Another Cycling Boiler Problem

[Fudgefactor]

Hi
Sorry guys - another cycling boiler problem, and it's driving me crackers!

We have Glow-worm 50FF Fanned-Flue boiler. For the last year or so, when the boiler comes on to heat either the H/W or C/H or both, the boiler stays on for 10 minutes continuously, and then goes off for approximately 2 minutes and then it starts this 90 seconds on 60 seconds off cycling pattern. The Grundfoss pump is set to position 2 (we live in a 3 bedroomed house with 9 radiators) and the pump remains on constantly during this cycling period. If I turn the thermostat up to max inside the boiler, the initial boiler fire-up period is extended to 15 or so minutes, but then still goes into the infamous 90/60 on/off cycle. Eventually, the boiler does go off totally and we have a nice tank of 50degreeC water and hot radiators. Obviously, as we are currently in our so-called summer, it's only occurring at the moment with the HW.

I have read all the forums I can find and tried most suggestions. I have even drained the system down, added Sentinel X400, flushed it and then added an X-200 & X-100. Everything seems to work fine, but I am now totally bereft of any other ideas.

I'm a DIY enthusiast, but by no means an expert.
Any ideas or suggestions would be truly appreciated. Many Thanks.

 

[D_Hailsham]

For the last year or so, when the boiler comes on to heat either the H/W or C/H or both, the boiler stays on for 10 minutes continuously, and then goes off for approximately 2 minutes and then it starts this 90 seconds on 60 seconds off cycling pattern.

Did you have any work done to the system just over two years ago - new rads, TRVs etc? When was the boiler last serviced?

The Grundfoss pump is set to position 2 (we live in a 3 bedroomed house with 9 radiators) and the pump remains on constantly during this cycling period.

Have you tried the pump on position 3?

The boiler has a pump overrun, which keeps the pump running for a few minutes after it goes out. As the boiler is relighting before the overrun stops, the pump will run continuously.

If I turn the thermostat up to max inside the boiler, the initial boiler fire-up period is extended to 15 or so minutes, but then still goes into the infamous 90/60 on/off cycle.

The longer time is to be expected as you are asking it to heat up to a higher temperature.

Eventually, the boiler does go off totally and we have a nice tank of 50degreeC water and hot radiators.

Is it a fully pumped system?
What other temperature controls do you have, i.e room thermostat, cylinder thermostat?

Hot water in the cylinder should be kept at 60°C. This is to prevent the growth of legionella.

 

[Fudgefactor]

Thanks for the quick reply. The boiler was serviced 18 months ago and was briefly checked by a heating engineer 3 months ago he said it was spotless. To be honest, the cycling problem was never really a major concern at that time or previously, but it's now a complete irritation because I know something's not working properly.

I've had nothing changes since the system was installed in 2000. Same rads and trv's. We have a wheeled thermostat in the lounge and a cylinder thermostat that WAS sat at 50, now on 60 (thanks!). This cycling problem occurs if the room stat is set to 26, 23, 18 - whatever.

I haven't tried the pump on 3 yet, but will tomorrow. We do hear air in the system now and again, but I release this with the threaded valve that sits 12" or so above the mid-position valve. We only have the one pump, but I'm not entirely sure what you mean by fully pumped - sorry!
Thanks again.

 

[David937]

Fully pumped AIUI means that the whole system is pumped, i.e. the primary DHW pipework is pumped and not gravity fed.

Don't take my word for it as I am a pretty much clueless DIY'er.

 

[twgas]

We do hear air in the system now and again

how often is this? frequent "airing" usually means more serious problems, such as wrongly configured pipework and/or a build up of sludge. The sludge generally builds up, mainly, in the cold feed/open vent area and gives rise to pumping over which accelerates the sludge problem.

 

[Fudgefactor]

Oh, I see. In that case, we have a gravity fed system.

 

[twgas]

I think you're confused, you say you have a mid position valve - which would point to a fully pumped system. you're description of gravity fed, I'm assuming, means it is fed from a F&E tank in the loft, if that is the case it is usually refered to as an open or open vented system.

 

[Fudgefactor]

twgas
Most daily visits to the airing cupboard valve will remove a small amount of air, but not much. Most mornings, when the HW comes on, air can be heard in the one of the two pipes (if not both?) above the boiler. Air can be also be heard (and felt) bubbling in the HW pipe that runs from the bottom of the HW cylinder up to the mid-position valve. During our recent drain-down, the water was virtually clear, just slightly grey. I assume that the Sentinel X-400 would have only cleaned the CH part of the system and not the boiler itself, right?

Also, one of the two pipes above the boiler has a drain cock in it, but when I loosen this, it just unscrews and virtually drops out – no air or water escapes at all (???)

A larger HW cylinder was installed by British Gas in 2001. Is it really possible that they would have configured the pipework incorrectly?

 

[Fudgefactor]

Yes, we do have a F&E tank in the loft.

 

[twgas]

daily bleeding = problem.
how long was the X400 in the system?
Was it circulated round the rad circuit (most people forget the CH in summer)?
The drain cock washer is probably stuck to its seating, another sign of a possibly dirty system - it may just be overtightened or old age.
BG wouldn't have altered the cold feed/open vent configuration when changing the cylinder - unless it was discussed and you agreed to pay for it. Do you have a service contract with BG?

 

[Fudgefactor]

Mick, thanks for your perseverance - it is appreciated.
No, we don’t have a contract with BG. Only a small amount of air is released, but there’s definitely air there for sure. The x400 was in the system for about 3 hours with all the radiators on full blast and very hot. We then drained the system and noticed how surprisingly clear the water was. I certainly hadn’t considered the washer being stuck to its seating. I tried undoing this to see if any air or muck would come out here. The boiler was a lot quieter after the X400, but still cycled

Could there be a problem with the boilers own thermostat?

 

[twgas]

X400 should be in for about 4 weeks 3 hours wont do alot.

I wouldnt rule out a problem with the boiler stat but I still think a partial blockage, around the feed/vent area, is more likely.

 

[Fudgefactor]

4 weeks?......DOH!
I remember reading 2 hours on the back of the Sentinel carton - but having just been to the Sentinel website, it does say 4 weeks.

As it's August and we don't have the rads on, I'll tollerate the cycling for a few weeks and stick another X400 in mid-September. Like you, and judging by the noises above the boiler, I'm also beginning to think that there maybe a blockage somewhere.

You've certainly given me something to work with Mick.