Wood stove usage and sizing for highly insulated, airtight home

[Whirled Peas]

Hi all,

I live in Southern Vermont and am halfway down the track with pricing out a new home construction. Specifically, I'm looking to potentially purchase this home shell and have it finished by a local builder: https://unityhomes.com/home-plans/varm-eda/

The home is ~2600 sf, R-60 roof, R-35 walls, slab on grade, triple pane tilt turn windows and typically results in an ACH50 of ~1. Air source heat pumps as primary heating. The stove would be a corner installation in the living room with an interior chimney chase up the corner. The manufacturer generally tries to get people to go all electric and forgo wood heat as a backup, but I live on acreage and have access to cut/collect/produce however much hardwood I want for the cost of my time (which I enjoy!). Outages are a thing here. Approximately 1/3 of owners install woodstoves anyways. I'm aware of OAKs and the challenges of having a wood stove competing with the ERV and other venting appliances and believe I can handle that. I'm sorry that I don't have energy modeling to share yet.

I currently live in a drafty 1790s cape and am being well served by a Woodstock Absolute Steel and previously ran a Jotul F400 for years, so I'm familiar with hybrids as well as secondary stoves.

My question is -- what would your strategy be for choosing a stove for this particular situation? Large cat stove that I run on low all the time (BK Ashford 30?)? Small secondary burn stove (PE Alderlea T4?) that I only burn when needed? Something in the middle (WS Keystone/Palladian?)?

I'm worried that even a medium secondary burner would overheat the space and I'd be constantly running tiny loads. Alternately, with a larger cat stove, I'm afraid the thermal mass will just be too much.


Thanks!

edit: adding floorplan to original post

 

[EbS-P]

How big is the stove room and how is connected to the rest of the house?

It will need an outside air kit.

 

[barnaclebob]

Your biggest issue may be heating the other rooms. You are right to be concerned that you'll be baked out of the living room if you try to size the stove for the whole house but cant move air around. A sketch of the layout off the house would help. Single floor? Its best to try to send cold air to the stove and not warm air to the rooms.

Given how well your house is insulated its likely you'll want a small cat stove that you can run hard if needed.

 

[Whirled Peas]

How big is the stove room and how is connected to the rest of the house?

It will need an outside air kit.

There is a floorplan on the link I included. It's an open floor plan kitchen/living/dining/stairs/mudroom that is ~1,300 sf plus a ~330 sf bedroom suite through a doorway and ~1000 sf upstairs.

Definitely planning for outside air.

 

[Whirled Peas]

 

[Whirled Peas]

 

[barnaclebob]

Ah, i see now, thats actually a really good layout for providing heat to the whole house. With the stove in the far corner you may need to help get some movement of cool air down the stairs.

My house isn't too different and when you open a door on the second floor thats been closed awhile you can feel the cool air flowing down the stairs.

I don't think a big stove with good heat control would bake you out too much. Bedroom 1 upstairs would probably be the coldest place in the house but that wouldn't bother me since I like a cold bedroom at night.

 

[Whirled Peas]

Ah, i see now, thats actually a really good layout for providing heat to the whole house. With the stove in the far corner you may need to help get some movement of cool air down the stairs.

My house isn't too different and when you open a door on the second floor thats been closed awhile you can feel the cool air flowing down the stairs.

I don't think a big stove with good heat control would bake you out too much.

Thanks. Looks like our posts crossed paths in time. My proposed layout is a little different and I've now added the floorplans. Definitely more of a bottleneck through the kitchen to get heat upstairs.

 

[EbS-P]

Good first floor layout. How many tons of hvac are they proposing?

I have heat pump here down south. I run it until the temps drop below 35. At that point I can run my F400 on low. It goes out at night and I re light in the morning. I could see a BK working quite well with the exception that sometimes I just want a hot short fire. Take the morning chill off. A fire and drink in the evening. Secondary stoves are good at that.

Decided if you want sized for normal winter time use where the heatpump can/will run some, or if you want it sized to heat the whole house.

A pacific energy T4 might be an option.

Has anyone discussed fresh air ventilation / ventilating dehumidifier with you? AHC 1 is tight!

 

[Whirled Peas]

I'm not sure on the mechanical specs yet. I've engaged this first layer of design and once I get my septic plan and rough GC cost estimates back, they'll convert this to actual construction drawings and do the mechanical equipment sizing.

I think you're right on with the 'take the morning chill off' vs 'heat the whole house' goals. Honestly, this home is so different from the one that I live in now that I'm not really sure what my ultimate preference would be. I do have some concerns about power outages and staying warm, but perhaps even an undersized 'take the chill off' stove would keep it acceptable during winter outages.

I will have an HRV system that is ducted throughout the house to provide balanced make up air.

 

[begreen]

It looks like the stove will primarily heat the open space of the first floor. The setup is not ideal for convection to the second floor, but not terrible either. Some heat will get up there. What size is the heat pump? How much of the time do you want to heat with wood? Will the wood be purchased?

If you like having a fire and will burn most of the time then a BK stove makes good sense. If instead it's nights and evenings when it's very chilly, then the Alderlea T4 makes sense. The choice is in large part a matter of lifestyle.

 

[EbS-P]

will have an HRV system that is ducted throughout the house to provide balanced make up air

You should think how much AC you will use and how often the windows will be open. How often the house will be closed when HRV is running and it’s more humid outside than in.

Your AC will not run enough to remove the moisture load of the moist fresh air in the summer. Make sure your HVAC contractor has experience with homes that tight with that much insulation.

 

[Whirled Peas]

It looks like the stove will primarily heat the open space of the first floor. The setup is not ideal for convection to the second floor, but not terrible either. Some heat will get up there. What size is the heat pump? How much of the time do you want to heat with wood? Will the wood be purchased?

If you like having a fire and will burn most of the time then a BK stove makes good sense. If instead it's nights and evenings when it's very chilly, then the Alderlea T4 makes sense. The choice is in large part a matter of lifestyle.

Thanks @begreen .
- not sure sure on the heat pump size yet
- How much time do you want to heat with wood? I think this is my actual decision, which I realize you all can't help me with. I've been a 24/7 burner for ~5 months/year for a long time now out of necessity, so it's hard to imagine not needing to run one. But maybe I need to imagine it.
- I'll continue to produce my own wood. I burn ~3 cords/year and have ~10 stacked up.

What about a smaller 20.2 BK stove? Is there some 'best of each' combination out there that's possible for low btu needs? I see the Ashford 20 has a max of ~33k btu and I'm anticipating the whole house demand will be less than that, so I'd have the top end covered.

 

[Whirled Peas]

You should think how much AC you will use and how often the windows will be open. How often the house will be closed when HRV is running and it’s more humid outside than in.

Your AC will not run enough to remove the moisture load of the moist fresh air in the summer. Make sure your HVAC contractor has experience with homes that tight with that much insulation.

Thanks for the heads up. I'll add it to my list of questions for the builder. Honestly, one of the attraction of the Unity Homes product is that they've got dozens (hundreds?) of them out there that are built this exact same way, so I think they've figured out the mechanical equipment. In fact, they spec the actual pieces of equipment and provide that to my builder (although I suppose I could ignore their suggestion, like I might with the woodstove!).

 

[begreen]

Thanks @begreen .
- not sure sure on the heat pump size yet
- How much time do you want to heat with wood? I think this is my actual decision, which I realize you all can't help me with. I've been a 24/7 burner for ~5 months/year for a long time now out of necessity, so it's hard to imagine not needing to run one. But maybe I need to imagine it.
- I'll continue to produce my own wood. I burn ~3 cords/year and have ~10 stacked up.

What about a smaller 20.2 BK stove? Is there some 'best of each' combination out there that's possible for low btu needs? I see the Ashford 20 has a max of ~33k btu and I'm anticipating the whole house demand will be less than that, so I'd have the top end covered.

If the goal is heating 24/7 with wood, the larger Ashford 30 will provide longer burns if that is relevant. It has a larger fuel tank. You may find that wood consumption drops to 2 cords or less a year depending on the efficiency of the heat pump and how much it is used. On the flip side, as you age you may find yourselves liking the house a few degrees hotter.

 

[Whirled Peas]

 

[Whirled Peas]

Hey @begreen @EbS-P . I'm back with my Manual J calculations (see post immediately above). Design demand is 23,790 btu for the whole house which includes 11,106 for the primary living space on the first floor and another 4,709 for the adjacent bedroom.

Fresh is exchange is via an Lifebreath 267Max HRV. The outdoor heat pump is a single, 30k btu 3-zone Mitsubishi. One heat pump in the living/dining area, one in the downstairs bedroom and a third ducted unit (located in the mechanical room) that serves the three upstairs bedrooms.

For either of you, would this change your above recommendations?

It's a tight house! Looks like my cooling load is greater than my heating load.

 

[EbS-P]

Hey @begreen @EbS-P . I'm back with my Manual J calculations (see post immediately above). Design demand is 23,790 btu for the whole house which includes 11,106 for the primary living space on the first floor and another 4,709 for the adjacent bedroom.

Fresh is exchange is via an Lifebreath 267Max HRV. The outdoor heat pump is a single, 30k btu 3-zone Mitsubishi. One heat pump in the living/dining area, one in the downstairs bedroom and a third ducted unit (located in the mechanical room) that serves the three upstairs bedrooms.

For either of you, would this change your above recommendations?

It's a tight house! Looks like my cooling load is greater than my heating load.

Umm how do you have a higher cooling load? I’m thinking I’m reading that table correctly? Something seems off?? Lots of south facing winds that are really thermally insulating????

I cool 2000 sq ft plus a basement on 25k btus of single zone cooling in the hot and humid south. (Expect when I go over design temp then I need my full 36k btus

Your heating design temp is probably close to -5F and cooling is close to 86. (Using Burlington as the only VT data point in my table).

I’d be asking some questions? The heating load seems average. I think it depends on how much you want to burn. Do you want to load a small stove 4 times a day and have the heatpump off. Or load it twice and let the heatpump keep it warm over night?

Or don’t want to let the heatpump keep pump run until it’s cold enough to load a big stove 2 or 3 times a day. Or do you just want to get a Blaze king know it’s lowest output is 10-12 k btus and run 24 hour cycle then 12 hour cycles when it’s really cold.

2600 sq ft on less than 3 tons of heat is impressive. The cooling calc is whac.

 

[Whirled Peas]

Yeah, I'm also having a hard time wrapping my head around cooling > heating. Design calcs were run twice. Once with -6df and 88df and the other with -12df and 93df.

Windows are all triple glazed tilt-turn Logic windows. They're U-0.15 (R-6.7). Some are pictures that are 0.13/R7.7. The length of the house faces SSW.

edit: @EbS-P I just got a clarification from the designer on the heating vs cooling loads: "Yes, this is because it is documenting peak load, not typical load. You've got a large southern exposure, and second floor southern exposure with windows. This increases cooling load, and lowers heating load. While the load may be higher for cooling- you're unlikely to be cooling more than you're heating over the course of the year in VT." Sounds like they feel confident in the modeling.

 

[EbS-P]

Yes, this is because it is documenting peak load

And they can do this because it’s variable speed equipment. Over shooting one impacts your cycle time at the lowest output. Not a big deal. You need the rated output for heat anyway.

you asked questions got a good answer (last time I asked when I got a quote the answer was “because that’s what the software said”. My reply was don’t you think 2 tons is a bit much for 1000 sq ft when I’m heating all 3000 with 3 tons? His reply was “well that’s what the software said”. I didn’t choose them. You contractor has better answers.

Back to stoves….. do you want 24:7 wood heater by potential at the risk of a hot stove room on average cold nights or do you want to supplement with two cold starts a day? Or do you want the consistent output of a BK.

I like my heatpump and tube stove. I don’t want to be a slave to the stove. I’m a busy guy. I’m really good at top down fires. Light it close the door and set a 10 minute timer and a second or third after that and come back 4 -6 hours later to reload if I want to.

 

[begreen]

I stand by my original recommendation. A low output cat stove from BK or Woodstock or a ~1.5 cu ft non-cat will be sufficient.

 

[DBoon]

Hi Whirled Peas,

I have a similar situation (but proportionally smaller house). I have a 1750 square foot house, <1.0 ACH50 (tested), Lifebreath HRV, in an ~8000 degree day heating climate. My heating load is 18,000 BTU/hour at 0 degrees F.

Note: I personally did the airsealing in my building shell once the contractors finished the rough framing remodels (older house), and I personally put 50 of the large cans of spray foam in the walls, ceilings, gaps, etc. after the spray-foam contractors "finished" their work. I did this to achieve <1.0 ACH50. If you are not doing this work, don't count on anything near 1.0 ACH50, and also demand a real test of the building envelope to make sure you get what you are expecting to get.

Your calculated heating load sounds realistic and accurate, based on how similar your house is insulated compared to mine (it might be even calculated a little on the high side, but not by much). I find that Manual J calculations are usually ridiculously fudged (too high, with bad assumptons), so nice to see someone do it accurately.

When I install cellular shades this fall my heating load should drop to 14-15,000 BTU/hour when they are closed. My house is about 1000 square foot on first floor and 750 square feet on second floor, pretty open floor plan (5' openings separating most rooms, 3' opening to back entry/pantry area). I have some reasonable south-facing glass, but nothing crazy, and honestly - it is pretty cloudy here in the winter, so this is not a big contributor for me in terms of solar gain. Anyone who lives near the Great Lakes understands that it can be 0 degrees outside and cloudy - you probably don't have this problem in Vermont.

I run my woodstove from mid-November to mid-March. I use about 2 full cords of wood a year. I don't bother with the woodstove in the early fall and early spring because I have net-metered PV and a geothermal heat pump with radiant floor heating that I don't mind using then for 100% of my heating needs.

I have a Woodstock Keystone. It's on the smaller side (~1.6 cubic foot). I never planned on it providing 100% of the heat. I'd say it does about 80% of the heating load when it is 0 degrees outside. I load it three times per day, mostly with hickory, when it is that cold. This keeps the downstairs at a comfortable 69 to 70 degrees, and the upstairs would only be about 60-62 degrees if I didn't run my radiant heat up there to help maintain a more comfortable temperature. So the 20% of the heating load the woodstove doesn't handle would be for the upstairs, mostly. If it is 20 degrees or warmer, the stove can heat the entire house (three loads a day at 20 degrees, maybe 2 loads a day at 30 or so degrees and more).

I like the stove, but it would be too small for your house. If you went one size up (2.2 cubic feet or so, like a Woodstock Fireview or the medium Blaze King) you are probably in the kind of situation I am in (covers most, but not all of your heating needs loading 3 times a day when it is 0 degrees outside, with supplemental heat needed for the adjacent areas off of the main living space). A bigger stove (as BeGreen suggested) gives you more flexibility BUT you probably still need the supplemental heat in those adjacent rooms.

I would definitely go with a CAT stove so as to never have to cold start (burns long, low and slow), and the OAK (as you indicated you would). I would never get a tube stove in a well-insulated, tight house.

I wouldn't go with a smaller (1.6 cu ft) stove in your house - it will be too small. Frankly, it is on the edge of too small for me (packing it in tight requires a lot of planning when wood cutting and sometimes "split adjustments" to pieces with an axe on my front walkway). It's not annoying, but if I had a medium stove I could just fill it without those minor headaches and it would be plenty full for the heat I needed. Having said that, the plan was never to have it provide 100% of my heating needs, but once you have it, well, you know what happens...

A medium stove (2.2 cu ft) would give you what I have (80% of heating needs, three loads a day when it is really cold). If your solar gain in winter was really great, it might reach 100% (or close to it) a bunch of the time.

A larger stove (2.8 to 3.0 cu ft) would give you more burn flexibility (two loads a day when it is really cold) at the expense of maybe too much output in the later spring or early fall (you indicated your burn 5 months a year). If you really had nice, sunny winters and good solar gain, that would probably tip me back to the medium stove and away from the large stove.

You'll probably only burn 3 cords a year, maybe a little more. What's not to like about that?

 

[Whirled Peas]

Thanks @DBoon.

Prior to my original post above, I was leaning towards something all catalytic. Nice to get a couple of nods that way.

Another thought I’ve had is that I could install my flue and then move the Jotul F400 I have sitting around from here to my new house. That would give me a way to test out a tube stove in the new home without too much expense. Maybe that’ll push me towards a new catalytic stove, something like the T4, or maybe it’ll work just fine and I can save the money.

 

[EbS-P]

Thanks @DBoon.

Prior to my original post above, I was leaning towards something all catalytic. Nice to get a couple of nods that way.

Another thought I’ve had is that I could install my flue and then move the Jotul F400 I have sitting around from here to my new house. That would give me a way to test out a tube stove in the new home without too much expense. Maybe that’ll push me towards a new catalytic stove, something like the T4, or maybe it’ll work just fine and I can save the money.

How many seasons has it seen? I love mine but with it was bigger. But if you haven’t 3 zones. I bet it does a ok job. With enough kindling top down fires are super easy. Its clearances will be significantly larger than a BK. So if you haven’t a particular location it will need to stick out further than other models.

 

[Whirled Peas]

@EbS-P its a ~2008 double door that was in service as a primary stove from 2008-2022.

I thought about the clearances issue, but my flue would a pretty tall, interior, top vent so I think a single wall offset wouldn’t be too hard to install for this ‘test drive’ plan?