Gas furnace Combustion air... (HVAC)

[Member 5605]

I have a gas furnace in our house. It's located in our garage that's been converted to living quarters by previous owners.

I have one hell of a draft because the furnace combustion air is drawing from in home air. There is a vent above furnace (in a closet) that's supposed to be the 'fresh air' inlet for the furnace so it's not pulling air from in the house. Problem is this fresh air inlet is cold as ice and cools the whole house. When combustion fan is on it will blow a candle out because of the amount of air it's pulling through. With fan off it's still sucking air but not nearly as much.

I'm wanting to isolate the combustion air that's drawn in by making a 1/2 (3 sided) box to slip over the front of the furnace where the burners are and make a pipe system to go straight up to the air inlet vent. I plan to use the ducting tape to seal corners and tape it to front of furnace so if access is needed later on I can undo 3 strips of tape and slip the cover off to repair etc. I cannot seal off the area where the furnace is as it's a closet with accordion style doors thats also used to store tables and chairs.

Are there ANY downsides to making a slipover box for front of furnace and piping it straight up to the fresh air vent? I'm removing the vacuum from inside the house that's caused by the combustion fan. Essentially I want to force the furnace to draw air from outside the living space, thus reducing my heating of outside air and losing house air up the exhaust vent.

I know that wood stoves use fresh air inlets now days and new furnaces also have a fresh air inlet... looks like they skipped this when the house was remodeled. The house is a 1968 version and $50 in materials could have solved this problem a while ago.

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[Joe13]

They make a ducting glue that's almost like plumbers putty. Seals well but comes off easy enough. Something to think about.


As to your question, I'm no pro but if your redoing the ducting to make the furnace work as it should then I can't see any down sides personally. Just don't block anything important on the furnace and your golden.

This is from an amature but I have done a few so take it for what it's worth.

Good Luck!

 

[Member 5605]

They make a ducting glue that's almost like plumbers putty. Seals well but comes off easy enough. Something to think about.


As to your question, I'm no pro but if your redoing the ducting to make the furnace work as it should then I can't see any down sides personally. Just don't block anything important on the furnace and your golden.

This is from an amature but I have done a few so take it for what it's worth.

Good Luck!

Yeah, it's stupid that there wasn't a system put in place on this furnace to draw from outside.

My whole house is under such negative air pressure it's forcing air down my range vent, up from crawl access and lord have mercy if you crack a window while the furnace is on... you are liable to have a tornado inside.

I'm thinking tape vs putty so I don't have to worry about it slipping off if the putty dries out. I may hinge it but don't want to screw anything to the furnace.


Essentially I would be blocking the entire front access cover to the furnace because that's where the burners draw air from but I would be boxing that in and forcing that to be drawn from outside.

Of course all easily undone for access if needed to be worked on.

 

[bbbass]

Furnace draw is engineered/designed/calculated. Sounds like you have the original furnace. You may be messing with danger of low combustion air that detunes the mix, increasing monoxide, and also causes backdraft, allowing combustion products to escape back into the house at the dilution shelf. I would highly recommend you consult with a professional.

 

[308]

This is a problem with residential type stuff. Same goes for those move-and-cool AC units we get suckered into buying...they recirc air from the space for the cooling side and suck all the same air out of the house on the exhaust/condenser side where the "supplied" exhaust duct and blankoff panel attach to the window. The friggen things move about 600+cfm and that has to come from somewhere, and that somewhere is through every crack and every bathroom/kitchen/gas fireplace exhaust vent and pulls all that nice unconditioned air right back in where you don't want it.

Bottom line is this...your/our/my house needs to be neutral to slightly positive to atmosphere or we are simply wasting energy on a big way.

If it is possible with your space, then run a piece of duct (from Home Depot) from the makeup air opening directly to the furnace where the combustion and combustion exhaust is occurring. Remember, air will always take the path of least resistance, so try not to use the aluminum flex duct...it has a very high duct friction loss, meaning that the air resistance is high. Use a smooth duct and fittings if possible.

 

[Member 5605]

Furnace draw is engineered/designed/calculated. Sounds like you have the original furnace. You may be messing with danger of low combustion air that detunes the mix, increasing monoxide, and also causes backdraft, allowing combustion products to escape back into the house at the dilution shelf. I would highly recommend you consult with a professional.

View attachment 330446

House was burned 3 years ago, furnace was replaced at that time. Mfg date is 2/07/2013 per sticker inside. The idea as well is to add a larger inlet piping than exhaust. CO is heavier than air, I run CO detectors on the floor and with my room here being lower than the rest of the house by 8" it's the natural settling point for CO and with that, there is a vent at the crawl access that's open so CO would flow down and under the house first before filling up and triggering the alarm in the room. Room is also complete opposite end of the house from sleeping quarters.

If I can make a box, check draw and see how much of a difference it makes on the draft inside the house I'll make sure it's done properly.. I highly doubt any professional will stamp it with their approval because it's a modification.

 

[Member 5605]

This is a problem with residential type stuff. Same goes for those move-and-cool AC units we get suckered into buying...they recirc air from the space for the cooling side and suck all the same air out of the house on the exhaust/condenser side where the "supplied" exhaust duct and blankoff panel attach to the window. The friggen things move about 600+cfm and that has to come from somewhere, and that somewhere is through every crack and every bathroom/kitchen/gas fireplace exhaust vent and pulls all that nice unconditioned air right back in where you don't want it.

Bottom line is this...your/our/my house needs to be neutral to slightly positive to atmosphere or we are simply wasting energy on a big way.

If it is possible with your space, then run a piece of duct (from Home Depot) from the makeup air opening directly to the furnace where the combustion and combustion exhaust is occurring. Remember, air will always take the path of least resistance, so try not to use the aluminum flex duct...it has a very high duct friction loss, meaning that the air resistance is high. Use a smooth duct and fittings if possible.

Correct. Smooth walled pipe, 45* angles if not less can get me to where I need to go. I could almost run it straight down because I'm not opposed to making a new hole in the ceiling to run feed air line straight down and one 45* into box.

 

[308]

Correct. Smooth walled pipe, 45* angles if not less can get me to where I need to go. I could almost run it straight down because I'm not opposed to making a new hole in the ceiling to run feed air line straight down and one 45* into box.

Good. Use as large of duct as possible and make it easy for the exhaust to pull from there rather than elsewhere. Home Depot has decent enough duct fittings and transitional pieces. Should be fun to sort it out. You can also wrap it with the aluminized bubble wrap for a nice finished look.

 

[jbett98]

Could you enclose the whole furnace with the required spacing (front, back & sides) with framed walls and a solid sealed door?
The new regs require combustion air vents within 12" from the ceiling and the floor.
I did this once with 2-1/4" steel studs and 5/8 fire rated sheet rock to keep the footprint as small as possible

 

[Member 5605]

Could you enclose the whole furnace with the required spacing (front, back & sides) with framed walls and a solid sealed door?
The new regs require combustion air vents within 12" from the ceiling and the floor.
I did this once with 2-1/4" steel studs and 5/8 fire rated sheet rock to keep the footprint as small as possible

I considered that but it would be really.. REALLY tight in that space. I don't think I could enclose because I have about 2" of space from furnace to door when they are closed. It's a really piss poor design in general how they did it. The previous owner went as cheap as possible after the 'accidental' fire so they could pocket as much as they could then offload the house onto somebody else. Love finding inefficient things down the road like this.

I look forward to when we can replace the system with an electric one like the mini-split I had in the last place.

 

[bbbass]

I highly doubt any professional will stamp it with their approval because it's a modification.

Poor install design. Lots of mid-efficiency furnaces have the PVC inlet open to the house. The furnace should have been replaced with one that has an enclosed system. However, if you check with the manufacturers manuals, you may be able to figure an allowable modification. Many HVAC companies do modify at the time of installation. They may still be a good source for an improved installation design for you.

 

[tarster]

is there a possibility that you could run a pipe just into the attic and down instead of going all the way through the roof? Or possiblity that you said the crawl access is close by, can you run the pipe down into the crawl space?

 

[Member 5605]

is there a possibility that you could run a pipe just into the attic and down instead of going all the way through the roof? Or possiblity that you said the crawl access is close by, can you run the pipe down into the crawl space?

Current hole in ceiling is to attic space. It's about 4" wide by 10" long and let's a huge downdraft in. Going to under the house would be a pain since it's on concrete and the access hole is small enough already without a 5" pipe competing for space. Much easier to go up than down.

 

[jbett98]

How about an electrical powered mechanical damper on the make up air duct that opens when the furnace turns on and closes when not needed.
That would solve the always open duct draft issue.

 

[Member 5605]

How about an electrical powered mechanical damper on the make up air duct that opens when the furnace turns on and closes when not needed.
That would solve the always open duct draft issue.

Thought about that, there is a company that I saw online that makes them. The downside is its still allowing cold air from outside into the house.

 

[jbett98]

In a commercial setting, the make up air duct would have an electrical strip heater built into it, plus the mechanical damper.

 

[bbbass]

This whole conversation worries me. Again, furnace systems are designed/engineered. ANY installation that does not comply with manufacturers instruction manual is DANGEROUS. IMO non-professional should not be messing with it. IMO opinions listed here by non-professionals are dangerous. IMO no professional would give you an opinion without inspecting your installation first. This is a life safety issue that would be gigged by any qualified home inspector upon resale of the home.

 

[Member 5605]

This whole conversation worries me. Again, furnace systems are designed/engineered. ANY installation that does not comply with manufacturers instruction manual is DANGEROUS. IMO non-professional should not be messing with it. IMO opinions listed here by non-professionals are dangerous. IMO no professional would give you an opinion without inspecting your installation first. This is a life safety issue that would be gigged by any qualified home inspector upon resale of the home.

Noted, appreciated and respected.

Now here is a thought...

Since I have louvered and two bifold doors that access said closet what's to prevent me from blocking off backside of louvers? Insulating back side of doors to prevent fresh combustion air from entering house through doors?

Same principle that I was attempting by boxing in furnace just on larger scale.

 

[Joe13]

Mine is a closed system....

The cold air intake should be a cold air "return" from the house, not taken from outside the house.

If done right that will give you a neutral air pressure.

Maybe I just read y'all posts wrong though.

 

[Member 5605]

Mine is a closed system....

The cold air intake should be a cold air "return" from the house, not taken from outside the house.

If done right that will give you a neutral air pressure.

Maybe I just read y'all posts wrong though.

Combustion air... gas furnace burns this air to heat element. This should be pulled from outside of house.

You didn't read it wrong.