Sizing a stove to a home using heat loss analysis data

[joephu]

Last year, I had an energy auditor conduct a heat loss analysis and blower door test (air infiltration) using HVAC residential standard “Manual J”. He concluded that our 3,160 sq/ft Colonial required 40,000 BTU/hr to heat. Now that we are looking at getting a wood stove for the main floor (inside chimney, open floor plan), I’m trying to use this heat loss data to properly size the stove to provide as much of the heat as possible and using my main system only as a supplement or backup.

I was looking at the Jotul F600 wood stove and have some questions. Here’s the specs:
Max output - 81,500 BTUs
Capacity - 2500 SqFt
Overall efficiency - 71%

I have a few newbie questions I was hoping to get answers too:

1. I assume MAX output is a raw number that must be adjusted with the efficiency rating of 71% to get the actual heating potential of the stove. So 81,500 BTUs X 71% = 57,800 BTUs of available “heat to the house”? Again this is probably a best case scenario too, meaning burning at ideal temps with a full load of wood, right?
2. I notice the EPA states this model with a 63% efficiency and a heat output of 11,600-32,500. Why such a discrepancy?
3. If EPA states 32,500 BTUs, then this stove would not produce enough heat in the dead of winter since I need 40,000 BTUs. So I need a bigger stove?

Thanks in advance! Very confused.

 

[stoveguy2esw]

the epa numbers come in lower as they were achieved with a controlled charge of douglas fir dried to a certain percentage,(i think 20%) this load is not comparable with an average 'real world" load of seasoned wood.

i hate those danged epa tags , they can be misleading as far as btu outputs are concerned.

 

[titan]

joephu,did your audit specify at what outdoor temp you would require 40000 btu's/hr to heat the house?

 

[Corie]

The EPA testing procedure involves a very specific fuel type, loading pattern and burn type. Often times the BTU output of a stove during EPA certification testing do not even come near what the stoves true potential would be with seasoned hardwood.

 

[Hogwildz]

The EPA testing procedure involves a very specific fuel type, loading pattern and burn type. Often times the BTU output of a stove during EPA certification testing do not even come near what the stoves true potential would be with seasoned hardwood.

Bay window pellet stove? Small wood/coal burner??? WTF
How bout the 5+CF OMster burner? Damnit man

 

[Hogwildz]

At that square footage, your going to want a large, don't say I didn't warn you.

 

[begreen]

The F600 is a large stove Hog. Sounds like an ok fit to me. If joephu is ok with a catalytic stove, the VC Defiant should be good here too. Or perhaps consider a big soapstone stove like the Hearthstone Mansfield.

 

[joephu]

Well, that's good to know the EPA ratings are ultra conservative. So with a EPA max of 32,500, I would probably get a higher real-world output with seasoned oak.

Titan, "Manual J" is the HVAC industry standard for sizing heating equipment and considered the only accurate way to do this. It takes into account house construction, insulation, air infiltration, sq/ft, etc. The 40,000 BTU is the amount of heat my house would require on the coldest recorded day in my town. For most heating days, the heating load would be much less.

This leads me to manufacture sizing recommendations. It looks like Jotul states this stove is sized for homes up to 2,500 sq/ft. My heating load is about 13 BTUs per SqFt (40,000 BTUs /3160 SqFt). So what do manufactures use as the "average" BTUs per SqFt? I'm thinking they use roughly 33 BTUs SqFt, that is take the Max BTU (81,500 in this case) divided by 33 BTU, which is about 2,500 SqFt in the case of the Jotul.

Now I’m not saying stove salespeople don’t know what they are talking about, but I guess I don’t want to buy more stove than I need. Can you still maintain ideal burning efficiency with a partially loaded firebox? If that’s the case, maybe I’m over analyzing this and should just go bigger. Dunno?

 

[babalu87]

Take a look at the Morso 3610 as well. Heat loss data aside your going to need a big ass stove because that is a big assed house.

 

[elkimmeg]

The J manual has 3 possible BTU ratings given the insulation home and window glazing I use 40 btu per ft doing inspections and a rule of thumb 13 is extremely low for my area of New England

Let me say this the manufactures over state the BTU outputs there are x btus that are in the wood that loads in your fire box that is the heat that one produces. to get 80000 buts that firebox need a conveyor attached to your wood pile to constantly load it and air being fully open. Realistically you might get 2/3 of that running you stove at optimum cordwood real world conditions.

EPA now assigns the default efficiency numbers 63% the 71 is a non certified number the manufactures post. To size a stove the only really factors are the BTUs in the wood the fire box can hold, the larger the fire box the more BTUS and assume the differences in efficiency is all about the same . My first question would be what size it the Jotul fire box from there we can determine if it will do the job

I think you have to have realistic expectations stoves are area heaters not whole house central heating systems. The far rooms will always be colder the further away from the heat source

next is at what point are you really hoping to accomplish carrying the heat load down to 10 degrees then some assistance from you central system Or are you trying to heat for those days under zero. Most homes are designed to be 70 degrees when it is zero outside

next consideration do you have 5 or 6 cords of dry seasoned wood ready for prime time. to push that stove you will need that much wood to heat that area

 

[begreen]

Stove ratings are similar to stereo amplifier ratings. Some companies are very conservative and only publish the wattage that produces a clean undistorted sound. Others, typically the cheaper amps, publish the maximum theoretical output, regardless of how distorted it would sound at that power level. The max can be a useless spec, yet you see it all the time, especially now with home theater amps and receivers.

Stove manufacturers are the same way, but unfortunately price is not a guide for accurate BTU output ratings. But it's safe to say that one should pretty much ignore the maximum btu rating. There is no way that one will be able to or want to run a stove at maximum 24/7. I usually divide the max by half for average btu output because the stove is warming up and cooling down between peak burning.

PS: Elk is right on about needing actual firebox sizes. IMHO, Jotul is really lacking in this regard and is responsible in large part for people purchasing undersized versions of their stoves. There is such a large varying degree of expertise in dealers that it is folly to expect them to get the size right all the time. With actual firebox cu ftg specifications, the guesswork is removed. Elk, if I get a chance, I will try to visit a large Jotul dealer this winter with a tape measure and will record the cu ftg for the site.

 

[elkimmeg]

I take flak for mentioning a certain manufacturer but here what I find interesting manufacturer x only publishes 55000 buts for a 3,45 cu ft fire box and manufacturer y published 97000 in their 3.0 cuft fire box to the un informed the 97000 but claim is a no brainer it produces more heat ,but the 3.45 will out produce hands down. but it is listed for 55000 btus which reflects a more realistic approach to real world conditions. No wonder this confuses the new buyers?

 

[Gunner]

I remember it being posted here that the same stove will produce 100x more heat at 600 deg than at 500deg. I have no idea if that is true or not....

#1 factor IMO on BTU's/hr is how hot you are willing to run the stove. Mfg numbers are irrelevant, the hotter stove wins.
#2 firebox size/ surface area of the stove
#3 how much air you can pass by the stove-fans,blowers,convection.

The f600 will probably carry most of the heating load most of the time, hard for ANY 1 stove to heat 3200sqft on the coldest of days.

 

[begreen]

This is a place where the stove industry needs improvement. It might even help sales. I know I don't recommend Regency as frequently because there is so little real information about the stoves published. They are nice stoves, but their website sucks. It's more about designing a look than making heat.

On and off Craig and I talk about a new stove ratings system. It would be great to have a form where things like build quality, specification accuracy, durability, ease of use, efficiency, customer service, etc. are rated on a 1-5 star basis. If it was done right, it could become the industry standard that stove makers strive to achieve. I can see it in the advertising now -"The new Englander 23NCL just achieved a perfect 5 star rating at hearth.com."

 

[joephu]

Ahh, now things are starting to click. So based on your replay (Thanks Elk & BG, good stuff), I could expect the F600 to produce roughly 50-67% of stated BTUs; roughly 40,000-54,000 BTUs. My 13 BTU/Sq is lower than my neighbors because I paid more upfront to build (insulation, window glazing, and air sealing). My air infiltration was below federal minimums that I installed an fresh air exchanger (HRV). Regardless, I know heating the whole house is not practical with this type of wood stove. My main level is very open, so I only see a problem with the bedroom over the garage. I can live with that since it is on it’s own heating zone.

I agree standards are lacking. With your stereo analogy, I can at least compare amplifiers using RMS watts versus Max Watts. Stoves should have an RMS equivalent, darn it!

I didn’t realize EPA used the same % for similar type stoves (cat models vs non-cat models). So it sounds like it really comes down to firebox size - you need to load x-pounds of wood to get x-BTUs. I’m starting to lean towards the “oversizing” route.

So, I still have a question I was hoping someone could answer. If you loaded a firebox half full, 1) will heat output be roughly half of a full load? 2) can the stove still burn just as cleanly (extract max energy for the given load) as a full load?
You guys are great.

 

[begreen]

Grey areas abound with stoves and stereos. RMS watts help, but at what THD? For some 1% is considered acceptable, for others that would be awful.

Same for stoves. To answer the last question, almost, it depends. That is, as long as the partial loads are large enough to take the stove up to temperature to fully heat the stove and where there is good secondary combustion. During fall and spring, sometimes efficiency and clean burning get compromised due to smaller fires. A catalytic stove or a soapstone stove are a bit better at this.

 

[G-rott]

Joephu,

Just remember, the btu's per hour are on that theroretical coldest day of the year as you stated. While a heating system sized by these numbers will work all year long don't assume a woodstove will be the same. A heating system cycles on and off from 0 btu output to 40,000 btu in this case. Your wood stove cycles along based on how you fire it but while running it never shuts off the heat output say 10000-25000 per hour. So smaller stove will still introduce a lot of heat into your home. Oversizing a stove works, you can build a smaller fire but your effency seems to suffer, becaue of the "hotter the stove" argument.

I would look into the large stoves if your going to be able to move the clod are out of the upstairs areas and heat the whole house, If you only end up heating the main floor (open) area the stove could be much smaller.


Good luck

Garett

 

[begreen]

Good point. Draft can be poorer on warmer days too.

How large a space will the stove really be expected to heat?

 

[joephu]

Joephu,

I would look into the large stoves if your going to be able to move the clod are out of the upstairs areas and heat the whole house, If you only end up heating the main floor (open) area the stove could be much smaller.


Good luck

Garett

Are you saying go bigger if I can get the cold air out of the upstairs areas? If so, I see your point otherwise no room will be comfortable (either too hot or too cold).

BG, I guess I'm still deciding. Ideally, I would like to heat as much of the house as possible but the 1st floor should be warm and cozy. If I need to walk around in boxers, sans shirt, just to get some heat upstairs, that would defeat the purpose. My house is tight, so heat will not readily migrate to the second floor without mechanical assitance.

 

[begreen]

How large - in sq ftg - a space do you expect to heat? Maybe it would be better if you posted a simple diagram of the house layout. I am having problems visualizing this.

 

[joephu]

It's called a shingle style home, basically like a 2-story colonial, 3160 SqFt total. Cut it up into 4 quadrants:

Main Floor / 2nd Floor Layout

Front Left - DR / Bedroom 1
Front Right - LR / Bedroom 2 & Small corner office
Rear Left is Kitchen / Bedroom 3 (Bath between Bed 1 & 3)
Rear Right Family Rm / Master Bath & Walkin Closet
To the right of the house - attached 2-car garage / Master Bed

The fireplace is in the center of the house facing the Family Room. This main level is very open, about 1380 SqFt
The second floor is basically a main hallway with the rooms off of it. All rooms are closed off completly with doors closed. The second level is about 1380 too, plus add about 400sq feet for the master bed above garage.

 

[Gooserider]

A few random thoughts...

1. How much of the heat load do you want the stove to carry? You can get a monster size that might be able to do 100% of your heat on the coldest days of the season, but that might overheat you on warmer days. A smaller stove might need some supplemental help from your regular system on really cold days, but be more comfortable during the "shoulder season" Some folks suggest the optimal size is one that will carry you for 75-80% of the time, others will go larger, it's a tradeoff.

2. One of the things that I haven't really seen mentioned that a large stove will get you is longer burn times when run at less than full output. There are limits on this, as you can only turn a stove down so far, so "low" on a big stove might well be hotter than "medium" on a little one, but in general the larger the box, the longer the burn.

3. If you look for posts where people complain about the size of their stoves, it's about 90% - 10% ratio of people wishing they had gotten the bigger stove.

4. I don't think it works well for people that only burn intermittently, but soapstone is great for those wanting to burn 24/7, as it gives a more uniform heat output.

5. It is easy to put a small fire in a big box, you can't but a big fire in a little box.

Gooserider

 

[babalu87]

My house is tight, so heat will not readily migrate to the second floor without mechanical assitance.

The heat WILL get upstairs with no help from you, heat rises. What you may need to try and figure out is how to get some of the cooler air into the room where the stove is. Can you sketch up a floor plan?

 

[BrotherBart]

My house is tight, so heat will not readily migrate to the second floor without mechanical assitance.

The heat WILL get upstairs with no help from you, heat rises. What you may need to try and figure out is how to get some of the cooler air into the room where the stove is. Can you sketch up a floor plan?

Yeah. I went through the fans, blocking downstairs room doors and all of the other crap for a few years. Then I figured out that when the rooms down stairs get warm it rises right up not only through stairwekk but through ceilings. The joint has been evenly warm ever since.

Nature. Ain't it a groan?

 

[Hogwildz]

Its great to research before purchasing, but trying to compute using heat loss calculations & numbers for a conventional heating system is not the best way to go about it.
Conventional heat comes on and goes off with a thermostat and puts out heat upon demand as the thermostats call for it, cycling. Wood burning depends on the wood, how dry it is, how the user burns it etc. Its about getting a stove to temp, then heating at that temp for a period of time, then it slowly drops in heat output. Applying a manual J geared towards conventional heating methods for wood burning purposes is just not going to work the way it does for conventional heating.