently opening the basin hot tap a little. This should provide just enough extra flow to keep the switch in and keep the shower hot. You should only need it open a touch but do ease it open bit by bit, if needed to maintain hot to the shower. Please confirm the boiler stops cycling and there is sustained hot water to the shower with basin tap ajar and shower on?

Luckily, once we prove this, the solutions are cheap and easy.

Do you happen to have any details for the bar mixer? Brand/model? And model of the boiler?

Cheers.

Hello!thanks so much for replying!

Yes between me sat up in the loft and the kids helping with taps/showers that's exactly what happens

The boiler is cycling on and off when the shower is on

If I run the tap on the bathroom sink tap on hot then the shower temp stabilises and stays on

I remember the plumber saying he took the flow regulator out of the bathroom sink tap because of a similar issue - that the flow was not enough to call for hot water from the boiler

The shower bar is a Bristan Artisan about 2 years old

The boiler is a Baxi potterton Heat Max Combi 24 HE

I'm pretty handy DIY wise so will have a go at fixing most things 😅

Thank you for carrying out the tests. We now know the root cause and just need to correct the issue.

(You also now have a (slightly wasteful) contingency method to have a warm shower.)

Can you please answer...

1. The shower previously provided stable temperature control?
2. The issue has occurred recently?
3. The issue has occurred since the aforementioned work was carried out on your boiler?

Cheers.

@youngdinosaur
Sorry to hijack your thread, but I was wondering if could help us, too. Our shower - which is connected to bath taps - is running hot and cold, too. When you watch our combi boiler fire up, all the hot water bars rise up as it reaches temperature, they then disappear completely, which is when the water gets cold, then they appear again and start rising as it gets warmer, and so on throughout the duration of the shower.
I've tried running the kitchen and bathroom sink hot water tap during the shower as mentioned above and it makes no difference to the shower running hot and cold.
Thankyou in advance if you can advise.

@youngdinosaur

1. The shower previously provided stable temperature control?

Yes albeit it never got really hot - if it wasn't turned to maximum passed the anti scald it was cold

(I recall on the old shower we had before this one - also a Bristan artisan I think - unless the flow setting was just right it would go cold. Didn't cycle the boiler though. It did stay very hot - it didn't have an anti scald button though)

1. The issue has occurred recently?

This issue yes. Previously the shower was consistent temp just consistently warm but not hot hot (I put it down to the plumber being overly cautious with the anti scald setting)

1. The issue has occurred since the aforementioned work was carried out on your boiler?

Yes. The plumber changed the heat exchanger, the actuator, the diverter and the electrodes I think due to various issues. The shower now gets hotter than it was but doesn't stay hot obviously due to the boiler cycling

I'm wondering if there is unequal pressure between hot and cold?

I have tried changing the shower head to one which can give variable flow speeds - it was worse actually than the small head I've put back on it

Hello

With this information the obvious fault will be with the inlet filter restricted for the hot feed.

This may not be your exact model but will give you a good idea...

https://www.showerspares.com/bristan_shower_spares/bristan_all/bristan_artisan_bar_artisan/

The filter can seen in the explosion diagram at the top. These need to be removed, cleaned and reinstated. When you are there you might find NRVs fitted in the inlets. These can be inspected too for buildup/damage/debris/etc. I cannot tell you to do this but if it were my house I would not be reinstalling there NRVs as they are simply an over-the-top measure installed to meet WRAS requirements in all cases and there is no benefit to your installation - in fact the opposite is true and they create an unnecessary restriction to the flow.

So...

1. Turn the water off.
2. Blip the shower on to release the pressure and confirm the water is off.
3. Use an adjustable spanner to undo the nuts on the two inlets.
4. Gently lift the bar mixer away from the wall.
5. Remove and clean the filters
6. Remove the NRVs and do not reinstall them.
7. Put the bar mixer back up.
8. Do up the nuts but do not overtighten - you are better to have a slight weep/leak that you can nip up another 1/8th of a turn than to overtighten an damage.
9. Turn the water back on.

10. Check for weeps and leaks.
11. Run the show and see how it performs.

There will likely be lots of green, white and/or metallic flakes in your filters. A toothbrush is pretty good for this job. Old one ideally but at least use your least favourite family members one.

Let me know how you get on.

Cheers.

Similar but the setup is different as you are fed from the bath taps(?).

A picture of this installation would be helpful.

@youngdinosaur

Thanks so much for that! I'll give that a go over the next couple of days and report back!

(Need the kids out of the way before they all decide to stand in the bath and "help" me !)

@youngdinosaur
Sorry if I didn't describe it very well.. It's just a shower hose that connects to the bath taps, so the water is coming through the bath taps and through the hose. So the water is going hit and cold when we use the bath taps. It happens a little bit when we use the kitchen taps too, but it's far more noticeable when using the bath taps.
Hope that makes sense

@youngdinosaur I have been searching the internet because I have a very similar fault but I have an internal Bristan shower. We have replaced the thermostatic valve with no effect.

Does the advice you gave to the person with the bar shower apply to internal showers too please and thank you?

We had this recently.

Plumber told landlord it needed replacing but he refused. It had a leak and they fixed the leak and then this issue developed. It was because the insides where clogged with gunk/limescale so the water doesn't move as fast as it should.

This causes the temperature inside the boiler to sky rocket and the automatic shut off cut in. Then the temperature drops and once its below the thresh hold it kicks in again, then it sky rockets, cuts off, drops again and so on and so on.

As an update in the end I had to just replace the boiler - no amount of replacing parts fixed it - I also installed a new thermostatic bar shower with the max flow I could get within my budget -m

Sorry to hear that OP. So does that mean because you replaced the shower AND the boiler, you never worked out which one was the problem?!

@Stringervest

It was definitely the boiler....decided to change the shower first to one with a bigger flow as the suggestion was that the flow previously was too low for the boiler to register it hence why it stopped calling for hot water.

New shower made no difference - plumber said we could keep changing parts on the boiler but at 10 years old it was end of life

Thank you, that's helpful. Well, it's not what a want to hear because I won't want to buy a new boiler but it's helpful!