Wood stove backdrafts
I just built a house of about 1500 s.f. It is double stud framing w/ spray foam covering the outer studs, and spray foam around all lighting fixtures in the ceiling and at the rim. We have an HRV. I got a Vermont Castings high efficiency wood stove (model CDW27007) in the basement w/ a 4″ diameter fresh air vent. I have run the stove about 6 or 7 times since we moved in. Three of those times it has backdrafted. The first time was after about 10-20 minutes I was just sitting there enjoying the heat/flames w/ my 2 year old and all of a sudden, the fire goes out and smoke starts billowing out from all corners of the stove (also a strange dripping like water on the top of the door coming out of the air intake???!!! Condensation or something?). The second time, it was running for about 10 minutes when my wife turned on the bathroom fan (panasonic). Again, smoke billowing out. I had her shut off the fan and it returned to normal. Third time, the stove had been running for about 20-30 minutes. We were up reading books with my 2 year old and the smoke alarm started going off. I run downstairs and once again, the fire is out and smoke is billowing out mostly from the top, next to the air intake lever. I can feel air coming in the fresh air vent 1 foot away. After 30-60 seconds of this it just sort of stops and the fire picks up again.
It starts right up initially every time and seems to draft wonderfully. I checked out side and it didn’t seem very windy, so I don’t think it would be any sort of wind preventing the draft suddenly. It seems baffling to me. The place that sold me the stove, suggested it was off gassing from the new paint, or a cold chimney. These are both incorrect because the chimney goes up the center of the house and I could see/smell the paint offgassing and it was not the smoke billowing out of the stove.
Does anyone have any ideas on why this is happening and how I can fix it? I would like to use the stove because we have access to free firewood.
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Sounds like the outside air duct is not connected directly to the firebox. Is that an option? If so, it might make all the difference. Your house is probably fairly tight, and it can make wood stoves difficult to burn. Other questions: how tall is the chimney pipe? How dry is the wood you have?
The most likely possibilities are:
1. The chimney doesn't extend above the ridge of your house.
2. The outdoor air duct is restricted or too small. (According to some experts, if you wood stove has a 6-inch-diameter stove pipe, then you need a 6-inch-diameter fresh air vent.)
3. Your house is under negative pressure due to exhaust fans, which might include a range hood fan, bathroom exhaust fans, a clothes dryer, or a power-vented water heater -- or a combination of these appliances.
For more information, including anecdotes from homeowners who live in very tight houses with wood stoves -- some of which operate successfully, and some of which have experienced the problems you describe -- see All About Wood Stoves.
The first (an probably biggest) problem is that with the wood stove in the basement, the outdoor air intake ducting for the stove is several feet above the firebox of the stove, which is a direct code violation. See section: R1006.2:
In particular, note the phase that reads:
"...nor shall the air intake be located at an elevation higher than the firebox. "
The reason for that section of code is as follows:
When the air intake is above the firebox, it behaves as a flue- the stack effect forces cause the hot combustion products to rise up the the air intake, condenses- the wet stuff dripping into the firebox you observed.
With both the main flue and the (parasitic) air-intake "flue" fighting against each other it doesn't take much wind current at all to de-pressurize the air-intake side of the house sufficiently to make it oscillate, puff, or temporarily back up, causing exactly the symptoms you describe.
Odds are pretty good that if you disconnect the air intake at the stove and let it draw its combustion air from conditioned space it would operate more consistently. If you do that, block the unused air intake piping, since it would still act as a flue depressurizing the basement, heightening the risk of backdrafting the stove into the room. In general it's better to simply avoid installing a wood stove below grade, since the combustion air is then of-necessity drawn from an elevation higher than the firebox (whether ducted, or drawn from the room.)
Thanks for all these great comments and answers. I would like to provide the additional information requested in my continued quest to understand the problem and in finding a solution:
Dave: The outside air is not connected directly to the firebox. The stove does not accommodate for that. There only appears to be a small slot on the front above the door w/ sliding air adjustment lever, and a very tiny port on the back about 1" square. The chimney is masonry block and is two stories plus 7' through the attic, plus about 3-4' above the roof. I am burning very dry kindling and hardwood.
Martin: The chimney does extend the code required 2' above the ridge of the house. Your point about the outside air being too small makes the most sense to me so far out of all the research I've done. I am a little baffled how it could be, though because the air going into the stove through the little ports on the front and back certainly do not equal the 4" dia duct to the outside. It does have a barometric damper on it and I wonder if that restricts air flow a little. My builder put it on, but I may swap it out with one I can either fully open or shut when running the stove. I can imagine the stove using slightly more air than is being allowed in, hence it works great for about 15-30 minutes, but then perhaps it reaches a point where the negative pressure in the house becomes greater than the draft pressure and the draft reverses directions therefore bringing the smoke in the firebox and cold air from outside down the chimney, through the stove and into the house. This could explain the moisture I saw forming where the cold air from outside was collecting on the door? There was still air coming in through the outside air vent. I guess it makes sense it was still coming in if there was negative pressure in the house, as the air would come in any way it could. Regarding your third point, I believe this to be true. The house is pretty sensitive to pressure and the second time it back/down drafted was due to my wife turning on the bathroom fan and reversing the flow, so we'll just have to be conscious of that if we are running the stove. So from your perspective, I would either want to exchange my stove for one that has outside air directly to the stove, or somehow have a greater supply of outside air to the stove?
Dana: I am not really following your reasoning for the the outside air elevation. And it is too bad the code section you cited directly conflicts with the Vermont Energy Code Handbook, which allows for a wood stove below grade, see Section 2.2b:
"Must not be located higher than the firebox. Where a woodstove or fireplace is installed below grade (in a basement), the combustion air intake on the home's exterior may be located above the firebox provided that the combustion air supply to the firebox drops below the firebox adjacent to the solid fuel burning system and the combustion air intake on the home's exterior is greater than 15 feet below the top of the chimney for the wood stove, solid fuel boiler or furnace, or fireplace."
This is what I referred to when laying out my ductwork. I am a structural engineer and am aware of codes and can generally follow them pretty well. However, I am sure the IBC put that in there for a reason, one of which I may be why I am experiencing difficulties. Your last point implies that the outside air is directly connected to my stove, but it is not and I fear plugging the duct would only make the problem worse. There is never a suction at the outside air duct. There is always air coming in when the stove is running (and even when it is backdrafting for that matter).
Thanks. Is my only option to open a window to allow greater air flow into the building? I would be a little disappointed if it was. Maybe there is a better stove out there for a tight house?
The pressures in your house (and most houses) are such that air leaks in near the bottom and leaks out near the top. I'm pretty sure that air is coming in through your outside air duct even when the stove is not burning, and that if you open the stove door when it's not burning, you'll feel cold air that's coming down the chimney. That's the way "stack effect" pressures work. Your chimney may be functioning as an air inlet whenever it can (i.e. unless you overcome the pressure with a roaring fire). Whenever the range hood, dryer, bath fans, built-in vacuum, etc., are in use, the problem is worse. In addition, your HRV may be unbalanced and causing a negative pressure.
When i read your original post, I assumed that you had a metal chimney, but you have a masonry chimney. Is it lined with metal pipe? All of the woodstove installs I have done use metal pipe, usually double-wall stove pipe on top of the stove, converting to class A pipe before leaving the room through the wall or ceiling. In those installations, it doesn't take long to heat the pipe enough for a solid draft. It may be that your masonry chimney is taking longer to heat, and that burning it hotter for longer will solve part of your problem.
Your description of the air intake on your stove matches what I have seen. Even stoves that have a 4" or larger duct connection on them have a much smaller opening. A large opening would allow the fire to burn much too freely.
In any case, I think your best bet is to install a different stove with a direct outside air connection (Dana's comments about elevation should be taken into account, as should your local inspector's). You need to remove house pressures from the mix here. Your house is sucking on your stove. I have a tight house with a woodstove, a Morso with a direct outside air connection. It burns completely independent of any use of the range hood, dryer, bath fan, etc. I can get the house to almost -30 pascals with the exhaust appliances running, and a stove without a direct air connection would never work here. If exhaust appliances are in use when I light the stove, I have to open the front door a crack until I get a draft going and close the stove door. Once it's drafting, nothing can stop it.
One thing you could do is experiment with your HRV. Turn it off for a period and burn the stove. If it drafts more reliably, that's a clue. Another thing you could do is get someone with a manometer (probably a blower door technician) to check things out. In the end, my money is on a different stove.
David made a good suggestion: "You could get someone with a manometer (probably a blower door technician) to check things out." If your house is under negative pressure, it's going to be hard to operate a wood stove.
Many homeowners in your predicament just end up opening a window when their stove is running. When you combine the energy penalty associated with an open window, along with the energy penalty associated with continual air leaks through your masonry chimney (and the energy penalty associated with the fact that your masonry chimney is a massive thermal bridge), the logic of having a wood stove in a tight house is compromised and undermined.
Here's the bottom line: it's hard to get a wood stove to operate in a very tight house. That's one reason that many Passivhaus consultants recommend strongly against wood stoves.
My prior comments were predicated upon a presumption that the ducted air was connected to the stove, which is a common configuration. If the ducted combustion air is a proximity duct with output near the floor, that changes some of the factors. (It's clearly not behaving as a second flue to the firebox if it's not hooked up.)
A masonry chimney on an exterior wall could cause high levels of condensation in the flue liner in the initial firing period, as could a masonry chimney without an insulated right-sized flue liner (right-sized for the BTU output of the stove. The backdrafting is related to the location an size(s) of your air leak(s). The HRV is probably going to be the single largest dedicated opening, and it's intake/exhaust port location(s) may be a driver of your overall house pressure.
The chimney is lined with 8" dia clay. I really don't think the problem is due to a cold chimney since it is in the middle of the house and warm for 16'. There is a draft even when no fire is going (I have lit a match in front of the open door, and the smoke moves quickly into the stove).
When nothing else is running in the house besides the HRV (so HRV running, but not bath fan, range hood or dryer) the barometric damper on the outside air intake for the stove is vertical, indicating little to no air flow and that the HRV is balanced pretty well.
I will look into a Morso stove, thanks for that suggestion, I can't do much about the location of my outside air intake, though, but I really don't think the "stack effect" will be an issue for the outside air duct. I like the idea of sealing the whole stove vent/draft as a separate system from the house, making it impervious (in theory) to the other draws (dryer, bath fan and range hood).
Yes, thermal bridges are...inconspicuous at times. I was never going for passivhaus standards, just something that would not require much energy. Last months gas bill was $55! That includes domestic hot water. I was pretty happy regardless of my stove woes.
During design, I weighed my options and had to make some choices on what to spend our limited budget on. The stove is technically a backup heat source, but I have access to wood, so it would be nice to use it to offset heating costs (and for the ambiance!).
Thanks for all the input.
I couldn't find a true spec or manual for the CDW27007 online. I did find this sales-page from a vendor:
The "29,300 Btu's" number implies a fairly small stove, with a BTU output a bit lower than appropriate for an 8" flue. Is the flue collar connection something like 6"? Smaller than 6"?
Most modern woodstoves work best when the chimney flue is between 100% and 150% of the flue collar's cross sectional area. (Standard practice in retrofits is to use a stainless flue liner of the same size as the stove's flue collar) A 6" flue is about 28 square inches making an 8x8 more than 200% of the cross sectional area, and near the maximal safe limit of 300%, even in an air-leaky house. Unless the flue collar on the stove is 8" (unlikely, if it's a ~30KBTU/hr stove), your chimney flue is oversized for the stove, particularly in a tight house.
If the flue collar on the stove is 5" it is grossly oversized, and would need a narrowing liner even in a house that isn't very tight.
BTW: The barometric damper on the air intake is indeed likely to be contributing the problem, since it requires a higher pressure differential than an unrestricted air intake to become fully open. You can probably test that propping it open for a test burn.
Thanks for that. I know my builder was not aware of appropriate flue sizes relative to the collar size, and just thought "bigger is better". The chimney flue is 8" round, making it about 50 sq. in. and about 177% the size of the 6" round flue collar. Still slightly more than ideal from what you are saying. I'll try another test run this weekend. I also wonder if a rain cap or vacu-stack of some sort would take another factor (wind pressure) out of the mix. My house is mid-slope of a moderate hill of sorts. The slope is high to the south, low to the north. There are trees all around, but not within 75' of the house. I have been reading a little about wind being a factor at times. The stove has run fine other times, and there was little to no wind the times it did backdraft. I will take off the barometric damper and give it another go and crack a window if I have trouble to see if that helps.
Venturi-type stack vents do quite a bit for reducing backdrafting related to wind (without depressurizing the house and over-firing the stove in a big way during a high wind the way a turbine-vent would.) You can probably drop one of these on there to fix the flue oversizing issue without it force-drafting the stove too heavily:
Stainless is the preferred material for the chimney cap, though galvanized can work if wood is the only fuel ever contemplated. (Propane/gas/oil absolutely demand
I've know a family in a hill-top location that had severe backdrafting issues on an open hearth fireplace who solved it with a turbine vent for a chimney cap, but that would be extreme overkill for a wood stove, and could push it into an over-firing state even at modest wind speeds.
I think that stove would perform completely differently with a 6" class A metal chimney in the 15-20' tall range, but that doesn't help you now. There may be a way to sleeve the clay pipe with a metal liner, but you would need a chimney person to do it.
Slipping a 6" stainless liner into an 8" round flue is neither the stuff of rocket science nor Swiss watches, but just the liner itself is not cheap, at least compared to a ~$140 stainless venturi vent cap ($32 if you cheap out and go with galvanized) that could be a DIY install. It's probably worth risking the $140 (or $32) to see if it fixes it sufficiently before narrowing the flue.
At only 177% sizing, on a chimney located mostly within the thermal envelope of the house it probably won't take much, even if it is a pretty tight house.
Dana, I just ordered a galv. syphon ventilator from your link. At the very least it will eliminate a factor and safeguard against windy days. I'll wait on drastic measures like getting a new Morso stove or a flue liner until I fully vet out the other (cheaper) possibilities. I am still thinking the main issue is make-up air and a buildup of negative pressure...
I've been monitoring my systems the last few weeks. Here is what I have discovered and my thoughts:
I bought a monometer. My bathroom fan and dryer draw .075" of water (19 Pa +/-?) each and my range hood draws 1.25" of water on high speed. The defrost phase exhaust fan draws .075" of water.
I have run the stove several times successfully (if you can call it that) by opening the window every 30 min for 5-10 seconds. It only backdrafted once. I happened to be walking by and heard the defrost cycle kick in, then the stove backdrafted. I rushed and opened a window and problem solved. I was not aware that the defrost cycle actually circulated air, but was under the impression that it was an electrical element that defrosted the core. I am kind of amazed that they would engineer a negative pressure cycle into the HRV knowing the kind of houses it is meant for. I may contact Fantech to discuss it with them though I would have pretty low expectations. It would make sense that this was the reason the stove backdrafted two times initially (seemingly at random times) since we were running the stove because it was particularly cold out (-10 F). The temperature difference creates condensation/frost in the core and that would trigger a defrost cycle.
Solution. I still don't know of a good solution to reliably prevent backdrafting other than not using the stove and saving it for when the power goes out. I did find a Fantech dryer vent booster fan with a positive pressure sensor. The positive pressure activates the fan for 10 min. I can see installing this in-line with the outside air vent to pump air into the house if there is a sudden negative pressure (and positive pressure in the vent) in the house to enable the fan. However there is no information telling how much pressure enables the fan. I could plug that fan into a switch that I could flip on (and energize the fan) when using the stove (so it isn't pumping air in at every random negative pressure, only when it is needed to prevent stove from backdrafting) and add a shut off damper to the vent to close it off completely when I don't need it. It would be a $300 +/- experiment. I am skeptical that the fan could produce enough air to depressurize the house fast enough before the stove backdrafted, especially if there was more than one fan running in the house at the same time. I wish I could find a cheap (under $40) thermostat type device, except for pressure (pressostat?) to set a range of negative pressure that an inline fan would turn on or off by. Perhaps I should have contacted the patent off before posting here and giving away my idea...And this doesn't even address the fact that it would be pumping outside air temp make-up air into the house.
I don't see many other ways of automated depressurization of our house. The defrost cycle is completely random. We only really want to use the stove when it is very cold out, and that is when the defrost cycle is likely to activate.
I suppose next I should record the draft pressure of the stove by taking readings from the manometer while the stove is in use.
It's too late to contact the patent office; the device you seek exists. It is called the Exhausto EBC 14. Here are two relevant links:
In 2008, I wrote an article about the device for the January 2008 issue of Energy Design Update. In that article, I wrote, "The EBC 14 consists of three components: a control box, an indoor pressure sensor, and an outdoor pressure sensor. The control box includes an LCD display showing the indoor room pressure. The control compares the indoor air pressure to the outdoor air pressure; if the indoor space becomes depressurized, the control energizes a variable-speed supply fan. The control modulates the fan speed in order to supply just enough fresh air to the house to correct the depressurization. (The EBC 14 can control a single-phase fan directly, or a three-phase fan indirectly, by means of a variable frequency drive.)
"The outdoor sensor is sold separately. It must be installed on a roof or exterior wall, and connected to the indoor control with a 50-foot length of silicone tubing. The installation instructions advise, “Select a mounting location as free as possible from rooftop obstructions. … Install the probe on an existing structure, like a pole, radio or TV antenna mast. Alternately, the L-shaped bracket can be attached directly to any wall or rooftop. … Obstructions such as trees, chimneys, signs, and buildings all cause turbulence, which results in abnormal and thus inaccurate static pressure. Position the probe as far from the sources of turbulence as possible.”
"The main disadvantage of the EBC is its high price; it lists for $1,500, although contractors will pay significantly less. Moreover, a complete makeup air system will require several other components in addition to the EBC 14 control: the outdoor sensor, a supply air fan, and, in some cases, a heating coil to condition the makeup air."
That might work, but what about just installing a stove with an outside kit? This problem is exactly what outside air is designed to solve.
Martin, dang, I thought I was onto something! Unfortunately, $1500 is not in my budget right now so I simply have been turning off the HRV while I have a burn going (4 hrs and heat for the day and night!).
I guess, I would probably go with Dave's suggestion of swapping for a stove with a direct outside air kit next. This isn't really in the budget right away either, so I'll make due for this winter. Maybe next winter. I could probably recoup most of my money from this stove via craigslist and put it towards a small Morso.
I just want to say thanks for all the input. It has been helpful.
Just one more comment. I've been keeping my builder apprised of the situation, since he is the one who installed the HRV. He had initially suggested installing an underground fresh air line (50' long, 6' deep, same concept as horiz geothermal heat tubes) to bring the air temp up to 40 degrees (warmed by the earth) or so before it got to the HRV, and in theory, negate any defrosting requirements, and in our case the defrost cycle and negative pressure from the HRV. It may have solved the problem, but unintentionally. I was skeptical when he presented the idea, and it got VE'd out during costing, but has anyone heard of this being done. In all my research on frosting/defrosting/air intake, this was never a solution.
Q. "My builder ... suggested installing an underground fresh air line (50' long, 6' deep) ... to bring the air temp up to 40 degrees (warmed by the earth) or so before it got to the HRV. ... Has anyone heard of this being done?"
A. Yes. This is called an earth tube. Some installations are somewhat successful (see, for example, Katrin Klingenberg's house), while others are disastrous (see, for example, Belgian Passivhaus is Rendered Uninhabitable by Bad Indoor Air).
Few experts recommend the use of earth tubes any more. Builders experimented with them in the 1980s, but many developed moisture problems and mold.
For more information, see the article on the Belgian house, as well as these links:
Earth tube for make-up air
Buried fresh air intake
Sorry for weighing in late on this.
Any HRV installed where there is a solid fuelled appliance should be equipped with recirculating defrost - i.e. the HRV is defrosted by recirculating the exhaust air back through the fresh air side of the HRV. In this arrangement, the house will not be depressurized when the HRV goes into the defrost cycle. Instead of replacing your stove, replace the HRV. Although, the market for used stoves is probably better than that for used HRVs.
In addition, you should also have heated, powered air make up interlocked with any significant exhaust appliances - dryer, bath fan, range hood.
Both of these should have been considered by your HVAC designer.
If you connect the combustion air duct directly to the stove, and a backdraft is caused by wind effect, you may run hot gases through the combustion air duct. It sounds far fetched but it has happened. In Canada, this practice has fallen out of favour because of documented occurrences of this phenomenon.
David, thanks! I was anxious to resolve the issue and had purchased a small Morso stove with direct air intake already. I have installed it and it is working much, much better. The dryer runs without any issues, but the bathroom fan seemed to still suck a tiny bit of smoke out of the stove. It hasn't been cold enough lately for the defrost cycle to turn on and test that issue, and I haven't tried the range hood yet because we have a newborn and toddler in the house. Waiting for limited occupancy to do the testing. So, the direct vented Morso stove has not completely resolved the issue.
Of course all of these do not heed your advice of heated make-up air. Can you recommend a heated powered make-up air unit for 1500 s.f. residential application that can be interlocked with my 6 year old dryer, already installed bathroom fan and rangehood?
For a little background, not that anyone really cares: I was acting unofficially as the owner/contractor (the bank wouldn't allow an owner to be the contractor) but my dad was the contractor. He is a capable fellow and told me with relative certainty that he thought he could figure out the HRV. Well, we were VE'ing pretty heavily at that point and needed to cut any costs we could so we didn't have an HVAC design performed, as it was not required by the bank, and that is the only person who seemed to matter at the time (as opposed to the bank requiring a licensed plumber and electrician). Unfortunately, we couldn't have built the house if we didn't VE out everything we did. We just didn't have the money and the bank wouldn't give us any more. If I had a chance to change how I did things, would I? The answer is I didn't have the budget to; we made some compromises. If I had the budget, of course, yes. I would have an HVAC design performed, I would have put a roof over the front deck, I would have built a garage, I would have built 4 bedrooms instead of 2, I would have a back deck, I would have paved the driveway, I would have backfilled with sand and not native material, I would have done 8" of sprayfoam instead of 4, I would have poured a concrete slab upstairs for the radiant heating and glued on hardwood flooring...you get the point.
I suppose I could have an HVAC audit done by a reputable firm that is fluent in super insulating techniques and tight house practices and see what they say. But even still, how can I trust I will be getting the right advice?
I have really been appreciating the advice on this column (but not holding anyone responsible, since it is just a blog...) There have been several different suggestions. I got a new stove, I got a syphon chimney cap thingy (don't remember the exact name). However, all in hindsight, which is never best. I am probably going to lose about $500 on my stove exchange, I wouldn't be able to replace the HRV and modify the ductwork without significant cosmetic demo inside, and sounds like I would need yet another hole in my envelope for general make-up air for dryer/bath/ranghood pressure mitigation. I would like to know the percentage of homes completed where there are zero issues and modifications due to hindsight revelations. I am sure it is getting better these days, but I feel like it would be impossible for someone like me to be able to compile all the data required, all the mitigation techniques, all the little notes to prevent any issues with a tight house. This turned into another tangent which none of you have time for...:-)
I am getting a little frustrated, but I shouldn't be. I love our new house. It is energy efficient, especially compared to most other homes. Do I wish I could have built something right the first time? Of course!
David Baerg, I've asked my dad to put a little pressure on the Fantech suppler for not tipping us off to the negative pressure issue the HRV produced. They should be informed about that right?
I'd like to point out that there are so many different expert opinions (David Mieland said new stove w/ outside air kit, David Baerg said make-up air for all appliances) with different solutions for seemingly the same challenges. It's hard to make the right building choices when everyone has their own definition.
Out of curiosity, did you drop in a narrowing liner compatible with the Morso collar sizing, or are you still using the 8" round with an Empire Syphon cap to help it along?
The combustion air duct behaving as a secondary flue when the air inlet is higher than the air box is a real effect, but can be mitigated by optimizing the draw on the REAL flue. A narrowing liner with poured rock wool or perlite between the liner and masonry would draft better, since the 8" terra cotta is most likely even more oversized for the Morso than it was with the bigger stove.
FWIW: Houses are never really "done", and the perfect house has yet to be built. If the stove & flue sizing is the biggest design error made in this brand new house you must be bordering on GENIUS!
I agree with Dana, this is the other piece of the puzzle.
Dana, I will put this up on the priority list. Unfortunately, funds are running low for experimental remedies. If it was only going to cost $100 and I could do it myself, I would probably try to get it done. What would you estimate the cost would be and could you point me to some products (page links) showing specifically what you had in mind, for now or for my records? How do I integrate the clean-out at the bottom of the chimney? How do I integrate the through wall connection from my stove pipe? I am not very familiar with this aspect of the design. Though the stove is drafting/starting well, I don't want to risk a backdraft of hot flue gases into a light gage outside air duct, so I realize the danger risks and well as stove performance issues. I haven't plopped the Empire Syphon cap on yet, simply because I don't own a ladder. The next time my dad comes up with his truck I'll have him bring his.
I also did some research on the HRV per David Baerg comments and there is an identical model (size, cfm) to what I currently have, except with a recirculating defrost cycle (as opposed to exhaust only) that would be a pretty easy swap. I think my dad would be interested in the old one so I might get that solved without significant cost...why we didn't get that in the first place is beyond me. Why do they even make the other one (negative pressure inducing defrost!?)?
I did this research last night at 3am because I was running the stove through the night with the HRV turned off (it was -11 out and the defrost cycle was sure to kick in) to prevent any midnight fire drills. Well the defrost cycle went on anyway! WHAT!!!??? The HRV was still plugged in and on stand-by mode, but there were no programmed cycles set and it still went into defrost cycle! At 2:30am!!! Of course causing a negative pressure and sucking smoke out of the stove pipe joints. So it is clear that that is an imminent issue. Or don't use the stove when it is below 20 degree...OY! A never-ending saga. Do you think I could make money off a book? It could be novel/documentary/educational.
Installing a flue liner can be a DIY project- watch a few vidis on the topic until you get comfortable with it:
The cost depends on the diameter and length, but you're talking a few hundred, not a grand. (From a randomly selected web-store: http://www.rockfordchimneysupply.com/liners_only_chimney.php ) There are some wrap-on insulation products for flue liners, but they can sometimes make installation a bit more awkward when it's a snug fit. A 5" liner with a 1" wrap is 7" in diameter, which could bind a bit trying to thread 35 feet or whatever down an 8" round flue, so poured rock-wool would be an easier install. But if it's a 3"-4" liner a wrap insulation might work. Poured rock wool or perlite is likely to be significantly cheaper though. Some people use fiberglass blowing wools, but in the event of a chimney fire fiberglass can melt, whereas the rock wool and perlite/vermiculite won't. If you can't find it locally, web stores that sell liners also sell pourable insulation.
For flue cleaning it's common to install a Tee where it meets the back of the stove with a removable cap on the unused port as a place to hook up the vacuum when sweeping the flue liner.
I would talk to your local HVAC supplier for the powered make up air. I'm in Northern Ontario; so, what we have available here is likely different from what you can get there.
I too suffer from hindsight syndrome on my house, too.
Enjoy the kids! It won't be long before they're giving you lip.
Where is the stove pipe damper positioned. If it is too closed off then the fire and smoke will undoubtedly be drawn to the fresh air.