I have a baxi genesis 80 combi.

It will not fire up. No gas flow.

If I take off the outer combustion box cover, gas will flow and it will
spring into life and ignite.
As soon as I replace the outer combustion box door, it dies.

What could be wrong?

The fan runs.



Steve..

On Sun, 01 May 2005 11:15:16 -0700, dog-man wrote:

I have a baxi genesis 80 combi.

It will not fire up. No gas flow.

If I take off the outer combustion box cover, gas will flow and it will
spring into life and ignite.
As soon as I replace the outer combustion box door, it dies.

What could be wrong?

The fan runs.



Blocked flue.
Blocked tubes to Air Pressure Switch.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

In message .com,
dog-man writes
I have a baxi genesis 80 combi.

It will not fire up. No gas flow.

If I take off the outer combustion box cover, gas will flow and it will
spring into life and ignite.
As soon as I replace the outer combustion box door, it dies.

What could be wrong?

The fan runs.

So it sounds like the air pressure switch isn't operating (I presume
you're not getting a spark)

Sounds like a blockage or a leak somewhere preventing the APS from
getting sufficient pressure differential

--
geoff

Yes, there is a spark and it will fire up if the outer combustion box
cover is off.
If it is on, it will not even spark.


Steve...

Stripped it down and blew through pipes etc.

There was no obvious blockages or large amounts of debris, but after
reassembling, it fired up and worked as it should.


Thanks for the advice.


Steve...

Hmmm, after working perfectly yesterday and most of today after my
fiddling with it, the fault has returned again.

I am therefore guessing that it is the air pressure switch that is
faulty and my fiddling must have persuaded it to work correctly for a
while.

Unless someone says I should not, I shall order a new air pressure
switch in the morning.


Steve..

dog-man wrote:
Hmmm, after working perfectly yesterday and most of today after my
fiddling with it, the fault has returned again.

I am therefore guessing that it is the air pressure switch that is
faulty and my fiddling must have persuaded it to work correctly for a
while.

Unless someone says I should not, I shall order a new air pressure
switch in the morning.


Have you tried shorting it out of the circuit to see if the boiler fires
without it?

IANAP

Rob

No, how would I do that?


I have tested for contuinity across 2 of the three terminals as
suggested by the handbook troubleshooter and it passed the test.


Steve..

dog-man wrote:
No, how would I do that?


The air-pressure switch was faulty on my boiler but it took a long time to
diagnose as the problem was intermittent. Anyway, the plumber connected a
wire across two of the terminals which bypassed the switch entirely
and the boiler worked. This narrowed it down to a flue or air-pressure
switch problem. Couldn't see anything wrong with the flue so the
air-pressure switch was changed and the boiler's been fine ever since.

Obviously this should only be done to diagnose the problem and not as a
permanent fix!

Cheers

Rob

But which terminals to bridge?

There are 3 terminals.

P1, P2 and one other.



Steve...

dog-man wrote:
But which terminals to bridge?

There are 3 terminals.

P1, P2 and one other.

Steve...

Sorry, can't help. There was a circuit diagram on the inside of our boiler
casing but I'm guessing there isn't on yours.

Cheers

Rob

I do recall a circuit diagram on the inside of the casing. I will see
if it gives any clues.

I seem to have the same intermittent problem you had, as I have now
replaced the outer combustion cover and it is working fine.

The replacement switch is only about =A326 so I would not watse too much
if I was wrong.


Steve..

In message , Rob Summers
writes
dog-man wrote:
Hmmm, after working perfectly yesterday and most of today after my
fiddling with it, the fault has returned again.

I am therefore guessing that it is the air pressure switch that is
faulty and my fiddling must have persuaded it to work correctly for a
while.

Unless someone says I should not, I shall order a new air pressure
switch in the morning.


Have you tried shorting it out of the circuit to see if the boiler fires
without it?

You can't just short out an APS for most boilers, you have to do it just
after the fan is powered . I can't remember the boiler in question, but
it's important for the pcb to see it switching into the normally open
position (If the contacts were welded together for example, the pcb
could think that the fan was running when it wasn't)

--
geoff

In message om,
dog-man writes
But which terminals to bridge?

There are 3 terminals.

P1, P2 and one other.

Initially, common to normally closed (NC) and then after the fan starts,
common to normally open (NO) - look on the APS

The proper way to do it is to check the resistance of the common to NC,
it should be a short circuit (less than an ohm). You need to hold the
DMM probes together and take a reading so you can factor this out. You
then have to get a bit of tubing and suck or blow (depending on the
boiler) GENTLY to operate the APS and check the resistance from the
common to the NO contacts. You should kink the tube so that it holds the
pressure and wait for a number of seconds in order to test the integrity
of the diaphragm

You should also test the integrity of the intermediate wiring

--
geoff

I feel that it has to be the aps that is faulty. (intermittently)

With the casing off it will operate 100%, but put replace the casing
tightly and it will die.
I can get away with slackening the screws and it will operate
correctly.

The flue is unblocked and there is not a build up of soot / debris.

Hmmm, whilst typing this my wife called me to say that there was no hot
water.

Upon checking , usual symptom, boiler would not fire up but you could
hear fan running.
Removed cover which normally causes it to fire up. Nothing!
Removed front pipe on aps and blew, boiler fired up.

I have now left that pipe unconnected to the aps to keep the boiler
working. I will get a replacement aps tomorrow.



Steve..

The problem is in the flue. It is not getting enough air into the boiler.
It is VERY dangerous to operate the boiler with the casing slack.
It should NEVER be left to operate like this.
You are putting your and your family's lives at risk!

"dog-man" wrote in message
ps.com...
I feel that it has to be the aps that is faulty. (intermittently)

With the casing off it will operate 100%, but put replace the casing
tightly and it will die.
I can get away with slackening the screws and it will operate
correctly.

The flue is unblocked and there is not a build up of soot / debris.

Hmmm, whilst typing this my wife called me to say that there was no hot
water.

Upon checking , usual symptom, boiler would not fire up but you could
hear fan running.
Removed cover which normally causes it to fire up. Nothing!
Removed front pipe on aps and blew, boiler fired up.

I have now left that pipe unconnected to the aps to keep the boiler
working. I will get a replacement aps tomorrow.



Steve..

There is no blockage of the flue.

The casing is now firmly in place.

I have left one pipe disconnected from the aps temporarily, which has
allowed the boiler to function normally.

As soon as the replacement aps arrives (Tuesday hopefully), I will
exchange it and replace the pipe.

If the problem is still there then I will have no option but to employ
someone to sort it out.


Steve...