Domestic Hot water question

Hi, can anyone please help me with this whilst I wait for a service engineer to return next week?

Our hot water is contained in an 150 Litre unvented cylinder and heated by a gas boiler. We recently noticed that the back up electric immersion heater was switched on and upon switching off we were unable to heat up the water via the gas boiler ie the boiler didn’t appear to fire up when requested. Boiler is working fine for the heating. A visit from a service engineer (under maintenance contract) resulted in him changing a faulty component (not quite sure what) and the boiler now fires up when hot water requested, however it doesn’t result in hot water at the tap?? The engineer apparently didn’t wait to confirm that the water was heating up. We are now back on the immersion heater until the engineer can return.

Any ideas/suggestions would be much appreciated. Thanks

Does the motorized valve move when you switch from central heating to hot water?

Hi, thanks for the reply. There are two motorised valves I think?? Are they the ones that can be set to Auto or manual open? If so then yes they move back to Auto when I move them to manual open.

I’m not at home right now so am relying on my wife to do any troubleshooting whilst we wait.

That’s not really answering the question. Do they move when a hot water demand is put on the system?

Try getting her to look at this. She should see the lever move.

Forgive my ignorance but Shirley you don’t want the boiler firing up every time you open a tap. The whole point in a hot tank is to store the hot water is it not? Then heat it at a convenient/cheap time.
Or does a gas heated tank continuously heat as the temp drops?

Forgive my ignorance but Shirley you don’t want the boiler firing up every time you open a tap.

Yup you’re right. I was assuming the problem was with the boiler but it could be the immersion heater or thermostat is jammed.

You’re absolutely right. There’s a thermostat on the tank which is also used by the controller to demand hot water heating.

So to check this, the tank needs to be colder than the thermostat setting i.e. either turn off the immersion heater for a while or set the thermostat on the tank to max.

Simple things first.
#0. 2 port valve near DHW tank. Get someone to trigger HW demand on boiler and listen / feel if valve is opening. If the tank is already hot the thermostat may be stopping the valve open. IF its not opening turn up this thermostat TEMPERARALY. See if that makes a difference. Make sur eyou revert the thermostat. If its has opened are you feeling hot fluid flow into / though the pipe into the coil.

If its not opening
#1. 2 port valve near DHW tank. Put into flush mode with the lever to manual. Trigger boiler to run either DHW or CH. Feel to see if pipes are getting hot and flow.

From that you can ascertain is it operation of valve or flow into the coil.

P.S. How old is the tank and is it a very hard water area?

hot water tank manufacturer here…….

and the boiler now fires up when hot water requested,

your zone valve is working in that case.

(for clarity: the timer sends a signal to open the appropriate zone valve. the zone valve opens and as it reaches its fully open state, it makes a small microswitch that send a signal to the boiler (and possibly a seperate pump) to fire. this signal is sent via the cylinder thermostat – if the thermostat is satisfied, it doesnt pass on the signal to the boiler/pump.)

check:
for how long does the boiler fire once requested? if the tank is cold, the boiler should fire for 20-30 minutes at least.
what is the thermostat on the side of the cylinder set to? if it has been set too low then it will be satisied at a low temperature and turn the boiler off.

happy to chat if you PM me your phone number.

Thanks for all the replies. There’s a couple of things suggested that I can ask my wife to check out. eg how long the boiler is on for once fired up and the cylinder thermostat setting. Some things I’ll have to wait for until the engineer returns. Hopefully they’ll get to the bottom of it. The cylinder and boiler were a new install about 10 years ago. I’m in the West of Scotland with soft water.

For some background, I’ve always been confused as to why my utility bill was so high but I hope that this explains it, using electric rather than gas to heat the water!! Annoyed at myself for not checking sooner!

First thing has the overheat stat tripped.

Then look at thermostat, then motorised valve.

Should be a really easy diagnose for a semi competent plumber / engineer.

Any news?