The value of outside combustion air for gas furnaces 

Thank you so much for this great explanation. Seventeen years ago, when my house was being built, the installer asked me if I wanted inside or outside air for the furnace intake. Not knowing, I incorrectly reasoned that inside air would be "more efficient" since it is already warm. Over the years, I suspected that I made the wrong decision, and for the reasons you state, I clearly did. Today, I had the A/C replaced and also paid (yes, the same company) to install a concentric vent in order to change the venting to draw air from the outside. In the past, my wife has complained that our house is "drafty" and "dry" in the winter. We now look forward to seeing the effects of this change!

There are requirements for room volume to supply combustion air. The amount depends on the size of the appliance. These are only used if the appliance takes combustion air from the room. Essentially, the requirement is depending on building envelope leakage to supply combustion air. Thanks for the comment. GFM

I think the point about outdoor air preventing the premature rusting/corroding of the heat exchangers and keeping the combustion box components clean is such an awesome point. Great video!

Great topic and great video. Hopefully everyone knows this is why a 90 plus exhaust vent can be 1' minimum below a window with outside combustion air & the minimum distance below a window jumps to 4' if you use inside combustion air.

Good tips and u raised a good point about how tight people make there house. I service in the northeast. Ive had calls where the house is tight and after heater runs awhile pilot goes out a heater burns a lot of air. If u have a tight house and your pilots goes out it could be u need a vent allowing out side air to come in. If its in a foom make sure u have 2 vents to that room upper and lower or a door with slats

You are right about flame Spenser's and outside air. I work on commercial equipment and never have failures on package units that burn outside air. The flame Spenser never wears out.

I would use a digital vacuum gauge first to find out what the vacuum is. If you know the vacuum it should have, you can compare to decide if the switch is bad. A heat exchanger problem can also cause this. Pressure switches are the messenger telling you to look farther. Make thorough check of the heat ex. If it happens to be a Goodman GMP model, the heat ex separates and is hard to tell if its failed. I did a series on these. Look up Goodman GMP in the search box. Hope this helps. GFM

Sounds like a good idea. I will work on it. Thanks for the thoughts. GFM

I realized, after having a new high efficiency (heat pump over propane) furnace installed, that my radon levels have gone back up. I am suspecting, that because the HVAC company did not install an outside air pipe to this furnace, it is pulling air from wherever it can (including the subfloor). I will install the pipe myself soon and see if this fixes the increasing radon levels. Thanks for a great video!

Very well done!! I learned so much when I had our 95%+ furnace installed and you are right on the money!

GFM, I enjoyed and learned from this video. Thank you very much for posting it!

That furnace is what we used to call the "80%" furnace. This was before the federal ratings came out. The rating for this furnace was called steady state efficiency, meaning 20% of the heat went out the vent. It did not include losses from startup and shut down, draft hood losses, off cycle losses, etc. When the ratings came out, it would be considered more like 60%.

Also doesn't code stipulate that the room with the make-up air has to have so many cubic feet . This is why I use make-up air drawn from the outside. Great video. Thanks for sharing.

I have an 80% gas furnace and you answered my question. Thank you

excellent video,God bless,from Milwaukee Wisconsin

I like the electrical switch installation. very profesh.

You can draw outside air on a 80%. Tie a fresh air into the return on the furnace and put a register on the supply directly above the evap coil. Every time that furnace runs you get outside air for combustion. Most guys up here in Northern Minnesota still do it that way new construction with modulating equipment. We can get 30" + of snow in 24hrs up here, a 2" pipe fills up with snow quick, and even if a 6" combustion air does the house is still drafty enough to keep that furnace running and keep a tech from having to leave his family in a snow storm at 11pm.

Installation quality is the number 1 problem in HVAC, in my opinion.

My thoughts would be to duct an air intake to near the burners with an electrically operated valve that opens with a call for heat. Hope this helps. GFM

Thank You.

Good show sir

it looks like i have the same furnace with a pvc outside air supply, but i also have a "skuttle" outside air supply (make-up air) with a counterweighted damper going into the cold air plenum before the large filter. Wife is complaining of dust in the house, can I shut off the "skuttle" or put another filter on it?

So what would be the ideal situation to get outside air to an 80%? My outside intake was insulated flex to right before the air filter. Recently a service tech told me to have a cold trap to the room but not directly ducted to normalize the temp( in Minnesota).

I have an 80% furnace with an 8 inch Field controls barometric damper make up air supply which works perfect. All passive, no wirings whatsoever When the furnace turns on it very easily pulls in make up air. The damper swings open a couple of inches. When I put the dryer on it opens up almost 3 inches.

Hello GFM, I was servicing a old 80% furnace. The pressure switch doesn't close. Switch appears to be working ok. And I checked chimney is also clean. I also removed the flue pipe and check the exhaust flow. The inducer blows good. Why would be the switch would close. I also checked hose was clean. Do I have a dirty or cracked heat exchanger.

No code for BX wire ?

The laundry room dryer will also compete for outside air with the heat.

Thanks for the knowledge. I'm better informed but realize I know so little and crave knowledge in my quest to reduce costs of the bills I must pay the rest of my natural life. Money buys less and we are more efficient than ever, but my wife refuses to live underground ("In a cave") and sell or lease our conventional home with it's greater overall costs. I found your video seeking to use the tankless water we have as a heat source in winter making it dual purpose. I think it could possibly help but have not found the idea in use. All of my ideas have good intentions but this one has to have been tried by someone. Folks use wood and oil to heat the water that transfers it into the home. Sorry for the book if you read this but I have to broach the subject to someone as it could be just another failed idea and a waste of money not worthy of even your time it is the most valuable thing we have. We the People...

open a window (2) for cross ventilation?

Quick question: My parents have a furnace with PVC intake pipe that draws air from the crawlspace right below the furnace (or maybe the pipe under the house draws it from outside). So, right on top of the furnace where there is a pipe joint to feed the intake air into the furnace, the top of that joint is open so that the furnace is at least partially drawing inside air for combustion. The furnace is probably taking the majority if combustion air from indoors, because the pipe is just open on the top. I took a piece of plastic wrap and sealed the top of that pipe with a rubber band so that the furnace draws 100% air from the crawlspace or outdoors. Is this the ideal way to fix the problem, cheap?

Outside Combustion air is by far better than indoor air agreed however in the snow belt part of the world combustion air can get plugged with snow and ice shutting off the furnace and most customers are too lazy to go out and check for obstruction !!! If so much snow plugged it up then chances are roads are pretty bad to drive on too ! Just my two cents ! Great video GFM

hi, anything negative on converting a 90 % lp furnace from an open atmospheric burner with door louvers open to seal it up and use opening in top sheet metal to breath sealed off air from outside. taping/seal door access and big enough pvc to outside of course

I used the feel the same way about the outdoor air intake being a better install however after years and years of service calls do to a blockage in the exhaust or intake piping on the outside I changed my mind. If an animal climbs in boom service call, burst of wind boom service call, ice/snow build up boom service call. I no longer pipe the intake outdoors unless the furnace is located in a laundry room or there is not enough combustion space where the unit is located. This will eliminate nusince calls by 40% and save the customer a lot of money on unnessisary service calls. Pipe the exhaust outside pitch to furnace keep intake inside whenever possible that's a better install in my opinion. Or just stick with 80% furnaces they are considerably more reliable for a home owner. For a tech or dyi guy go with the condensing furnace for energy savings.

Don't you love auto correct? Ditto on the flame sensor. GFM

I was just looking at a house to buy, and had seen this little one foot pipe sticking up with the elbow in the basement and I had never seen that before. Thank you for clearing that up! Looks like if I buy the house I’ll need to get it fixed. Do you recommend a split intake that allows for combustion air and return air mix? I live in the Midwest and wonder if the mixed air would cause the furnace to work harder?

Inside air dosent freeze so in the north Canada where we get minus forty intakes freeze up resulting in customers making emergency no heat call ..installer goes there chips yhe ice off the Intake takes all of 2 mins bam 75 dollar charge to customer ...so idk what do you think

I have one PVC pipe vent going out my roof. This is a 30 year old furnace and Im replacing it next week. Can I run a fresh air pipe out the side of my house while using the existing vent going to my roof? Thank you.

Inlet to the combustion chamber is what sucks the air IN. The vent pipe going to the big round fan would be the exhaust being pushed out................... would I be right?

They only installed one pipe on wife system put in 2018 and I notice Doors Close when furnace kick on if Door Is Cracked. Cause home has all new windows and Sprayed insulation Attic an Blown wall insulation.

Outside winter air is drier so would there be much moisture in the secondary heat exchanger to condense to extract the extra heat, and if there's no moisture in the secondary, wouldn't that also bring the efficiency rating way down?

Yeah, but installers can really screw things up. I once rented a space for my small business in a one story, brick commercial building with a center cupola in the roof which vented the attic for all of the business in the building. Small vents right below the parapet let air into the attic to flow out of the cupola. The whole attic was wide open (no walls in the attic between offices) with only roll insulation on top of the suspended ceiling tiles to insulate each office. The old forced air furnace in my office, which was in the bathroom and drew combustion air from inside, conked out. The landlord hired an HVAC company to install a new furnace which was the newer type which used outside air for combustion. Instead of putting a sealed pipe from the attic into the combustion chamber they installed a 4" galvanized pipe and left it suspended two inches above the outside air opening on the chamber. As you can imagine, cold air roared down the pipe and we were playing freeze-out in the office. We might as well have had the outside door propped a quarter of the way open. This caused the furnace to run more to try to heat the office. Since I was paying the utility bills, I removed the galvanized pipe and installed a new ceiling tile as the old one now had a 4" hole. When the owner of the HVAC company stopped by a few days later to check his employees' work he asked why "they did not finish the instillation?" He was told what had happened. They never did come back and put a proper size PVC pipe on the furnace. The office heated just fine.

This is why I'm installing an intake on my 90 plus unit this weekend. You'd think it's common sense, right?

If your install is in an attic that has gable vents are you ok with not running the intake pipe?

Does it effect CO output of combustion?

We just had a new furnace installed and now water is dripping from our bathroom fan. The intake is not vented outside..so the very cold is coming back down the the fan vent and causing condensation. Should we vent the intake outside..?.

I have always thought that inside air was a cheap way to go and why would you buy an efficiant furnace to make the rest of your home les efficiant through infiltration loss.. outside air is such a better option.

If they're installing the exhaust it seems so cheap to skip out on the supply. I specified that I wanted inside and outside fresh air piping installed with our new furnace. Wouldn't have it any other way. Also, isn't COLDER air also better for combustion? You know, the reason behind why guys install "cold air intakes" on their engines? Cold air = better performance over hot air? Could the same argument be made for using colder outside air (during the winter months) versus warmer inside air? Of course the warmer inside air isn't 150 degrees like it would be in an engine compartment.....

Is there any danger of having the furnace use inside air? There’s a video on RU-vid that claims it’s a major issue because it pulls air away from the nearby gas burning hot water heater, causing improper burn toxic gases to circulate inside the house. Or, is it simply a question of using inside air not being as efficient as using outside air?

Draft furnace with no inducer would be around 60% efficient

It should say sensor. Stupid auto correct.

www.rezspec.com/en/na/products/product-air-handlers-caua Here is one of the few 80% furnaces that I know of that takes combustion air from outside.

To say the chemicals in laundry rooms is causing the rust is 100% speculation and I refuse to believe this is in anyway necessary to the equipment

these furnaces pull toxic fungus and mold pollutions allergens into home my water tube tested positive for several toxic things which means it blows through vents also