Would it be beneficial to pre-heat intake air?

[CountryBoy19]

I have a Kozyheat ZC fireplace which must be installed with an OAK (according to installation manual).

I did just that but I veered from the manual just a bit because there was no easy option for installing the OAK in my house without going up with it. So my OAK is routed right along the chimney, and terminates in the attic space.

When my secondaries ignite (it takes them a while) they never burn on the first 4 inches or so of the tubes. I think that is because the Kozyheat does not pre-heat the secondary air, it just comes straight in, and it is cold. While working in the attic over the weekend it occurred to me, my intake terminates right beside the chimney in the attic space. Would it be beneficial to harvest the waste heat coming off the outside of the class A chimney to preheat the intake air?

It could potentially increase heat output by a slight amount because the intake air won't be as cold and the burn will be more efficient.

My thought is to put a 12" duct around the chimney leaving the top open (just like the insulation shield only higher) and connect the OAK to the duct. This would pass the intake air over the surface of the chimney before going to the fireplace.

Any thoughts/discussion? Am I wasting my time? I just can't stand to see good heat (coming off the outside of the chimney) go to waste in the attic space when there is a potential to improve the heating to my home.

 

[maple1]

This is something I've wondered about for a while. I've read lots of PR about stoves & furnaces pre-heating incoming air - but I thought colder air was denser & contained more oxygen - therefore should be better for combustion?

 

[CountryBoy19]

This is something I've wondered about for a while. I've read lots of PR about stoves & furnaces pre-heating incoming air - but I thought colder air was denser & contained more oxygen - therefore should be better for combustion?

Yes, cold air is denser and contain more oxygen per specific volume, but it doesn't contain any higher percent oxygen, it's still roughly 19% oxygen. You can just fit more air into the same volume, but that shouldn't really matter any because the stove will be heating the air up changing the volume anyways. My idea is just to pre-heat the air so my secondaries will burn more fully, and the air won't be sucking so much heat out of the firebox when it enters.

Think of it this way: for every BTU that your fire spends heating up the incoming air, that's one less BTU spent heating your house. If I can replace some of those BTU's with wasted BTU coming off the chimney then it should help things out a bit right?

 

[FyreBug]

Stove MFG's understand pre-heating secondary air helps in getting a cleaner & more efficient burn. It is done by extending the channels that feed the secondaries so the air can warm up. There's other tricks.

One that will work but not a great idea is to use energy to warm up the air (ie. electric element). Because any efficiencies gained on the unit is lost powering up the device warming up the air.

 

[CountryBoy19]

Seeing no objections I think I'm going to try it. Worst case scenario I get no benefit out of it other than feeling good about not wasting that extra heat and I will have a few minor out of pocket expenses.

I will report back on my findings. Hopefully it results in much more reliable, and faster/longer secondary ignition.

 

[mhrischuk]

Only possible problem is if you heat up the air in a vertical pipe it will want to rise. It could quite possibly become a real issue trying to fight with the chimney.

 

[CountryBoy19]

Only possible problem is if you heat up the air in a vertical pipe it will want to rise. It could quite possibly become a real issue trying to fight with the chimney.

I have a really strong draft (almost too strong) so that won't be a bad thing I don't think. But thanks for bringing it up, I hadn't considered that.

 

[mhrischuk]

This is the reason an OAK can be questionable. Since most OAK piping materials are not designed to handle heat, any reversal in air pressure could cause heat to go out the OAK. I think I'll pass for now.

 

[Todd]

The thing that concerns me is your OAK goes up and that dense cold air is pushing down into your stove verses the stove drawing it in. This could lead to short burns or could even be the source of your strong draft.

 

[CountryBoy19]

This is the reason an OAK can be questionable. Since most OAK piping materials are not designed to handle heat, any reversal in air pressure could cause heat to go out the OAK. I think I'll pass for now.

However unlikely that may be, I ran non-combustible OAK material for that very reason. I'd rather be safe than sorry.