Shower goes cold when hot water turned on elsewhere

ordnancesurvey84

Hi - could anyone advise what the problem might be?
We have a combi boiler in the loft. Until about a year ago, if hot water was being used elsewhere in the house - this would cause the shower flow to reduce - but it would remain hot.
But then for some reason this stopped happening, and if the hot water was used elsewhere (e.g. bathroom sink or kitchen), the shower would go cold until the other tap was turned off.
Our plumber came to do the annual boiler service in October - and somehow this solved the problem...
...for a couple of months, and now it's doing it again.

ThisIsWeird

Make and model of boiler?
Have you adjusted the DHW temp on your boiler over this time?
Have you noticed if the incoming mains flow has reduced or changed? Do you know what it is, in litres per minute?
And, make and model of shower? 
I'm assuming the latter is a thermostatic type, and has some temperature settings on it - 38oC, for example? Thermo mixers are designed to keep the required showering temp steady, and also to prevent scalding. So, if a second hot tap is used during a shower, the hot supply to the shower will drop as a consequence, and the thermo responds by reducing the cold a similar, balancing, amount, and this gives you the response you used to have - the shower temp remains steady, but the flow rate drops.
Similarly, if a cold tap is used or a loo flushed, the hot side is reduced to match the reduced cold, so the temp stays steady, but the overall flow reduces again.
Taken to its extreme, if enough cold is being used elsewhere so that not enough is available to the shower mixer to steady the temp, the shower should stop flowing altogether.
That's how it should work. I suspect yours doesn't...
Tests:
1) when a hot tap is being used elsewhere now, you say the shower goes cold - but does the flow drop as much as before?
2) when you turn on a cold tap instead/flush a loo during a shower, what does the temp do? Does it remain steady, or go ouchouchouch?
I think the most likely causes will be a faulty shower mixer thermo cartridge, or else your incoming mains flow rate may have fallen a bit? Or possibly just down to the colder incoming mains meaning that your boiler is struggling to heat as much DHW as before, so you have less hot to begin with.

ordnancesurvey84

Thanks @ThisIsWeird
It's a Vaillant ecoTEC Pro 24 boiler
The shower fitting is a Solepearl thermostatic bath shower mixer - bought on Amazon.
No adjustments to the DHW since boiler serviced in October.
Haven't noticed any reduced incoming mains flow - I've just done a rudimentary test using the cold water tap in the kitchen and the flow is about 13L/minute.

Question 1: No, the flow doesn't drop as much as before, it's a pretty strong flow, just cold.
Question 2: No, I haven't noticed the temperature getting very hot when someone else is using cold water only - but I'll double check this.

I assumed it would be something to do with the boiler rather than the thermostatic bath shower mixer, seeing that the problem was temporarily resolved immediately following a boiler service.

ThisIsWeird

ordnancesurvey84 said:

I assumed it would be something to do with the boiler rather than the thermostatic bath shower mixer, seeing that the problem was temporarily resolved immediately following a boiler service.

Possibly. Or possibly pure coincidence

"Question 1: No, the flow doesn't drop as much as before, it's a pretty strong flow, just cold." Ok, if you compare what used to happen to what is happening now, there are seemingly two differences. One is that the shower runs cold, and the other is that the shower flow doesn't drop away as obviously.

I think we can assume that the boiler is happily running properly in both cases, since the hot tap that's been naughtily turned on is presumably providing a nice flow of hot water? Ie, the boiler's DHW output hasn't changed.

Before, when you reduced the hot to the shower by using another tap, the cold was throttled back by the thermo to keep the mix the same = a noticeable overall drop in flow, but temp stayed the same. Now, when the hot flow is reduced to the shower by a tap being turned on, all that happens is that the shower receives less hot water, but the cold flow remains the same = nice healthy flow, but chilly.

That's the most likely bet, I think - the thermo cartridge is kaput.

Do you have hard water? How old is the shower mixer? Are you a decent DIYer?!

ThisIsWeird

If you have hard water, it's quite common for the thermo cartridge to scale up and prevent it from working properly. So an 'easy' first step would be to remove it and immerse it in limescale remover.

Do you have the instructions it came with? That might show how to do this, but it's a pretty standard style so there's plenty of info out there.

If that doesn't sort the issue (assuming it's the cause, of course), then it looks pretty much as cheap to replace the whole mixer as buying a new cartridge, but that's depending on any current deals.

ThisIsWeird

I'm going to stick my neck out and say that this is identical in every way, and not a 'copy': https://www.ebay.co.uk/itm/314099014952

Or, https://www.ebay.co.uk/itm/403740538205 (select 'Shower Mixer Valve Cartridge' at £14)

But obviously either would have to be your decision. And we don't yet know that the cartridge is the issue.

JohnB47

We're on our second Worcester Bosch combi boiler and we always assumed that anyone turning on a cold tap (or flushing the loo), while the shower was running, would drop the pressure in the shower and lead to a period of cold water coming shortly after. Anyone turning on a hot tap would only drop the pressure.

That was the way it was for the first boiler and we assumed it would be the same for the second. So we both learnt not to turn on hot or cold water while someone is in the shower and that's how we do things now, even though the new boiler might not behave the same as the first.

ordnancesurvey84

ThisIsWeird said:

ordnancesurvey84 said:

I assumed it would be something to do with the boiler rather than the thermostatic bath shower mixer, seeing that the problem was temporarily resolved immediately following a boiler service.

Possibly. Or possibly pure coincidence

"Question 1: No, the flow doesn't drop as much as before, it's a pretty strong flow, just cold." Ok, if you compare what used to happen to what is happening now, there are seemingly two differences. One is that the shower runs cold, and the other is that the shower flow doesn't drop away as obviously.

I think we can assume that the boiler is happily running properly in both cases, since the hot tap that's been naughtily turned on is presumably providing a nice flow of hot water? Ie, the boiler's DHW output hasn't changed.

Before, when you reduced the hot to the shower by using another tap, the cold was throttled back by the thermo to keep the mix the same = a noticeable overall drop in flow, but temp stayed the same. Now, when the hot flow is reduced to the shower by a tap being turned on, all that happens is that the shower receives less hot water, but the cold flow remains the same = nice healthy flow, but chilly.

That's the most likely bet, I think - the thermo cartridge is kaput.

Do you have hard water? How old is the shower mixer? Are you a decent DIYer?!

Thanks - this sounds like the explanation.
Yes we do live in a hard water area, and it's already getting pretty hard to turn the thermostatic dial due to it feeling stiff - presumably with limescale.

ThisIsWeird

ordnancesurvey84 said:

ThisIsWeird said:

ordnancesurvey84 said:

I assumed it would be something to do with the boiler rather than the thermostatic bath shower mixer, seeing that the problem was temporarily resolved immediately following a boiler service.

Possibly. Or possibly pure coincidence

"Question 1: No, the flow doesn't drop as much as before, it's a pretty strong flow, just cold." Ok, if you compare what used to happen to what is happening now, there are seemingly two differences. One is that the shower runs cold, and the other is that the shower flow doesn't drop away as obviously.

I think we can assume that the boiler is happily running properly in both cases, since the hot tap that's been naughtily turned on is presumably providing a nice flow of hot water? Ie, the boiler's DHW output hasn't changed.

Before, when you reduced the hot to the shower by using another tap, the cold was throttled back by the thermo to keep the mix the same = a noticeable overall drop in flow, but temp stayed the same. Now, when the hot flow is reduced to the shower by a tap being turned on, all that happens is that the shower receives less hot water, but the cold flow remains the same = nice healthy flow, but chilly.

That's the most likely bet, I think - the thermo cartridge is kaput.

Do you have hard water? How old is the shower mixer? Are you a decent DIYer?!

Thanks - this sounds like the explanation.
Yes we do live in a hard water area, and it's already getting pretty hard to turn the thermostatic dial due to it feeling stiff - presumably with limescale.

That would add to its likelihood, but of course not certain.
Are you up for some DIY? :-)

ordnancesurvey84

Thanks @ThisIsWeird yes - I reckon I could do it myself. Is it as simple as turning off the mains water supply, unscrewing the nuts, removing the whole shower mixer bar and dunking it in a bucket of limescale remover, or replacing it with a new one if that doesn't fix the problem?

ThisIsWeird

ordnancesurvey84 said:

Thanks @ThisIsWeird yes - I reckon I could do it myself. Is it as simple as turning off the mains water supply, unscrewing the nuts, removing the whole shower mixer bar and dunking it in a bucket of limescale remover, or replacing it with a new one if that doesn't fix the problem?

Worst case scenario is that you remove the whole unit - easy, it just needs a couple of nuts - and replace it all, perhaps with the one I linked to.

Best case is you remove it as above - to make life easier - remove the cartridge, soak it in scaly, and replace. £2.

Do you know if the shower has isolating valves, so you can turn off the supply to it without stopping anything else?