[Closed] Central Heating Failure

heating is either a sealed system or with a vented expansion tank in the roof.

Only reason for low pressure would be a fairly major leak.

ill very much doubt its your low pressure from the mains.

does your pump run ? is there a supply to the pump ? are there motorised valves on the system - are they working ?

Thanks for the replies. Not sure if the pump runs. It's pretty old but gets serviced regularly. There's hot water from the taps, and the boiler fires up if I turn up the water thermostat. The air thermostat in the hall is working, and requesting heat, but the boiler doesn't turn on.
I don't think there are any motorised valves. I didn't have much time to investigate this morning. It just seems too much of a coincidence with the low water supply pressure.

oil you say - you have checked the fire safety valve hasnt tripped? and that you have sufficient oil right ?

your hot water is probably still hot from last time it was heated because you have a tank and thats why you have hot water - the two are probably unrelated

trail_rat,
Yes oil present. It fires up ok if I increase the thermostat on the boiler (which set the water temp), but not from the room thermostat.
There are two rads (utility room and bathroom) that are warm, which I think come directly from the boiler, all the others are cold.

There are two rads (utility room and bathroom) that are warm, which I think come directly from the boiler, all the others are cold.

I'd start with the pump

Isn't there a low water pressure switch on the pump?

Pump, or, zone / mid pos valve gone south, my guess.

Does the boiler cut out rather quickly, when you do turn up the HW stat? Or does it happily run for a good long while?

If it does run for a long time, the pump is probably working ok and is circulating water from boiler to HW cylinder, that would suggest the valve.

I'm not a plumber, etc. etc.

Boiler runs fine. i should really learn more about CH systems!

I would look at the motorised valves if you have any.

If the CH valve wont open, then the boiler wont fire up - it is designed this way so hot water isn't pumped if there is nowhere for it to go.

It would also explain why the boiler fires for the hot water but not CH, there is normally a separate valve for each.

Valves are about £40 and can be replaced without draining the system as long as they still make the valve.

My CH has been playing up a bit recently, had no idea what it was till I caught it in one of it's moods and found the pump is getting confused every so often. 19/20 it starts up fine but 1 time in 20 goes it just sits there with the LEDs flickering dimly and won't pump. Most times the first I know is when the shower is cold as it didn't heat the water at 6am in the morning, but I finally caught it out during the day when I was about to investigate.

As others have said, CH is a closed system so the mains water pressure shouldn't matter.

I would check that the CH has pressure, should have a pressure gauge on the boiler, and it should be between 1 and 2 bar.

I would then check the boiler fires up, when you switch the heating on.

I would then check the pump is running (put a screwdriver on the pump, and test if you can feel the vibration against your ear)

I would then check any motorized values and manually check they are open, and open them if they aren't and see if that helps

You could also, attempt to bleed a single radiator, and see if it gets hot as you are bleeding it as that would also imply a pump problem.

Essentially, though if you have pressure, the boiler is firing, the pumps running, the valves are open and you still don't have heat, and the bleeding trick doesn't work, at this point I would ring a plumber.....

I would then check the pump is running (put a screwdriver on the pump, and test if you can feel the vibration against your ear)

Really?

I can hear the pump and the water moving in the pipes throughout the entire house. Mind you it is a beefy little number which self regulates so back off as the TRVs close off.

Really?

Its a very easy and quick way to check that a plumber told me, and has served me well for years, and works regardless of the pump size or volume of water 🙂

what he means is - it should be glaringly obvious if the waters flowing as youll hear it being forced round the rads.

But the zone valves are where id be looking first.

If low water pressure is a problem, any reason why I can't just short out the low pressure switch? (assuming it's a switch).

If someone went to the trouble of fitting a low pressure switch, then they don't want their system run with low pressure. So why would you force it to do that?

it does sound like a surefire way to bugger the pump seals to me. infact ill be surprised if there are not labels warning against this on the pump- both mines have them.

Thanks everyone, sounds like there is a fault so I'll call someone out.

Thanks again, now fixed.
It was the room thermostat. The 'heat' icon on the LCD display was going on, but the relay wasn't switching, so the boiler wasn't tuning on.
Batteries are ok in the thermostat (Drayton Digistat 3), but taking it off the wall seems to have made it work.

Cheers all, now warm.