No hot water,
but the heating is fine
A customer messaged me the other week to say the water was still cold. We had been on a video call an hour before that, sorting out the pressure on her boiler, and as far as she could tell nothing had changed. Nothing was wrong. She just needed to wait.
Hot water and heating come from the same boiler but they take different routes to get to you, and a fault on one side will leave the other working normally. Some of the reasons are not faults at all, and two of those are worth ruling out before you call anybody.
Have you just topped up the pressure
If you have a system boiler with a hot water cylinder, and the pressure had dropped low enough to stop the boiler, then the cylinder has been sitting there going cold the whole time the boiler was off.
Topping the pressure back up is the right fix. What it does not do is give you hot water immediately, because the boiler now has to heat a cylinder full of cold water from scratch, and that is two or three hundred litres of it. Give it about an hour. It could be less and it could be more, depending on the size of the cylinder and what else is going on in the house. But in about an hour you should see a difference, and if it is not hot it will at least be warmer.
This catches people out because everything looks fixed. The pressure is right, the boiler is running, the radiators are warming up, and the tap is still cold.
Is the hot water on a timer
The other one that is not a fault. If your hot water runs on a time programme it will not heat up until the next scheduled period comes round, however healthy the boiler is. Putting it on manually, or using the boost, answers that in a few seconds.
Between them, those two cover a fair proportion of the "no hot water" calls that turn out to need no repair at all.
The zone valve
If it is not either of those, then on a system with a cylinder the most common cause in our experience is the zone valve.
Most systems with a cylinder have valves that direct the hot water from the boiler either to the radiators or to the cylinder coil, depending on what has been asked for. If the one feeding the cylinder fails, or its motor gives up, the boiler will happily heat your radiators and never send anything to the cylinder. That is exactly the pattern you are seeing, with the heating fine and the water cold.
On a combi, the diverter valve
A combi has no cylinder. It heats water as you draw it, and it uses a diverter valve to switch between sending heat to the radiators and sending it to the tap.
There is a test for this you can do yourself in about a minute. Run a hot tap, then go and feel a radiator. If the radiators start warming up while you are asking for hot water, the diverter valve is not switching properly, and that is where we would be looking.
Diverter valves are common faults in most boilers in our experience, and that is regardless of the brand. In some instances it shows up the other way round, with the hot water working and the heating doing nothing.
What we check
Beyond those, we would be looking at the cylinder thermostat and the wiring to it, the controls, and on a combi the hot water side of the boiler itself.
"Ravinder turned up when he said he would, did a very thorough service (most plumbers just look around then disappear with your money). Asked before replacing a seal as it was not required by manufacture and was additional cost, I did it regardless. Also spotted that the diverter valve would need to be replaced at some point. Utterly trustworthy and reasonably priced."
Quick answers
Why do I have heating but no hot water?
How long does a hot water cylinder take to heat up?
How do I know if my diverter valve has failed?
I topped up the pressure and the water is still cold. Why?
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