Bosch 24Ri Boiler gas valve

[Bob Minchin]

I'm in the process of installing one of these boilers and notice there are two coils/solenoids on the gas valve. They look similar but one is

177 ohms and the other 373 ohms. The boiler output is supposed to be able to be modulated between about 25% and 100% of full output. I'd have expected some sort of stepper motor to modulate the gas flow but presumably one of these coils does the job? Anyone know how it works please? The fan motor is full of electronics and so presumably is speed controlled to adjust airflow. Does the gas valve work a bit like a carburettor ie more airflow draws in more fuel?

There is no monitoring of combustion products just a series of binary sensors for excess flue pressure, excess heat exchanger temperature and excess flue temperature all normally closed contacts connected in series. A single electrode into the combustion chamber presumably looks for ionisation of the gases indicating the flame is present and a NTC resistive probe for flow water temperature.

Enquiring minds just need to know how it all works.

TIA

Bob

[YAPH]

Yup, AIUI

[Bob Minchin]

Ok Thanks John. In that case, do you know what the function of the second coil on the gas valve is please. One coil can turn it on and off but the other one....?

Bob

[Tim+]

Some boilers still use a pilot flame to light the main burner. Unlike pilot flames of old this one is only lit as part of the main burner ignition sequence. Gas valves for these boilers have two solenoids in sequence, the first one supplying the pilot flame and the second one opening once ignition is detected.

Tim

[Bob Minchin]

Hi Tim, I guess that would give a 'soft start' and have the added advantage of not filling the combustion chamber too full of gas if it does not ignite first time.

I'm still at the installation stage and my gas safe man will do all the gas connections once I've done all the wet bits. Once he has been and gone, I can put some tell-tale leds on the solenoids and see what is going on. My aim is to make sure I understand what it is supposed to do whilst it is new and working so I might have a chance of diagnosing any faults in the future.

Bob

[Tim+]

We could have done with that when our boiler started igniting explosively on Christmas Eve! Turned out the second solenoid/valve was sticking open partially allowing the boiler to fill with gas before the pilot lit. On reflection though, it was a mechanical fault rather than electrical so perhaps LEDs wouldn't have helped. Knowing that it had a two-stage ignition sequence would have speeded up diagnosis though.

Tim

[YAPH]

I haven't looked closely at the gas valves on Greenstars but on their older standard-efficiency (non-condensing) appliances Worcester Bosch always used to have 2 solenoid valves in series, and my understanding was that the PCB alternated between using one or other of them first to shut off the gas, then by detecting if they still sensed flame after the valve had been told to shut off they could tell if a valve wasn't closing properly whilst still having the other valve available to shut off the gas as a backup.

But in that arrangement I'd expect the two coils to be the same resistances.

[Bob Minchin]

My boiler installation was commissioned to day by a very friendly Gas Safe man who was very happy to discuss the inner workings of the Bosch 24Ri. I put my theory to him about the modulation being achieved in the manner of a carburettor. He said yes this was indeed how they used to work but now an output from the motor - which is stuffed full of electronics, passed a varying current through the second, higher resistance, coil in the gas valve which is cunningly matched to the fan speed to maintain the gas/air mix. The first coil has binary action switching on about 20% of the max gas volume for the purposes of ignition and thereafter the second coil has an analogue action and tops up the gas flow to between 25% and 100% of maximum according to demand.

On a related point that I had made in another thread at a similar time, more powerful models do indeed have return water temperature sensors in an attempt to maximise the use of condensing mode but not on these basic models.

Commissioning took about an hour and a half and he charged £80 plus vat which I thought was pretty reasonable.

Bob