Boiler/heating issues

[sleaterkinney]

I've only ever had combi or ancient solid fuel systems so I'm at a loss as to what's going on.

We have boiler and a water tank. See pics. On the boiler there is a nest thermostat and it's set to come on once a day to heat up the water (I am presuming it heats up the tank). Whenever this happens the radiators also come on.
The boiler and radiators also seem to come on at random - what could be causing this?.

 

[Sprocket.]

Nine times out of ten a plumber will say it’s probably the motorised valve that’s sticking and charge you £££ to replace it. On your system you could have what’s called reverse flow, if the return from your hot water cylinder is teed into the central heating return line and not a separate return to the boiler.
Do your radiators get very hot or warm up slowly. This points to a pipe work issue rather than a motorised diverter valve problem.

 

[sleaterkinney]

They warm up slowly, but what besides the thermostat could make the boiler come on?

 

[platinumsage]

They warm up slowly, but what besides the thermostat could make the boiler come on?

Hot water demand? Another thermostat you don’t know about?

 

[danski]

Hot water demand? Another thermostat you don’t know about?

Yeah, cylinder stat.

 

[Sprocket.]

They warm up slowly.

Sounds a possible pipe work issue. If you feel the nearest radiator, you can feel which side is hotter. The side with the shut off valve should be the return, but will be hottest if pipe work is a problem.

 

[sleaterkinney]

Yeah, cylinder stat.

And that comes on whenever?

 

[StoneRoad]

Possibly, the Cylinder stat is overriding the nest thermostat. (try turning it down to 55degress - ours can get knocked or nudged upwards).

I suggest checking that the motorised valve is operating correctly, there should be a manual trip to get it to motor over. During the summer, low demand for room heat can make it stick on HW only, or only partway across - I had a period before we changed the boiler when a series of valves died, (one was replaced under guarantee) with this problem. Especially if your pipework has a bit of "sludge" floating around.

 

[danski]

And that comes on whenever?

If the programmer is in a period calling for hot water then yes. Whenever the water temp drops below that which the cylinder stat is set at it will cause the boiler to fire.

Something that can cause a boiler to kick in randomly is a 'preheat' setting although that might be just for combis. It basically keeps the boiler warm with little bursts so that when a big burst is needed, it doesn't have to heat up so far, if that makes sense.

 

[WouldBe]

And that comes on whenever?

Whenever the temperature in the tank is below the set temperature.