I draw outside air with my p38. I Bought it the end of last year. I did not notice any frost on the stainless flex but the outside temp was not that cold I believe 20+ F. Glass stays pretty clean here and It makes sense to use cold outside air for combustion instead of already heated inside air. More 02 in the cold air also I guess its supposed to give 2% more efficiency using cold air. I don't know about the PVC thing Considering the heavy design of the outside air kit seems like they are worried about a fire issue. The outside dryer style shroud is thick and heavy stainless on my kit would need a cutting torch to get through it...

Here are three things I thought about before deciding to go with outside combustion air -

1. Because you're not drawing outside air into the cracks of your house to feed your fire, the air in your house will not get as dry.

and.

2. As already mentioned, why waste internal heated air?

and.

3. Using inside air can create cold drafts near cracks.

I'm definitely going outside air. Do it once, forget it.

I don't understand, by the way, the frost issue. On my setup (for an insert), I'm going to have a pipe sticking out of the house to bring air into the fire. If it gets frost or whatever on it, so what?

The only thing that I can figure on the frost issue is that maybe Murf1 is seeing frost on the flex where it ties to the intake of the combustion blower in the home and that below freezing air is hitting room temp and condensing on him. My flex was real cold but no condensation here.

Yeah, you're probably right. But even so, what's the issue? Little frost means you know that it's working.

The problem with the frost is : the stainless flex pipe from the wall to the stove is about 2 ft.- about 18 in. of that gets frost build up, the rest stays clear, therefore theres a transition area where some melting occurs, the water from this ends up following the outside of the flex pipe towards the stove, now theres water at the combustion intake area of the stove, this not only will cause rust, but there are also electrical components in that area.

the reason you are getting frost on the OUTSIDE of the pipe is simply because the cold air inside the pipe cools the metal, thus allowing moist indoor air to condense/freeze onto the pipe, quite common actually. yes, if you used pvc you could potentially eliminate this problem (depending on how cold it gets where you live); this is one of the reasons pvc is used for high efficiency nat. gas furnaces that used outside air. however even when using pvc, it would be a good idea to use two pvc pipes, one inside the other, and run them both to the outside of the house and then seal both end spaces of the larger pipe, thus creating an insulating dead air space. it would probably be advisable not to run pvc all the way to the stove, by that point the warmth of the stove should prevent any condensation anyway in the last foot or so.

Never used it, but not sure how PVC is going to make a difference.

Insulation should work. Like a two pane glass window, condensation shouldn't occur on the outside.

I guess I'll know more in 2 months. I plan to use regular tin ducting, but if I have the same issue as you, then I'll have to do something different.

"Like a two pane glass window, condensation shouldn't occur on the outside."

condensation is not occuring on the "outside" of the home.
the condensation IS occuring on the "outside" of the pipe, when it comes into the home.

This has nothing to do with a two pane glass window and that comparison cannot be made.

also, the reason i suggested pvc is because does not allow fast thermal transfer. this is where the problem is, metal allows the transfer of heat from inside the home and the associated moisture in the air to cool rapidly on the outside surface of the metal cold air intake, this causes condensation and frost build up, pvc will prevent this from hapening simply because of its much lower abilty to allow this heat transfer, insulating may work, however it depends on what type of insulation is used and how air tight it is. as i mentioned previously, i have personally seen this many times before in many different situations.

after re-reading your initial posts, i realize that this pipe is only about two feet long anyway, thus my reccomendation to you would be to simply try wraping the pipe with some fiberglass insulation and see how it works out, wrap it tight and then go buy some foil backed hvac tape (available at ur local hardware store) and wrap the tape around the fiberglass, this will help seal out any unwanted airflow and allow it to be more asthetically pleasing; it could even then be painted to your likeing, if you so wish.

The two pane glass is a perfect analogy.

We had single panes in our last house.

Every winter, we would have significant ice and water accumulation on the inside.

I ripped them out and put in two pane insulated units.

Thereafter, no more frost or condensation.

It will be the same with your pipe. Wrap it up, and you shouldn't get condensation on the outside of the insulation.

We are in the process of putting in a wood stove this month, so was wondering about bringing in outside air. What size pipe should we install for this?

Thanks, Elsa.

i don't mean to hijack the thread, but i'm also wondering about the combustion air intake. the plans for the masonry heater i'll be building are here:

http://www.geocities.com/RainForest/Vines/4095/masonry/

under the "combustion air intake" page, it says i could use two 3x3 metal downspouts. that seems like a lot of outside air, but what do i know. just wanted a second and maybe a third opinion. i'm not seven sure if i could find those kinds of down spouts around here. 2" galvanized round pipe might be as big as i can get. thanks, Reuben

Hi, I am also planning on installing an outside air duct for my masonry fireplace this weekend. Might be quite an undertaking since I'll have to drill through about 8" of masonry with a core drill to install it. I have an ID 2" pipe (OD is 2.375), and I plan to drill a 2-5/8" hole. I have used the Bernoulli equation to calculate and estimated airflow capacity trough this pipe assuming a preddure drop of 5 Pascals, and a length of about 2' - THEORETICALLY, it should take 125 liters/second based on these assumptions. Even if you cut the estimated flow in half, it should be sufficient to fuel the fire - and even if it's not, it will still decrease the sucking of warm air from my house into the fire/chimney. I have two questions however: will it be OK to have the pipe come in the rear of the fireplace and how far out from a brick house should the pipe stick to minimize the risk of fire in the case of flow reversal? Thanks.

A little condensation is nothing. When it gets really cold here in Alaska, you should see what happens in my living room. I have outside air for combustion and another outside air intake that brings air around my firebox and into my blowers which blow it out the front of the unit.

When it gets really, really cold, the condensation from the second air intake builds up and the blowers actually blow snow out of the front of my fireplace!

I installed my 2" pipe and I'm pretty pleased with it. it clearly reduces combustion air being used from inside the house (as evidenced by less candle flame leaning toward the fireplace representing draft from inside the house, and by the obvious air draft through the pipe from outside), especially with the glass doors closed. And it was a fun project.

lol feathersnfins! A fireplace and snowblower, all in one!

We have a pipe in the back of our firebox that brings in outside air. I can't imagine the drafts we would have inside the house, if we didn't have that, 'cause it still gets drafty in the house.

I wonder how much cold air that pipe lets into the house, when the fireplace is not in use?? We have a screen over the outside end of it, so little animals don't get any ideas.
I wonder if that creates much of a resistence to the airflow?
Also, when we added on to our house, behind the fireplace, we had to have a couple 90 degree angle joints put into that pipe, to reach the outside. Is there a problem with angles? Thanks for helping me with all these questions!

bends in the pipe increase resistance to flow substantially. The screen probably does too. But if everything is burning well, who cares? I have a 2 3/8" pipe about 2' long, which supposedly is not near big enough, but it works well for me. If I had to do it over though, I would have put two pipes in. I no longer have any problems with smoke or smell in the house whether the fire is going or not. Previously I did.

One thing I do is plug the pipe when the fire is not burning. you could do the same, and if you did, you couldl remove the screen.