Small fire in large firebox VS burn time - Choosing an insert

[Runner50]

Quick Question -

How does making a smaller fire in a larger firebox affect burn time?


Longer Story -

Hi, new here. I grew up with a wood stove as the primary heat source, left home for school/work and am not back in a region cold enough for wood heat (worked in Louisiana for a couple years). I bought my house last October and used between 16,300 and 20,900 BTU/hr to heat per month (monthly average) during last winter. We kept our first floor between 60F and 65F and the second floor bedrooms around 45F because those 2 bedrooms were not being used. If we increased the temperatures to 68-74F and heated the second floor, I would expect to need a heat output around 25,000-33,00 BTU/hr (added 21% for the overall increased temperature and 30% for properly heating the second floor). The house was built in 1864, is 1 ½ story (1200ft2 first floor with a 650ft2 second floor build into the attic), and has a full basement (unfinished and uninsulated). Insulation is decent for the age and we just replaced all the old (not original), single pane windows. I will be adding more attic insulation, but it will always be limited in areas due to 2x4 roof rafters behind a slopped ceiling.


The two inserts I’m going back and forth on are the PE Super and PE Summit. Based on PE’s heat output number and the calculations on ChimneySweepsOnline.com, the Super should average 32,700 BTU/hr when fully loaded w/ average wood and burned for an 8 hr duration. Whereas, the Summit should be around 48,300 BTU/hr. Those numbers tell me that the Super should be able to keep up our heating needs with a ‘low’ setting, allowing 8 hr burns, except during unusually cold spells. With the estimated average heat output of the Summit, I rarely see having to fully load it or set it above the ‘low’ setting. That brings me to my initial question… I read lots of people say “go big, you can always do a small fire in a big firebox”, but what are the effects on burn time when going with a smaller fire? I would expect the burn time to be decreased, but I don’t have an appreciation for how much it may decrease. I don’t mind the extra expense of the Summit over the Super, but I wouldn’t want to get the Summit and get sub-Super burn times because I’m not fully loading it. The fireplace is large enough for either.


Indianapolis temperature averages around 25F (minus 4C) during the winter.


Thanks

 

[Grisu]

Welcome to the forum!

At least in my experience, the effect of the air control setting on heat output in the PE Super insert is minimal. I can dial down the air to the lowest setting and the insert will eventually end up at 650 to 700 F stovetop anyway. Thus, most of the BTU in the wood will be released during the first 2 to 3 hours of the burn and then it becomes gradually less. Please don't expect that you will get anywhere close to a constant 30,000 BTU/hr.

I (conservatively) assume I get 100,000 BTU per cu ft of mixed hardwood. Three loads of 2 cu ft each will then give me ~600,000 BTU per day. If I need more than that I try to add in another load. Pine works well for that as it burns quick, hot, and with little coals and ashes.

Based on your calculations I assume the Super will be fine. Maybe on the occasional cold night you will need to supplement a bit of heat. We have electric baseboards that sometimes come on in the early morning hours when the insert just does not deliver enough heat anymore. Of course, I am not striving for using wood as my only heating source. I don't really see the benefit in getting a large insert that I can fill maybe a few weeks per year but most of the time has to run on partial loads. I find partial loads more of a chore and believe they are also more inefficient as it takes longer for the stove to get up to optimal operating temperature again. However, that is a personal choice. Others may feel differently.

I feel the Summit will be overkill for you. If you are worried the Super may not be enough: Have you looked at the Enviro 1700 series? They have a 2.5 cu ft firebox which would just give you enough spare capacity that the insert will be fine even during the coldest nights. I don't remember ever reading a bad word about them.

 

[begreen]

FWIW, I had this same question before getting the T6. Tom Oyen and I had a couple long discussions on the topic. The conclusion was that both siz stoves will do the job well but the larger firebox will have more reserve capacity for colder weather and offers a bit more flexibility for loading. I have a friend with a Summit, freestanding in a well insulated 1600 sq ft home. It is fine burning a partial load as is our T6 when the weather is milder. We both load the stove full when the temps are 35F or below.

Just to clarify, will the insert be on the main floor or basement floor?

 

[Runner50]

Have you looked at the Enviro 1700 series

The Envrio 1700 was actually the first stove I looked at, then drifted towards an inexpensive Drolet, and then to PE Super/Summit. The only thing that stands out to me about the 1700 that I dislike like is that the warranty states it must be installed by a professional and the warranty card is partially completed by that professional. I will be installing it myself (damper already removed). PE doesn't appear to have the same professional installer requirement.

Just to clarify, will the insert be on the main floor or basement floor?

The insert will be on the main floor. Last year the unheated basement stayed a mild 45F to 50F. The house isn't a modern open concept layout, but the insert will be a good location relative to most of the rooms.

 

[begreen]

Can you post a sketch of the floorplan or describe the first floor plan? The insert will be an area heater. If the stove room is closed off by narrow doorways then it can be a challenge to make outlying rooms comfortable while not having the stove room bake.

 

[Runner50]

Here is a rough sketch of the floor plan. I'm not at home so dimensions are not perfect, but should be within few feet. I envision some difficulty in the master bath and the upper bedrooms.

Also attached is the fireplace dimensions. The chimney is almost 25' with a 13"x18" clay liner, so I plan on ordering a 25' pre-insulated flexible liner.

https://www.firesidechimneysupply.c...ex-basic-pre-insulated-chimney-liner-kit.html

 

[begreen]

It may take a little creative fan work or a ducted fan to move the heat to the kitchen and master bath, but possible. The upstairs maybe ok, especially if it is preferred to be a little cooler for better sleeping temps.

 

[prezes13]

Just my two cents here. I am heating 1200sf of living space in a fairly well insulated house built in 1994 2x6 construction with an insert of a 2.2 cf fire box and I wouldn't mind having a bigger fire box. Don't get me wrong the insert does a fine job but the advertised 10 hours burn I can achieve when it's 35 or higher outside in a dead of winter like last year 4-6 hours is more likely. So my personal opinion is go as big as you can.