Furnace Outside Air Intake

[greentree]

I built my dads house last year and last winter he disconnected his furnace's outside air intake because he felt the air was so cold outside (0- -20F+) that the furnace was consuming excess propane to heat it. Seems like something you shouldnt have to do, the HVAC man said he was ok to do that but he's not real privy to the latest in building science. So, is my old man right or is he unknowingly destroying something?

Travis

[Jon Blakemore]

Re: Furnace Outside Air Intake

I'm no expert, but this could create a dangerous back draft situation, especially if the water heater is propane as well.

If the furnace cannot get combustion air directly from outside then it will have to come indirectly from the outside. In other words, you know that a volume of air is going up the flue when the furnace is firing, so that air has to be replaced by air leaking through your wall, traveling through cracks in the building envelope, or being sucked from the flue pipes of other propane devices.

If the air is coming through leaks in the house, your sacrificing comfort and performance of your house for a potential savings in fuel (I'm not sure, but you may not save anything at all). If the air is being pulled out of the flue from the water heater, you could have a potentially deadly situation because the combustion gases from the devices are not being properly vented.

[frenchie]

Re: Furnace Outside Air Intake

Even if it doesn't create a backdraft, he also might be "starving" the furnace of combustion air: makes for very poor efficiency & shortens the life of the furnace.

[BigLou80]

Re: Furnace Outside Air Intake

greentree,
I am no expert on the physics and chemistry of combustion by any means. I would make an educated guess that cold air going directly to the unit has little or no effect on the flame temperature. they fuel NASA rockets with -400 Liquid hydrogen doesnt seem to lower the flame temp.Its the cemical process of combustion it self that creates the heat.

Air drawn directly in to the fire is going to get heated at 100% efficiency, with no mechanical or heat exchanger loss. Cold air drawn indirectly in to the house and heated by other means is going to be at far less then 100% efficiency. There are also all the reasons mentioned above about why drawing air directly from the house is a poor idea

Lou

[greentree]

Re: Furnace Outside Air Intake

I'm no expert, but this could create a dangerous back draft situation, especially if the water heater is propane as well.

If the furnace cannot get combustion air directly from outside then it will have to come indirectly from the outside. In other words, you know that a volume of air is going up the flue when the furnace is firing, so that air has to be replaced by air leaking through your wall, traveling through cracks in the building envelope, or being sucked from the flue pipes of other propane devices.

If the air is coming through leaks in the house, your sacrificing comfort and performance of your house for a potential savings in fuel (I'm not sure, but you may not save anything at all). If the air is being pulled out of the flue from the water heater, you could have a potentially deadly situation because the combustion gases from the devices are not being properly vented.

The water heater is power vented and the furnace I believe is something similar. There is no chimney in the house, gases are vented through pvc pipe out the walls, seperate 2" for water heater and furnace has 2 - 4" pvc plus this 6" outside air intake. There are alot of gas furnaces in this neck of the woods that have no outside air intake, granted these are mainly older homes.

So:
1. Older homes are leaky enough to support gas furnaces without direct outside air?

2. New homes (built tight) need outside combustion air because there is not enough available indoors otherwise?

Obviously people sustained life in my parents home, there is O2 in the house.

I want to understand the principles behind this and tell him to hook it back up if there is any serious danger to his big idea. This house is approx 5400 sq ft conditioned air.

Travis

[greentree]

Re: Furnace Outside Air Intake

greentree,
I am no expert on the physics and chemistry of combustion by any means. I would make an educated guess that cold air going directly to the unit has little or no effect on the flame temperature. they fuel NASA rockets with -400 Liquid hydrogen doesnt seem to lower the flame temp.Its the cemical process of combustion it self that creates the heat.

Air drawn directly in to the fire is going to get heated at 100% efficiency, with no mechanical or heat exchanger loss. Cold air drawn indirectly in to the house and heated by other means is going to be at far less then 100% efficiency. There are also all the reasons mentioned above about why drawing air directly from the house is a poor idea

Lou

He said that his furnace was working much less after his disconnect than before with the freezing air coming in.

[Martin Holladay]

Re: Furnace Outside Air Intake

Greentree,
Here is a link to a free JLC article that should answer your questions about combustion makeup air:
http://www.jlconline.com/cgi-local/v...177f0000010572

[greentree]

Re: Furnace Outside Air Intake

Martin,
Thanks for the link, that is exactly what I was looking for. Now the article may have made me dangerous, for what I have gathered is his furnace is sealed combustion (2 pvc vent pipes) and the duct he disconnected was for exterior makeup air (duct goes into cold air return I believe) however his furnace location easily qualifies as unconfined space.

There is no door to the basement and the floor plan is so open the furnace will have almost 8000 cubic feet of air if contained by a yet to be built seperation wall in the basement and currently has over 29000 cubic feet of air if all the available doors were shut and airtight which neither are likely to be the case. So his furnace has 29000-49000 cubic feet of air to use at any given time and at the least, once the space is walled off and confined will have 8000 cubic feet of space. So according to the article what he did shouldnt be a problem. The water heater is power vented and does not pose a backdraft problem.

Any of this incorrect?

Travis

[James Eggert]

Re: Furnace Outside Air Intake

"duct he disconnected was for exterior makeup air (duct goes into cold air return I believe).....wrong.

The direct vent/sealed combustion units require makeup "COMBUSTION" air, and the reason they use an outside vent is because many homes today are built much tighter than years ago. Your cubic foot analogy is only partially correct, the furnace requires make-up air(usually from the extra duct to the outside) and if the duct isn't hooked up to the exterior, the combustion air drawn in is replaced by......air infiltration thru windows, doors, and other leaks! So, instead of using the furnace the way it was designed, your father would rather repaint walls in rooms where the air leaks in, and streaks the walls??

The hvac guy isn't up to todays technology, but says its OK?? Did he install this furnace, or is he the current bargain emergency serviceman in case the heat goes out?

I see absolutely zero coorelation between disconnecting the combustion make-up air and the furnace running less! One has nothing to do with the other!

By the way, there are two distictly different make-up air duct scenarios...one is simply an open duct into the room which provides a pressureless input of air, ,and then there is the make-up air connected directly to the burner shroud. I'm not sure which you disconnected??

[johnny watt]

Re: Furnace Outside Air Intake

Carbon monoxide (CO) is harmful to people because it bonds to your blood with 10,000 times more affinity then O2 does. That is why it takes only hundreds of parts per million in the atmosphere to kill you. Low O2 levels in the atmosphere are seldom the problem.

People are notoriously poor at observing occurrences and tabulating statistics in their heads. I would put no confidence in someone’s perception that their furnace is running less often.

Is it ironic that dad lives in a rather large house but is concerned over a few extra dollars he might save on heat bills a few months out of the year?

Propane will burn efficiently with –20F air. Look in the chamber, if the flame is blue it is burning properly. On the other hand, it is VERY unlikely that he has created any problems. There should be plenty of air infiltration, even in a house half that size. Either way, you are still bringing in the same amount of –20F air. His way simply allows warmer air to enter the burn chamber. The installation manuals usually allow the option of not piping the makeup air based on things like you have mentioned.

There are vacuum switches that will shut the furnace down if it becomes starved for air as well as spill switches if the vent is not venting. The power water heater will have a spill switch too. Even if the water heater did not have a spill switch, it is unlikely that it would produce enough CO to be dangerous. Water heaters are usually 30,000 BTU/hour just like the un-vented gas stoves in many kitchens. Water heaters usually recover in half an hour but it is common to turn the kitchen stove on much longer then that. The flame on stove ovens comes very close to impinging on the metal, which is a sure way to produce CO.

BTW, they love to say that CO is colorless and odorless but I have never been in a house that has dangerous CO levels when there was not an accompanying obnoxious odor present. Co is produced from unclean burning of the gas and that almost always produces smelly aldehydes.

[greentree]

Re: Furnace Outside Air Intake

Is it ironic that dad lives in a rather large house but is concerned over a few extra dollars he might save on heat bills a few months out of the year?

That's my old man and he's a spitting image of his dad. I will probably start to morph into his mold someday as well. His life of full of little ironies such as this that make little sense to anyone but him, and thats how it is.

I think he's actually a good portrait of an average consumer. He sees a furnace and a furnace to him doesnt need a duct to the outside because the furnace in his house he built in 1975 didnt have a duct to the outside so why should this one..ect, ect. And hes not asking about any consequence to dissconnecting the duct, I am and I would probably have to argue with him to get him to reconnect it if I thought he should.

James Eggert, you realize people do open doors in homes. "So, instead of using the furnace the way it was designed, your father would rather repaint walls in rooms where the air leaks in, and streaks the walls??" He probably would because he honestly doesnt believe that will happen, and now that you mention it I hope he doesnt reconnect the duct and I hope it does start to streak the walls so I can learn more about air infiltration, I really want to see how this plays out because I dont think he's the first person to do this.

Somewhat off topic, could my dads setup create a negative pressure and when an exterior door is opened there's a vacuum intake of fresh outside air, more than would normally come in?

The furnace guy who said ok was the hvac installer and his technical knowledge was poor but his price was great. Thanks for the replies, to me the correct relationship of the heating and cooling systems in a home are the most complicated to understand and I appreciate the sharing.

Travis

[Martin Holladay]

Re: Furnace Outside Air Intake

Travis aka Greentree,
None of us can be sure what type of duct you are talking about until you provide more information.
"His furnace is sealed combustion (2 PVC vent pipes)." Well, I hope he didn't plug or disconnect the PVC pipe that provides combustion air to his furnace.
"The duct he disconnected was for exterior makeup air (duct goes into cold air return I believe)." If he disconnected a duct that supplies outdoor air to the return plenum on his furnace, then the duct was probably part of his mechanical ventilation system, and had nothing to do with combustion air. This type of ventilation system is called a "supply ventilation system"; it depends on the furnace blower and furnace ductwork to supply fresh ventilation air to the occupants of the house.
It's beginning to sound as if neither you nor your father is exactly sure what type of ductwork has been disabled. That's not a good sign.

[BigLou80]

Re: Furnace Outside Air Intake

Travis,
If you can argue with your dad and get him to do something, you are steps ahead of me. My father sees things from the 1970's-'80s point of view. NO amount of arguing, science or engineering can change his mind. My father in law is even worse. I think his(my dads) mentality is todays actions are tomorrows research. To his credit he has seen a lot of new things tried and failed in his 35+ years in construction so he doesn't venture far from the tried and true methods and materials .
Lou

[johnny watt]

Re: Furnace Outside Air Intake

Your furnace guy is correct, I have put in a few dozen and the instructions gave the option of getting the combustion air from inside the home based on room size house size tightness etc.

You can always pay someone to do a tightness test on the house and prove that there is enough infiltration.

You are creating a nominal vacuum on the house with a fraction of a horsepower draft inducing motor, to small to notice when you open a door. Less then 1 psi.

They say heat rises. When they mean hot air rises. But it doesn’t actually. Cold air sinks
and pushes the hot air up. Anyways, this happens in a home and creates pressure differential between outdoors, upstairs and downstairs. When you open an upstairs window on a winter night you do get more air movement then if you open a downstairs window but it is nominal. Worst case, that is what you would see when you open a door and the house is under vacuum because of your fathers situation. Nothing noticeable.

BTW My question on irony was rhetorical. Hee hee if I could save a few bucks, I would do it too, but in this case I cannot think of a scientific reason to indicate he is.

[greentree]

Re: Furnace Outside Air Intake

Travis aka Greentree,
None of us can be sure what type of duct you are talking about until you provide more information.
"His furnace is sealed combustion (2 PVC vent pipes)." Well, I hope he didn't plug or disconnect the PVC pipe that provides combustion air to his furnace.
"The duct he disconnected was for exterior makeup air (duct goes into cold air return I believe)." If he disconnected a duct that supplies outdoor air to the return plenum on his furnace, then the duct was probably part of his mechanical ventilation system, and had nothing to do with combustion air. This type of ventilation system is called a "supply ventilation system"; it depends on the furnace blower and furnace ductwork to supply fresh ventilation air to the occupants of the house.
It's beginning to sound as if neither you nor your father is exactly sure what type of ductwork has been disabled. That's not a good sign.

Martin,
You should reread post #1 and post #5, those posts make it murky clear what pipe he disconnected, a 6" duct that goes to the return plenum.

I'm a little hazy as I haven't looked at his setup since last fall and am going on memory, but I'm not an HVAC installer so my knowledge is basic. If anyone is dying to know I'll take a picture next time I'm there and post it.

I assume one of the PVC pipes supplies combustion air. Is makeup air the same or an additional supply?

Travis