Strange problem with hot water suddenly turning cold. Any experts about?

[Wiifitmama]

The last week or so, we have all noticed that when we shower (two different bathrooms so two different showers) the water suddenly goes cold for a moment before returning to hot. We have checked that no-one else is using any taps in the house at the same time, no machines like washing machine or dishwasher are on, etc.

We have a combi boiler which we had major work to a month or so ago - the expansion vessel and pressure valve were replaced and it was serviced. All seems to be fine since then.

What on earth could this be? Nothing else has changed in our house - no refits to bathroom or anything.

Hopeful bump!

This used to happen at mine, but after three or four different engineers looked at it, they all said the boiler was failing. It hasn't happened since we got a new one.

Have you got whoever put the new parts on back to check it over? I think that'd be a logical next step.

Ugh! Thank you for answering but that is exactly the response I did not want! We had the head of the company check everything over after one of their engineers made a cock up so I am fairly sure everything is as it should be, unless of course, as you say, it is failing.

Is the pilot light on your boiler going out?

Hmmmm....well, I don't think so but I suppose someone would need to stand and watch the boiler while someone else is in the shower. Might try that this weekend when everyone is home!

Ok, I have done some further testing tonight while running the bath. If anyone can read this and advise I would be very grateful! Maybe @pigletjohn ?

When we turn the hot water on in the bath (or anywhere) the light to indicate the heating is on goes off and instead the hot water one goes on. Surely they should both be on as our heating is on?

Then, the temperature steadily rises on the display until it hits 90. During this time you can hear the boiler and the little light indicating the pilot light (burner light) is on. At 90, it cuts out, the little light goes off and the temperature drops down to about 65 at which point it turns back on again and rises. This continues in a cycle. Presumably this is what we are feeling in the shower as the temperature goes from hot to cold.

Our boiler is a Potterton Promax Combi HE Plus. It recently had a full service (supposedly) and had the expansion vessel and pressure valve replaced as the pressure was rising too high and leaking water.

I used to get this when i turned the water temerature up on my boiler. It stopped ehen I turned the water tempersture back down Coukd it be that?

Oooo....interesting! No idea, but worth a try thanks!

This used to happen when we let our pressure run too low in the boiler. Our boiler is quite an old one and can't get parts for it anymore but the landlady is holding onto it for dear life. Don't blame her really, it works fine apart from the pressure drops and we have to tweak it every now and again. Boilers are costly things!

It's definitely not the pressure. We had the expansion vessel and the pressure valve changed recently because we had pressure issues. The pressures hold steady now exactly where it should be.

Potterton aren't great boilers. We just replaced ours and to be fair to it it was 16 years old and came with the house so not done too bad I felt. How old is yours?

"When we turn the hot water on in the bath (or anywhere) the light to indicate the heating is on goes off and instead the hot water one goes on."

It is quite normal for a combi to give priority to HW when a hot tap is turned on.

A bath or shower tap may require all ther power that the boiler can provide, so it can't heat the radiators at the same time.

With luck, you will turn the tap off before the radiators have time to get cold enough to notice.

Also, you say the temperature rises to 90C. This indicates that the boiler is getting excessively hot, the amount of heat provided by the gas burners exceeds what the water flow can carry away. So the emergency overheat cutout is operating and turning it off.

This suggests two faults (1) the boiler is not modulating down its power to match the requirement (all modern boilers do that, so perhaps yours is faulty or very old) and (2) the water is not circulating through the boiler fast enough, for example if the heat exchanger is clogged with sediment or limescale.

We had something similar recently and they came out and changed a part for us. It might have been the hot water exchange or something? It certainly solved the problem. And with a combi boiler the heat does go off when the water comes on.

As you can tell, I am not any kind of an expert, hence why we have a care plan for the boiler.

The boiler gets too hot and the emergency overheating cut-out kicks in. I like my baths red hot so I turn the hot water right up and then it starts cutting out. If I turn the hot water down it doesn’t happen. But then my bath is too cold!

Thanks so much to everyone that has replied! This morning I tried what a few people suggested and turned down the temperature of the hot water on the boiler before I showered. DH stood and watched the boiler while I was in the shower. I did not feel any cold spells at all. The temp fluctuated a little in it's hotness but never went cold at all. DH reports that the pilot light stayed on the whole time and the temp fluctuated between 65 and 72.

So, what does this tell us? Is it as simple as we had the water temp turned up too high? Or is there still a fault with something and this is just mitigating it a little? I would love any further insight please!

To answer one question, the boiler is about 8 years old. We replaced it when we moved in to this place. We just spent about £500 on servicing (what does that even mean?!?) and having the expansion vessel and pressure valve replaced. Sooooo don't want to have to buy a new boiler now or spend more on it!!

sorry I realise I put the wrong temperatures. It was between 75-82

82 is about the maximum temperature that your boiler should ever reach.

Thank you - that is useful to know. I will turn it down slightly more then to make sure it does not go above that.

Does the fact that turning down the temp seems to have stopped the problem tell you anything about what might be happening?

it should have a temperature control to prevent it ever reaching overheat. Perhaps the temperature control is faulty, or perhaps the heat exchanger is clogged with limescale so it can't carry away the heat fast enough.

We had the same problem and have a new heat exchanger, pump and diverter valve I think.our boiler was overheating due to water scale cutting out then restarting repeatedly when running the hot tap.

For anyone who is still reading this - and I really do appreciate all the comments as they have really helped! - when DH showered I turned the water temp down just a little more and it stabilised. Instead of fluctuating between 75-82, it stayed steady at 79 pretty much the whole time. Result!

I am just wondering if this means there is actually no fault but that we just had the temperature turned up too high? Or is this a temporary fix and we still have a problem?

Hi @Wiifitmama glad youve sorted it. As I said earlier, it sounds like you (someone) had simply turned the temperature up too high (mine never goes above 72). Do you have children? Had they turned it up or was it the gas engineer who carried out the service? Maybe explain to everyone in the house that turning the temperature up equals hot and cold showers!

We just had this problem. It did this with the hot water for a year and then the heating started cutting out. The boiler was overheating, as time went on the problem got worse until it stopped working altogether. It turned out to be the heat exchange clogged up. Since this was replaced we've had no problems.