fluff29

If boiler stays on full flame past the point when flame size should decrease, safety component kicks in and shuts the burner down. Ignition then goes to sleep for three minutes.

Also, in some cases if temperature rise is too rapid, burner can still shut down (having swamped the modulation process) forcing the boiler to sleep for three minutes.

DH plan to check gas consumption is good, to make it a little better read the meter sooner, say every 20 seconds. When flame on full burn, meter units every 20 seconds will be the same. When the boiler modulates, meter will show lower reading.

I would suggest leave the plumbing as is. Some installers insist 8mm is no good. I say the system worked before, should work when root of problem rectified. If the new rad then shows excessive temperature differential, then repipe. Why run up a bill on guesswork.

thanks for your vote of confidence on cleaning out. Is there something wrong with boiler if it doesn't modulate? Should I have seen it modulate by now on either HW or CH? I even took the boiler front off to watch the flame but it never changes.

\you do not need anyone to clean out your system. You can do it yourself. You seem competent enough. Hane got some of my clients to do this without any problem. Most likely you will do it better.

If burner does not downsize when things start getting tad hot, replumbing will not cure the problem of boiler going to sleep for 3 minutes

We are thinking at the moment that we should get the big radiator repiped and get the boiler flushed with the cleanser (will get someone local to do it). If this doesn't fix things then we are thinking that you would be our best bet for trying to solve the next part of the riddle.

What niggles me is that even if I was to switch the problem radiator off, and leave all other working rads on, unless the boiler is at no. 7, the other rads would only be at a low heat as the boiler would cycle more often. I would have thought with such a big boiler that if I turned it back to a lower number for CH, it would still give out a good output but it doesn't.

On my old worcester bosch 28i junior, my boiler only sat at a low setting the majority of the winter and was only turned up if really cold. And yet when the heating came on, the rads all heated up very quickly and warm and the house stayed constantly warm even thought the boiler cycled.

Thanks again for your help. It is greatly appreciated.

my email is [email protected]

1 BTU is needed to heat one pound of water by 1 degree F. If cold water at 15 degree C is raised to 50 degrees C (converted to degrees F 63), Flowrate of 11l will weigh 24.2 lbs
63x24.2x60 gives 91476 BTU.
Your boiler is rated at 95500
Therefore, main heat exchanger puts out 95500 BTU. HW heat exchanger will soak up 95500 BTU.
If you reduce the flowrate then 95500 BTU out of main heat exchanger will not be used up by HWHE. Two things could happen. 1) hot water rises by not 35 degrees but much more. If heat starts to back up as main heat exchanger is producing excess heat, something needs to happen to exercise control. The burner gets downsized (called modulation) On some boilers it was step modulation (Full flame to low flame) on others like your boiler, I think it is full modulation (like cruise control that keeps track of what temperature you are asking). So, in boiler cruise control, burner size will result in heat output at main heat exchanger that is matched to what the HWHE can ‘sink’

To heat water is difficult. To heat radiators, boiler does not even need to get out of bed as it will be able to achieve that while ‘snoozing’ at lowly 12kw instead of massive 28kw

Water that leaves your boiler on its way to the radiators, enters the radiator and starts rising to the top and starts spreading out across the top. Cold water that is displaced, leaves through the opposite valve to go to the boiler to be heated. Cycle continues.

Radiator output is directly proportional to amount of water it needs- weight of water carries the heat>> larger radiator need more water, smaller less water. Pipe length and bore have to be matched to radiator size (need for right amount of water)

Radiators are fitted between two rails (flow and return). As one would expect, the radiator closest to the pump and smaller in size may get a deluge of water thus using up pump power needlessly leaving larger radiator some distance away insufficient flow.

Way I set the radiators/ balance them is by use of dual clamp thermometer. Traditionally (cast iron boiler) were set for 11 degree differential. Low water content like yours can be set up for anything between 11 and 20 degree C. Condensing boiler- better set for 20 differential
From above, you will appreciate, each of you radiators is balanced correctly, will show a temperature difference of, let us say 15 degrees C. If the new radiator pipe was undersized, then that radiator would be starved of heat but remaining rads would all heat correctly.
If you can get someone to stick a monometer on the gas valve burner pressure should swing between high 11.7 to low 2.5mbars. If the gas valve was not set correctly, of for some other reason the burner stayed at full flame, burner would be knocked out and boiler would be forced to cool down for 3 minutes (pump keeps running to dissipate heat)

Hope above is of some help.

Fluff, I suspect the diverter to be the culprit. You could either put some magic soup (Fernox DS40) which will certainly clear the debris where circulation exists, but also reveal weaknesses in the system.

I am in Bearsden, have in the past traveled to exotic places like Edinburgh, West Calder, Kilcregan etc to carry out repairs. But this is at cost of £1.00 a mile round trip surcharge. If I were to attending, I would come prepaired to strip the boiler and check the components for buildup of crud.

Who so ever looks at the boiler might want to look at the 14 pages of discussion

I wrote this to Chris. Have met him a few times.

Hi Chris,

Referring to your lovely two drawings, the port left of apple shuttle, is the return to the heat exchanger from CH.
The pipe with item 19 fitted to it is the return from HWHE

While it is early days just now, looks like HW loop if clear but there is a distinct possibility of there being a blockage/ build up crud where the pipe fits the diverter (left of shuttle)
Went to similar fault on a Alpha CB, where the bore on the diverter was drastically reduced
Will follow the thread and diagnose remotely to conclude where the blockage lies.

Dan