Hot water cycling between hot and cold

[WastingMyYoungYears]

Our hot water is cycling between hot and cold - does anyone know what might be wrong? It's literally a few seconds of hot, then a few seconds of cold (then this repeats).

I should have mentioned that a plumber was fiddling with the boiler yesterday after a leaking pipe.

I am guessing you have a combi boiler.

Does this happen on all taps, including the bath?

Yes, it's a combi boiler. The hot-cold thing only happens with the bathroom taps (sink and bath), not the kitchen taps.

I think it might be that the flow of hot water through the combi is not enough to keep the flow switch turned on, though in this case the bath tap, or perhaps bath and basin hot taps turned on together, usually overcome it. Maybe the flow switch is worn (it is replaceable) or maybe the combi heat exchanger is clogged (for example with limescale or sediment) or maybe there is a pipe obstruction.

Does the water in the bathroom taps get unusually hot?

Are the bathroom taps mixers, possibly rather worn, and possibly having ceramic cartridges (quarter-turn levers or joysticks)?

If you fill a container at the troublesome taps, do they deliver less flow of hot water (litres per minute) than the kitchen tap?

Might the stop cock have been closed recently, and not fully opened, or might a new piece of pipe (possibly flexible) or service valve have been fitted between the boiler and the bathroom?

I'd be on the phone to the plumber, as it is clearly something caused by his "fiddling".

Cointreau, yes, that's my thinking too! The plumber's adamant that it can't be anything he did though.

PigletJohn, hopefully the following will answer your questions!

Before the plumber came to fix the central heating leak, both the central heating and the direct hot water all worked okay. Only minor annoyance was that for the kitchen tap, the flow rate would drop off a lot as the water was heated up, and you'd have to keep turning the tap up to keep it going. But the bathroom tap seemed fine.

Then the floor guy sliced open one of the central heating pipes. We had one day where the hot water appeared to work okay, but I noticed the pressure was low and the house wasn't being heated. I tried to re-pressurise the system, at which point water started running down the wall. We then called a plumber. He drained the central heating and the boiler, replaced the damage pipe section with a new bit of pipe, and the refilled the central heating and boiler.

Afterwards, we noticed that the bathroom water was oscillating between hot and cold, and that we couldn't run a hot bath. Since then, I more carefully checked the kitchen tap and that is also oscillating between hot and cold, but to a lesser degree (i.e. it's hotter overall, and less variable, but still varying). We tried turning down the hot water temperature control, and it made no difference. We also tried running the hot tap slowly, and it stops oscillating, but just runs cold instead.

To answer those specific questions:

• The water in the bathroom taps does not get unusually hot.
• We have a mixer tap in the kitchen but not in the bathroom.
• The flow rate in the bathroom has always been lower than the flow rate in the kitchen, and this remains the case.
• The stop cock has not been closed recently AFAIK, but I've asked the plumber to tell me if he did.
• None of the pipes associated with the hot water system have been changed recently (the plumber only touched the central heating system).

Just to add, if you run either (or both) hot taps upstairs at full blast, the water doesn't heat up at all.

Okay, I've just measured:

Kitchen 10L/min
Bathroom bath 10L/min

" if you run either (or both) hot taps upstairs at full blast, the water doesn't heat up at all."

that's very odd. You mean the boiler doesn't fire up? Or that is does fire but the water is cold? And you just have a combi, no water tanks?

Were you in the house last winter?

10lpm is quite low, but should be enough for the flow switch. I imagine it is slightly higher from the cold taps.

Try it again during the night, or 6am, when the water pressure will be higher (it increases overnight when not much is used)

If you have the cold taps and the hot running at the same time, the boiler may not fire due to reduced hot flow.

Do you have a regular person who services the boiler?

Hi PigletJohn - thank you for your continued help, and apologies for my delay in replying.

We were in the house last winter, and didn't have any issues with the boiler.

The boiler is due to have it's (annual?) gas safety check - is that the same as a service, or is a service something over and above a gas safety check?

I discovered this morning that the bath fills up with hotter water if I only turn the tap half on.

I'll answer your other questions as soon as I can.

Oh yes, we only have a combi boiler (no water tanks).

"I discovered this morning that the bath fills up with hotter water if I only turn the tap half on."

That is normal for a combi.

I had something similar on a year old Worcester combi about a year ago. First noticed it in the shower when the flow went from consistently hot to cycling cool then very hot, then cooler and so on.
The boiler was in the bathroom so I could hear the change in the sound it made and I could hear the boiler seeming to switch itself on and off.
Got Worcester out and they tested it and claimed not to be able to discern any change in the water temperature, claimed the change in the boiler cycling sound was the way it was supposed to work.
Anyway they changed the heat exchanger under guarantee and serviced it.
Seemed fine for a bit then a few weeks later it started again.
I pressed the reset buttone on the boiler a couple of times and that seemed to bring it back to normal.

About 6 months ago it started again and I did the reset button trick and again it worked - back to normal service.

I don't know why it occurs and nor do Worcester but maybe it will help you too if you haven't already tried it.

Thanks bungalow, we'll try to re-set it again.

PigletJohn, you made me laugh out loud. I meant that you get an acceptably hot bath if you only turn the tap half on, as opposed to an unacceptably cool bath if you turn the tap fully on.

yep, quite normal for a combi.

It will be worse in winter, when the incoming water is colder.

But it was fine last winter! And it's still doing the hot/cold thing.

I think it is either obstructed flow, for example limescale in the heat exchanger, or possibly the flow switch is not recognising the flow. Limescale will build up over time.

Test the flow (litres per minute) at the bath and sink cold taps and compare it with the flow from the hot taps. A clogged heat exchanger will only reduce hot flow.

or it might be something else I haven't thought of, for example an electronic control, or the thermistor that measures hot water temperature.

If it is not an easy DIY fix like a half-open stopcock, I think you are going to need a boiler mender.

It might help to know the make and model of the boiler. A 30kW boiler should be able to heat 10 litres per minute without the temperature being cool.