Boiler will not tolerate fewer than 4 radiators on

[gowe]

Old Chaffoteaux Celtic Plus boiler in house I have moved into. Various problems fixed by BG and boiler working reliably for some time. However have noted CH works fine with all radiators on but if then start turning some off, as soon as only 3 radiators left on, boiler flames go out leaving only the pilot. Pump continues and rads gradually cool. Turning a fourth rad on then causes the boiler to come to life and rads heat up normally. DHW is normal throughout and hot. Rads normally hot or very hot and warm up quickly. Boiler pressure and temperature in normal ranges. BG unable to suggest solution.

Connections to rads are microbore.

Would be grateful for any ideas.

 

[Bambergaspipe]

Old Chaffoteaux Celtic Plus boiler in house I have moved into. Various problems fixed by BG and boiler working reliably for some time. However have noted CH works fine with all radiators on but if then start turning some off, as soon as only 3 radiators left on, boiler flames go out leaving only the pilot. Pump continues and rads gradually cool. Turning a fourth rad on then causes the boiler to come to life and rads heat up normally. DHW is normal throughout and hot. Rads normally hot or very hot and warm up quickly. Boiler pressure and temperature in normal ranges. BG unable to suggest solution.

Connections to rads are microbore.

Would be grateful for any ideas.

Sounds like gas valve not modulating have it checked.

 

[heatingman]

Is it the very old chaff, a photo would be good. Rattly white case, large drop down flap and dark blue panel with grey knobs I think.

 

[gowe]

BG replaced the 3 solenoids around gas valve which fixed erratic DHW. Also replaced pressure switch above boiler. BG then said gas valve working correctly, correct voltages measured etc.
Regret unable to photo. However boiler looks as you describe but with dark brown large glass drop down door, revealing grey rather than blue metal panel with dark brown control knobs.

Fault I described is repeatable, not intermittent.
Area is hard water but boiler is quiet without kettling (although it was starting to screech some months ago. I put in descaler through radiator bleed hole and later flushed system, since when it has been OK.). Is it possible that with fewer than 3 rads there is insufficent flow through the boiler to maintain the minimum flow rate required ? Due to scaled boiler and/or the use of microbore pipes ? As I said, there seems to be good flow through all rads as they heat up quickly. There is no external by-pass fitted. Thanks for your thoughts so far.

 

[heatingman]

Possible areas to check, been a long time since worked on a chaff so cant be to precise.
The return isolation valve has a mesh filter fitted in it. It is the large 1"ish nut with what looks like a smal bowling pin sticking out. The pin pushes in and screws shut, shut the other iso valve and the filter can be cleaned. Need to undo 1"ish nut. On the l/h side of machine is a large braided hose. This hose will be blocked,due to the loop and angle its fitted. Remove it, poke,bang and wash thru to clear crud. You hopefully will have good flow again. In its day it was a very good boiler.

 

[gowe]

Thank you Heatingman. Will look into the points you raised.

 

[Slugbabydotcom]

There is no external by-pass fitted.

Its a while since I played with a Chaff as well but I feel that the lack of a by-pass could be part of the problem.
I dont know if this has a built in by-pass. If so it could be blocked.
Opening the lockshield valves on the three working rads could simulate a by-pass and prove the point at the risk of throwing the system out of balance.

The chaffs that I have played with often seem to be prone to collecting a lot of system debris so heatingman could well be on the right track.

A common denominator could turn out to be sludge so it might be time for a flush.

 

[gowe]

Re heatingsman's post of 17/2, BG has confirmed it has checked these points and they are OK.
The manual imples that there is no internal bypass, saying that an external bypass is required if TRVs are on ALL rads, so that the even if all TRVs are closed the min flow rate for the boiler is achieved. I think the system may be scaled up although I have put descaler in and flushed - a powerflush might work but not worth the money on this old boiler. An external bypass might fix it, but for now I think I will just live with the problem.
Thanks for suggestions.