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

On Fri, 28 Jun 2013 16:41:40 +0100, Bob Minchin wrote:

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?

Yup, AIUI


--
John Stumbles -- http://yaph.co.uk

"I used to think correlation implied causation.
Then I took a statistics course and now I don't."
"Sounds as if the statistics course helped."
"Well, maybe."

YAPH wrote:
On Fri, 28 Jun 2013 16:41:40 +0100, Bob Minchin wrote:

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?

Yup, AIUI


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

Bob Minchin wrote:
YAPH wrote:
On Fri, 28 Jun 2013 16:41:40 +0100, Bob Minchin wrote:

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?

Yup, AIUI


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

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

Tim+ wrote:
Bob Minchin wrote:
YAPH wrote:
On Fri, 28 Jun 2013 16:41:40 +0100, Bob Minchin wrote:

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?

Yup, AIUI


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

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

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

Bob Minchin wrote:
Tim+ wrote:
Bob Minchin wrote:
YAPH wrote:
On Fri, 28 Jun 2013 16:41:40 +0100, Bob Minchin wrote:

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?

Yup, AIUI


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

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

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

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

On Sat, 29 Jun 2013 10:17:54 +0100, Bob Minchin wrote:

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....?

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.


--
John Stumbles -- http://yaph.co.uk

I'm sure I've had this feeling of Deja Vu before

Bob Minchin wrote:
YAPH wrote:
On Fri, 28 Jun 2013 16:41:40 +0100, Bob Minchin wrote:

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?

Yup, AIUI


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
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