passive air intake for oil burner vs direct vent

crazy907ak

For years in my area (fairbanks, AK) i've seen people in many different homes closing or blocking their boilers air intake holes. holes in the wall seemed counterintuitive do keeping homes tight and keeping the home warm. Ive personally installed many make up air intake systems using a duct in the exterior wall with a elbow going up to the ceiling, the thought being to passively let in combustion air while discouraging cold air from spilling into the home. Seemed efficient enough. Lately i have seen oil burners having their own make-up air intake. Are the passive air intake setup the way i described being discouraged now? If so what are the reasons? Can anybody point me in the direction of some literature that might give me some incentive to install a make-up air intake directly to my boilers burner?



Thanks

A newbie :)

icesailor

Combustion Air:

Back in the Pliestocene Era when I went to school in the LA City School System, we had a fire demonstration, twice per year in shop from the 7th grade to the 12th grade. They explained to us that it takes three things to support a fire. Heat, fuel and air. In any order. Take one away and you have no fire. Colder air is more dense than hot air. That's why hot air rises. So, if you set up any kind of fuel burning device, when it is warm outside (40 degrees) for maximum efficiency, and it becomes cold outside (0 degrees), the air fuel ration will be off and there will be more air when the fuel is mixed. We have learned that if you set up something with the best air fuel ratio, CO (Carbon Monoxide) should be the least. But if the A/F ratio goes above or below, it should make CO. Therefore, many don't like outside air introduction directly through or into a burner, especially if the incoming air is extremely cold.

However, if you block off the "make-up" or combustion air, you should make CO. I remember (hopefully) that it takes 19# of air to burn a gallon of fuel oil. That air has to come from somewhere. If you have a very tight building envelope, you need outside air for combustion. They do make heat exchanging inside air/outside air devices.

If you block off the free access of air for combustion, there are unintended consequences. "No good deed goes unpunished."

Be careful.

icesailor

Combustion Air:

Back in the Pleistocene Era when I went to school in the LA City School System, we had a fire demonstration, twice per year in shop from the 7th grade to the 12th grade. They explained to us that it takes three things to support a fire. Heat, fuel and air. In any order. Take one away and you have no fire. Colder air is more dense than hot air. That's why hot air rises. So, if you set up any kind of fuel burning device, when it is warm outside (40 degrees) for maximum efficiency, and it becomes cold outside (0 degrees), the air fuel ration will be off and there will be more air when the fuel is mixed. We have learned that if you set up something with the best air fuel ratio, CO (Carbon Monoxide) should be the least. But if the A/F ratio goes above or below, it should make CO. Therefore, many don't like outside air introduction directly through or into a burner, especially if the incoming air is extremely cold.

However, if you block off the "make-up" or combustion air, you should make CO. I remember (hopefully) that it takes 19# of air to burn a gallon of fuel oil. That air has to come from somewhere. If you have a very tight building envelope, you need outside air for combustion. They do make heat exchanging inside air/outside air devices.

If you block off the free access of air for combustion, there are unintended consequences. "No good deed goes unpunished."

Be careful.

Jim Davis_3

Combustion air

It should be known that cold air is the worst combustion air for combustion.  Many industrial plants pre-heat their combustion air and can run at O2 less than 2%.

Cold air is denser but the molecules are so close together there is very little surface area to mix with the fuel and it actually takes more air when cold.  I don't like the cold air chilling the oil in the drawer assembly either.  Cold oil doesn't burn very well. 

Trapping passive combustion air can minimize excess infiltration when not needed.

D107

same problem for natural gas?

Does the cold air effect natural gas as much as oil?

Jim Davis_3

Combustion air - gas vs oil

I would say cold combustion air affects all fuels similarly, but it would not be as noticeable on natural gas because it is already a gas vapor.  Cold combustion air does require you to add more excess air than normal and lowers your efficiency 2%-3%.

D107

cold air

Thanks, so a heat exchanging intake/exhaust (concentric I assume) as Ice Sailor suggested is the only way to do this right? or an intake pipe that does a 90 el and goes to the floor, then up a few feet then down again (creating a trap for the cold air,  allowing the room temp to modify the incoming air without making the room itself so cold. (as a simple hole in the wall would do.)?? As I've read many times on this site, concentric vents have their own issues.

Jim Davis_3

combustion air

Concentrics can be restrictive and cause recycling of flue gasses.  I teach that inducers on furnaces are designed for maximum piping lengths - in and out.  If we don't add some restriction to the outlet and the inlet, the flue gasses fly through the heat exchanger to fast and we lose efficiency.  The only way the flue can be restrictive is based on the length of pipe needed to vent outside.  However the intake can be restricted regardless of the length.  Even with a one pipe installation the intake needs to be restricted.

If two pipes are used, I recommend an open tee on the intake right inside the wall to help temper the air, even with a concentric.  If the pipes are less than 20 feet then I recommend a ball valve on the intake to compensate for the short pipes.  This has proven to be a solution for many problems in the field on condensing equipment.

What most don't realize is how much combustion air a 90% furnace actually needs.  It needs a maximum of 15cfh per 1000 btus input or 1/4cfm per 1000 btus.  So a 100,000 btu furnace needs only 25cfm of combustion air.  I doubt this would depressurize a house, but like everything else we are learning today in home performance, there is a balance point between infiltration and exfiltration.

crazy907ak

Passive air

Systems like the one i'm using right now might still be the best choice in our cold region then. I get temps as low as -50f, and just the other day i had to pull off an air intake on a toyotomi om-180 which had a built in concentric air intake/exhaust so that it would stay running.