Sizing a stove for an energy efficient home

[ERTupper]

I'm building a home in Central Maine (climate zone 6) and have been struggling to land on the right size stove to heat the place. Here's some details:

• Our walls are insulated to R-30 with an R-5 thermal break with Rockwool batts and foam boards
• Our ceilings are insulated to R-60+ with cellulose
• The home is being built with a lot of attention to airtightness. I don't have a blower door test yet, but the sheathing is sealed to the foundation, the mudsill is sealed to the foundation with caulk and sill seal, ZIP sheathing with taped seams, all penetrations are sealed, our electrical boxes all have flanges that the drywall will be sealed to, etc. We're adding balanced mechanical ventilation throughout the home which can be temporarily set to just bring in supply air in case we need more combustion air.
• The house is just a basic box with a 6:12 roof and a 28'x 36' footprint. On the first floor, the kitchen, living room, and dining room are all open to each other (about a 14' x 36' space). The entryway on the first floor is open to the second story to allow an easier path for heat to move up. We have about 1,800 sq ft of floor space.
• The long end of the house is situated due solar south. On the summer solstice, sunbeams run the length of the living/eating/dining room. We get a fair amount of solar gain given the placement and our slab floor
• The stove will be located in just about the dead center of the house on the first floor in the living/eating/cooking area.
• I calculated our heat loss at -20 F (we very rarely get days colder than this, but most winters we get at least a few days that get this cold) to be about 46,000 BTUs/hr -- I'm not an HVAC specialist or anything, so it's very possible I may be off on this estimate, but I took into account the projected airtightness, the real u-factor of the walls and ceilings, the projected solar gain at our latitude, the size of each room, the u-factors, solar heat gain coefficients, and square footage of our windows, and the heat from our bodies. Even if I'm missing something, I feel like this is a lot closer to accurate than any rule-of-thumb estimate that the stove shop would use to size our stove.
• We will have a heat pump as an additional heat source, but given our occasional extremely low temps and the fact that power outages happen multiple times every winter, sometimes for several days, it can't be our primary heat source.

I went to the local stove shop which supplies Jotul, Hearthstone, Rais, and Lopi stoves. The people I've worked with there seem knowledgeable and attentive. The shop recommended we get a Lopi Evergreen (non-cat) which has a heat output of between around 12,000-70,000 BTUs. The shop workers recommended against getting a catalytic stove, and this was the only non-cat stove they had that also qualified for a tax credit. Here are my questions:

• I seriously doubt I would ever burn this stove as hot as it gets. Should I be concerned about creosote build-up from underfiring? The shop workers have said that because the stove is so efficient, that's not something I should worry about.
• I've also been considering a hybrid stove from Lopi with a lower max output (the Rockport), but I've never operated a stove with a catalyst (I grew up in a wood-heated home). Are hybrids less finicky than straight-up cat stoves? Am I right to be concerned about the longevity, maintenance, and use difficulty of cat/hybrid stoves?

 

[begreen]

Considering that a properly-sized, high-efficiency heat pump will easily cover most if not all of the heating above 20ºF, the advantages of a low and slow-burning cat stove probably are not relevant in this case unless the goal is to use the heat pump only on rare occasions.

The Evergreen would work ok, but for this cold climate my preference would be for a bit more reserve capacity for the extreme weather and power outages. I prefer a N/S loading stove so that one gets to use the full firebox capacity without concern of wood rolling up against the stove door glass. And if a cat stove is desired, then Blaze King makes heaters that will do the job. In that case, I would select either the Princess or one of their 3 cu ft models.

I strongly recommend the heat pump system have strip heaters for the very cold weather. If the HP has strip heaters for backup, then using that combo together with the Evergreen stove will keep up with the worst cold weather.

FWIW, I would not let the tax credit be the final determining factor. Get the stove that is best for the job.

PS: What is the heat pump make and model going in?

 

[DickRussell]

I suspect your heat load calculation is far too high. By comparison, my house also is highly insulated ( double-wall R-40, R-60 in attic, slab and foundation insulation, triple pane windows, very tight). My house in NH is two levels on a 2000 sqft footprint, although the lower level is walkout in front but into the hill in back. So conditioned space is about double yours. You are right about "rules of thumb" - don't do that. I did my own highly detailed heat loss spreadsheet and, for an outside temperature of -3 F, I got 22,000 BTU/hr. From the best but limited set of data I've been able to collect, the actual loss is more like 19,000 BTU/hr. This fits well with the numbers on the GSHP used for heating. That is a two-ton unit with second stage output of 25,000 BTU/hr, yet even when it's well below zero the system keeps the house at 70 F inside in just first stage.

In the basement level living room I installed a Quadrafire Millenium 2100 woodstove. There are two reasons I picked that one. The first is because its heat output, according to the tag on it, was 11-22 KBTu/hr. The other is because it has a connection in back for an outside ducted air supply. Air tightness for a woodstove is not perfect, but this was better than having the system open to the outside after the fire burnt out. As for performance, we actually used just the stove, fired up from mid-afternoon to perhaps 11pm, to heat the entire house during the 2010-11 winter, before the heat pump was up and running.

I hope this data helps you. My opinion is that you want a small woodstove; a large one could cook you out of the place if overfired. We use ours mainly in the evenings from say November to April. I don't build huge fires in it, but I do run it hot enough so that the glass is kept clear. If you see the glass get fouled with creosote, then the stove is not running hot enough. As an aside, if you have a lot of solar gain from south-facing windows, you may want some way to shade some of that. Highly insulated houses can suffer from too much solar gain.

 

[ERTupper]

I suspect your heat load calculation is far too high. By comparison, my house also is highly insulated ( double-wall R-40, R-60 in attic, slab and foundation insulation, triple pane windows, very tight). My house in NH is two levels on a 2000 sqft footprint, although the lower level is walkout in front but into the hill in back. So conditioned space is about double yours. You are right about "rules of thumb" - don't do that. I did my own highly detailed heat loss spreadsheet and, for an outside temperature of -3 F, I got 22,000 BTU/hr. From the best but limited set of data I've been able to collect, the actual loss is more like 19,000 BTU/hr. This fits well with the numbers on the GSHP used for heating. That is a two-ton unit with second stage output of 25,000 BTU/hr, yet even when it's well below zero the system keeps the house at 70 F inside in just first stage.

In the basement level living room I installed a Quadrafire Millenium 2100 woodstove. There are two reasons I picked that one. The first is because its heat output, according to the tag on it, was 11-22 KBTu/hr. The other is because it has a connection in back for an outside ducted air supply. Air tightness for a woodstove is not perfect, but this was better than having the system open to the outside after the fire burnt out. As for performance, we actually used just the stove, fired up from mid-afternoon to perhaps 11pm, to heat the entire house during the 2010-11 winter, before the heat pump was up and running.

I hope this data helps you. My opinion is that you want a small woodstove; a large one could cook you out of the place if overfired. We use ours mainly in the evenings from say November to April. I don't build huge fires in it, but I do run it hot enough so that the glass is kept clear. If you see the glass get fouled with creosote, then the stove is not running hot enough. As an aside, if you have a lot of solar gain from south-facing windows, you may want some way to shade some of that. Highly insulated houses can suffer from too much solar gain.

It's super helpful to hear from someone with the same kind of home. For 0 F, I got a heat loss of 19,000 BTU/hr, so it sounds like we have very similar heating needs.

I don't think there are any vendors local to me for your stove, but it's really useful knowing what heat output worked for you. Do you think you would have been comfortable at -20 F or so using your stove as your only heat source?

Thanks for the tip about the windows. We're planning on having shades to keep out some of the summer sun. Most of the year the window overhangs are enough, but a hot day in September can overheat it a bit.

 

[mcdougy]

We have a house insulated to similar levels. Our HLC came to 30,000 btus due to sq footage and over glazing. We installed a PE FP30 on main floor of a ranch home) It heats the house entirely (basement included) with the remote duct running to the basement. I wouldn't be able to do that with any smaller of a firebox. In shoulder season we have a small fire once a day. In the dead of winter it runs 24/7.
I would recommend going bigger than smaller with a firebox. Too small of a stove is just that, too small. You can put in less wood in a big stove to maintain desired temps.
Seriously consider a cat stove that can have low output for long durations if your only wanting to supplement your HP. If you want to heat only with a woodstove, consider a non cat.
My .02 cents.
Congratulations on your new home.

 

[EbS-P]

Look up your actual design temp. A small fire can be lit in a big stove. Given your tight house you need to consider stove location so you can install an outside air kit (OAK). Basically the stove needs to be on an exterior wall. Yeah it’s another penetration but I think it’s the best.

Will you have an ERV, HRV or ventilating dehumidifier? Balancing air flow in a tight home with a wood stove is important and an OAK makes balancing less critical for stove opperation but you still don’t want smoke rolling out every time you open the door.

Don’t go smaller than 2.4 cu ft. You don’t want to have to load the stove every four hours when it gets really cold.

 

[kborndale]

We have a house insulated to similar levels. Our HLC came to 30,000 btus due to sq footage and over glazing. We installed a PE FP30 on main floor of a ranch home) It heats the house entirely (basement included) with the remote duct running to the basement. I wouldn't be able to do that with any smaller of a firebox. In shoulder season we have a small fire once a day. In the dead of winter it runs 24/7.
I would recommend going bigger than smaller with a firebox. Too small of a stove is just that, too small. You can put in less wood in a big stove to maintain desired temps.
Seriously consider a cat stove that can have low output for long durations if your only wanting to supplement your HP. If you want to heat only with a woodstove, consider a non cat.
My .02 cents.
Congratulations on your new home.

What's wrong with a cat stove if you want to heat only with the stove? A cat stove can go low and slow but can also burn hot and fast.

 

[joop]

in my opinion medium cat,with that kind of insulation i don't think you'll be using it much on full fire.make sure it has an oak attatchment

 

[Ashful]

What's wrong with a cat stove if you want to heat only with the stove? A cat stove can go low and slow but can also burn hot and fast.

I'm not mcdougy, but suspect he was referring to the notion that those burning hotter and faster aren't going to reap one of the primary benefits of a cat stove.

But with a well-insulated home, it's likely the OP will have plenty of days where low and slow fits the bill. Someone should be knocking on @Poindexter's door, with any thread like this.

 

[EbS-P]

I think cat Vs non cat depends how much wood you want to burn.
Running a cold climate heat pump in a tight home down to say 20 degrees would take care of a big chunk of your annual heating. It may cost more $$$ but you have to figure cost of firewood if you are buying it and value of your time if harvesting yourself.

Personally I don’t see any benefits of a cat stove in a new tight home. I do cold starts probably 1/2-2/3 of the days I burn. (Stack it, light it, set 10 minute timer, adjust air, 10 more minutes set air. ) so I do see the advantage of slow and slow but the extra wood consumption just wouldn’t be worth it to me.

 

[Ashful]

Good points, EbS-P. But I'm also not sure these finer points on experience gained in North Carolina is that relevant to someone heating a home in Maine! Both homes need heat, but let's acknowledge a pretty austere difference in climates.

 

[begreen]

A well-insulated place in Maine will still see a lot of 20-40º winter weather. A properly sized, high-efficiency heat pump in a well-insulated house will cover those temps easily. The difference that good insulation and glazing make is really amazing.

 

[EbS-P]

Good points, EbS-P. But I'm also not sure these finer points on experience gained in North Carolina is that relevant to someone heating a home in Maine! Both homes need heat, but let's acknowledge a pretty austere difference in climates.

Fair… but my cross over temp is 40 degrees and I have r14 in my attic. The principal is the same. Run the heatpump when it makes cent$ to do so and burn wood once the temps drop below what ever value threshold you have set for one’s self. If the house really does well with solar gain it could easily not need any heat for several hours a day. Anyone turned thier BK off at 10 am and back on at 4pm? But you can light a fire at 6 am and another at 4 or 6 pm. Or just not bother while t stat keeps it a perfect 72 degrees.

I lived in Maine and woke up one morning to -30 air temp. oil burners were not shutting down that night.

There is no reason a new 1800 square foot house that is build tight can’t be heated well with a heat pump down to design temp. It will be expensive. But cheaper than oil and electric baseboards. So it comes down to money. To be honest a top end stove and chimney professionally installed will cost $6k that’s a lot of electrons. And were not even figuring in wood cost. My point is what’s everyone places value differently. And what one values should figure in to how much you want to run a stove, and how much you run a stove figures into the cat vs non cat decision.

 

[begreen]

My sister's house in mid-state NY is super-insulated. It uses silly little heat. She just uses the pizza oven most of the time. The house holds the heat really well. The average winter wood consumption is around 2 cords for about 2300 sq ft. There's a wood fired boiler for the really cold weather, but it's only fired up for a week or two usually.

 

[Poindexter]

So much win in this thread! You guys are killing it without my help just fine.

1. OP is taking about almost exactly the forever home Mrs. P and I want to build in PGH zone 8. We are planning wheelchair accessible ground floor ~1000 sqft, with 2 beds and 1 bath upstairs, cause writing comps on 3/2 homes is easy and not very risky for the appraiser.

2. With a centrally located woodstove on the ground floor of a 2 story home, there is going to be plenty of flue height to run a cat stove low and slow.

3. My next/last home is going to have some kind of ERV/HRV system, but our wood stove is going to have a standalone air circuit, with an Outside Air IntaKe/ Kit (OAK) feeding outdoor air directly to the firebox, and an ordinary chimney carrying combustion products up and out. There is no good reason to feed the fire with conditioned indoor air, the more heat you need to make, the more obvious this becomes.

I am hoping for a basement, or at least a partial basement, in our last home to store canned goods, and am willing to put a penetration somewhere in the foundation or rim joist to let a OAK through to feed the woodstove in the middle of the floor plan. I hadn't previously considered under slab, but I would very likely do that if the lot topography required it.

4. The Pretty Good House movement actually started in Portland, ME. Maybe contact someone in the PGH world now so they are expecting OP's call when the air door test is complete. There is a comprehensive chapter in the PGH book I have about calculating heat loss, but it really looks like voodoo to me. It is one thing to say, as an engineer or etcetera, this is what the house will do if no one is living in it. I believe those numbers. But as soon as you move people in there, especially if there are any kids, just forget that first number except to do multiplication on it to find out what your actual BTU usage is going to be.

5. Another advantage to centrally located stove in snow country, I like having the chimney penetration of the roof as close to the ridge pole as possible. I am not going to cut away part of the ridge pole for structural reasons of course, but I want the smallest possible glacier trying to shove my chimney off the roof every spring.

6. One thing the OP and similar should look at is the relative cost of energy. Imagine I can buy a laundry basket full of cord wood for $1. In Fairbanks, I can buy the same number of BTUs as boiler oil for $2-3. In Fairbanks (land of the unicorn electricity) the same number of BTUs as electricity is $10. So for me, a heat pump running in the goldilocks zone at 300% efficiency, if I am running it from the grid, is still more expensive BTU for BTU than oil, and at least double the price of cordwood.

Someone two blocks away from a large dam on a big river may be able to buy electricity (BTU for BTU) cheaper than they can buy cordwood.

One possibility here is on site solar generation. This will be especially helpful in the summer months to run (potentially off grid) heat pumps to make AC, but will have very little impact on heating in the short days of winter.

7. Heat pump technology is moving fast lately. The one thing I will insist on is whatever heat pump(s) I choose are going to have live outdoors year round. My willingness to climb up on a ladder every fall to take down the minisplit outdoor sections and store them in the garage over the winter is zero point zero zero, and carrying them back up a ladder every spring to reattach the piping is a very negative number. If I was building today I would choose something that is well proven but maybe a little bit behind the leading edge, accepting that my next heat pump(s) will very likely be more efficient than what I buy today. I am willing to give up a few points of performance for low maintenance and durability.

8. I am very much in agreement with begreen 's first post in this thread that a Blaze King Princess or a Blaze King 30 box, Ashford 30, Scirrocco 30 or etc, with air door test pending and an unknown number of kids in the house, is probably a good choice - esp given frequent and prolonged power outages with onsite solar capacity in cold weather likely to be negligible.

Once the kids are up and out and OP has an airdoor test I might suggest a smaller stove, but I cannot imagine needing a stove bigger than those. There may be other similar stoves from other manufacturers that could also fit the need.

I would also consider having 2 types of wood onsite. I would have a great deal of spruce-pine-fir (SPF) for really cold weather, but down in climate zone 6 in a really tight home I would want to have a fair bit of fuel with long coaling stage for when I want the stove to just be ticking over ever so gently. Oak, Beech, Walnut, Hickory, hard maple, all of these can provide extremely long burn times at low output with their long coaling stages. Maybe some birch. But when the power is out and the heatpumps are offline, I would be prepared to be ripping through SPF to keep my wife wearing short pants around the house.

9. As far as cat v- non cat stove, the last non cat stove I owned was built in 1999. I replaced it with a cat equipped Blaze King Ashford 30 in May 2014. I will, please Jesus, never again have to ride the non-cat temperature swing 'rollercoaster.' I am a firm believer in catalytic woodstove technology. I can, for most of the winter, set my stove at the heat output I want for whatever weather I have and come back in 12-24 hours to reload the stove. Non-cat secondary burn technology has come a long way since 1999.

10. I think the main things here for the OP to get done is to get return pings from someone in the PGH world, and get back to us once he has given airdoor results to someone deep in PGH and has a new updated BTU/hr number. We will also need to know how many kids he has at home that have never paid their own utility bills to offer better resolution on stove sizing.

 

[Poindexter]

the notion that those burning hotter and faster aren't going to reap one of the primary benefits of a cat stove.

I have zero data as far as CO2 concentration in my flue gas or PM2.5 ejected.

I have a data confound as the last of my long haired daughters moved out (with two laundry baskets of hair product) the same summer my catalytic stove moved in.

With a (2014) catalytic stove I am burning 10-40% less wood (best guess ~30% less) than I was with a (1999) non cat. I am taking less motrin. I have more free time to sit in front of the wood stove. I am storing less wood on the property.

Whether or not I am getting full benefit, I have zero doubt the combustor in my stove, at wide open throttle, is converting some smoke to BTUs for the house and some CO2 instead of unburnt VOCs for the chimney.

Catalytic stoves kick butt.

 

[Todd]

I burned more wood with my Blaze King! The temptation for a 24 hour burn schedule was too strong during the shoulder seasons when I probably could have gotten away with a smaller load or just run the furnace for a couple hours a day. 😂