Barometric Dampers and Cold Air Infiltration

Im scared. After the last several posts I almost understand,

This subject can get sooooo deep, I'm just enjoying the conversation. I will add one little tid bit here. The atmosphere is trying to balance itself out, like displaced water trying to reach the surface. The house has warmer inside air, and is displacing the cooler outside air. A similar example: Take an empty five gallon pail, boat, or what ever. The further you push it down in the water, the more resistance. Put a hole in the bottom, and water comes in (stack effect). Bigger hole = more volume coming in at faster rate, as the water tries to seek it's level. A hole in the bottom of the house is letting air in, which is seeking it's level in the earths atmosphere (stack effect). The cooler the outside air, and warmer the inside air, the stronger the stack effect. And this subject goes on, and on.

warminmn wrote:Im scared. After the last several posts I almost understand,

I know right? Isn't this great? It happened to me one day. I was standing in the basement trying to make sense of it all and it was like a light bulb turned on in my head. I was thinking about pressure and how it would be the main driver in everything we use to control our coal stoves and all of a sudden it all made sense. I'm not claiming to be an expert in any of this stuff, but merely trying to explain how it all makes good sense to me and hopefully it will make sense to others too. And if anything seams glaringly wrong, please suggest it so we can work it out and find the right answer..

northernmainecoal wrote:Are you sure that when you opened your primary to increase CFM more the baro didn't starting taking in less?

I had to think about this for a minute, but I'm gonna say confidently that the baro's intake would stay the same or increase. Here's my explanation why. The baro needs air intake for two things to occur #1 it satisfies some of the negative pressure in the chimney and #2 the cooler air offsets the hot exhaust from the coal to lower the temp difference between chimney air and outside air. Both attributes reduce the draft pressure. If the baro were (forced somehow) to take in less with added primary air, what would happen? The negative draft pressure would strengthen and result with the stove growing hotter than we want it to. I've noticed that when my fire is burning hot (more primary air), the baro door is open more also. But there are two reasons for that, its colder outside and more hot coal exhaust is going up the chimney creating more need for stabilization via the baro.

If all this is true then why is it the baro door closes when I open the ash pan door or the load door? Because openings that big under the baro exceed the volume capacity that the chimney can handle and satisfy most of the negative pressure being generated by the chimney. That's why sometimes we see the mano reading get close to zero but if you hold a lit cigarette in front of one of those open doors you see a ton of air being pulled in. This is where draft and pressure go their separate ways, meaning the mano pressure reading isn't united with the air volume escaping the chimney anymore. It's a breach in the hull of the ship.

franco b wrote:As long as both openings of the manometer are open to the room when zero is adjusted, then zero will be accurate and not change no matter where the manometer is placed, inside or out will read the same zero because whatever the pressure, even if different from inside it is pressing on both sides of the liquid. A manometer reads differences in pressure and not absolute pressure as a barometer does.

Good call franco, I would have answered that the same way..

titleist1 wrote:Or would you have to leave the primary open that same amount to have the stove burn hotter than you'd really like because a lot of stove heat would be rushing up the chimney......

In an air tight stove, it doesn't matter.. Heat can't rush up the chimney unless air (that isn't contributing to combustion) is coming in to replace it. This is why the guys with automatic bi metallic intake regulators don't require a baro or MPD, although some use them anyways which I would think is fine.. Their air volume intake regulator is on the other side of the combustion. They don't care if their draft is -.08 or -.04, the automatic intake regulator takes care how much air volume is allowed into the stove based on the heat output that is desired..

hotblast1357 wrote:Ok now what about this curve ball.. I have a fresh air pipe 1/1/2 next to my furnace, that is bringing in fresh air, for my stove and baro to use. It is about 3 feet away from both inlets, and yes cold air is coming in constantly.

Now this here is a real can of worms..
My thought on this is that it isn't needed unless the chimney is struggling to draft. Running a dedicated combustion air supply can have some huge consequences that normally people wouldn't consider. For example, letting in fresh air at the bottom of the house will influence the neutral pressure plane by lowering it. So how could that be a bad thing? Lets examine the big picture. Well, since the NPP is lower now, this presents more positive pressure at the top of the house which means air is escaping faster there (as demonstrated in the first post of this thread). This results in more air turn overs in the house per day, which means more outside air is entering the home that needs to be heated, which ultimately equals more coal consumption. It gets worse than that too. The warm air escaping at the ceiling could be carrying a lot of moisture (with a humidifier running). This moist warm air will hit colder parts as it makes it's way out at the ceiling level and condense causing harm to the roof and joists. This is why its thought to keep the neutral pressure plane as high in the house as possible. It helps alleviate moisture problems in the attic.

For the hell of it, I just opened and closed my primary air a couple times. I observed that the baro jittered but then assumed it's previous position no matter if the primary was open or completely closed. This demonstrates that the baro's actions are independent of the primary air's position and more dependant on how much heat is going up the chimney to create negative pressure.