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Miller Gas CMF2 combustion venting issue

Posted: Fri Jan 23, 2015 5:04 am
by Pafrmboy
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.


Re: Miller Gas CMF2 combustion venting issue

Posted: Fri Jan 23, 2015 5:55 pm
by Greg
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.


Re: Miller Gas CMF2 combustion venting issue

Posted: Sat Jan 24, 2015 3:20 am
by Pafrmboy
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

Re: Miller Gas CMF2 combustion venting issue

Posted: Sun Jan 25, 2015 12:20 am
by 1987Commodore
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.

Re: Miller Gas CMF2 combustion venting issue

Posted: Sun Jan 25, 2015 2:15 am
by Pafrmboy
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?

Re: Miller Gas CMF2 combustion venting issue

Posted: Sun Jan 25, 2015 2:35 pm
by jpingram5
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.

Re: Miller Gas CMF2 combustion venting issue

Posted: Mon Jan 26, 2015 7:07 pm
by Pafrmboy
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!


Re: Miller Gas CMF2 combustion venting issue

Posted: Wed Jan 28, 2015 5:21 pm
by jpingram5
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.

Re: Miller Gas CMF2 combustion venting issue

Posted: Fri Jan 22, 2016 3:23 am
by Pafrmboy
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!

Re: Miller Gas CMF2 combustion venting issue

Posted: Thu Mar 08, 2018 12:54 pm
by stevieb
This is what I would have done. Actually we are about to convert to a Miller propane furnace also from kerosene.. I would put screening over the end of the pvc to prevent mice from climbing in.When our kero furnace was replaced 10 yrs ago the installers did the same thing as you had happen.
I will probably have to do the job myself as you did. I think our firebox is cracked as we have soot on allot of surfaces.
May I ask a few questions? Did they replace the roof jack or the cap? Also I hate to ask but what did they charge you for the entire job? We will need another tank as we have propane now for 4 other appliances but it would not be big enough. We also have a new ROTH fuel tank for kero that was $2400.00 I will try to sell that. Thanks!

Re: Miller Gas CMF2 combustion venting issue

Posted: Thu Mar 08, 2018 5:46 pm
by Pafrmboy
Yes, the roof jack was replaced with the new furnace. However, this was done prior to my purchase of this home. I think the previous owner paid $1800 for all. Now as for the combustion venting....I did that. It's the 2nd winter this year and it has been perfect. You cannot have such a large opening, flooding the home with cold air. It was like having a window open!! Crazy. Now the furnace pulls all combustion air through the 2" PVC pipe I put down through the floor. No cold draft!