Central heating on standby Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[AdrianS]

Hi all,

Sorry - long first post coming up!

We've just bought our first house which has an FER Falcon II combi boiler. I'm seeing what seems (to my ignorant eye) to be a strange effect. I've swapped the old fashioned mechanical room thermostat for a programmable one (a [url=http://www.salus-tech.com/programmable-room-thermostat.html]Salus RT500[/url]). It's has zero volts at the contacts, so I believe it's compatible with the boiler.

However I've noticed that even when the thermostat is 'on' and demanding heat, the boiler often keeps the central heating on standby and only fires the burner for a fairly short period of time - sometimes as little as 15 seconds or so. The house certainly isn't cold - the thermostat is reading 19C but it's set to 21C, which it never actually reaches - presumably because the boiler goes back to standby before getting there.

I'm not sure if this is normal behaviour with a combi or not, but it certainly seems a bit odd.

In case it's relevant, we replaced a radiator last weekend, which entailed a partial draindown (we used freeze spray so only lost a radiator full or so) but put the TRV on the return rather than supply side (schoolboy error) so will be swapping that round this weekend. My father-in-law also let a little pressure out of the pressure vessel before realising it wasn't necessary to do that to de-pressurise the system.

The system is about 1.1 bar when cold and rises to about 1.8 when hot.

There were also 2 occasions this evening when the boiler made a horrible racket when the CH kicked in. Opening a tap resulted in spurts of extremely hot water, which then ran cold as the burner didn't ignite. I had to turn the boiler off and on again to get it supply hot water once more.

I'm now a bit confused about the standby query and worried about the behaviour this evening. I'm comfortable with almost any other DIY but confess central heating is something I've never had to work on before.

Cheers,
Ade

[The Heating Doctor]

ok a couple of things here, are you sure your combi requires a volt free contact? Not all do, some need 240v others need 24v and others require the volt free connection you have indicated. I've never came across your manufacturer before so good luck when it comes to spsre parts, however to your thermostat, some electronic thermostats have adjustable dead bands, all thermostats have a dead band this is the area where the thermostat will not switch ie if you set your thermostat to 20 deg C set point) and the stat has a dead band of 2 deg C this means that from the set point the thermostat will cut in at 19 deg C and will hold the heating on untill the thermostat reaches 21 deg C and then will cut out, this is called a cycle. I would say a 2 deg C dead band is the minimum dead band you would want to programme in, any less and the boiler will switch on & off very quickly which is not good, some boilers have a maximum number of cycles per hour exceed this and the boiler will just switch off.

[AdrianS]

Hi. Thanks for the reply. The instructions for the combi state (ambiguously) "[i]Use a room thermostat (24V) without voltage to the contacts[/i]". Not very clear! However, looking at the circuit diagram, it shows the thermostat as a simple switch and that's certainly all the old mercury switch thermostat was.

The dead band doesn't appear to be adjustable on this thermostat, but the odd thing is that the boiler goes into standby even when the thermostat is showing the room temp as 4-5C below the set temp. However, maybe that's just my lack of understanding about how a combi works. I guess the heat exchanger could have reached the required temp and turned the burner off, even though the room is still heating up.

I have a suspicion that the radiator in the room where the thermostat is located might simply be too small to heat the room properly, hence why the room temp never actually reaches the set point.

Am I right in my understanding of the way a combi works? Would it be normal for the burner to shut off for a while, even if the thermostat is still demanding heat, if the heat exchanger is sufficiently hot?

Thanks again,
Ade

[The Heating Doctor]

Yes you are correct, however not just a combi all boilers will fire to the dictates of their own in built control thermostat, is the control thermostat on your boiler set to maximum? If not that could prevent the radiator(s) getting to full temperature.

[AdrianS]

If you mean the temperature control for the central heating water on the front, it's set at about 70C, which isn't quite the maximum but is most of the way there. The radiators certainly get too hot to touch.

I'm rapidly concluding that the radiator is simply too small for the room that contains the thermostat. Online calculators that I've used seem to suggest it needs about 6000BTU and I guess the current single panel convector radiator (600mm x 950mm) is throwing out about 3500BTU.

I was planning to move that radiator anyway, so looks like I'll have to replace it with a double panel version of similar size.

Ade