Baxi Solo 2 PF Constantly Ignites - Help Please

[stardanny]

I have a baxi solo 2 50, it's still work. When it was installed to replace potterton neatheat 10/16, I've noticed that it's cycling off and on too short during cold winter. Upgrading 2 radiators to double seem to cut the cycling.

Dan.

 

[acmedone]

The other design issue with Solo is that the water temperature sensor is in the output side. It should be in the input side so that when the burners switch off, the sensor doesn't see a large temperature drop. There is still a separate cut out to protect the thing, after all.

I am afraid that you dont know much about boiler design!

The single sensor has to be on the output so that the boiler is controlled to a set OUTPUT temperature! That set temperature is what the user can use to control the heat output from the rads.

I presume you are an installer/repairer with minimal design/scientific background?

My betting is that keeping the setting sensor on the output side is just a tradition from the days when one sensor doubled as protection and sensing. Moving the sensor would not change the ability to set the water temperature. The dial is marked 1 to 6 (or whatever) so there's no calibation against temperature anyway.

In spite of what you think, the Baxi Solo 2 was also made in 12 kW and 9 kW models.

Fair enough. It was some years ago. I know of a 20kW version installed in a smaller house.

Its a pretty basic design and like all boilers rather uses older design concepts on its control circuitry. But that made them more reliable.

Cough...

Micro processors only became common on boiler PCBs since about the year 2000. Even now most PCBs still use mechanical relays instead of triacs or solid state relays. Probably because they are still cheaper but possibly because they are thought less likely to stick on when controlling the gas valve.

Micros replace the logic or decision making parts of the circuit; they do not replace the function of relays or SSRs, which switch the larger currents needed by the various actuators.

 

[Agile]

My betting is that keeping the setting sensor on the output side is just a tradition from the days when one sensor doubled as protection and sensing. Moving the sensor would not change the ability to set the water temperature.

Never bet on anything that you are likely to lose.

If the boiler does not know the flow temperature then it has no ability to turn off the gas when the set temperature is reached!

Even worse if you turn off the rads then the boiler will continue to fire at maximum power and potentially explode or catch fire.

Tony

 

[acmedone]

Even worse if you turn off the rads then the boiler will continue to fire at maximum power and potentially explode or catch fire.

That is why it has a safety sensor. If I turn the pump off it boils and trips within a second or two; there's no question of it quietly simmering away under the control sensor.

 

[Agile]

An overheat stat is intended to be a final last resort protector and not something which would operate every day.

I am sorry to disappoint you but boilers do require temperature sensing on the flow!

Tony

 

[acmedone]

An overheat stat is intended to be a final last resort protector and not something which would operate every day.

If every radiator had a thermostatic valve and all were satisfied before the room stat, the effect would be no different to turning the pump off (unless the 3 way valve allows sufficient to leak through the HW circuit).

I am sorry to disappoint you but boilers do require temperature sensing on the flow!

Tony

I have to accept that people at the sharp end of boiler design know what they're doing. It would be interesting to get their reasoning.

 

[acmedone]

Over sizing boilers results in excessive cycling and less efficiency.

I have just found this
http://www.idhee.org.uk/calculator.html

This is talking about cycling that occurs when there is no demand but the request to the boiler has not been cancelled. It is not the reason why the Solo cycles (at least, not mine).

This might be a reason why a sensor is thought to be better on the boiler output side. But an installation that has no demand but does not remove the request is, frankly, an incompetent one and should trigger the safety stat to get the installer back to fix it. (Once upon a time cylinder stats weren't common, for instance, so the boiler stat ultimately determined the HW temperature. No longer, I hope.)

The Solo will still rapid cycle to modulate the output when demand is present and water is flowing. A charactistically short cycle time (down to a few seconds, often) is the essence of the complaint. It is the inevitable result of the flow temperature changing by several degrees in slightly delayed sync with the burners.