There seems to be a lot of debate around fresh air / outside air intakes for wood and pellet stoves. As someone who owns a Regency Greenfire GF55 pellet stove I wanted to see how much (if any heat) I’d save.
I first started off with an anemometer to measure the air speed going through the intake pipe. Since this pipe intakes even when the blower is off it’s safe to assume the air is going straight to the combustion chamber and then the flue. My anemometer has a fan diameter of exactly 1″.
During startup the GF55 intakes 6.0 to 6.5m/s of air. Once the stove is lit and the blower turns on it drops to 5.0m/s. For the sake of calculations I’m going to use the 5.0 number since the startup sequence is less than 5 minutes and the 1.5m/s of air difference in this time should be negligible.
Using this calculator we can determine that 5.0m/s though a 1″ duct is 21.47CFM.
My pellets are Cubex Extra rated “up to” 9,000 BTU/lb. Since we live in the real world and my pellet stove isn’t perfect I’m going to use 8,500 BTU/lb. This is inline with other brands of pellet outputs.
Using this chart air at 32°F 90% RH weighs 0.0806lb/ft3 and we’re intaking 21.47CFM which comes out to 1.730482 lbs/minute of air we are in-taking. I’m using 32°F as the starting point because even though we’re taking in 68° room temperature air, the house as a whole will be under a vacuum and indirectly in-take 32° air from cracks, doors, windows, etc.
Now by using this calculator we can plug our numbers in with the assumption that we are warming to 68F or a 36 degree difference. After all is said and done 15,753.6 Joules / minute of energy is being removed from the house.
Now we can convert 15,753.6 Joules to BTU which comes out to 14.931.
Ok, we’re getting close. We’re removing 14.931 BTU / min from the house and a single 40lb bag of pellets will last roughly 24 hours. So over 24 hours we’re dumping 21,500.64 BTU outside though the air intake.
Since the bag is 40lbs and we’re assuming each pound produces 8,500BTU this comes out to 340,000BTU / bag. With intake losses of 21,500.64 each bag is now 318,499.36.
That comes out to 7.4% loss per bag or 2.52lbs of pellets per bag being wasted.
There’s definitely a lot of hand waving here since the outside air temperature, pellet stove efficiency, individual pellet efficiency, and intake airspeed on various other stoves are variable. With that said using preheated inside air during the intake & combustion process will overall increase your pellet consumption and lower the stoves efficiency.