Woodstove - Heat output question

[bczoom]

OK, why is it that:
When it's 15 degrees outside, my wood stove (by itself, no furnace) keeps it 66 degrees in the house (51 degree increase) but when it's 30 degrees outside, it only raises it to 69 degrees (39 degree increase).

Why the difference from 51 degrees to 39?

 

[fogtender]

Well the colder it is outside, cold air is "heavier" than warm air and as your stove burns and the hot gases are sucked up the stack, colder denser air creeps/pushed in through all the house's cracks and needs to be heated up and will make the house cooler because of it unless your stove is red hot all the time.

In your case, the air is not that cold and the outside air isn't being forced into the house like it would in the extreme cold like we deal with here, the hot air going up the stack isn't being pushed out of the house as fast either when it is warmer outside, so it doesn't produce as much heat.

When it is 20 above, there isn't much of a draft as at -40, then you would think the stove had a supercharger on it.

To solve that, put ducting from outside to the back of the stove so the cold incoming air hits the hot metal and is heated up, and it stops/slows the other house cracks from letting cold air in, your stove will breath easier and heat the house a lot more in both cases.

When it is -20 and colder, you can walk around your house and see where all the air leaks are at because of the frost around where the cold air comes in. You seal the house and the stove can't breathe either.

Also putting a fan behind the stove will help as well since it allows the air going past the stove to heat up faster, verses the cold air coming in cooling things down.

May not make a lot of sense the way I am discribing it, but that is what happens and I may not be saying it right, but I understand me!

 

[snow dog]

OK, why is it that:
When it's 15 degrees outside, my wood stove (by itself, no furnace) keeps it 66 degrees in the house (51 degree increase) but when it's 30 degrees outside, it only raises it to 69 degrees (39 degree increase).

Why the difference from 51 degrees to 39?

Is it possible, that the output of the stove is not the same on differant days.

 

[tommu56]

there's more draft on colder days

tom

 

[jpr62902]

Well the colder it is outside, cold air is "heavier" than warm air and as your stove burns and the hot gases are sucked up the stack, colder denser air creeps/pushed in through all the house's cracks and needs to be heated up and will make the house cooler because of it unless your stove is red hot all the time.

In your case, the air is not that cold and the outside air isn't being forced into the house like it would in the extreme cold like we deal with here, the hot air going up the stack isn't being pushed out of the house as fast either when it is warmer outside, so it doesn't produce as much heat.

When it is 20 above, there isn't much of a draft as at -40, then you would think the stove had a supercharger on it.

To solve that, put ducting from outside to the back of the stove so the cold incoming air hits the hot metal and is heated up, and it stops/slows the other house cracks from letting cold air in, your stove will breath easier and heat the house a lot more in both cases.

When it is -20 and colder, you can walk around your house and see where all the air leaks are at because of the frost around where the cold air comes in. You seal the house and the stove can't breathe either.

Also putting a fan behind the stove will help as well since it allows the air going past the stove to heat up faster, verses the cold air coming in cooling things down.

May not make a lot of sense the way I am discribing it, but that is what happens and I may not be saying it right, but I understand me!

In other words, cold air has more pressure than hot air, due to density, all other things being equal.

 

[tsaw]

OK, why is it that:
When it's 15 degrees outside, my wood stove (by itself, no furnace) keeps it 66 degrees in the house (51 degree increase) but when it's 30 degrees outside, it only raises it to 69 degrees (39 degree increase).

Why the difference from 51 degrees to 39?

The reason is BTU's It takes x*u/wood (BTU output) ^ by outside temp. So, it will always seem that way.

 

[Cowboy]

OK, why is it that:
When it's 15 degrees outside, my wood stove (by itself, no furnace) keeps it 66 degrees in the house (51 degree increase) but when it's 30 degrees outside, it only raises it to 69 degrees (39 degree increase).

Why the difference from 51 degrees to 39?

Just a wild guess , but if you dont have a fresh air intake from the outside to provide fresh air to your woodburner, you are at times sucking the heat out of the area you are trying to heat therefore defeating the purpose of using it .

But I may be way off base too , it happens a lot .

I would go with whatever Fogtender suggests. as he deals with real cold temps & sure as hell knows more about it then most of us IMO .

 

[loboloco]

Really silly question, but do you have a pic of your stove? Different stoves work slightly differently and if we saw it maybe we could give you an idea. off hand. I'm going with heat loss and less than optimal air flow.

 

[Snowtrac Nome]

i would love to see a pic of fogtenders idea

 

[pixie]

I've noticed the same thing. Some days I wonder if it's the barometric pressure as the stove doesn't seem to draw as well no matter the outside temp..

I also think it has to do with heating the mass of the house. My garage with a cement floor stays about the same temperature no matter how much hot air I run thru it or whether it's 40 degrees or 10 outside.

 

[snow dog]

My pellet stove is plumbed to bring in outside air for combustion, arn't wood stoves done the same ?

 

[bczoom]

I'm not sure I'm following. Draft control is done by me on the intake side of the stove. I keep it the same setting, regardless of the outside temp. I keep it where there's just a small flame so there is no "turbocharger" flow.

Here's a pic of one of the stoves as we were working on installing it.. Actually, it's an insert.

 

[muleman]

Foggy is right on the money. A piece of 1 1/2 pvc pipe coming in from the outside will balance the airflow and let the stove breathe. Today houses are sealed almost too tight for good combustion. All the higher efficiency furnaces require fresh air makeup to function properly. I have 4 " pvc pipe into my root cellar off the basement to provide air for the boiler. Have fixed a lot of sooty oil burners that were not getting enough air to burn right by installing a pipe from outside to give the burner air. Clothes dryers are another air robber that most folks don't think about. They exhaust inside air to outside and pull drafts from wherever they can.

 

[pixie]

I don't have make-up air ducted to my woodstove but there is lots of air leakage around the garage doors and there is usually a window open a crack. The furnace does have an outside air intake mostly because it has a Powervent.

Many woodstoves have air control on the front door(s) which means the outside air wouldn't be contained or controlled. I have an old Vigilant stove with the bi-metallic spring controlled air intake on the rear so I could box in a duct.... I might try it (next time I feel like drilling a big hole in the wall )

 

[fogtender]

Re: Wood stove - Heat output question

I'm not sure I'm following. Draft control is done by me on the intake side of the stove. I keep it the same setting, regardless of the outside temp. I keep it where there's just a small flame so there is no "turbocharger" flow.

Here's a pic of one of the stoves as we were working on installing it.. Actually, it's an insert.

Well the inserts aren't generally as efficient as a stand alone stove is, but they still suck the warm air out of your house without a inlet vent from outside.

In this case, I would have built it like you have it, but put a duct in from the outside under the stonework (if you are building from new) and put a register vent right in front of the stove, that way it would suck the cold air from outside directly into the stove's intake instead of sucking the warmer air from the room up the stack. The fire gets it's air and the warm air of the house isn't sucked outside.

By the way, you cover the outside inlet pipe with a screen so the bugs don't come in during the summer! I would make it a pipe that goes up a few feet, then an 180 degree elbow so it points down so the rain and snow don't run down it and it is above the snowfall level that way. The register vent can be closed if you aren't using the stove so cold air don't flow into the house.

 

[fogtender]

Re: Wood stove - Heat output question

Just a wild guess , but if you dont have a fresh air intake from the outside to provide fresh air to your woodburner, you are at times sucking the heat out of the area you are trying to heat therefore defeating the purpose of using it .

But I may be way off base too , it happens a lot .

I would go with whatever Fogtender suggests. as he deals with real cold temps & sure as hell knows more about it then most of us IMO .

Nope, you are right, some cabins keep the stove so hot to make up for all the cold air they are sucking in from outside.

At my cabin out on the lake, the wood stove would heat a large house, but because the air it uses was made up from cold air being sucked in from outside which keeps the cabin pretty cool if I hadn't put a vent to that from the back of it. It had a place you could put a fan to blow air in between a heat shield and the stove, will I ran a 1 1/4" pipe and flange up to that and now it sucks cold air up from outside under the cabin into the fan's hole mount and pre heats the air before the stoves sucks it in to be burned. So now the stove just idles and keeps the cabin really warm and I don't have frost building up along all the cracks!