PDA

View Full Version : Outside combustion air intake on furnace



jdunmyer
12-17-2013, 07:35 PM
Modern high-efficiency furnaces seem to all have outside combustion air intakes, even wood and corn stoves. How much efficiency does that add as opposed to using inside air? To put it another way, how much efficiency does one lose if one were to disconnect the outside intake pipe and just let it suck inside air?

I've heard of at least one furnace installer who swears that the furnace is more efficient if it's using inside air for combustion. It seems to me that the furnace designers should know better than an installer, but...???

Lu47Dan
12-17-2013, 08:01 PM
Drawing air from the outside to supply a furnace with combustion air was started to improve the draft of the furnace, with houses being built tighter and tighter. It is harder to get a furnace to draw properly, so the engineers came up with drawing cold air from the outside to allow the furnace to draw properly. Then they compounded the problems by making the furnaces more and more efficient, pulling more and more heat out of the combustion gases, making the gases too heavy to rise up the chimney without assistance. So the draft inducer fan was invented to cure the problem.
Any installer that believes that using inside air in more efficient than drawing in colder denser air, (more O2 / Cu.Ft.). I would not hire him to work with me. :confused:
We installed 8 high efficiency boilers to heat a grade school building to replace 16 older less efficient boilers. we ran pvc piping under the boiler room slab to draw in air to them from outside the building to make sure they would burn correctly.
Dan.

bob_s
12-17-2013, 08:05 PM
John:

Remember Carnot cycle thermal efficiency = (Thot - Tcold)/Tcold

The greater the temperature difference between hot and cold the greater the thermal efficiency.

ironmonger
12-17-2013, 08:38 PM
Modern high-efficiency furnaces seem to all have outside combustion air intakes, even wood and corn stoves. How much efficiency does that add as opposed to using inside air? To put it another way, how much efficiency does one lose if one were to disconnect the outside intake pipe and just let it suck inside air?

I've heard of at least one furnace installer who swears that the furnace is more efficient if it's using inside air for combustion. It seems to me that the furnace designers should know better than an installer, but...???

Every cubic foot of natural gas requires about 12 cubic feet of air. 1000 BTU per cubic foot, 100,000 BTU input furnace uses about 100 cubic feet of gas per hour times 12 = 1200 cubic feet of combustion air per hour of run time.

Mine is doing about 50% run time per hour now so with inside combustion air you are dumping 600 cubic feet or air you already paid to heat into the burners. Not to mention that the air is coming in from outside through the walls and windows... and is probably cold air :)... which is why the furnace is running now...

Use the outside air...

paul

SteveF
12-17-2013, 10:49 PM
In addition to what Dan posted, if your furnace (with no outside supply) with a fan starts running and you have something like a gas water heater, the furnace will use the water heater vent pipe as an air supply and suck the water heater exhaust back into the house. Breathing carbon monoxide is bad.

Yes, the furnace designers are FAR smarter than this particular installer.

Steve

dp
12-18-2013, 12:19 AM
If you burn inside air it all has to go up the chimney and outside and of course all that air has to be replaced with cold air from outside. It's like having a nasty draft in your house. Systems that use coaxial exhaust/intake pipes can preheat the incoming air with waste heat which also improves efficiency.

boslab
12-18-2013, 01:14 AM
The balanced flue system is much safer, there was an old famous case over here of the Ascot gas water heater, a wall hung water heater usually found in bathrooms, lots of people were found dead in their bathtubs, a few were deemed to be murdered!, some hangings no doubt! It turned out to be a deflector plate in the boiler, when corroded dropping down into the boiler flame nd shunting heat out of the flame, the boiler stopped burning exothermicaly and started endothermic, flame going from blue to yellow, always abad sign in a gas flame, although there was plenty of free air, it was still producing CO, killing the unwitting bather.
It was later that a test for death by CO was found by the new post of Home office Coroner, the first time the government had employed one, He was Bernard Spillsbury, later Sir Bernard.
He was apparently the insperation for Sherlock Holmes!
It was noted that pricking the finger of the victim and squeezing a drop of blood that the colour of the blood was scarlet orange red, realy vivid, the result of the binding of heamaglobin with CO.
It was later determined that many had died with the window open and plenty of fresh air, thats when they worked out that CO has about 15 times more affinity with heamaglobin than oxygen, you can be poisoned and die in a well ventilated room!
Since then thousands have fallen victim to this silent killer, several this year in this country, mostly children, all the result of faulty gas boilers and fires.
Drawing air from outside removes a passageway from the boiler to the room, encasing the flue gas with another tube had little to do with heating the incoming air as a failsafe to keep flue gas out.
The fan assist is connected indirectly to the gas valve py barometric pressure switch to the combustion chamber, if the fan is not running and the pressure inside the combustion chamber is not lower than the air pressure outside the appliance the gas valve cannot open and the flame will not ignite, another safety measure, strong wind into the flue can also close the gas valve.
CO is deadly, if you walk into a cloud of it the first thing that happens after a couple of lungfulls is your legs turn to jelly, you drop like a sack of shie, concous but unable to move, pain in your neck oddly is a common symptom, you will slowly die and know your doing it, fortunately you will loose conciousness shortly after exposure, not long after.
If you work in areas that CO is known or suspected of a gas moniter should be worn, they go off at 80 parts per million!
Keep the flue and intake outside, its safer
Mark

JRouche
12-18-2013, 01:26 AM
Every cubic foot of natural gas requires about 12 cubic feet of air. 1000 BTU per cubic foot, 100,000 BTU input furnace uses about 100 cubic feet of gas per hour times 12 = 1200 cubic feet of combustion air per hour of run time.

Mine is doing about 50% run time per hour now so with inside combustion air you are dumping 600 cubic feet or air you already paid to heat into the burners. Not to mention that the air is coming in from outside through the walls and windows... and is probably cold air :)... which is why the furnace is running now...

Use the outside air...

paul

We dont have a "Like" button so. :cool: Like! Good explanation. JR

vincemulhollon
12-18-2013, 08:22 AM
One fun way to solve problems is to optimize in many tiny little steps. So start with the idea of burning inside air, well, the next tiny step is opening a window upstairs to get a good draft, after all that air needs to get in somewhere either an open window or air leaks. That gets a little chilly upstairs, so you open a window in the basement instead. Well that makes for a chilly basement and my shop is in my basement so thats a non-starter... you could use a plain old HVAC duct running from the open basement window to the furnace and then both the upstairs and downstairs will be warm and comfy. But now you're right back to where you started with a pipe from the outside supplying air!

In my experience side vents will never clog if the attached machine burns every hour or so under even the worst conditions, but under the worst imaginable conditions something like an instant hot water heater can clog if it doesn't run for perhaps 12 hours (like, ice storm followed by 18 inches of blowing drifting snow, which for the Canadians is probably a nice warm July day but locally is about once a decade)

Baz
12-18-2013, 08:48 AM
In the UK regulations require new installs of stoves to have an enormouse fresh air vent about as big as the chimney permanently open not allowed to have a means of closing off when fire is not used. In the face of that permanent icy blast a direct feed is highly desirable and the problem is wanting to retrofit an older design that doesn't have the facility.
All my bedroom coal fires (almost new house built 1911) were originally fitted with 1 ft squ grills going to 4 in drainpipe to feed in under the grate. House on top of Dartmoor (another Sherlock Holmes connection) so vents blocked off probably during the first winter.

jdunmyer
12-18-2013, 09:31 AM
I learned first-hand about the amount of combustion air that a gas furnace uses:

When we built our house in 1971, it used electric baseboard heaters. This worked well and was inexpensive at the time, due to special rates for heating. About 1974, the gas company extended their main past our place, so I had a gas furnace installed, along with the appropriate ductwork. The furnace is in the garage, where the main plumbing also resides. All of a sudden, we had trouble with the pipes freezing when the weather was really cold. Of course, the problem was the furnace's air exchange, and largely went away with the installation of the new high-efficiency furnace in about 1997.

We had a problem recently when I discovered water on the floor around the furnace during a very cold snap that was coupled with high winds. As near as I could figure, the exhaust pipe froze up and was at least partially blocked. The water seemed to be dripping from the heat exchanger.

At that time, I discovered the outside air intake pipe was disconnected; we had a new roof installed this past Summer, and the workers apparently dislodged the pipe. I fixed that, the weather warmed slightly, and the puddle went away. I'm hoping that the exhaust pipe wasn't really blocked, as that's a bit scary to think about.

RandyZ
12-18-2013, 11:16 AM
What about the fact that the flame has to heat the colder air during combustion? If you are losing BTUs doing that, there is less to heat, the heat exchanger.
I think that it is a wash either way. You burn x number of cfm's of gas the heat air one way or the other.
Also the inside draw furnace exhausts some of the foul inside air. My garage has an inside draw furnace which seems to clear the air of cleaning tank odors. My house has an outside draw furnace as the inside air is not as smelly.

dian
12-18-2013, 12:54 PM
it depends on where the furnace is located. if its in the basement and there is an old wooden door in the room, that does not seel anyway, it will get enough air. if it is however a fireplace in the living room and you have modern windows in the house, then an exterior air supply helps. (but not by much, as i have found out.)

ironmonger
12-18-2013, 06:50 PM
What about the fact that the flame has to heat the colder air during combustion? If you are losing BTUs doing that, there is less to heat, the heat exchanger.
I think that it is a wash either way. You burn x number of cfm's of gas the heat air one way or the other.
Also the inside draw furnace exhausts some of the foul inside air. My garage has an inside draw furnace which seems to clear the air of cleaning tank odors. My house has an outside draw furnace as the inside air is not as smelly.

A cubic foot of dry air requires .240 BTU / degree to heat. Lets ignore the humidity... makes no net difference.

Assume 70 degree rise = 16 BTH / cubic foot

1200 cubic foot from my previous example = 19,200 BTU to heat the air in the fire box for outside combustion air.

If you draw inside air, that 1200 cubic feet air required for combustion, which comes from outside anyway, still has to be heated. That won't change... just no cold drafts to worry about if you use outside air.

If you have odors to deal with, try an air to air heat exchanger instead. Some chemicals, if burned in your heat exchanger can lead to corrosion of the heat exchanger resulting in possible CO leakage into the building.

paul

layman's three laws of thermodynamics.

1 You can't win
2 You can't break even
3 You can't change rules 1 & 2

rws
12-19-2013, 06:49 AM
In commercial buildings using boilers, the boiler room is made with outside air louvers for combustion air. The louver controls are linked to the t-stat, when a call for heat happens, the louver(s) open the boiler fires. Louver shuts when the boiler shuts down. You can't burn fuel without air, it has to come from somewhere.

I run a wood stove in the basement, and my outside basement door doesn't have any gasketing at all and doesn't close very tight. My wife asks why I don't install gaskets......

Lu47Dan
12-19-2013, 08:36 AM
A buddy built a new house after years of living in a house, that homeless people would avoid. When he was in the planning stages he told me he was going to have the shell of the house spray foam insulated. The basement walls were ICF's and the attic insulation was above R-50 with the way it was insulated.
I told him if he was going to continue to have a wood stove for heat he would have to put in a combustion air intake for it as the house was going to be so tight that the stove would not draw properly. When the basement was built, he had a sleeve installed to allow for the installation of combustion air intake piping to be installed. He also ran a piece of 4" PVC pipe under the living room slab to where the wood stove would be.
Once the house was finished, he moved in, installed the stove and built his first fire. The stove seemed like it was not burning well to him, he called me and asked me what I thought and I asked him if the fire burnt better if he opened a door or window a little. He said he would try it. He called back a little later and said yes it did. I told him he needed more air to the stove and I would be over to figure it out.
With the wood stove burning, I held a match over the 4" PVC pipe and nothing happened. We went down into the basement and looked at it. They had never finished installing the piping to get the outside air.
He had a drawing to go by so we finished up the piping. Now his stove burns well and he is happy with his new house.
His old house had so many air leaks into it that the stove would burn a full loading in under 3hrs if the temperatures dropped into the teens.
Dan.

davidwdyer
12-19-2013, 08:51 AM
How about my mother who, when fall came, turned on her high efficiency furnace as she went to bed.

Sometime during the night her CO detector started screaming.

Luckily she had one, since the fireplace had been converted to gas.

The painters, that summer, had taped over the exhaust outlet to the furnace and so everything was

coming back into the house.