Boiler stops and starts before getting up to temp

[RodneyJ54]

We have a British Gas 330 condensing boiler, which is a customized version of a Glowworm hxi as far a I can see. I think it's about 10 years old.
We've recently had the radiators replaced by convecting models, which obviously required the system to be drained. Since then the boiler takes ages to bring the rads up to temp as it initially turns on and off, taking the boiler temp quickly up to something around 50degs, then dropping immediately back down to below 20, waiting for a few minutes then doing it again. This can go on for in excess of half an hour, very gradually building the temp up until it starts working continuously and staying on until a decent level is reached. The system has been bled several times, and there are no obvious cold spots on the rads.
Occasionally it gives up in the fluctuating stage with an error code of F26, which the manual tells me is 'max delta temp'. Stopping and starting the boiler begins the whole process again and the error code goes away for a few days.
Any advice would be very welcome.

 

[D_Hailsham]

We've recently had the radiators replaced by convecting models

Did anyone check that there was sufficient spare capacity in the boiler to take care of the additional heat given off by convector rads?

Were the new radiators balanced?

Do you have TRVs on the radiators?

 

[RodneyJ54]

There are TRV's on all rads except one, and they have been balanced. There was no discussion of the capacity of the boiler to handle the different type of rads. Once it's past the fluctuating stage it operates OK, would it do that if there was a capacity issue ?

 

[JohnD]

does one of the radiators get hot before all the others?

Who did the balancing, and how long did they spend on it?

 

[mickyg]

boiler probably needs rating down, poor circulation, pump issue yada yada.

 

[RodneyJ54]

Thanks guys for your interest in my problem.
The rads appear to come on together, I can't be sure how long the fitters took on balancing them as I was not around. If it makes a difference, we now have some of the rads turned right down (not completely off) as the rooms are unoccupied. How would unbalanced rads lead to the boiler fluctuating as it does ? In the first half hour or so there is very little heat in any of the rads as the boiler spends little time at a decent operating temperature.

 

[JohnD]

I was thinking there might be short-circuiting through one rad (or a bypass) leading to the boiler quickly getting too hot as it is only circulating a small amount of water; yet leaving everything else cold.

Did sediment come out of the old rads when they were removed?

 

[RodneyJ54]

I'm not sure about the sediment, nothing was said about any unusual amount. If it makes any difference, it seems to me that the system starts working properly more quickly when the hot water is on as well as the heating.

 

[JohnD]

look for the three-speed switch on the pump. What number is it on? It will be a small round plastic thing that you turn.

I too think it is a problem with circulation.

How often do you have to bleed the radiators? When it was refilled, what chemical did they add?

 

[Agile]

In order to get any further with this you would need to measure the flow and return temps at the boler during th period when the short cycling is occuring.

Tony

 

[RodneyJ54]

Thanks everyone for your inputs. The pump is on maximum (setting 3), and this morning I've opened up all of the TSV valves on all of the rads to 5 (max) and the system has got going after only a couple of start/stop phases. Does this tell us anything new ? I will get hold of a digital thermometer tomorrow and check the temps at the rads looking for the 12deg difference, and on the flow and returns to the boiler, as recommended.

 

[spareshunter]

poor circulation F&R, pump issue partially blocked H/exchanger, thermistors.