Rads randomly not turning on downstairs

DvB

Howdy all,
We have a gas fuelled boiler (replaced 4 years ago & fully maintained since) and 2 zones for our radiators (updstairs & downstairs)
Last winter we had a couple of occasions where the central heating was timed to come on before we got up & the rads upstairs were fine (nice & hot) but the downstairs ones stayed cold & didnt work at all. However If I adjusted the thermostat dial up until I heard a small click they'd start to heat up again (i.e. turn it up from 18/20 degrees to 24 degrees or suchlike... other times it worked perfectly and both zones came on as normal.

Now at the start of the summer we turned both thermostats down to zero so the boiler just heated the water for us in the mornings, but since turning the rads back on a few days ago we've had the same problem with the downstairs rads not heating. They dont come on in the morning but if I go down & adjust the thermostat up (& hear the small click) they'll start to come on & work again. However if i leave it set at the higher temperature they dont come on, it appears to be the physical adjustment thats starting them working again.

I suspect this has something to do with either the thermostat itself or the motorised zone valves up behind the hot water cylinder but not sure if its one or the other or both that have a problem.

Obviously with winter in the post I'd like to get this sorted ASAP and was wondering if this has any obvious issue thats easily fixed or if I'll need a plumber in to look at.

Any help greatly appreciated.

Wearb

Have you any other (room thermometer?) of measuring the room temperature before you turn up the thermostat? Perhaps the room is up to temperature when the heating comes on and therefore the thermostat isn't calling for heat. If the room measures way below what the stat is set for, then it's very likely a faulty stat.

DvB

Wearb wrote: »

Have you any other (room thermometer?) of measuring the room temperature before you turn up the thermostat? Perhaps the room is up to temperature when the heating comes on and therefore the thermostat isn't calling for heat. If the room measures way below what the stat is set for, then it's very likely a faulty stat.

Must check that so. Thanks.

However at 5am this morning I'd have been surprised if downstairs was any warmer (18 degrees) than upstairs & both were set to the same temperature.

Can I assume with the adjustment allowing the rad to turn on the fault lies with the thermostat & not something else in the system?