Miller Gas CMF2 combustion venting issue

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Pafrmboy
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Joined: Fri Feb 21, 2014 6:20 am

Fri Jan 23, 2015 5:04 am

Hi
I have a 2 year old Miller gas furnace. It was a replacement for an old Miller oil burner. The furnace runs fine and the ductwork for the heat has all been inspected, sealed and insulated. The problem is with the venting for the combustion air. Currently there is a combustion air duct that extends down under the home next to the heat runs.( I believe it is the same duct that was there for the oil furnace). It appears to be open and be connected to a few metal knockouts in the pan of the actual furnace. From there the 2" hose that connects to the inducer/combustion blower, simply sucks air up from those knockouts, as the hose just lays on the furnace compartment floor.

Well the cold air just fills the furnace compartment when the furnace is off and then spills into the house! Is this the correct install for the combustion air? Is the combustion air hose just supposed to lay next to the knockouts in the furnace floor pan?

I am thinking about closing off the "combustion air duct", steel taping the "knockouts" and actually connecting the 2" hose to a PVC pipe setup to the outside edge of the home. I would drill a 2 " hole in the floor to put down the hose and then seal around the hose. This would eliminate the free flow of cold air into the furnace compartment and home.

Your thoughts?

Thanks so much.

Todd


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Greg
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Fri Jan 23, 2015 5:55 pm

Todd, Codes for mobile homes state that gas appliances must use outside air for combustion. This applies to gas fired water heater & furnaces. For some reason they are worried about oxygen depletion (they obviously have never lived in one). I know many people have chosen to close off the outside air, and have not had a problem. I can not endorse the practice, codes are there for a reason although some don't seem to make much sense.

Greg
"If I can't fix it, I can screw it up so bad no one else can either."

Pafrmboy
Posts: 28
Joined: Fri Feb 21, 2014 6:20 am

Sat Jan 24, 2015 3:20 am

I do understand about the outside air need for combustion. What I'm trying to do is stop the cold air infiltration at the base of the furnace, by adapting an alternative method, (tighter sealed) way to get air in.

Is the air intake hose just supposed to lay in the furnace cabinet and draw air from the knockouts? If so, it seems like a poor way.

What I am purposing is to seal up the furnace floor duct and ADD a separate PVC pipe down thru the floor to the outside. The induction hose would be secured on the piping and draw air through the short PVC pipe directly into the combustion chamber. No cold air would then seep into the house.

Does this sound like a valid option? The pipe would be less than 3 feet in length.

Surely others have had this cold air issue.

Thanks, Todd

1987Commodore
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Location: Steuben County, NY

Sun Jan 25, 2015 12:20 am

I believe you are on the right track. It sounds like the installers did a poor job. The intake hose definitely should go outside, and the existing holes should have been sealed.

Pafrmboy
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Sun Jan 25, 2015 2:15 am

Well, I plan to use some 2" PVC piping to facilitate the venting. I'm curious though, how is the typical fresh combustion air actually set up in most of these gas furnaces?


jpingram5
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Sun Jan 25, 2015 2:35 pm

Typically anything 90%+ will have it's on vent pipe to draw outside air in. With the mobile home unit though what you are seeing does not only serve a purpose as a fresh air vent for the furnace but its idea too is to provide fresh air into the entire home. Basically they want x amount of fresh air exchanged because a lot of these mobile homes are sealed pretty darn tight.
2009 Skyline Sunwood Premier 14 x 80

Pafrmboy
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Mon Jan 26, 2015 7:07 pm

Yes...this is an 80 percent, but I feel that there is enough air exchange elsewhere. I'm going to provide this furnace it's own private vent I guess. The cold from under the furnace is just way tooooo much! It chills the house in short order. It's obviously from the knockouts. I will keep you posted!

Thanks!

jpingram5
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Wed Jan 28, 2015 5:21 pm

Yeah, I know they are not as tight as they think but its just one of those they have to have it deals. You would probably be fine if you just eliminate it all together.
2009 Skyline Sunwood Premier 14 x 80

Pafrmboy
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Joined: Fri Feb 21, 2014 6:20 am

Fri Jan 22, 2016 3:23 am

Well, I closed off the vent access duct from under the home. I drilled a hole in the floor outside/next to the furnace cabinet and inserted a piece of PVC pipe through to exit the belly. I then attached the combustion air hose directly to this pipe and it works PERFECTLY! The cold draft from the open vent under the furnace certainly needed to go! I did have to drill a hole in the side of the furnace cabinet to exit the combustion hose and get it to the new hole/pvc pipe in the floor. All sealed up with foam and caulk and I must say that the difference is profound!

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