These four sound like completely separate faults and they're usually the same thing:
If any of those is what you've got, the diverter valve is where I'd look first.
If you've a combi boiler, it makes heating and hot water from the same heat exchanger, and it can only do one at a time.
The diverter valve is the part that decides which. Run a tap and it switches everything to hot water. Close the tap and it goes back to the radiators. It does that hundreds of times a week, every week, for years, which is exactly why it's one of the parts that wears out.
When it sticks, it gets stuck pointing one way. Whichever way that is decides which of those four complaints you end up with.
Stuck on hot water: taps are fine, radiators stay cold. The boiler fires, it just sends all the heat to a tap nobody is running.
Stuck on heating: radiators fine, hot water is lukewarm or cold.
Somewhere in the middle: the worst version and the most confusing. You get some of both and neither properly, and it comes and goes, which sends people chasing thermostats and pressure for weeks.
Passing when it shouldn't: radiators get warm when you run a tap, or the heating never quite goes off, because the valve isn't sealing.
They stick in all sorts of positions and there's no pattern to it worth relying on. Most people only ever hear about the ones that stick towards heating, because that's the version that leaves you with no hot water and sends you looking for an answer. Nobody rings anyone because their radiators went slightly warm while the shower was on.
A stuck one can often be freed off, and that'll get you going again for a while. It isn't a fix, but it can buy you time, which matters if it's December.
This one catches people out.
The boiler isn't in any distress. It's lighting properly, heating water properly and running as designed. It simply doesn't know the water is going somewhere other than where it was asked to send it.
So the display often shows nothing at all, which makes people assume it must be something else entirely. A fault with no code is still a fault.
Not much, honestly, but these are worth doing before anyone comes out:
Write the answers down. Those four answers tell an engineer more than anything else you could say, and they're the difference between a quick job and an hour of hunting.
Use a tap for this rather than the shower. A shower has its own blending valve that fails on its own, and if that's what's gone you've got a shower running cold rather than a boiler fault at all.
The valve is inside the boiler, behind the casing, on the water side of a gas appliance.
That's not a job for you and it isn't a grey area. Taking the front off a boiler is engineer work, and everything useful about diagnosing this properly happens after that point.
So the honest answer is: gather the four answers above, then hand it over.
Worth knowing, because it's where people lose real money.
A diverter valve is a repair, not a replacement boiler.
Usually about an hour's work plus the part. Most of the time it's the cartridge or the actuator rather than the whole assembly, and that comes in around £150 to £250 fitted. Where the full valve has to go, it's dearer, and you'll see £300 to £350 quoted for that. Neither is a rip-off. What matters is which of the two jobs you're being charged for, so ask whether they're replacing the whole valve or just the part that's failed.
It also gets misdiagnosed both ways. I've seen a plate heat exchanger replaced at £425 on a hot water fault that turned out to be the diverter. And I've seen diverters replaced on faults that were never the valve at all.
If you've been quoted for a new boiler off the back of these symptoms, that's worth a second opinion before you agree to anything.
Straight away, really. There's nothing here you can safely do yourself beyond the four checks.
Get someone if:
Past that point it's about getting the casing off and proving which way the valve is actually sitting, and that's what decides whether you need the whole assembly, just the cartridge, or nothing at all. Getting that order wrong is how people end up paying for a plate heat exchanger they never needed.
I'm Carlo Cennerazzo, twenty-three years in the trade. Point your phone at the boiler, run through those four checks with me, and I'll tell you what it's likely to be and roughly what it should cost.
£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.