Why does my boiler keep locking-out?

[Davey]

Got a 9yr old kettling GW Micron 50FF boiler which's had the reset light come on and the boiler stop working of late, caused by a "non-volatile lock out" according to manual(!). Turn the boiler off/on, comes back to life.

If I turn the boiler temp down to 3/4 it stops the boiler locking out. I dosed the system last week with fernox boiler silencer for the 2nd time which had stopped the kettling for a time and the boiler ran ok on max temp until yesterday. The kettling has come back but not as bad as it was.

I was thinking the boiler is locking out due to overtemp cutout due to air pockets in the heat exchanger or is this a red herring?

I also have an Alpha+ pump which lowers the speed of the pump according to pressure (got TRV's fitted to all rads but one). Could the pumps (sometimes) low speed be exacerbating the problem?

[Roger Mills]

How is the pump connected - is it connected in parallel with boiler demand, or does the boiler control it in order to provide pump over-run (i.e. to keep the pump running for a bit after the boiler stops in order to carry away the residual heat)?

I suspect that your boiler needs pump over-run and that either the pump hasn't been connected correctly or that there's not a big enough flow path when the TRVs are closed to allow the boiler to dump its residual heat. The fact that it doesn't happen when you turn down the boiler stat reinforces that view.

Is there a by-pass circuit and, if so, is it controlled by an automatic by-pass valve? Smart pumps and automatic by-pass valves are often not very happy bedfellows. I've had to put my Alpha pump on a fixed speed setting to overcome just the problem you describe - which somewhat defeats the object of having a smart pump!

[Davey]

I don't think the GW micron 50FF's have pump over-run. The larger models do I believe.

Indeed. I have taken the pump off the auto adapt setting and set it to one of the proportional settings to see what happens. There is no auto bypass valve in my stystem. The bypass consists of a gate valve opened a small amount.

Is there a module available I can wire in that will keep the pump running for a minute or two once demand has stopped? Surely this sort of thing would be relatively simple with just a discharging capacitor keeping the pump going for a short while?!

[cynic]

Its an ac pump a capacitor, even a very big one, will be no use in this situation. If the problem is recent then it is possible that the performance of the pump is inadequate, the pipes are coated with an obstructive coating, your gate valve bypass is obstructed (count how many turns to close it, then open wide with pump running, and reset to where it was, may clear this). Your thermostat may be "tiring" or your limit stat may also be drifting.

[Roger Mills]

Yes - but you can't do it with a capacitor - you need a timer.

I have done mine - in *addition* to what the boiler already does - by using a timer designed to keep a bathroom fan running after the light has been switched off.

You'll find a wiring diagram appropriate to retro-fitting your system on Page 13 of

[Once you've loaded the PDF document and found the right page, right click the diagram, and then click on 'rotate clockwise' to make it easier to read]

[Mr Sandman]

Sounds like inadequate flow through the heat exchanger or scale build up in there causing it to overheat and lockout. The alpha will shut itself right down when the rooms are up to temp, so your flow -will- drop and you will kettleing and over heating. If it works fine at 3/4 id leave it at that, im sure your rads will be hot enough at those temps.

Otherwise you have to run the pump on full and have a bypass, but as you say, that expensive pump will not be doing its job in that case. When does it kettle? when the rads are up to temp or all the time?

Steve