Central heating rapid cycling

Have been having the lockout fault on a Potterton Suprima 50L, so had PCB replaced with 5111603. Fixed the lockout problem, but rapid cycling has become apparent since.

When starting from cold, takes about 10 mins to reach temperature, all radiators getting hot, albeit probably not balanced properly as some slightly slower than others, but they all do get very hot after 20-30mins. Thereafter, boiler will heat for about a minute and a half, and then rest for 3 or 4 mins, and then heat again. Obviously getting hot at boiler as can hear slight hissing, i.e., water close to boiling point, before burner turns off. Would suggest the boiler is right to think it has reached the maximum temperature before turning off.

Is this down to badly balanced radiators, an over-specified boiler for the job (6 radiators in a flat), or is the thermistor needing replacement? Or something else entirely?

I want to reduce the number of cycles to help extend the lifespan of the PCB, so any advice on how to reduce the number of cycles would be gratefully received.

Thanks.

I would check the pump.

This could have been responsible for the lock outs before, and the new PCB could have a closer tolerance.

Not all Suprima faults are the PCB. Just most of them.

Ok, thanks for the pointer.

Pretty sure it was the PCB as the boiler was behaving erratically before it was changed - it was ok for 2 weeks and then locked up 3 or 4 times a day. Changing the PCB has at least given me a boiler which turns on and off reliably.

However, I'll check the pump is spinning freely tonight by opening up the screw and turning with a screwdriver, but having quickly put my hand on it when running, it felt like it was spinning ok.

If the pump were at fault wouldn't the radiators take ages to heat up, if at all?

Is there any benefit changing the pump speed - it's on 2 (middle) at the moment?

Could well be you have 6kW of rads on your 15kW boiler, so 1.5 minutes in 4 sounds about right - you can't get the heat out!
Turn the boiler temp down and it'll be off longer, as the dissipation will drop from the rads.

Thanks. Will try turning the boiler temp down, and also the pump flow rate as I assume that will give the water a longer time flowing round the system and thus longer to cool.

BTW, I'd assumed that the temp control on this boiler just set the cut-off temp for the CH water heat, say 60C or 80C. Would dropping it from a cut off of 80C down to 60C lengthen the cycle time? I would have thought that once it hit 60C, it would then cycle as quickly as it does around 80C - have I misunderstood what the temp control does?

Cheers for the suggestions.

Optimus said:

Thanks. Will try turning the boiler temp down, and also the pump flow rate as I assume that will give the water a longer time flowing round the system and thus longer to cool.

BTW, I'd assumed that the temp control on this boiler just set the cut-off temp for the CH water heat, say 60C or 80C. Would dropping it from a cut off of 80C down to 60C lengthen the cycle time? I would have thought that once it hit 60C, it would then cycle as quickly as it does around 80C - have I misunderstood what the temp control does?

Cheers for the suggestions.

Click to expand...

As a physicist, not a plumber, I agree with Chris R's first comment. My system matches a 14kW boiler against 10 kW of radiators and cycles at an appropriate ratio - about 3 minutes on, 2 minutes off.

With regard to Chris R's second comment, if the water takes longer to cool (because the radiators are cooler) the boiler will still cycle a lot but might change from being 1.5 on 4 off to, say 1.5 on 6 off - but of course
your flat will be colder.

Balancing the system maximises the heat output from the radiators and would reduce the cycling a bit.

Turning the pump down would reduce the output from the radiators and might increase the cycling? The return will be cooler, but because the water flows slower through the boiler, the flow will be hotter.

I guess the ideal boiler heats up and then modulates its output to keep the flow temperature constant regardless of the return- don't suppose such a
boiler exists though???

Is there a real issue with the PCB or are you being paranoid
(That's not an insult, I've become very paranoid since being landed with
a dodgy CH installation).

Mixture of paranoia and inexperience methinks

PCB was definitely stuffed - lost track of the number of reset pushes it needed. Touch wood, none since the PCB was changed.

I'm not really sure what 'normal' is for cycling to be honest, as the boiler never worked properly since I bought the place. I just read a few other posts which suggested the cycling I was seeing was high and it concerned me. Given how much the PCB cost, I was erring on the side of caution as I don't want it wearing out too soon!

Having taken the earlier advice, I have checked the pump, temp control, balancing, and it all seems to be fine. Also, from what you guys have posted it doesn't sound like the speed of cycling is abnormal when you compare it against your systems.

From what's been said it just sounds like the boiler is a bit too heavyweight for the installation and there is no real issue - phew

Thanks for your help

He`s not paranoid about the pcb`s in them, I`ve replaced 8 of the suprima boards on a new build site up the road from me. The other 2 houses got so fed up they`ve had combis put in!

Unfortunately this sounds like a classic case of an oversized boiler.

It will aparently come as a surprise to the physicist that all modern boilers modulate by a factor of about 4:1 to match the heating load.

That does not help those with old non modulating boilers. Fiddling with pump speeds will not help much. The problem is simply that his boiler is massively overrated for the heating load.

Take a design criteria of an outside temperature of say -1° C. A correctly sized boiler would run continuously to meet the target internal temp of 21° C.

As the external temperature increases less heat is lost and the boiler operates on lower duty cycle until at the extreme with an external temperature of 20° C the boiler would come on for a few minutes every half hour!

Unfortunately the Suprima is not intended to be operated at a lower power than its rating. Reducing the burner pressure would reduce the heat output though.

Tony

Agile said:

It will aparently come as a surprise to the physicist that all modern boilers modulate by a factor of about 4:1 to match the heating load.

Click to expand...

Well physics has got me a long way understanding the deficiencies of my
system since the designers and installers of my ropey CH system refuse
to tell me what is wrong with it. However, real-world knowledge is also
essential and this forum has been very helpful.

I have a Micron 60ff which I guess does not count as "modern" even
though it's only 2 years old - fitted by a cheapskate house-builder

The lack of modulation means that the output of radiators is below spec,
since when the boiler is off the radiators receive much cooler water. As
TRVs in warm rooms switch off, the boiler is off for longer and longer
periods, and the cold rooms get colder and colder...