Boiler cycling on/off, but everything checks out

[kpmatthews]

Hi

I have a precious boiler it seems, a Baxi Solo 3 PF 30.

When calling for heat or hot water it will run fine for 15 minutes, and then it starts to cycle on and off at one minute intervals until the control stops calling.

If I unclip the thermostat from the pipe it will run fine without cycling so the thermostat is sending the right messages to the PCB and doing it’s job. The pipe is hot to touch. I thought blockage but radiators heat fast and fine with no cold spots. All radiators are less than a year old as is the hot water cylinder.

And the pump too for that matter. Have tried pump at faster speeds, and it does over run.

What else is there to check?

Thanks, Kris

 

[John D v2.0]

Is the return pipe about as hot as the flow pipe?
Maybe it's cycling because the demand at the set flow temperature is lower than the minimum burner modulation.
Is it causing an issue?

 

[kpmatthews]

I’d say the return pipe is not AS hot. Nothing in the install/test manual about burner modulation, is there another term for it?

I gave the thermostat and pipe a wipe with a scourer earlier and it hasn’t cycled since. Not counting chickens just yet.

 

[Hot&Cold]

Nothing in the install/test manual about burner modulation,

Edit,could just be cycling due to the radiator heat demand,it is winter and the heating load could need topping up more frequent.

Design of the heating system will also have an effect

 

[John D v2.0]

All modern boilers domestic modulate, some further than others, but they all have a minimum limit. If it's a relatively basic Combi the minimum will be high, hence it will cycle. That's normal.
On some there's an eco mode where the pump also stops for some time, that can be a pain especially with plinth heaters which have low water content. In your case it doesn't sound like that's happening though.

 

[Hairymelon]

Just saying it isn’t a combi and it doesn’t modulate. You need a registered gas installer to check the gas input etc etc

 

[D_Hailsham]

If I unclip the thermostat from the pipe it will run fine without cycling so the thermostat is sending the right messages to the PCB and doing it’s job.

Which "thermostat" are you talking about? There are two temperature sensors, next to each other, on the flow pipe. One is connected to the temperature control; the other is the overheat safety device.