Oil boiler randomly switching on

[Darthy_P]

Hi all,
Just moved to a house with an ancient Grant oil boiler supplying a hot water tank and the heating, via a Danfoss timer connected to a thermostat in the hallway for the heating, and one in the tank for hot water.

Everything appears to be in working order, except the heating keeps coming on even when it's set to be off.
It heats the radiators and hot water fine when it's set to be on, but also comes on intermittently throughout the day.

I've tried turning the thermostat way down, didn't make a difference. I've tried manuall setting the heating and hot water to "off" on the Danfoss controller and it still fires up.

I've tried to find evidence of a frost thermostat in the boiler or somewhere in the house (as the weather's been chilly) but couldn't find one, it's 12 degrees outside right now and the boiler is indoors so I don't think it's that.
I suspect the A/B valves controlling whether the water goes to the hot water tank or the radiators may be broken, but I don't think that could cause it to fire up even when both are set to off?

Anyone got any ideas? Any input much appreciated.

 

[CACKATALLTRADES]

I am really totally ignorant here and you've already covered most of my thoughts.

I'd definitely be trying to find that frost thermostat because it sounds to me like the most plausible explanation for it firing up when it's set to off. Can you identify any wires from the programmer that indicate its presence?

Failing that I would think a new programmer would be worth investigating (as in, cheaper than calling out a heating engineer) assuming you're confident you'll get all the wires in the right place (which may or may not be where they are at the moment )

Hope someone else can advise you better

 

[Darthy_P]

Been all around the inside and outside of the house, plus the loft, looking for a frost stat. Can't find any traces of it, so I don't think one exists.

The thermostat "clicks" when energised by the timer and doesn't click when the timer is off. Yet the boiler still fires up. So I think it must either be the timer or the boiler.

The other thing that jumps out is the pump appears to be running even after the boiler has shut down, I've tried manually closing the valves and hear a "glugging" sound. And the pipes near the boiler are super hot yet the radiators are lukewarm, so I wonder if it's either some internal frost protection or if it's somehow sending water round itself in a short loop...

Have posted a job on Checkatrade as I don't think I'm gonna get any further myself.

 

[CBW]

I highly doubt it’s frost protection, so I’d suggest you stop thinking it is. I would look again towards the motorised valve(s) that you refer to as A/B.

 

[ianmcd]

Many oil boilers have a small thermal store for the HW if yours has one it will fire to keep it topped up unless you can disable the setting , you dont say which boiler you have so I am just guessing

 

[Darthy_P]

Thanks, it's a Grant Vortex Eco indoor oil boiler. There's a separate hot water tank that's on a timer, so I assume there's not an internal thermal store?

I'm increasingly thinking it's an A/B valve problem, I'll update once a professional has had a look!

 

[ianmcd]

It could be an external control problem, but your boiler also has an inbuilt frost thermostst, this could also be faulty