Sizing a Wood Burning Insert for this home & floorplan

[Guess52]

Hello,

TLDR- please peek at the floorplan with fireplace highlighted in living room, how big a wood burning insert would you consider? Thanks so much!



More detail:
We will soon settle on a home whose fireplace / chimney failed it's level II inspection. The cost of joint mortar repair of the terra-cotta flue and damper installation ($7,217.25) doesn't make sense just to recover a drafty old fireplace from the 40s vs. installing an an efficient wood-burning insert and running a stainless flue within the existing chimney. The property includes a 1.5A wooded lot that we can put to use for firewood.

The house has an oil furnace and forced water radiators that I'm sure we will end up using, on 5 different control zones, colored above (one for the 1940s main brick house, 3 for areas of a 1990s addition, and one tiny zone to keep pipes in the garage bath from freezing). There is also an AC system - which I presume could be run "fan only" with one zone for each floor, but am not 100% sure since we don't have possession of the house yet. I don't know where the main intake(s) for that system are located.

We're all very new to heating with wood (aside from small cabin camping) and having been learning a ton from the forums. I'd like to size our unit appropriately to contribute meaningfully to heating, but not force us from the living room. My question are:

1) Based on the floorplan below - how much area should we realistically be thinking about the insert being able to "cover" - is that even the right way to think about it? I don't have historical oil usage to estimate BTUs and what % we might be able to replace with wood (that's the other way I would think about it, that would better reflect how good / poor the insulation is)

2) If we run the HVAC in fan mode, would that dramatically change our ability to distribute heat? Is fan mode low enough power consumption that it makes sense as a strategy from an overall energy usage standpoint? Obviously if we do that we lose some of the ability to finer control areas not in use at that time.

Perhaps these questions are moot if it's hard to get a big enough insert in our existing masonry fireplace for the total sq footage, but on the large side you have something like the Buck 91 with a giant 4.4 cu ft box that can probably put out enough heat for the whole place (assuming we can distribute it). More realistically we're gravitating towards something with a ~2.5cu ft box. We're only looking at stoves that are tax-credit compatible, not only for the $ but also as a convenient cut-off for efficiency.

Thanks so much!

P.S. the connections between living, foyer, dining and sunroom are pretty sizable

 

[stoveliker]

I think very little heat will spread so a small insert is max what I'd do. Not larger than 2 cu ft.

And HVAC ducting generally does not work well to spread the heat especially if it's running through uninsulated (attic) areas.

 

[gruesome]

If all the ducts have individual dampers I'd think as long as the room the stove is in has a large enough duct feeding it you can use the central A/C fan to distribute the heat. We had all our duct work replaced last year (to add A/C), and the installer was quite adamant about putting a damper on each duct, in addition to the register dampers. The central fan does of course use some non-negligible power, but if you dial up the flow to the room with the stove (dial down the other ducts), redistributing the heat that way should work just fine. Added side benefit is the air filtration you get from running the HVAC fan.

I agree with stoveliker that the ducts should be insulated. I was initially worried that R6 on the ductwork was not enough until I did the numbers: there's just not that many square feet area in duct work to give a huge loss.

 

[begreen]

If all the ducts have individual dampers I'd think as long as the room the stove is in has a large enough duct feeding it you can use the central A/C fan to distribute the heat. We had all our duct work replaced last year (to add A/C), and the installer was quite adamant about putting a damper on each duct, in addition to the register dampers.

The duct dampers are for the initial balancing of the system, not for user tweaking. An imbalanced system can change the design static pressure putting premature wear on the blower. The ducts must be well insulated in order to work for recirculting stove heat. The heat loss in the insulated ducts depends not only on the insulation value, but also on the ambient temp around the ducts and the length of the run(s). The plenums should also be fully insulated.

I don't think the areas in green and blue are going to benefit from the wood insert. They will still need independent heating.

It looks like a fair amount of heat will make it to the stairwell and upstairs. Depending on the insert I would be looking in the 2.5 cu ft range, especially if this is in the colder part of the state. Regardless of choice, get firewood now. It takes 2 yrs for a lot of hardwoods to season well and modern stoves really need fully seasoned firewood to perform at their best.

 

[gruesome]

The heat loss in the insulated ducts depends not only on the insulation value, but also on the ambient temp around the ducts and the length of the run(s).

Absolutely! What I calculated was temperature difference times sum of duct lengths times circumference times pi (i.e. the total exposed duct area in the attic) divided by the R-value of the insulation, to get the heat loss rate. It turned out the duct surface area is quite small compared to the other loss-inducing surfaces, so it's a few percent effect in our case.

 

[Guess52]

Thanks everyone for the replies.

@stoveliker - can you elaborate a little on why you think very little heat will spread? I recognize it'll never be a house where the stove can do all the heating, but I guess I was hoping that between the larger room connections and the stairs right outside the room I'd have some decent heat flow both out of the living room and up to the 2nd floor.

@gruesome - interesting approach to calculate the surface. Our ductwork is insulated, but does run through unconditioned attic space. I don't know the R-value. My (naive) intuition going into this was that if a system is well designed for AC (50-55F air through a hot attic under the sun, maybe ~110F) that it would probably be OK for warmish air through a cold attic. I am starting to recognize some of the nuances is trying to use the HVAC system.

@begreen - Fully agree we should get wood ASAP. My thought was to simultaneously buy "fully seasoned" for this coming winter, regular for the one after, and start harvesting for year 3. Of course, I don't have a great predication of our consumption yet.

I was already leaning Blaze King, but the uncertainty in heat distribution makes a solution where I can really dial back the hourly output, even if a 2.5 cu ft box is oversized, extra appealing.

I was at my local BK dealer today, they strongly recommended removing the existing 9x13 terra-cotta flue because it is likely too tight a squeeze to put a pre-insulated 6" liner through it. They've never used oval liners and seemed skeptical. I've read accounts here where folks have been successful both ovalizing a regular liner or getting factory oval. If we're confident we're not going back to standard masonry use, what are the downsides to destroying it? The salesperson did mention possible damage to the furnace flue, but that was before he learned it already had a stainless liner.

Unfortunately my chimney inspection did not include good measurements of the interior of the existing fireplace, so we will have to wait until we're in the house to finalize decisions and place orders. It only had size of hearth extension - which is a bare 16" and will need to be enhanced for any insert

Thanks again.

 

[begreen]

I was already leaning Blaze King, but the uncertainty in heat distribution makes a solution where I can really dial back the hourly output, even if a 2.5 cu ft box is oversized, extra appealing.

It looks like enough heat will convect upstairs help distribute the heat. If a fan is placed on the floor in the dining room, pointed toward the stove room and run on low speed, the heat will be more evenly distributed. Blaze King inserts are 2.5 cu ft but that won't b an issue. They can run low and slow for hours. I don't think it will be an issue. If the weather is mild just build a smaller fire and let it go out.

I was at my local BK dealer today, they strongly recommended removing the existing 9x13 terra-cotta flue because it is likely too tight a squeeze to put a pre-insulated 6" liner through it. They've never used oval liners and seemed skeptical. I've read accounts here where folks have been successful both ovalizing a regular liner or getting factory oval. If we're confident we're not going back to standard masonry use, what are the downsides to destroying it? The salesperson did mention possible damage to the furnace flue, but that was before he learned it already had a stainless liner.

If it's 9x13 ID, an insulated 6" liner should fit ok. If that is OD, then it could be tighter. A good shop will have an ovalizer or the liner can be bought pre-ovalized. Sounds like either the salesperson is not very experienced or their installer aren't. If you are lucky enough to be in bholler's work area, that's who I would hire.

 

[Guess52]

If it's 9x13 ID, an insulated 6" liner should fit ok. If that is OD, then it could be tighter. A good shop will have an ovalizer or the liner can be bought pre-ovalized. Sounds like either the salesperson is not very experienced or their installer aren't. If you are lucky enough to be in bholler's work area, that's who I would hire.

Unfortunately bholler is listed as central PA and I'm SouthEast (Bucks County). Elsewhere he declined someone who was 2.5 hours from them and their zip code is actually 30 min west of me. Happy to take any recommendations for my area if anyone has them.

I and the salesperson assumed it was 9x13 OD, but we both just have the inspection report to go on. Better measurements needed all around.

 

[stoveliker]

Thanks everyone for the replies.

@stoveliker - can you elaborate a little on why you think very little heat will spread? I recognize it'll never be a house where the stove can do all the heating, but I guess I was hoping that between the larger room connections and the stairs right outside the room I'd have some decent heat flow both out of the living room and up to the 2nd floor.

@gruesome - interesting approach to calculate the surface. Our ductwork is insulated, but does run through unconditioned attic space. I don't know the R-value. My (naive) intuition going into this was that if a system is well designed for AC (50-55F air through a hot attic under the sun, maybe ~110F) that it would probably be OK for warmish air through a cold attic. I am starting to recognize some of the nuances is trying to use the HVAC system.

@begreen - Fully agree we should get wood ASAP. My thought was to simultaneously buy "fully seasoned" for this coming winter, regular for the one after, and start harvesting for year 3. Of course, I don't have a great predication of our consumption yet.

I was already leaning Blaze King, but the uncertainty in heat distribution makes a solution where I can really dial back the hourly output, even if a 2.5 cu ft box is oversized, extra appealing.

I was at my local BK dealer today, they strongly recommended removing the existing 9x13 terra-cotta flue because it is likely too tight a squeeze to put a pre-insulated 6" liner through it. They've never used oval liners and seemed skeptical. I've read accounts here where folks have been successful both ovalizing a regular liner or getting factory oval. If we're confident we're not going back to standard masonry use, what are the downsides to destroying it? The salesperson did mention possible damage to the furnace flue, but that was before he learned it already had a stainless liner.

Unfortunately my chimney inspection did not include good measurements of the interior of the existing fireplace, so we will have to wait until we're in the house to finalize decisions and place orders. It only had size of hearth extension - which is a bare 16" and will need to be enhanced for any insert

Thanks again.

See how many corners the heat will have to take to do something upstairs. And going thru smaller door openings too.

I heat from the basement, heat coming up thru stairs. But I aid it with a dedicated small duct with inline fan that pumps cold air from the living room floor to the basement floor, significantly increasing air flow up thru the stairs.

Those pink rooms upstairs will easily be 10-15 degrees colder than the stove room if relying on natural convection.
If you have the stove room at 75 it won't be making those upstairs rooms very comfortable in my experience.

 

[EbS-P]

Three thoughts. Layout is not ideal for wood heat. It will be supplemental. If the AC is more than 10 years old consider a heat pump. Tax credits for qualifying stove install (and heat pump) are probably worth taking advantage of. BK makes good inserts that qualify.

 

[kborndale]

@begreen - Fully agree we should get wood ASAP. My thought was to simultaneously buy "fully seasoned" for this coming winter, regular for the one after, and start harvesting for year 3. Of course, I don't have a great predication of our consumption yet.

Unfortunately, buying fully seasoned wood is rarely an option. Every dealer says their wood is fully seasoned but in reality it is not. Even kiln dried wood does not guarantee dry wood.

 

[gruesome]

Absolutely! What I calculated was temperature difference times sum of duct lengths times circumference times pi (i.e. the total exposed duct area in the attic) divided by the R-value of the insulation, to get the heat loss rate. It turned out the duct surface area is quite small compared to the other loss-inducing surfaces, so it's a few percent effect in our case.

Give or take a factor of pi... ;-) That of course needs to be length times diameter times pi.

 

[Guess52]

@stoveliker - Thanks. Getting those pink rooms upstairs comfortable is my biggest open question since everything else is on it's own zone. Maybe radiator-level thermostat valves can help here? I'll need to figure out where the main thermostat for that zone is - I guess I'd want it upstairs.

@EbS-P - Thanks. Central air is 2016 and reportedly works fine. Will consider a heat-pump unit for sure if either it or the furnace fails. All the HVAC vents are overhead, however, so perhaps not ideal for heat.

@kborndale - Thanks. Curious about norms for buying seasoned wood. If people split and measure after 24 hours indoors and the MC is above 20%, do you ask for a discount? Just switch vendors for next time? (doesn't help you for that coming winter... I'd probably stock up on Bio-bricks or Envi in that event)

Another question - I've been reading the dozens of threads about block-off plates and they seem pretty universally recommended. Given my internal chimney potential heat-flow challenges, am I better off doing a metal plate for safety, but maybe *not* putting rock wool above it? I'm already leaning towards not insulating the masonry firebox to take advantage of the thermal mass balancing out temperature highs/lows and potentially passing some heat to the room behind it. Thoughts? Thanks.

 

[EbS-P]

@stoveliker - Thanks. Getting those pink rooms upstairs comfortable is my biggest open question since everything else is on it's own zone. Maybe radiator-level thermostat valves can help here? I'll need to figure out where the main thermostat for that zone is - I guess I'd want it upstairs.

@EbS-P - Thanks. Central air is 2016 and reportedly works fine. Will consider a heat-pump unit for sure if either it or the furnace fails. All the HVAC vents are overhead, however, so perhaps not ideal for heat.

@kborndale - Thanks. Curious about norms for buying seasoned wood. If people split and measure after 24 hours indoors and the MC is above 20%, do you ask for a discount? Just switch vendors for next time? (doesn't help you for that coming winter... I'd probably stock up on Bio-bricks or Envi in that event)

Another question - I've been reading the dozens of threads about block-off plates and they seem pretty universally recommended. Given my internal chimney potential heat-flow challenges, am I better off doing a metal plate for safety, but maybe *not* putting rock wool above it? I'm already leaning towards not insulating the masonry firebox to take advantage of the thermal mass balancing out temperature highs/lows and potentially passing some heat to the room behind it. Thoughts? Thanks.

Metal block off that is well sealed is the way to go. Keep all that insulation dust and dirt off the top of the insert.

 

[BKVP]

OP
For your insert considerations, remember you need 16" from door opening to front edge of floor protection. The Canadians require 18", a bit more is always better. The second thing to consider is clearances to mantel. Yours is tied-in to the entire shelving system, so verify this in the Owner's manuals of the inserts you are evaluating for your home.

BKVP

 

[Guess52]

@BKVP - Thanks. Always a good reminder. I will certainly have to extend the hearth for any insert - the current extension is only 16" deep, but plenty wide. I don't think I can set a shroud-free stove deep enough such that the front of the door is flush with the existing brick.

"Yours is tied-in to the entire shelving system" - Is that just a comment on it being harder to make something that looks nice if I have to alter or remove the mantel?

As I read the BK clearance specs, for the Princess 29, I'm coming up with 44" needed from floor to bottom of that center mantle support (26" + ref 18" from floor to top of door). For Sirocco or Ashford 25 I'm coming up with 39.5" needed (20"+ 19.5" ref from floor to top of door). In all cases the mantle should only be 12 inches deep. Does that sound correct?

 

[BKVP]

Correct on both accounts.

BKVP