Wickes Halstead 102 combi of indeterminate age.
Has worked just fine for three years since we moved in.

When DHW is demanded it keeps cutting out and back in – a couple seconds usually, but can be 10seconds. It’s also taking a while to fire up when DHW is demanded – it used to be instant, now 10-30seconds. It’s worsened gradually over last week.

Plumber will be called in morning, but before sucking of teeth and ‘you need a new boiler son’ response, is this possible some simple flow switch/thermostat/a few quid not a bank account emptying experience.

Can you see into a hole on the front of the boiler to see the pilot light and whether it’s firing up and then if the main burner is firing up too?

If so, is it firing when the demand is placed on it? And is the burner burning full and steady? Likely hood is a sensor or valve .

Had a similar issue on ours. It couldn’t heat water on full flow,back it off and it got hot.
There was debris in the heat exchanger.
It can replaced or cleaned out in the sink & refitted.
Or it could be sticky diverter valve or temperamental temperature sensor.

Soft water where you live Matt so less likely to be heat exchanger. If a combi then could be the diverter valve membrane. With the front of the casing off and DHW running you should hear a tiny click as the microswitch on the diverter valve comes on; if after half a minute you hear it click again then probably the membrane is goosed. Get a new valve on ebay and get someone to fit it.

My money’s on diverter valve too. Central heating company (surprisingly) recommended new boiler /Worcetser Bosch in my case) – I bought the diaphragm from the manufacturer for about £10, took a couple of hours to fit/reassemble; was quite happy to do it as not gas related.
If there’s a hole in the diaphragm, it will be reluctant to ‘fire up’, hence no hot water. Depending how handy you are, you could turn the boiler off from the mains, disconnect the wiring from the switch on the end of the valve and use a multimeter to see if the contacts closed when the hot tap is switched on. Probably.

n; if after half a minute you hear it click again then probably the membrane is goosed.

This is what it sounds like.

Thanks all.

Trouble shooting will be in service/installation manual as well as parts list.
Does it work in central heating or does it cut out?
Pressure gauge should be 1 bar or slightly above.
Does hot water have good flow rate?
Does the problem occur at all hot taps.

Halstead parts are expensive so make sure you are sitting down.

If your boiler is condensing, The Wickes Ace HE 102, it does not have a diverter valve. It may be the DHW flow sensor or thermistor. Have you tried to see if the central heating side is working? If it has the same fault when in heating demand, it will be something else, possibly the flame sensor, air switch, fan or??. There’s quite a difference between a plumber and a heating engineer. Some can do both, some can not. Be mindful of this.

Changed the Thermister and all seems well…

Toad ya.

Toad ya.