All radiators cold but boiler & hot water working fine

I have a "Systemate A Class" fitted in a 2 years old house. Yesterday I noticed that all the radiators are completely cold and the house is getting colder quickly (it's -1 celsius outside). At the same time, the boiler seems to be working fine and there is plenty of hot water in the taps. It's just the radiators.

I tried all sensible things I could think of, but could not restore the heating. The thermostat unit in the hall continuously displays a little flame icon, which according to the manual, means it requests radiators heating from the system. But no heating is happening.

I have made no changes and haven't touched the system in the last several days. Whatever it is, it seems to have happened on its own.

Any ideas what I could try before calling in the pros? I'm freezing.

Sadly builders warranty for the house (2 years) ran out last month :-(

Thank you,
Marcin

check your store temp if below 60 degrees heating pump wont run but will be hot enough for hot water.
mark.
www.gledhill-repair.co.uk

Thanks for your reply. The temp is higher than 60.

I have found the cause but I'm not sure what to do next. The cause is that midposition motorized valve (Danfoss HSA3) does not have enough oomph to open the valve when stat requests radiator heating. When I move the valve manually, the heating returns, until stat shuts it down again when target temp is reached. Then I have to manually open it again etc.

I have removed the valve head with motor in it (i.e. the white box), and on its own it rotates fine from end to end when stat requests it (also checked the voltages with multimeter - seem fine). As soon as it is put back on the valve, it only rotates a little bit and then stops.

This would indicate to me that the valve is stuck, but I can rotate it with my fingers (no tools, but through a cloth as it is hot), and I sense only small resistance.

Also, when I open the valve using manual lever, the motor return spring has plenty of force to move it back to original position.

So maybe the valve is not stuck, but it is the just motor that is too weak for some reason?

I intend to buy the valve head with motor first (about £40) and replace it to see if it fixes the problem. If not, it means the valve is stuck - but to replace it (another £30) I have to drain the system down, which I have never done before and I prefer to wait until temps outside go a little bit up before I start on this.

Question:
If I drain the system down, do I have to add inhibitor when refilling again? What inhibitor do I need and how do I put it in? I'll be refilling through the loop under my boiler. Also, I'll probably need to vent the air out when I'm refilling. How do I do that?

Thanks,
Marcin