Outside Combustion Air

The combustion air blower is rated at 45 cfm that must be taken from the living space. To help with air infiltration would it be acceptable to duct in the outside air directly to the blower?

Welcome to the forum partner! There are mixed opinions about dedicated outside combustion air. It really depends on how tight your house is. Using a sealed dedicated outside combustion air source means putting air into your stove that could be pretty cold which raises questions. Using an unsealed combustion air source could just be contributing more infiltration overall. My opinion is the the ladder..

Welcome to the forum and good question!

I installed a 4" pipe from outside to my combustion fan a couple years ago and noticed an immediate improvement in the draftiness and warmth at the one end of the basement which in turn raised the floor temp for the upstairs in that area.

The combustion efficiency of the coal is compromised somewhat due to the colder temps of the air hitting the coal bed. What percentage it declines is up for debate. I don't doubt there is some impact but in my case I think the air infiltration issue outweighed the combustion efficiency decline.

It could also help in cases where the house is sealed so tight lack of makeup air affects the burn characteristics.

I believe that make-up air from the outside is supposed to be brought into the general vicinity of the appliance, but not directly to the blower inlet.

My house was built to be electric only back in the early 80's. It's extremely air tight and bringing in out side air improved my stoves ability to heat better and more uniformly. I have a finished basement so a flex pipe was used to drag in the air.

Maybe I've spent too much time thinking in automotive terms, but I would expect the stove's efficiency to be improved when using colder, denser air. Cars will run better and make more power with cooler air intake temps, so I figured the same would apply here.

Another consideration, on a 1000+ degree coal bed, the difference in air intake temps inside vs outside would be maybe 10% difference in a worst case scenario.

Not sure if I'm right, but just putting this out there as food for thought.

Outside combustion air is my next project. I have been opening a nearby basement window in the family room to dictate where the "infiltration air" arrives.

LDPosse wrote:Maybe I've spent too much time thinking in automotive terms, but I would expect the stove's efficiency to be improved when using colder, denser air. Cars will run better and make more power with cooler air intake temps, so I figured the same would apply here.

Another consideration, on a 1000+ degree coal bed, the difference in air intake temps inside vs outside would be maybe 10% difference in a worst case scenario.

Not sure if I'm right, but just putting this out there as food for thought.

Yes and no. It's true for cars, up to a point, but stoves "breath" differently.

The cold air induction systems for cars do use cold air because it is denser. Car motors get their power by air being able to move into a cylinder during a small window of time. That window of time puts limits on how much power can be made by that cylinder. By using cooler, denser air for every cylinder fill they can get more air in each time a cylinder fills.

Coal stoves don't work on the stop/start movement of air during a very short time period that auto induction systems do, since the air flow is constant.

However, what both systems have in common is that the air/fuel mix has to be heated to get it to burn. Even cars with cool air induction have designed in hot spots to preheat that air/fuel mix before it gets to the cylinders.

Carbs and fuel injection systems can only vaporize the fuel to droplet stage, which can't affectively combine with oxygen to burn as efficiently as possible. It takes heat to get that vaporized fuel to the atomized stage so that it can burn. Get the incoming air/fuel mix too cold and you get a lot of unburned gas in the cylinders that then burns in the exhaust system instead of during the power stroke where you need it.

Cold air induction taken even further - with race motors there is a lot of fuel wasted because they can't preheat the air/fuel as much as a road car. That's partly why you see the flames coming out short exhaust pipes (that, and race cams with a lot of overlap ). But, they are not really worried about fuel efficiency as much as they are about usable power.

Like gasoline, coal needs heat to burn it's best - the more the better. The outside cold air systems that were being used back in the 70's were not as good as hoped. Later systems get better efficiency by using waste exhaust heat to preheat incoming air before it gets to the stove intake.

I've read that some heating systems up their efficiency by using a double wall pipe with the incoming air drawn in through the same pipe as the hot exhaust is going out.

Not using heated room air to feed a stove is a savings, but replacing it with cold outside air has to be done right, or you might waste more heat than you save.

You've likely read on here how some of the old stoves, like base burners, gain efficiency by making the lower part of the stove hotter to help preheat that incoming primary air, and on some even preheating the secondary air as well. If stack waste heat can be used to preheat the incoming air, then you stand a chance of increasing efficiency. But, I don't see how bringing in outside air would help stove heating efficiency if it lowers the air temp before the firebed.

Paul

I live in a tight electric baseboard house and once I brought in outside air the flame was not nearly as lazy and all the rotten egg smell went away immediately.

heatwithcoal wrote:I live in a tight electric baseboard house and once I brought in outside air the flame was not nearly as lazy and all the rotten egg smell went away immediately.

Like my house as well. Sometimes real world experience belies the nay saying of the others based on theory. In my real world experience, my efficiency and warmth was increase by bringing in a cold air induction.

heatwithcoal wrote:I live in a tight electric baseboard house and once I brought in outside air the flame was not nearly as lazy and all the rotten egg smell went away immediately.

You have demonstrated the importance of make-up air for combustion. In a tight home it is usually necessary to add a vent to the room containing the stove/boiler. The size of the vent depends on the BTU rating of the appliance.

No matter how you slice it, the air comes from outside. By using dedicated outside air the stove and chimney become a single unit, independent of what changes might occur in the house from exhaust fans, dryer exhaust, chimney effect of house, etc. Just a better system, but usually not needed.

I don't believe the colder air makes any difference at all. The pipe conducting it in will even pick up some heat from the room, just as a vent in the wall will do.

but usually not needed

I totally agree here, houses already have natural air infiltration happening. And to add to it, is just that, adding to it.. but in some cases of newly built air tight homes it can be beneficial instead of detrimental.

Which leads me back to - unless the chimney is having trouble drafting, dedicated combustion air source is just contributing to unneeded additional air infiltration.

Just my nickels worth...