This is one of the most common callouts in the trade and a fair number of them were never a boiler job at all.
People ring up saying they've no hot water, because the shower has gone cold. But depending on what sort of shower you've got, your boiler may play no part in it whatsoever.
Two minutes of checking tells you which conversation you're having.
This is the whole thing, so it's worth getting right.
Electric shower. A box on the wall, usually white, with a dial or buttons on it, and one small pipe going in. It takes cold water and heats it itself as it goes through. It has nothing to do with your boiler at all.
Mixer shower. A valve with a hot side and a cold side, blending the two. Either a bar across the tiles or a control set into the wall. This one does use your boiler's hot water.
Power shower. A mixer with a pump built in, so it also blends hot and cold, and also uses your boiler.
If you're not sure, look at how many pipes feed it and whether there's a big box on the wall with a pull cord or its own switch.
Then your boiler is innocent and no engineer needs to look at it.
An electric shower heats its own water. If it's running cold, it's the shower unit, its supply, or its element. They have a working life like anything else and they do just stop.
Worth checking the pull cord switch or its breaker before anything else, because a shower running cold with no power to the heating element is the same symptom as a failed element.
This is an electrician's job, or a shower replacement. Not a boiler repair, and not something to pay a heating engineer to come and look at.
Now your boiler is in the picture, but it still might not be the problem.
Mixer showers have a cartridge inside that does the blending, and they fail. When they do you get cold, or scalding, or one then the other. That's the shower, not the boiler, and it's the same fault whether your hot water is perfect or not.
When a thermostatic cartridge goes, it nearly always goes cold rather than scalding hot. Inside there's a wax element that moves as the temperature changes, and that's what balances the hot and the cold. As it ages it stops responding properly, and limescale narrows the hot side at the same time. Less hot gets into the mix, and you end up with a shower that won't get warm no matter which way you turn it.
First thing I'd ask anyone is to go and run a hot tap. The kitchen's usually the best one, it tends to be closest to the boiler.
If it runs properly hot, your boiler is making hot water and the problem is at the shower. Stop looking at the boiler.
If it's cold there too, now you've genuinely got a hot water fault and it's worth working through that properly.
On a combi, hot water gone while the radiators are still fine usually points at the diverter valve.
Do this before you ring anybody. It takes thirty seconds and it decides which trade you need.
That’s a different fault from a shower that won’t get warm, and again it depends on the shower.
Electric shower. It heats the water as it goes through, so it needs a steady flow. If the flow drops, the water gets too hot, a safety cut-out kicks in, and it runs cold until it resets. Limescale in the shower head and low mains pressure are the usual causes. Descale the head first. It’s the cheapest thing to try.
Mixer shower on a combi. If someone runs a tap or flushes a toilet, the balance of hot and cold changes and the temperature jumps. A thermostatic cartridge should even that out, and if it doesn’t any more, it’s worn. If the flow through the shower is very low, the combi can also drop below the flow it needs to keep firing, so it cuts out and relights, and you feel that as hot, cold, hot.
Mixer shower with a hot water cylinder. Usually the cartridge, or the hot and cold coming in at very different pressures.
Worth knowing, because it isn't a fault at all.
The water coming into your house is colder in winter, sometimes a lot colder. Your boiler or shower is doing exactly the same job it did in July, it just has further to lift the temperature, so it either gets less hot or the flow drops while it tries.
An electric shower in particular will feel noticeably weaker in winter, because it's heating water on the way through and there's only so much it can do.
If yours has got worse gradually as the weather turned, and it was fine in summer, nothing has broken. If that's not good enough, the fix is a different shower rather than a repair.
That last one is a perfectly good reason to ask. Telling someone which trade they need is worth more than most people realise, because paying a heating engineer to look at an electric shower is money gone before anyone has touched anything.
I'm Carlo Cennerazzo, twenty-three years in the trade. Point your phone at the shower and I'll tell you what sort it is, what's likely wrong, and which trade you actually need.
£20, no callout charge, same day when there's a slot free. A written report follows afterwards, so you've got it in black and white rather than trying to remember.
And if you've already had someone out and you're holding a quote you're not sure about, that's worth a call too. I've no interest in who does the work, so I'll tell you straight whether the price is fair and whether it's even the right job.
Smell gas, or hear your CO alarm?
Don't book a call. Leave the property, then ring the National Gas Emergency Service on 0800 111 999. In Northern Ireland it's 0800 002 001.
Burning smell, sparking, or scorch marks indoors?
Turn the circuit off at the consumer unit if it's safe to do so, and get an electrician out. If there's fire or anyone's in danger, ring 999.
Power cut, or a damaged cable or substation outside?
Ring 105. It's free, 24 hours, and puts you straight through to your local network operator.