Central heating question re thermostats, cut offs, etc

[jamma-plusser]

I'm puzzled.

I have an oil-fired boiler (a Danesmoor 15 /19) - as I've always understood these things, once the thermostat in the hall senses that the temperature has reached the preset value it will turn off the heating.

If the heating goes off (due to the thermostat switching it off) it should then be possible to make the heating come back on again by turning up the thermostat, correct?

My boiler isn't doing this - wasn't doing it before I had the (old, slightly faulty) thermostat replaced and the new thermostat isn't doing it either.

It will happily reach the thermostat's preset temperature with the radiators getting nice and hot, BUT it then switches off and even though the thermometer in the hall indicates that the temp has gone a few degress BELOW the thermostat's cut-in point, AND the radiators are cooling off markedly, the heating stays resolutely off until it gets really cold. And it's not the thermometer that's 'off' either, I can tell it's getting colder.

The thermostat isn't wrongly placed.

I SHOULD, as I understand it, be able to turn up the thermostat to, say, 30 (it's usually at about 22) and the radiators should start to heat up again. They aren't.

And yes, the boiler IS switched on. :)

[John]

Does it turn back on if you leave it off for a few minutes? Perhaps it gets a bit overheated somewhere and something needs to cool down before it will re-ignite. Probably a safety feature to stop it exploding if oil is injected into a hot combustion chamber.

[jamma-plusser]

Yes, it will turn back on if left off for a few minutes, and turning up the thermostat on the boiler helps too, but not all the time.

I do think that something is overheating somewhere, but if so why does briefly switching it off and on help? Surely if something has overheated it shouldn't switch back on so soon when done manually?

[Toby]

It could just be the latent heat in the boiler being enough to keep the boiler from actually firing - if you put your ear on the boiler just after you have re-triggered the room thermostat, so you hear the pump running?

[Allan Mac]

Your thermostat almost certainly controls a motorised valve which once open will run the circulating pump and fire the boiler. ( may take about 15seconds to open). If this valve has become sticky or the microswitch within become intermittent, this could account for your problems. If after about 15 seconds you hear the circulating pump but the boiler does not fire shortly thereafter, suspect the boiler. If the circulating pump does not run suspect the valve.

Allan

[John]

Latent heat - good thinking - the metalwork of the boiler gets hotter than the water - ant this can be enough to prevent the boiler from seeing a need to fire up for a while.

[jamma-plusser]

To answer a few questions, what I'm finding is that if I try and re-start it manually from the control panel, ie if I switch it off then on after maybe be off for 5 seconds, then you hear the pump start for a second or two then shut off.

If I switch it off at the control panel then leave it for a few minutes THEN switch it back on it fires up, but even then the heating won't necessarily get that hot.

[Allan Mac]

sounds like some kind of circulation problem, pump?Valve?blockage?air? if the boiler its self is roasting but the rads are not that hot the boiler will be short cycling on its own stat. Difficult to be more specific at a distance.

[jamma-plusser]

The boiler doesn't seem excessively hot, but I don't make a point of checking it so hard to be sure.

What is 'short cycling on its own stat'? Running for short periods under its own thermostat control?

Is it harmful to the boiler?

[Michael Chare]

The boiler will (typically) also have its own thermostat which can be adjusted. Have you tried increasing this?

Have you tried reducing and then increasing the room thermostat shortly after the boiler has fired to see if the boiler stops and starts and you do not perhaps have a sticking motorised valve.?

Are there any indicator lights on the boiler which might show if the boiler is getting power but not firing because it is hot?

[jamma-plusser]

I have now, seems to be helping thanks. :-)

However, should it really NEED adjusting? It's ALWAYS been on the second setting and now it's up to the third - why the need for an increase?

Haven't tried that but will do tomorrow.

None that I can see - it's a Danesmoor 15 / 19 - should it have any such lights?

Many thanks

[Michael Chare]

If for example you turned off some of the radiators then the boiler would reach its max temperature sooner and this stat would turn the boiler off. Increasing the setting would let you output more heat into the remaining radiators.

My stat (on a different boiler) is set to about 2/3rds of the maximum.

If increasing this stats causes your boiler to start then that is your 'problem'.

Pity!

[jamma-plusser]

So are you saying that it's not a 'problem' at all, simply a result of colder air (and water?) temps so meaning that the stat on the boiler needs to be set higher as a result?

[Michael Chare]

I am saying that increasing this stat makes the boiler use a higher output temperature.

What I would also add is that if the boiler stops because it has got hot enough for this stat to open, then (perhaps if the pump is running) I would expect it to have cooled down enough to restart within a few minutes. (i.e. more like 5 minutes - not 30 minutes)