Boiler cycling after balancing.

[Jamm89]

H all, I spent a good day balancing my CH heating system with a digital thermometer. Couldn't quite get the magic 12DegC diff on all the rads but they all get nice and hot now. Some at the far end of the system have the lock shields fully open.
The new problem I have encountered is once the rads are all upto temperature the return flow to the boiler increases and the boiler starts cycling. It fires up temperature ramps up to as high as 90DegC the shuts off. It's off for a minute or so then the cycle begins again. I've got a baxi due tec 2ga 40kw. Approx 9yrs old. I've got 8 rads and 2 towel rails on the system.
Any advice on how I can stop the cycling would be greatly received.
Thanks

 

[ShaunCorbs]

How many kw are the rads total eg load 33kw sounds massively oversized hence cycling

 

[Last Plumber]

I agree with Shaun. Sounds way too big. Your minimum output from the boiler is 5.7Kw so any requirement/demand below that will cause this issue.

 

[John.g]

Problems seems compounded now since rad balancing which has probably reduced the flowrate resulting in the boiler now not getting away with rads up to temperature, one minute off time hasn't a hope IMO of allowing the boiler temperature to fall enough to prevent it reaching SP+5C and burner trip/continuous cycling when initially firing at ignition conditions, perhaps increasing the anticycle time to 3/5 mins would help?.

 

[Jamm89]

I think the total load for the house is around 9kw. So yes the boiler is oversize. Im guessing since balancing the system I've strangled the flow a bit and the boiler can't reject enough heat to the system at the low end of its modulation. Would it be worth opening up all the lock shields and seeing what happens? The boiler is 8 years old and don't really remember having any problems until i had some new rads fitted 3ish years ago. Big vertical ones. If that makes any difference. I mains flushed the system last year as i kept getting debris blocking the plate heat exchanger and diverter valve. System is pretty clean now. How do i increase the ani cycle time? Had a look in the manual and i can't see it.

 

[John.g]

Don't know if the anticycle time can be increased, baxi indicate that it's 3 minutes, someone on here may know if it can be increased, I have seen other posts which seem to say the same as you in that it fires up after a minute or so. You could reopen the lockshield valves and see what happens,

What is surprising is that after it hits 90C and cuts out that it can then fire up a minute or so later which means that the boiler temp has fallen a few deg below the CH SP temp wich is probably 60/70C?.

 

[John.g]

Suggest monitoring and timing exactly ("stopwatch") the boiler temperature from the time the burner cuts out until it re fires again.