Why Is My Boiler Firing Up?

[chrissa1]

I'm sitting in my lounge with no central heating on in the house, no hot water has been used since this morning and my gas boiler fires up and stays on for about 5 minutes.

Can anyone tell me why please? I'm trying to save money.

[Hopkirk]

The thermostat on the front of our hot tank fell off once.

That made the boiler keep firing up, and the water got scalding hot.

[RevArkwright]

The stat on the boiler generally only controls the boiler shut down once the returned water has reached the temperature set.

It may be that the cylinder stat overrides that. Depends how it has been wired up. I would check the stat on the hot water cylinder, however, if your hot water is adequately supplied, that should be ok.

[RevArkwright]

Quite right Hopkirk, good idea to check it occasionally.

[buildersmate]

I assume the previous Answerers have established with you that this is a conventional boiler and not a Combi.

In which case this is almost certainly caused by the way the logic has been wired in your heating system. The system provides for both DHW and CH, when the thermostats call for it and when the times you have selected on your time clock are applicable. Outside of these parameters, the boiler should never fire up, as no heat is required.

However the boiler unit itself has a thermostat which controls the water temperature in the 'primary system'. The primary system water is that which flows through the CH rads and the coil inside the DHW tank. This water is sealed in and never changes nor comes out of your taps. If the logic of the system has been wired such that the boiler is NOT disabled when the rest of the system does not require heat (see para 2 above), the boiler will continually respond to a cooling of the water in the primary circuit. This is known in the trade as 'boiler cycling' and it just wastes energy.

Whether or not this can easily be fixed may depend on the controller you have. I wouldn't employ a heating technician to deal with it - I would DIY, but then I'm an Engineer. If anything, you need a tame electrician who is familiar with wiring heating systems.

[The Builder]

Take BM's advice Chrissa. I've only just seen this, and my first thought was "boiler cycling".

[buildersmate]

There are a few tests you can do to confirm my diagnosis. Do you know where the pump is that pumps water around the system? If the boiler is ever coming on when the pump is not pumping (grab hold of its body to see if it is vibrating), you have a boiler cycling problem.

The technical solution is typically to rewire the boiler such that it is wired (in parallel) to the same pair of terminals (in the control box) that determine when the pump comes on. That way, the boiler can only ever fire up when the system is calling for heat - not when it just feels like it!

You can't DIY this unless you know what you are doing. A handiperson who is familiar with working with electrical systems could possibly help you - or ask an electrician.

[chrissa1]

When I went up to bed last night, I found the thermostat on the front of the tank in the airing cupboard. It was sitting at 70-80 degrees. I turned it down to 60 degrees and since then.....silence.

I haven't run any hot water yet but I'll see if the water is hot enough when I do.

Thank you all for your help.

[buildersmate]

60 degrees is a good temperature for the DHW stat - humans need hot water at about 42 for a hot shower (once it is mixed with cold).
The temperature of the water in the primary system (the boiler stat) needs to be higher than the DHW stat by 10-15 degrees for max. efficiency.

[chrissa1]

I had a shower this morning and the temperature of the water was just a little bit cooler than before, without changing the heat control on the shower, so that's ok.

Used the hot water tap before lunch and the water was still hot but not "ouch" hot. The boiler clicked on for about one minute and then clicked off, as it should.

Thanks to all your help, I think I've got the right temperatures now. It hasn't fired up since. Cheers.

[Hopkirk]

If you have reduced the temperature of the hot tank, that should save you some money.

However, if someone decides to have a deep bath is there enough hot water left for the next person to have a shower straight afterwards?

[chrissa1]

Who cares Hopkirk. Lol.

I live by myself so it's hardly going to happen.

[Hopkirk]

If someone comes to stay, tell them they will have to share your bath.

[chrissa1]

That depends on Who comes to stay. Lol.