Shower temperature suddenly varying

[Hellenabe]

Any plumbing experts here? My shower used to get hot easily but now takes a while to just get reasonably warm (despite the tap being on max). Then it suddenly gets boiling then warm for a bit, then freezing. I havent a clue why/whether I can live with it!

Is someone else turning taps on elsewhere in the house or is the washing machine or something on?

How long have you had the shower? Is it a combi boiler?

Up to a couple of weeks ago the starting temp of the water was higher because it was warmer weather in general, a combi boiler heats the water as it passes through so can heat warmer water faster than it can heat colder water.

As it has now cooled down, you may need to run the shower a bit slower/less powerful flow so that the boiler can keep up with it and get it to the right temperature.

Is it an electric one on the wall? If so it's full of limescale and will probably need to be replaced.

Hi all,

Thanks for all the replies!

@CyberCritical I've had the shower for years and the boiler is at least 15-20 years (a valiant). I spoke to a plumber on the phone and he said check the taps downstairs and see if it's the same issue (it's not). The upstairs shower just seems to struggle to keep up with warming the water consistently. I used to turn it to max then reduce down. Now It starts freezing (at max), gets warmish, then gets boiling then id quickly adjust the taps and it would get to a better temperature. Are you saying this is to be expected and nothing is broken like a faulty thermostat?

It's really difficult for anyone to advise without knowing what kind of shower it is. Can you take a photo?

Is this running off the hot water tank (system boiler) or an electric shower or running off a combi boiler ?

It's hard to see how a shower running off a hot water tank (system boiler) would give this behaviour.

An electric shower might because there is a problem with the shower internal temperature sensing or issues with the flow rate.

A combi boiler might if the boiler was having problems sensing the water flow and output temperature. But this would probably affect every hot water output in your house (for example kitchen hot taps) as well as the shower. Which is probably why your plumber asked whether other hot taps in the house have similar issues.

My shower which runs off the combi boiler does this when the pressure on the boiler drops - this can happen if you’ve bled the radiators - which is an activity we’ve done recently - perhaps you have too?

OP do you have the boiler serviced every year?

If so, is a service due?

If not, I would book one ASAP.

We had a problem with a part in the boiler. Was clogged up with accumulated crap from the central heating system so the sensor wasn’t working properly. Central heating needed high pressure flushing out. If it’s that, you’ll need a plumber.

That sounds weird.

The heating circuits in a combi boiler are completely separate, so what happens in the CH loop shouldn't affect what happens in the HW loop.

I supposes there is a possibility that if the pressure is low in the CH loop it shuts down the operation of the boiler entirely and cuts out the HW heating as well.

Do you need to replace shower bar? Our shower started going luke warm, even though all other taps it came out piping hot. Bought a new shower bar (husband fitted it, but I’d be confident doing it and I’ve no experience) and now we have hot showers. Sometimes the cartridges get worn out is what I found in my research, but it was cheaper just to replace whole bar.

I'd say you've got a faulty thermostat in the shower.

We had this with our electric shower. One day the thermostat went poof and wouldnt warm up unless it was on max, and even then the temperature would fluctuate.

We had to replace the whole unit.

Even if it's not an electric shower, I would say that there does seem to be an issue with the thermostatic control if it's not affecting other taps in the house.

If it's electric, it sounds like the thermostat.
If it runs from the hot water system it could be a valve sticking in the boiler. I found out the latter after a shower like you describe and a radiator was warm when the heating was off. I think it's called a diverter valve.

Thanks all, its not an electric shower so I'm going to get a boiler check

What is your actual shower like?

As another poster said, if it's a bar then they have cartridges on both taps and sometimes the cartridge or the full bar needs to be replaced. It's quite an easy diy job sometimes - like for like replacement.

Have someone watch the boiler while you are in the shower to see if the boiler switches off as it goes cold and then comes back on as it warms back up.

It sounds like a blocked plate heat exchanger to be which is a boiler issue. Luckily it’s not a major repair!

Let me know and I can give more advice if needed.

I had this with a three year old shower (not electric, had the shower replaced (cheapest option first) and the problem went away. It was limescale related, very hard water here.

If it is an electric shower (cold-feed only, it heats the water itself) and you've had it for unknown years, then it's likely the shower has just reached the end of its useful life. If you're in a hard water area you may find that an electric shower has a useful life of 5-10 years. It's usually less than an hour to swap out the old electric shower for a new one.

Hi, update. Plumber let me down so I'm monitoring the situation till I get another. So my heating is on timer but I always had hot water in the past 24/7. Now it seems like when the timer goes off for heating, then the water does the same and goes cold. Does anyone know why this is?

That was the sign for us that our boiler was terminal and needed to be replaced. If you've had it 15+ years it's done well. You're getting 'accidental' hot water when the heating is on.

Without knowing whether it is a combi or a system boiler it is hard to say.

I think if it is a combi Doginatent may be right. The hot water is being heated indirectly from the heating loop heat exchanger. So it is probably quite a bit colder than normal.

Normally when you turn the hot water tap on with a combi boiler you can hear the boiler fire up.

Have you tried standing next to the boiler and listening to whether it fires up when someone somewhere else in the house turns a hot tap on ? There is normally a click and then it should make a quite roaring type of noise (the same noise it makes when you switch the heating on). When you turn the hot water off it should stop.

Even if it's not a combi boiler (i.e. it feeds a hot water tank) you get the same symptoms.

Not on my system boiler and I suspect on most. If the divertor value for the DHW is open and the boiler fires then you get hot water produced. If it is closed and the boiler fires for CH you don't. There is no opportunity for significant cross conductance of heat, the pipe arrangement doesn't allow it.

I guess if you ran the closed loop heating circuit close to some of the hot water tap pipes you might get some conductance of heat, in the same way some people sometimes get conductance of heat between cold water taps and hot water taps because of the pipe proximity. But it wouldn't be very much.

Hi, so today morning freezing. Then about 6ish really hot again. Heating has been on all day. What'd going on!?

Take a picture of your actual boiler and post it! I might be able to offer more precise advice