Thoughts on cold combustion air

[711mhw]

I just read Gasifier's thread on the effects of using cold wood and it got me thinking about my (sometimes) very cold outside air supply for combustion. I know in race engines that cold air is more dense and actually makes a slight (posative, faster)difference at a race track where a thousand of a second makes a difference. My thinking on the outside air supply goes in line with the newer high efficency gas and oil burners, and even fireplaces that do just that. I know that part of the thinking is that youre not using up your (warm) conditioned air that some where along the line is being replaced by (cold) outside air being drawn in some where else in your house.
I'm running a Wood Gun that uses a giant fan that sucks a lot of air and my home is very well air sealed and insulated FWIW.

 

[maple1]

That is something I always wondered about. I think all the manufacturers tout pre-heating the air, but cold air is more dense and has more oxygen - hence why my snowmobile runs a lot cleaner, crisper & stronger on a cold day. I have a smoke hood cobbled up on my furnace, right above the door which has the air intake in it. When the hood fan is on, it sucks air out, but when it is off, prevailing winds can direct cold air from outside out the hood right at my air intake if the wind is blowing the right way. I am sure the fire burns much better when that is going on, as opposed to when the wind is blowing the other way and warm air is being taken in from the room. I have no way of quantifying that - but seeing how mine works under these conditions is I think is enough to convince me that cold air will make your fire burn better than warm.

 

[Clarkbug]

My house and basement are so leaky, I didnt even worry at all about where my combustion air was coming from. If I had a newer house, I would definitely want to have some form of outdoor air intake rigged up so that I wasnt burning my nice warm house air. Also, with that big fan running, it will pull the air into your house from somewhere, and usually its where you dont want it to be. Better to be in control of where the air from outdoors is coming from in my mind.

In my house, it just happens to be everywhere

 

[TCaldwell]

for primary air cold is ok, but secondary air should be as hot as possible. cold primary air contributes to the initial combustion and is heated quickly in the firebox where temps rarely go beyond 1000degf. secondary air needs to be preheated usally through refractory and be able reburn combustables at a minimum of 1200degf and higher. with cold secondary air you could not maintain secondary burn. when manufacturers incorporate secondary air delivery nozzles in the refractory, that is also the preheat mechanism. most industrial biomass boilers incorporate a combustion air that is preheated using fluegas temps.

 

[ISeeDeadBTUs]

We need a "Thanks" button on Hearth.com.

This was a damn good thread

 

[maple1]

for primary air cold is ok, but secondary air should be as hot as possible. cold primary air contributes to the initial combustion and is heated quickly in the firebox where temps rarely go beyond 1000degf. secondary air needs to be preheated usally through refractory and be able reburn combustables at a minimum of 1200degf and higher. with cold secondary air you could not maintain secondary burn. when manufacturers incorporate secondary air delivery nozzles in the refractory, that is also the preheat mechanism. most industrial biomass boilers incorporate a combustion air that is preheated using fluegas temps.

That makes sense.

 

[DaveBP]

The internal combustion engine analogy keeps popping up around this question of intake air temperature.
I'm not sure the comparison applies though.
Piston engines are sealed combustion devices. When the intake valve shuts, the chamber has all it's going to get to burn. The more air and fuel in the cylinder, the harder it will push against the piston when it fires. It's about about turning combustion energy into mechanical energy. Turbochargers just intensify the process of increasing the charge, like loading more powder into that rifle cartridge.
But a wood fire is a steady process. There is no pressure blast to use to drive any mechanical device in a wood fire (not under normal conditions, back puffing is not useful). It's all about getting those last few chemical compounds to burn which don't burn until they get way above normal woodstove temps. Get those last chemicals to burn and you get to have their extra heat that you will lose up the chimney otherwise. Getting the combustion chamber in the boiler to maintain such a high temperature is tricky and pumping cold air into it at that point will only make it harder.
Does this hunch of mine make sense?

 

[hiker88]

I'm not sure the analogy applies either. I'm a pilot, so I really understand the benefit of cold air. When flying, dense air not only makes the engine perform better, but also the prop and wings.

However, the fire tetrahedon is composed of fuel, oxygen and heat. Add more of any (within reason) and you are going to get a biggerhotter fire.

I'm not an expert, but the reason for an outside air source on the boilers is for the safetly of the occupants in the home - not the performance of the boiler. I think the concern is that a roaring fire on a cold night in a completely sealed home can eat up the oxygen the humans in the house need - this is especially dangerous with infants or when the occupants are sleeping. This isn't as much of a concern in drafty house, but I still think it is code to have an outdoor air source.

My house is really tight so if I were using an internal air source for my boiler (I won't) I'd have multiple CO deterctors and especaially have them where people sleep.

 

[leaddog]

I'm thinking the reason the fire burns better on those really cold days is you are getting a better draft. If you need to pack in more air just put a bigger fan and turbocharge it.
leaddog

 

[ozzie88]

First,in a house useing up the warm indide air to burn a boiler just dont make sence. And if you have a oil boiler and wood boiler in same basement codes want the wood unit to have it own air suplly so both units dont fight for air. And with my boiler I made a unit on top to pre-heat the outside air before it goes into unit,works VERY good. And also when your running the boiler and electric goes off the flap control will shut off right away and fire will go out, Saftey first. Like the other guy said the secondary just wont burn as good with cold air, I accually have an air inlet at both ends of my scendary [just added] I found that just a little bit of new [warm] air right before it exits the chamber will make it burn a little longer up to 3rd chamber, complete combustion.