Gas furnace Combustion air... (HVAC)

[Stomper]

I'm a professional HVAC guy of 26yrs... the PROPER solution to this problem is you need a "direct-vent" furnace, but since this is an "induced draft" (power vented) 80%AFUE furnace, you COULD (probably) get away with cutting a 6" round duct into the side of the furnace cabinet where the burner compartment is, run the duct to outside of the house, and seal the louvers in the front of furnace cabinet with metal foil-tape so it only draws from the ducting. This would eliminate the draft you're feeling, yet supply combustion air to the burners.

Your other viable option would be to replace the existing furnace with a 90%+AFUE condensing furnace that uses separate PVC pipes to exhaust the flue gases and draw in th needed combustion air... but that's a WHOLE lot more than a $100 "fix".

 

[Jamie6.5]

oops, I took diameter not radius... uugh..

I will be doing that shortly. I was out running errands this am and now I'm in "get it done" mode at the hacienda.

Keep us apprised.

My next door neighbor is in the biz, and I'll ask him about this when he gets home this afternoon. My bet is, he recommends you extend the ceiling vent down to the level of the combustion air intake on the furnace housing, like is shown in the diagram bbbass posted.
Which also happened to be initial recommendation.

 

[Jamie6.5]

I'm a professional HVAC guy of 26yrs... the PROPER solution to this problem is you need a "direct-vent" furnace, but since this is an "induced draft" (power vented) 80%AFUE furnace, you COULD (probably) get away with cutting a 6" round duct into the side of the furnace cabinet where the burner compartment is, run the duct to outside of the house, and seal the louvers in the front of furnace cabinet with metal foil-tape so it only draws from the ducting. This would eliminate the draft you're feeling, yet supply combustion air to the burners.

Doesn't he still need a combustion air source for the gas water heater mounted in the same closet though?

 

[Member 5605]

I have seen furnaces and homes ruined by drawing moisture laden combustion air from the crawlspace. To reiterate, go completely to the exterior is best practice. And regardless if you have an opening to the crawlspace, you need to close it off!!!

I have a piece of foam that I cut last year that blocks of crawl access from the house. I have NO clue why the heck they put a vent in the access hole. Seriously... this stuff is what makes me wonder who put stuff in there.

I'm a professional HVAC guy of 26yrs... the PROPER solution to this problem is you need a "direct-vent" furnace, but since this is an "induced draft" (power vented) 80%AFUE furnace, you COULD (probably) get away with cutting a 6" round duct into the side of the furnace cabinet where the burner compartment is, run the duct to outside of the house, and seal the louvers in the front of furnace cabinet with metal foil-tape so it only draws from the ducting. This would eliminate the draft you're feeling, yet supply combustion air to the burners.

Your other viable option would be to replace the existing furnace with a 90%+AFUE condensing furnace that uses separate PVC pipes to exhaust the flue gases and draw in th needed combustion air... but that's a WHOLE lot more than a $100 "fix".

Let's say that what I'm doing is a temp solution for this year and next year. Had we not been paying out the bubblegum for attorney costs and ruining our disposable income this 80% furnace would have been replaced last year.

So yes, long term this is not going to be in here.

@Jamie6.5 and @Stomper, my gas water heater turns on maybe four times a day. When somebody showers it runs, when dishwasher used and if the washer is ran.

The H2O heater does tie into the other vent just below the ceiling and is like a 2" pipe.

 

[bbbass]

I have a piece of foam that I cut last year that blocks of crawl access from the house. I have NO clue why the heck they put a vent in the access hole. Seriously... this stuff is what makes me wonder who put stuff in there.

The H2O heater dies tie into the other vent just below the ceiling and is like a 2" pipe.

Yah well, you wouldn't believe the dumb stuff I've seen. Inspected close to 3000 existing homes and new construction as well. You expect new stuff to be perfect but some of it's not any better than the many remodels I've seen... all due respect to the guys out there doing it right!!

The 4" metal vent W/H tie in is for the exhaust flue. The 1" PVC pipe is the temp/pressure relief drain line (needs to go outside too. Many just exit to the crawl... not good if it actuates!) This W/H installation needs makeup air too. So the diagram and closing the louvers off IMO would be the best way to do it. I'll defer to Stomper tho.

 

[bbbass]

Interesting appendix: You nailed the low heat btu.

PDF Viewer

 

[308]

View attachment 330468

That's a good pic.

In that condition one could seal off all those louvers and put some sweeps under the doors to close up that gap. It doesn't take much of a gap/crack to ruin the pressure differential. Once sealed up and with the furnace in the startup mode where combustion is occurring and the power exhaust fan is running, the owner could crack the closet door just slightly and hold a feather or some light fabric, toilet paper...whatever, against the door and it should show a very slight inward to neutral air movement. This would indicate the OSA duct is sufficient to makeup all the combustion and exhaust air.

Closing off the louvers and adding sweeps could help with that cold air dumping into the house when the bathroom fans or kitchen hood exhaust startup. Those can add up considering a bathroom fart fan will usually be pulling 50+cfm from somewhere...usually from cracks and leaks we don't even know exist.

 

[bbbass]

As a home inspector, the minute I knew that that home had been remodeled/retrofitted, my antennas started quivering. God forbid the homeowner had done his own work!

 

[Member 5605]

As a home inspector, the minute I knew that that home had been remodeled/retrofitted, my antennas started quivering. God forbid the homeowner had done his own work!

Yeah, so glad this is a temp two~ish year until we replace the unit with a higher efficiency...

 

[PaulB47]

the louver doors should be blocked and an adequate outside air supply provided

Yeah. dedicate the closet to utilities, store your chairs somewhere else, and seal off the closet from the rest of the house. Or replace the furnace with a 90% efficient one (that pipes in exterior air). Or do a mini-split.

We have a wood stove. Heat pumps too, but the stove does the heavy lifting when it is cold out.

BTW the stove should have a manual, and the manual should show installation details. I don't think it is bad for a homeowner to do things like this, provided you can follow directions. Of course selling the home may add difficulty.

 

[Gaucho Gringo]

From your pictures I see the problem. Why not seal the room the furnace\water heater are in and vent the room for outside combustion air with properly sized duct. Get rid of the bi fold doors, reframe the opening for a solid and fully weatherstriped outward swinging 36'" door. That way you get a sealed furnace room with no inside air being pulled in and fresh combustion air being fed in from the outside.

 

[Jamie6.5]

Yeah, so glad this is a temp two~ish year until we replace the unit with a higher efficiency...

My neighbor looked at the picture and thinks the installers never finished that fresh air duct.
He says the problem is the warm air near the ceiling will naturally flow up through it and out via convection, like a chimney drafts. And once it starts it will just continue to suck the warmest air out of the house.
He says if you extend it down to about 16-18" off the floor the air down below will be cooler and won't promote as much draft, but if it's just made of sheet metal you should insulate it too.
He also agreed the opening at the bottom should be on the side that faces the furnace, not on the bottom. He said if there's still cold air flowing out of it, to hang a flapper door over it that will open when the furnace creates low pressure in the closet.

He suggested you make it out of this kind of stuff:
QuietR Duct Board - Residential Insulation - Owens Corning

But he also said you shouldn't completely close off the venting of that closet. All of it except about 6" of one louvered section towards the bottom is okay.
"Just in case" that fresh air source should somehow get plugged. He also suggested a hardware cloth screen over the end that's in the attic.

 

[jbett98]

 

[Member 5605]

My neighbor looked at the picture and thinks the installers never finished that fresh air duct.
He says the problem is the warm air near the ceiling will naturally flow up through it and out via convection, like a chimney drafts. And once it starts it will just continue to suck the warmest air out of the house.
He says if you extend it down to about 16-18" off the floor the air down below will be cooler and won't promote as much draft, but if it's just made of sheet metal you should insulate it too.
He also agreed the opening at the bottom should be on the side that faces the furnace, not on the bottom. He said if there's still cold air flowing out of it, to hang a flapper door over it that will open when the furnace creates low pressure in the closet.

He suggested you make it out of this kind of stuff:
QuietR Duct Board - Residential Insulation - Owens Corning

But he also said you shouldn't completely close off the venting of that closet. All of it except about 6" of one louvered section towards the bottom is okay.
"Just in case" that fresh air source should somehow get plugged. He also suggested a hardware cloth screen over the end that's in the attic.

Thanks.

I got sidetracked today she my dad stopped by and we were brainstorming everything. Tomorrow I'm picking up some 5" pipe and going to hang the fresh inlet lower so it's closer. Didn't get the chance to block off louvers. Will get some thin stuff to cover the doors that's proper when I get the ducting.

 

[Gaucho Gringo]

My stepfather had his house built in 1962 with a dedicated furnace \boiler\water heater room of about 130 sq ft.. It was completely sealed with outside fresh air for combustion. When it was sold a few years ago everything was still functioning as originally installed. Of course the house also has a 400 sq ft bomb shelter 24 feet below grade with 200 gal fresh water tank and other necessities for inhabiting it. I always thought with a vault door it would make one heck of a gun safe.

 

[Stomper]

OK, I have only perused this thread but have not read every (any) post "in-depth", but I did pick up on the question of "why is there an outlet at the ceiling and floor".

I didn't notice the water heater in the photo when I gave my "solution", but the average (standard) residential water heater is around 40,000-btu's and since it's currently "surrounded" (not isolated in a closet anymore) by the rest of the living space of the house I would wager there's enough cubic feet with enough "infiltration" that it'll be fine.



The answer to why "high & low vents":

Unless all equipment is "direct-vent", "The Code" requires a mechanical room to have a properly sized combustion-air inlet within 12" of the ceiling and one within 12" of the floor. This will develop a natural draft as the warmer air @ the ceiling escapes and the cooler air infiltrates in to fill the space vacated by the warmer air, thus achieving "air exchanges" and ensuring fresh air is provided for proper combustion at the appliance's burner(s).

There are many different requirements & approved methods of providing combustion air depending on the situation. A GENERAL "rule of thumb" is you need one square inch of free space per 1000-btu's, i.e. 10"x10" duct (100 sq. inches) for 100,000-btu's. If there are grills covering the openings, you GENERALLY have to add 20% more to allow for the restrictions of the louvers, so that makes it a 10"x12" required opening that you could transition to a 10"x10" duct, or just run a 10"x12" the entire way (even better).


REMEMBER, any combustion process that gets inadequate air provided to it "burns dirty", and produces carbon monoxide!!