Boiler working but rads not warming - sometimes

[hasti]

Hi,

I wonder if anyone can help me with this issue...

I have a Greenstar 42 CDi combi boiler with a DT20RF programmer. Often, the boiler comes on at correct time and heats the radiators as it should. But, a lot of times the boiler will come on but the radiators with be just slightly warm. The thermostat is set high (25C) and the room is cold (15C) and the central heating light is lit on the boiler, so I think the RF communication is fine (in fact, I have moved the thermostat right next to the boiler to be certain).

But then, if I turn the thermostat up a couple of degrees, the radiators warm up as they should. I should not have to do this as the rads should already be scorching hot according to the room temp and thermostat setting.

I am constantly checking radiators to see if they are warming up, and if not, fiddling with the thermostat to 'kick start' the heating.

Why would this happen, and how can I fix it?

I have bled the radiators and checked the pressure in the boiler - both are fine. The hot water from the taps is fine too.

Many thanks. Any help greatly appreciated!

 

[fraz101]

Do you have TRVs on Rads?

 

[hasti]

Do you have TRVs on Rads?

Yes, I have Thermostatic valves on each radiator. Most of them are set quite high, and they work fine - except when the problem I have described occurs.

 

[doitmyself]

How many rads in the house?

Do all rads have the same problem?

Remove the TRV head and check that the pin can be pressed down easily and springs up when released.

 

[hasti]

Thanks for your help...

There are 10 rads in the house. All rads have the same problem.

Should I remove all the TRV heads, or just the first one on the circuit? And, is it simple to do (I am not a plumber)?

Just to clarify: Often it works fine and the rads do get hot. But then (sometimes an hour into the 'on' period) they cool down, even though the central heating light is lit on the boiler. Turning the thermostat up a few degrees seems to get it going again, even though the room temp is nowhere near the thermostat setting.

 

[jtsplumbing]

Got a faulty transmitter unit , not reading correct temperature

 

[hasti]

Got a faulty transmitter unit , not reading correct temperature

Would it be the transmitter unit? The central heating light is lit on the boiler - so wouldn't that mean that the signal is getting through?

It's not like the central heating is going off (like it would if it reached the temperature set on the thermostat), it is still on, but not heating up the radiators.

Or am I being a bit thick?

 

[doitmyself]

Should I remove all the TRV heads, or just the first one on the circuit? And, is it simple to do (I am not a plumber)?

Remove all heads. It's very easy to do. Most TRVs have a chrome knurled nut holding the head onto the chrome body. Set the TRV to max then turn this nut anticlockwise (looking down) and the head will come off. (No water will escape). This will expose a pin sticking out of the valve body. Use something suitable to press the pin down the release it. The pin should do down easily then spring up.

Often it works fine and the rads do get hot. But then (sometimes an hour into the 'on' period) they cool down, even though the central heating light is lit on the boiler. Turning the thermostat up a few degrees seems to get it going again, even though the room temp is nowhere near the thermostat setting.

Don't replace the TRV heads. Run the system without them on and see if the problem goes away or is still there.

Just for info, which thermostat do you have?

 

[hasti]

Many thanks 'doitmyself'. I'll try what you suggest and get back to you.

The thermostat is a a Worcester Digistat.

(Incidentally, I don't even want a thermostat, but it seems I have to use one. I just want the heating on for a couple of hours morning and evening. During this period, it's never going to get so hot that the heating needs to be told to switch off. In the past I've had trouble with the RF signal, so I moved the thermostat to right next to the boiler. If I could bypass the thermostat altogether and just have the heating blasting away at the times I want, that would be a lot simpler.)