Re: SB: Outside air, was:breathability

[Norbert Senf <mheat@mha-net.org>]

At 08:22 PM 2001-10-28 -0700, Rick wrote:

>I thought the idea of sealing up a house was for tighter control of the 
>infiltrating air sources to save heating/cooling energy. Part of this was 
>to use an external source of air for combustion in a fireplace so you 
>weren't using heated air and the energy it took to heat it to make fire. 
>(snip)it appears you are more worried about creating a negative pressure 
>in the house.


>Could you do a summary of the major issues and the current thinking on 
>solving or handling them? If I have a reasonably sealed house, with less 
>breathable walls than I thought, what do I need to be aware of regarding 
>fireplaces, masonry heating, and combustion air?


Hi Rick:

One thing we need to get clear is whether we are discussing an open or a 
closed door fireplace. With an open fireplace, the air consumption is much 
higher, so it will depressurize the house and therefore spill smoke if the 
house is very tight. The solution to this is makeup air.

Outside combustion air can only happen once you put doors on the fireplace, 
so that you can put the air directly into the firebox.

The theory that I believe you and Rob are proposing is that if you bring 
the combustion air directly into the firebox (on a closed door fireplace), 
you save having to heat it up from the house. This makes sense on the 
surface. However, you have to heat that air up with the fireplace one way 
or another, whether it goes through your house or not. If it goes through 
your house, it becomes prewarmed combustion air, which gives you a slightly 
hotter and cleaner burn. Your fireplace gives you a bit more heat. If the 
air comes directly from outside into the firebox, you have chilled 
combustion air, which requires some energy from the fire to get it up to 
where the room air would be. So, you have a little bit less heat available 
to heat your house.

Also, when you calculate the actual air mass and the amount of energy we 
are talking about to heat it from outside temperature to room temperature, 
it is a small amount of energy.

On an open fireplace, if you place a 4" dia air duct in or near the 
firebox, as per building code, the fireplace doesn't particularly care. The 
huge amount of dilution air that an open fireplace requires overwhelms any 
possible effect from a 4" air duct.

To summarize:

- there is a big difference in air consumption between an open fireplace 
and a closed fireplace or stove
- there is a big difference in air consumption between a small and a large 
open fireplace
- if you depressurize the house more than -5 pa, the fireplace will smoke. 
A stove may start to spill carbon monoxide during the charcoal phase.
- a smoky fireplace is an annoyance. Spillage from an odorless charcoal 
fire can be a serious safety hazard.
- you can depressurize the house with a large open fireplace, a kitchen 
range hood, a Jenaire, etc. Jenaires are the worst culprit.
- if you depressurize the house, there is no science to support the 
argument that an outside combustion air supply to your stove will make you 
any safer. Don't depressurize the house.
- if you put in a Jenaire, etc., make sure that it has a makeup air supply 
so that it won't depressurize your house.

>  Is there a difference in requirements if I am closer to David in Tejas 
> than to Rob in the Upper Tundra?

Not sure. Tundra Rob is a smart guy if he installs an HRV in his house, 
which give him some additional envelope leaks and protection from 
depressurization. I have heard arguments that HRV's may not make as much 
sense in your climate, but don't know.


>Isn't it easier to just open a window a crack and mellow out in front of a 
>nice fire?

Yes, in Texas. Tundra Rob would feel a bit of a draft from that -20F air.

Best ....... Norbert

----------------------------------------
Norbert Senf---------- mheat@mha-net.org-nospam
Masonry Stove Builders	(remove -nospam)
RR 5, Shawville------- www.heatkit.com		
Quebec J0X 2Y0-------- fax:-----819.647.6082
---------------------- voice:---819.647.5092
			
		
				



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