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Subject Author Date
Furnace Inconsistant Light-Off Marc Anderson 12-31-2006
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Posted by Marc Anderson on December 31, 2006, 3:07 pm
Coleman residential (LP) doesn't alway light-off.
Novice at furnaces (but not ignorant) DYI'er (aircraft mechanic)

I changed the igniter first.
Local parts man believed (99%) this would be the fix based on the
appearance (arcing) and probable age (probably original).
Relatively cheap first try too :^)
Still doesn't always light off.
Cycles the igniter 3 times, then goes 'to sleep' just as it's supposed
to (I'm told).
Power off - then back on - try again.

Is the ignition/control unit next ?

I do have 24v to the ignition unit, but I don't get 24v output to the
gas valve. At least not every time. One time, when it finally opened,
it was toward the end of the igniter cycle (it was cooling down).
Is the gas valve on a time delay, or should the valve be powered
immediately ? How does the control unit know when it's time to open
the gas valve, since there's no temp sensor at the igniter.

This wouldn't be so frustrating or hard to diagnose if it wasn't an
intermittent problem. (we hate those on aircraft too !!!)
Usually, circuit-card controllers either work or they don't.
They don't usually "kinda" work.

Thankfully it's not cold around here right now (Cincinnati area)
Hate to have this sucker completely give up the ghost when it gets colder.

Watta y'all think...

Posted by Noon-Air on December 31, 2006, 3:29 pm

> Coleman residential (LP) doesn't alway light-off.
> Novice at furnaces (but not ignorant) DYI'er (aircraft mechanic)
>
> I changed the igniter first.
> Local parts man believed (99%) this would be the fix based on the
> appearance (arcing) and probable age (probably original).
> Relatively cheap first try too :^)
> Still doesn't always light off.
> Cycles the igniter 3 times, then goes 'to sleep' just as it's supposed to
> (I'm told).
> Power off - then back on - try again.
>
> Is the ignition/control unit next ?
>
> I do have 24v to the ignition unit, but I don't get 24v output to the gas
> valve. At least not every time. One time, when it finally opened, it was
> toward the end of the igniter cycle (it was cooling down).
> Is the gas valve on a time delay, or should the valve be powered
> immediately ? How does the control unit know when it's time to open the
> gas valve, since there's no temp sensor at the igniter.
>
> This wouldn't be so frustrating or hard to diagnose if it wasn't an
> intermittent problem. (we hate those on aircraft too !!!)
> Usually, circuit-card controllers either work or they don't.
> They don't usually "kinda" work.
>
> Thankfully it's not cold around here right now (Cincinnati area)
> Hate to have this sucker completely give up the ghost when it gets colder.
>
> Watta y'all think...

I think you need to call a pro.
LP isn't anything to mess with. and FWIW the primary cause is probably
*not* the ignitor or its controls. You can change parts all day long and
spend a lot of unnecessary time and money and still not correct the problem.
Your only looking at symptoms, not the cause.



Posted by Marc Anderson on December 31, 2006, 6:39 pm
The following message is written with respect to your profession !

As for the "LP isn't anything to mess with"...
Don't take this wrong, but that falls under the normal 'understood'
precautions. People might call me crazy, but I'm not stupid.
Is there something specific you're trying to warn me of ?

Let's re-phrase my "novice" status - I'm not going to pretend to be a
HVAC expert, but I'm a lot more advanced than your regular DYI'er.
(I'm an aircraft maintenance instructor - on commercial aircraft)
So, could you please respond on a higher level - please :^)
I know you all hate it when a non-expert looks at the schematic and
thinks he's got it all figured out.
Well, I'll confess that's partly the case, but how complicated can this
sucker be (I CAN grasp the concepts and understand system functions).
And systems troubleshooting (including electrical)is part of my job, and
I've got the test equipment.
Yes, at some point I do have enough smarts to know when I'm in over my head.

So, back to the problem ...
If it's not a control issue, then what's preventing the controller from
applying the power to open the gas valve ?
The vent blower switch and limit switch (which provide power to the
controller) are good - there's 24v to the controller. I don't see any
other inputs to the controller that it would use as logic for powering
the gas valve.
High voltage to the controller is good, and igniter works,
so, that leaves the low voltage side of the controller - to the gas
valve. So, what am I missing? I checked the voltages to/at the
connector on the controller - it's easy to probe from the back of the unit.

I'd just like to understand... :^)
but I also don't want to be lumped in the category from the
"every year, there's got to be a few of these idiots" thread :^{

Remember, I meant this all in a respectful way !!! :^)
Thanks Marc

Posted by Noon-Air on December 31, 2006, 6:44 pm

> The following message is written with respect to your profession !
>
> As for the "LP isn't anything to mess with"...
> Don't take this wrong, but that falls under the normal 'understood'
> precautions. People might call me crazy, but I'm not stupid.
> Is there something specific you're trying to warn me of ?
>
> Let's re-phrase my "novice" status - I'm not going to pretend to be a HVAC
> expert, but I'm a lot more advanced than your regular DYI'er.
> (I'm an aircraft maintenance instructor - on commercial aircraft)
> So, could you please respond on a higher level - please :^)
> I know you all hate it when a non-expert looks at the schematic and thinks
> he's got it all figured out.
> Well, I'll confess that's partly the case, but how complicated can this
> sucker be (I CAN grasp the concepts and understand system functions).
> And systems troubleshooting (including electrical)is part of my job, and
> I've got the test equipment.
> Yes, at some point I do have enough smarts to know when I'm in over my
> head.
>
> So, back to the problem ...
> If it's not a control issue, then what's preventing the controller from
> applying the power to open the gas valve ?
> The vent blower switch and limit switch (which provide power to the
> controller) are good - there's 24v to the controller. I don't see any
> other inputs to the controller that it would use as logic for powering the
> gas valve.
> High voltage to the controller is good, and igniter works,
> so, that leaves the low voltage side of the controller - to the gas valve.
> So, what am I missing? I checked the voltages to/at the connector on the
> controller - it's easy to probe from the back of the unit.
>
> I'd just like to understand... :^)
> but I also don't want to be lumped in the category from the
> "every year, there's got to be a few of these idiots" thread :^{
>
> Remember, I meant this all in a respectful way !!! :^)
> Thanks Marc

Like Mike said.... "once all the safties are proven"......
and *if* all of the safties are proven, then yes, look at the control
module....



Posted by on December 31, 2006, 6:58 pm

>
>> The following message is written with respect to your profession !
>>
>> As for the "LP isn't anything to mess with"...
>> Don't take this wrong, but that falls under the normal 'understood'
>> precautions. People might call me crazy, but I'm not stupid.
>> Is there something specific you're trying to warn me of ?
>>
>> Let's re-phrase my "novice" status - I'm not going to pretend to be a HVAC
>> expert, but I'm a lot more advanced than your regular DYI'er.
>> (I'm an aircraft maintenance instructor - on commercial aircraft)
>> So, could you please respond on a higher level - please :^)
>> I know you all hate it when a non-expert looks at the schematic and thinks
>> he's got it all figured out.
>> Well, I'll confess that's partly the case, but how complicated can this
>> sucker be (I CAN grasp the concepts and understand system functions).
>> And systems troubleshooting (including electrical)is part of my job, and
>> I've got the test equipment.
>> Yes, at some point I do have enough smarts to know when I'm in over my
>> head.
>>
>> So, back to the problem ...
>> If it's not a control issue, then what's preventing the controller from
>> applying the power to open the gas valve ?
>> The vent blower switch and limit switch (which provide power to the
>> controller) are good - there's 24v to the controller. I don't see any
>> other inputs to the controller that it would use as logic for powering the
>> gas valve.
>> High voltage to the controller is good, and igniter works,
>> so, that leaves the low voltage side of the controller - to the gas valve.
>> So, what am I missing? I checked the voltages to/at the connector on the
>> controller - it's easy to probe from the back of the unit.
>>
>> I'd just like to understand... :^)
>> but I also don't want to be lumped in the category from the
>> "every year, there's got to be a few of these idiots" thread :^{
>>
>> Remember, I meant this all in a respectful way !!! :^)
>> Thanks Marc
>
>Like Mike said.... "once all the safties are proven"......
>and *if* all of the safties are proven, then yes, look at the control
>module....
>

hell who knows it could also be a slow opening gas valve or
intermittant gas valve. might be a bad or intermittant grounding
issue. maybe itsa honeywell smartvalve issue.

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