Baxi solo 2 keeps cycling every 2 minutes

[Stiffkey]

Hi all, got a Baxi Solo 2 40 PF boiler that’s acting up. Whenever there’s demand on the boiler, it will fire up, run for 10 or so minutes and then shut off despite the DHW/heating not being up to temp. It will the proceed to cycle between firing up and shutting off every couple of minutes.

Tried adjusting the pump flow rate and boiler temp gauge but with no luck.

It’s (semi) recently had the PCB and air sensor replaced on an unrelated issue and the engineer pointed out that the PCB must’ve undergone a revision between when the original and the new unit. Is this just a feature of the new PCB’s to cycle more frequently rather than running for a long time? The boiler fires up fine every time it does cycle but concerned about damage to the boiler in the medium term.

 

[Johntheo5]

Generally, any modulating gas boiler will not cycle unless the heating demand is less than the boiler's minimum output, when this happens, the return temp and the flowtemp will start rising and when the flowtemp is 5C higher than the SP temp, the burner will trip, the circ pump will continue to run and the burner will refire when the anticycling time has elapsed providing the flowtemp is = or < than SP-5C.

If your heat requirements havn't changed and its cycling more often then looks like something hasn;t been set up properly after repairs and the boiler isn't now modulating to its former minimum output.
If you look up the MIs you should see its minimum output, if you then want to check for yourself that its achieving this, then, when you think its at minimum output, take a snapshot of your gas meter and EXACTLY 2 minutes later take another, Multipy the difference X 11.0X 30X 0.80 which will give a very good idea of its actual minimum output.

Yours looks as if its a non condensing boiler with a max output of 8.9kw and a minimum output of 5.9kw??

 

[stem]

Generally, any modulating gas boiler will not cycle unless the heating demand is less than the boiler's minimum output

It's a Baxi Solo 2 40PF so over 25 years old. Modulating wasn't a thing back then. The burner will either be on or off.

It will fire up, run for 10 or so minutes and then shut off despite the DHW/heating not being up to temp. It will the proceed to cycle between firing up and shutting off every couple of minutes.

That's how it will work. Despite its name, a boiler is designed not to actually boil the water. So with a non modulating boiler, its thermostat will switch the burner on and off such that the temperature of the water leaving it remains constant, at the value set on the boilers thermostat.

These type of boilers tended to be sized well in access of the heating demand, so yours as a 40PF will provide up to 40,000 btu's of heat (approx. 12 kW) If your radiators only require 6kW then the burner will cycle on and off accordingly, being on and off for equal amounts of time.

As the house warms up and the radiators are shutting down (if you have TRV's fitted) the the demand drops and the off periods will get longer and the on periods shorter.

Bottom line. The boiler thermostat is regulating the temperature of the water leaving the boiler, not the room temperature.

 

[Stiffkey]

It is indeed a non-condensing boiler. Based on the manual heat output should be 11.72 to 9.09kW.

I’ll give measuring the output a go. If it isn’t achieving the correct output, what would be solution to setting it up properly again? A case of varying the setpoint temp and pump flow rate?

 

[Stiffkey]

It's a Baxi Solo 2 40PF so over 25 years old. Modulating wasn't a thing back then. The burner will either be on or off.


That's how it will work. Despite its name, a boiler is designed not to actually boil the water. So with a non modulating boiler, its thermostat will switch the burner on and off such that the temperature of the water leaving it remains constant, at the value set on the boilers thermostat.

These type of boilers tended to be sized well in access of the heating demand, so yours as a 40PF will provide up to 40,000 btu's of heat (approx. 12 kW) If your radiators only require 6kW then the burner will cycle on and off accordingly, being on and off for equal amounts of time.

As the house warms up and the radiators are shutting down (if you have TRV's fitted) the the demand drops and the off periods will get longer and the on periods shorter.

Bottom line. The boiler thermostat is regulating the temperature of the water leaving the boiler, not the room temperature.

The concern I have is that the rate at which it is cycling has suddenly changed over the past week when the heating and water demand hasn’t been adjusted.

Previously it was cycling for longer periods of time (20 to 40 minutes etc) rather than the 2 or 3 mins that it’s doing now.

 

[Johntheo5]

It is indeed a non-condensing boiler. Based on the manual heat output should be 11.72 to 9.09kW.

I’ll give measuring the output a go. If it isn’t achieving the correct output, what would be solution to setting it up properly again? A case of varying the setpoint temp and pump flow rate?

Going from say 30 mins to 3 mins would seem gto indicate something other than that narrow band of power between 11.72&9.09kw so suggest taking those readings ASAP to see whats going on.

 

[jackthom]

When you say the heating is not getting up to temp do you mean the radiators are not getting as hot as they used to?

 

[Johntheo5]

If I based the above behaviour (losely) on my oil fired (non modulating) boiler and assuming the above has a fixed output of ~ 10kw.
then a 0.5kw demand will ~ be 1.58mins ON/30.0minsOFF, 5kw demand will be 3.0minsON/3.0mins OFF and 9.5kw demand will be 30.0minsON/1.58minsOFF so a ~ 30min cycle can mean practically no heating demand or a very big heating demand?.
So maybe the 2or3 mins ON and OFF is now correct on the above boiler??

 

[Stiffkey]

Measured the gas usage last night with a difference of 0.041m3 with an estimated power output of 10.824kW per JohnTheo5's suggestion - so the boiler is falling within its rated output.

When you say the heating is not getting up to temp do you mean the radiators are not getting as hot as they used to?

Essentially. By the time the boiler turns off after the first burn, the radiators have barely begun to heat up whereas previously you'd see the temperature start to creep up to the thermostat setpoint before the boiler would start cycling. Similar to the DHW, the cylinder tank hasn't reached temp before the boiler begins to cycle.

So maybe the 2or3 mins ON and OFF is now correct on the above boiler??

Yeah perhaps it is correct and on the old PCB the whole boiler was setup incorrectly (or dirty and not achieving the correct power output)

 

[Johntheo5]

Measured the gas usage last night with a difference of 0.041m3 with an estimated power output of 10.824kW per JohnTheo5's suggestion - so the boiler is falling within its rated output.


Essentially. By the time the boiler turns off after the first burn, the radiators have barely begun to heat up whereas previously you'd see the temperature start to creep up to the thermostat setpoint before the boiler would start cycling. Similar to the DHW, the cylinder tank hasn't reached temp before the boiler begins to cycle.


Yeah perhaps it is correct and on the old PCB the whole boiler was setup incorrectly (or dirty and not achieving the correct power output)

Its producing its rated output. If the on/off times are the same say 3 min on, 3min off, then the heating demand is only 10.82/2, 5.41kw. when you start up from cold then the boiler should IMO run almost continuously because the whole system acts as a buffer until up to temperature .
If the circulation rate is very low then the boiler dT will be very high leading to more frequent cycling. If a heat only boiler with a external pump what is its make/model/setting?.

 

[Johntheo5]

Won't go through the dreary calcs but assuming the boiler cuts out after 10 minutes from a cold start at 20C to reaching its cut out at 70C then even not accounting for the rads emitting any heat during that 10 minute heat up period and assuming a system contents of 85L then the circ rate would appear to be only ~ 5.0LPM with a boiler dT of 31C. Even if you just feel the boiler flow/return pipes you will easily feel that sort of temperature difference, flow/return ~ 70C/39C on burner trip. Normal circ flow would be ~ 12 to 15LPM