• Last Updated:
  • Feb 21st, 2021 8:12 am
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[OP]
Newbie
Feb 18, 2021
7 posts

Furnace backfire

Hi,
I recently had a new propane furnace installed for my house. It is now making a loud bang when starting. I would characterize this as a backfire.
I have videos but I don't know how to include them in a post. My question is ... Is this normal for this type of furnace ? Does this pose any risk of damage or safety ?
Our previous furnace was oil and it definitely did not make this type of noise.

Thanks,
Rick
12 replies
Deal Addict
User avatar
Sep 27, 2006
3542 posts
638 upvotes
Not so easy there Ma…
There should be a small viewing portal, It does it when the propane ignites? Ignition should take place several seconds after the thermostat initiates a call for heat. Or does it do it when the furnace fan turns on?
Sr. Member
Jun 4, 2020
561 posts
471 upvotes
Clarington, ON
Is this maybe ductwork expanding? You stated that it happens after the new furnace. More powerful fan/more aggressive ramp up?
Deal Addict
Jan 5, 2003
4268 posts
3698 upvotes
Toronto
Turn it on with fan only and see if you get the same noise. It'll at least help isolate whether it happens with the blower only or if ignition has something to do with it.

Could be the filter is loose and will bang against the furnace when it's turned on because of the air pressure.
Deal Addict
Jan 5, 2003
4268 posts
3698 upvotes
Toronto
I just remembered that one time I helped a friend diagnose a noisy furnace and it was just that metal cover was loose and made a loud bang (since it's metal banging against metal) whenever the blower started up.
[OP]
Newbie
Feb 18, 2021
7 posts
Hi fergy,

The sequence appears to be ... Thermostat requests heats. Exhaust fan runs and the furnace goes through a sequence of checks. This lasts for 30 seconds or so. The glow plug turns on ... I can see a red glow from the viewing portal. Give or take 3-5 seconds BANG and the furnace ignites. Shortly after the blower kicks in. Again. I have a video of the sequence but I don't know how to post it.

Rick
Deal Addict
User avatar
Sep 27, 2006
3542 posts
638 upvotes
Not so easy there Ma…
RickD1809 wrote: Hi fergy,

The sequence appears to be ... Thermostat requests heats. Exhaust fan runs and the furnace goes through a sequence of checks. This lasts for 30 seconds or so. The glow plug turns on ... I can see a red glow from the viewing portal. Give or take 3-5 seconds BANG and the furnace ignites. Shortly after the blower kicks in. Again. I have a video of the sequence but I don't know how to post it.

Rick
Check the exhaust pipe for obstructions, if none I think you should get the installer on the phone ASAP.
The viewing portals are limited, maybe there's an issue with one of the burners you can't see.
Member
Oct 19, 2020
322 posts
189 upvotes
The unit comes equipped for natural gas and the installer has to modify it with a propane kit.
Call the installer back asap -> fuel pressure is off, orifices are wrong or burners are misaligned.

Propane is not something to mess with, it's heavy and therefore more dangerous than gas.
Dirtier fuel too.
Member
Feb 9, 2011
256 posts
4 upvotes
Mississauga
We need to see those videos. I get that a little bit too when it's really cold out. I think it's maybe because the cold air being sucked in then once it ignites, it has a rapid expansion.
[OP]
Newbie
Feb 18, 2021
7 posts
All,

I have managed to post two videos showing the start-up of my furnace. One is a
single backfire and the second is a double.

To see the glow plug turn on in the video it must be watched in theatre
mode as it is very subtle in the youtube video.

The links to the videos are ....





also,

Thanks for the feedback.
Deal Fanatic
Jul 4, 2004
8679 posts
1624 upvotes
As the others have said, contact the guy who did the gas conversion to get verification whether this is normal operation or not. It sounds like to me the ignitor is a little slow to start to ignite the gas in the chamber hence the sounds heard in the video due to the amount of gas waiting to be ignited. Although the ignitor timing is controlled by the motherboard and may not be easily adjustable (if even possible), the gas pressure can be adjusted through the gas regulator.
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Member
Feb 9, 2011
256 posts
4 upvotes
Mississauga
RickD1809 wrote: All,

I have managed to post two videos showing the start-up of my furnace. One is a
single backfire and the second is a double.

To see the glow plug turn on in the video it must be watched in theatre
mode as it is very subtle in the youtube video.

The links to the videos are ....





also,

Thanks for the feedback.
That double one was a bit odd. The single one seems like how most furnaces light off. Maybe the manifold pressure is a bit too high on the gas valve?

Maybe even send those videos to Goodman custoemr service and maybe let a technical rep of there's see it and comment.
Deal Addict
Feb 10, 2006
2294 posts
309 upvotes
Kitchener
Op stop posting on the forum. That is not ductwork expansion. This was the best advice given.
insertname2020 wrote: The unit comes equipped for natural gas and the installer has to modify it with a propane kit.
Call the installer back asap -> fuel pressure is off, orifices are wrong or burners are misaligned.

Propane is not something to mess with, it's heavy and therefore more dangerous than gas.
Dirtier fuel too.

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