Sizing wood stove from oil furnace

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Chas0218

Chas0218

Minister of Fire
Sep 20, 2015
539
Beaver Dams New York
So is there a way to size a wood stove to a house based on a furnace? I am in the process of installing my wood stove (Summers heat SSW02 rated for 2400 sq ft. house.) into our home which measures 2900 sq. ft. I know my stove is a little small to heat the entire house but it has been recently remodeled with R21 wall insulation and R48 ceiling insulation and is tight.

Our heat is currently disconnected and during the overnight temps at 39 degrees and daytime temps in the upper 50's the lower level of the house was kept at 70 degrees using 2 infrared heaters and up stairs was around 60 degrees with no heat source and no drywall. I know this doesn't compare to the winter sustained temps of 30 degrees but I think this stove will be enough.

I'm trying range how much dried firewood I will need this season.

My oil furnace says 175,00 btu not sure if that is per hour or what.
 
I'm thinking 4.5 cords should get you there
 
I better get going then I have about 3 cord now cut and split and have another 2 cord that I have to get still.
 
Chas0218 said:
I better get going then I have about 3 cord now cut and split and have another 2 cord that I have to get still.
Click to expand...
Might be to late for the other stuff, that stove like dry wood <20% moisture content, I'm skeptical that you'll get splits in that range from log form by the time winter rolls in, I hope for the best though, but its been a fairly wet summer up our way.
 
Yeah this is dead standing ash so I'm thinking I should be good. I already split some and was 20% and lower

Sent from my SM-G930V using Tapatalk
 
The 175,000 has to be btu/hour

For comparison, IIRC, peak output of my wood stove is ~73,000 btu/hr, and that's large. It heats a 1500 ft2 insulated house with ~ 2 chords /year.

I'd double that for a house twice as large lacking insulation upstairs, but that's just a guess
 
Everything is insulated just not everything is drywalled right now. My plan is to have all the drywall up and finished before winter is locked in.

So the consensus is around 4 chord, not too bad I guess?
 
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Are you guys talking face cords or full cords? Just wanting to make sure.
 
Cords bro (1) cord 4x4x8 or 128 cu ft
 
kennyp2339 said:
Cords bro (1) cord 4x4x8 or 128 cu ft
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That was what I thought, I was thinking 4 face cord would be really low consumption for the 2.6 cu.ft. firebox on this stove. I know these new stoves are more efficient but man that is super efficient! haha
 
Getting that drywall up will help air seal things more too. Keep in mind that rated heating capacity of almost any stove is basically just pulled out of thin air by the Mfgr...sooo many variables. I bet that stove will keep your furnace turned off 90% of the time...maybe more.
As far as consumption, I'd guess 3 cords on warm winter and up to 5 in a cold year...again, a lot of variables. I'd stock up as much as you can...won't go to waste, just gets you ahead for next year...and dryer wood next year too...also keep in mind that many denser woods, like Oak, can take 2-3 years to dry properly...and only after it is cut/split/stacked...doesn't dry much in the round
 
brenndatomu said:
Getting that drywall up will help air seal things more too. Keep in mind that rated heating capacity of almost any stove is basically just pulled out of thin air by the Mfgr...sooo many variables. I bet that stove will keep your furnace turned off 90% of the time...maybe more.
As far as consumption, I'd guess 3 cords on warm winter and up to 5 in a cold year...again, a lot of variables. I'd stock up as much as you can...won't go to waste, just gets you ahead for next year...and dryer wood next year too...also keep in mind that many denser woods, like Oak, can take 2-3 years to dry properly...and only after it is cut/split/stacked...doesn't dry much in the round
Click to expand...
Yeah that's what I'm afraid of that the manufacturer just kind of said "ehh good enough". I have taken every step possible to get the house sealed tight caulking every seam and foaming every crack that is wide enough to stick that straw in before insulating and putting up drywall. The drywall should be done this week, if I can get myself motivated to finish it. The only species I have available is iron wood, ash, and hard maple. Most of my firewood is bore killed ash and 2 year seasoned maple. I have a bunch of dead standing beech from beech blight that I'm going to try and grab long as it isn't rotten. I like ash a lot burns nice in our little stove at our cottage. The only rounds that I have are about 8" across for burning whole. I'll split some of the standing ash and see how it checks out moisture wise before I try burning it. It is looking like this winter I will be doing some firewood out in the cold. At least it will be warming me a few times like it's supposed to. :)
 
Chas0218 said:
So is there a way to size a wood stove to a house based on a furnace? I am in the process of installing my wood stove (Summers heat SSW02 rated for 2400 sq ft. house.) into our home which measures 2900 sq. ft. I know my stove is a little small to heat the entire house but it has been recently remodeled with R21 wall insulation and R48 ceiling insulation and is tight.

Our heat is currently disconnected and during the overnight temps at 39 degrees and daytime temps in the upper 50's the lower level of the house was kept at 70 degrees using 2 infrared heaters and up stairs was around 60 degrees with no heat source and no drywall. I know this doesn't compare to the winter sustained temps of 30 degrees but I think this stove will be enough.

I'm trying range how much dried firewood I will need this season.

My oil furnace says 175,00 btu not sure if that is per hour or what.
Click to expand...

The 175000btu is usually btu/hr INPUT.

But because of heat distribution of a central system vs wood stove and the continuous output of an oil burner, it might be easier on you to take a look at the burner nozzle sizing which is gallons per hr rated. So if you have a 175000btu boiler, then you might have a 1.5 gph nozzle. You need to look at yours. So consider about 80% efficiency for your burner and then estimate how many times (and how long) if fires in an hour. So for instance, a 1 gph nozzle operating at 30 min per hr burns approx. 1/2 gallon/hr, and you get about 80% of that energy transfer to your boiler. These are really ROUGH numbers. . At 144000 BTU's/gallon that means you burn about 70000 btus/hr for heat. But, you only get about 80% of that into the system. Also, that's centrally distributed heat. Check this for several hours and you can get an idea of btu needed. Remember that woodstove companies often over rate their stoves---they don't off constant output. I have a 79000 BTU furnace using a 0.65 gpm nozzle. It probably runs 10- minutes per hr on a steady state scenario.. I have a stove rated at about 18000 btu's for supplemental heat. Due to central heat distribution of the oil burner, I'd estimate that the oil boiler is about 5 times more powerful than the stove. So consider how you distribute your heat--it's not just btu ratings and in the real world, their ratings are pretty far off. My stove is rated for ~800 ft2. I estimate it would be adequate as a primary heat source for 400ft2. You do have excellent insulation which is key in your case.
 
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louie said:
The 175000btu is usually btu/hr INPUT.

But because of heat distribution of a central system vs wood stove and the continuous output of an oil burner, it might be easier on you to take a look at the burner nozzle sizing which is gallons per hr rated. So if you have a 175000btu boiler, then you might have a 1.5 gph nozzle. You need to look at yours. So consider about 80% efficiency for your burner and then estimate how many times (and how long) if fires in an hour. So for instance, a 1 gph nozzle operating at 30 min per hr burns approx. 1/2 gallon/hr, and you get about 80% of that energy transfer to your boiler. These are really ROUGH numbers. . At 144000 BTU's/gallon that means you burn about 70000 btus/hr for heat. But, you only get about 80% of that into the system. Also, that's centrally distributed heat. Check this for several hours and you can get an idea of btu needed. Remember that woodstove companies often over rate their stoves---they don't off constant output. I have a 79000 BTU furnace using a 0.65 gpm nozzle. It probably runs 10- minutes per hr on a steady state scenario.. I have a stove rated at about 18000 btu's for supplemental heat. Due to central heat distribution of the oil burner, I'd estimate that the oil boiler is about 5 times more powerful than the stove. So consider how you distribute your heat--it's not just btu ratings and in the real world, their ratings are pretty far off. My stove is rated for ~800 ft2. I estimate it would be adequate as a primary heat source for 400ft2. You do have excellent insulation which is key in your case.
Click to expand...
Is the nozzle size marked anywhere on the exterior or only on the nozzle? I might not be able to easily heat the other side of the house with some creative duct work. I was thinking about running a duct line from the living area where the stove is over to those bedrooms with a booster fan on some sort of temp switch.
 
Chas0218 said:
Is the nozzle size marked anywhere on the exterior or only on the nozzle? I might not be able to easily heat the other side of the house with some creative duct work. I was thinking about running a duct line from the living area where the stove is over to those bedrooms with a booster fan on some sort of temp switch.
Click to expand...
If you do that then draw air from the bedrooms and blow it toward the stove room. That should be a bit more efficient. Insulate the ductwork well. The bedroom doors will need to be open or have a grille in the door or a gap under the door to allow for airflow.
 
begreen said:
If you do that then draw air from the bedrooms and blow it toward the stove room. Insulate the ductwork well. The bedroom doors will need to be open or have a grille in the door or a gap under the door to allow for airflow.
Click to expand...
I was thinking about adding a grate on both sides of the wall, do you think that would be sufficient? Also it would be better to suck from the room instead of blowing it into the room? THe fans can be wired either direction so I figured try one way if it didn't work do the other way.
 
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There are lots of ways to allow airflow. If you put a grille on both sides of the wall consider putting one high and one low so that light and sounds are baffled in the stud cavity. Blow colder air toward the warm area. That should be more efficient and comfortable.
 
begreen said:
There are lots of ways to allow airflow. If you put a grille on both sides of the wall consider putting one high and one low so that light and sounds are baffled in the stud cavity. Blow colder air toward the warm area. That should be more efficient and comfortable.
Click to expand...
Ok thanks, I haven't tried to heat 2 stories before with a wood stove so it is quite the learning process. Just trying to cut the minimum amount of holes in the ceilings, and floors.
 
Chas0218 said:
Ok thanks, I haven't tried to heat 2 stories before with a wood stove so it is quite the learning process. Just trying to cut the minimum amount of holes in the ceilings, and floors.
Click to expand...
That's a good plan. Sometimes stove location can also make a big difference. Can you post a floorplan or sketch of the floor layout that shows stove and stairwell locations?
 
Stove is the little red box on the 1st floor (right floor plan) the big area in the middle of the 2nd story is a living room with bedrooms to the left and right. The master is on the 2nd floor left top corner, connected to it is the bathroom and closet. Stairs are the L shaped area at the top of the 2nd story floor plan and little box at the top of the 1st story floor plan.
 

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Is the 178 sq ft box above the label 1st Story an open stairwell? If so heat may convect naturally pretty well with this open floorplan. I note a lack of supporting walls on the first floor. What carries the 2nd floor?
 
I would size the stove more off the seasonal oil consumption figures than the actual burner output. The oil burner could be way oversized.
 
Agreed. The additional insulation and sealing should drop oil consumption if it was done well. It could be that the furnace is now oversized for the house's heat loss.
 
We would go through about 300 gallons in a month and a half, so 300 gallons in 45 days (give or take a day). The furnace doesn't cycle for very long when heating the house it runs around 20 minute on 1 hr. off.

I will say before the addition and such we had next to no insulation in the roof. I'll see if I can find some picture of the house before and after.
 
Chas0218 said:
So is there a way to size a wood stove to a house based on a furnace?
Click to expand...
Your oil usage is a great way to calculate wood usage, but stove size is also dependent on the parameters of the space into which it is installed.

Assuming a furnace efficiency around 85%, and a wood stove average efficiency around 83%, calculation of wood usage should simplify to something on the order of this:

Cords = .005/gal in hardwood
Cords = .008/gal in softwood

These are rough numbers, as wood BTU/cord varies enormously, as does the efficiency of your stove throughout the course of every burn cycle. This would make your 300 gallons in 45 days come out around 1.0 - 1.5 cords per month, in that period, depending on wood species.

Translating this to a stove size is a whole other challenge, as there are numerous factors involved. There's issues of your floor plan, and distribution. There's the issue of weather that varies enormously through the burn season, from 55F shoulder seasons to sub-zero January nights.

One thing that you can do is look at how much wood you're willing to process, and how much you can put thru a single stove in a year. The number I see most often quoted here is up to 4 cords per stove per season, for us mid-Atlantic folk. In my case, my need is around 2600 gallons of oil per heating season, which would be over 12 cords of very good hardwood. This is more than I can physically process, year after year, so I chose my stoves based on what keeps those rooms comfortable without overheating. My furnace still runs to pick up the slack (I burn only 6 - 8 cords per year, now), but I'm cutting my oil usage by 60 - 70%, without killing myself.
 
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