Strong Draft and Flue Temps

[evergreenvt]

Hi everyone,

I am new to wood stoves this winter, and I've been leaning heavily on this forum as a resource for safe and efficient burning. Thanks for all of your insights because they've been very helpful in getting started.

My questions revolve around strong draft and safe operating flue temperatures.

Location is northern Vermont. Setup (from the stove/ground up):

• Lopi Evergreen NexGen-Fyre Insert (EPA certified, non-catalytic wood stove) set in a fireplace hearth
  • There is an aftermarket blower attached, ~260+ CFM (fireplaceblowersonline.com: part #9900123). It typically runs on medium to high while the stove is burning.
• Flue offset connector box bridging the stove to the bottom of the stainless steel liner (couldn't fit/connect without it)
• ~27' of 6" stainless steel liner (type 316) professionally installed by the stove retailer (Note: the liner setup is a bit strange, so I'll try my best to explain below)
  • The first ~5' of liner from the stove goes up through the old fireplace damper, which was removed to make space for the liner, and to the bottom of the prior owner's stainless steel metal chimney. This section is filled with rock wool I installed along with a block off plate following this website's guidelines and instructions.
  • The rest of the 6" liner runs through the Metalbestos (now Selkirk brand) Model SS 8" stainless steel chimney (pre-2100 UL rating, so rated for 1700). It has an offset (maybe 30 degrees?) halfway up in the middle.
    • Within this 8" stainless steel chimney, the first ~8' (closer to the stove) is uninsulated up to the offset because the 6" insulated liner wouldn't fit. The 6" liner just runs through the 8" insulated stainless steel chimney.
    • Past the offset, there's another ~8' of chimney up to the roof. In this section, the 6" liner is insulated (installers hand-wrapped and glued on faced insulation) within the 8" insulated stainless steel chimney.
    • The remaining ~4-5' of chimney is at the roof and above. The old chimney was ~2' above the roof, so as a part of the wood stove install, the installers extended with 3' of matching Selkirk 8" chimney to satisfy the 3-2-10 code. The 6" liner is insulated up to the chimney cap.
• Two thermometers magnetically attached
  • I have a Condor stovetop thermometer at the manufacturer's recommended placement on top of the Evergreen Insert
  • I have a Condor single wall flue thermometer magnetically secured to a screwhead that goes through the bottom of the 6" liner (~4" above the top of the stove, bottom of magnet essentially touches the flue offset box)
    • Because of the block off plate, I can't place this thermometer any higher on the liner
    • I attached to the screwhead out of convenience because it's magnetic. I've compared the outside of the liner temp to the thermometer on the screwhead with my IR gun, and they're directionally similar temps (only ~10-25 degree difference)
• I attached a few photos:
  • Photo #1 shows the stove in equilibrium, firing at ~500 stovetop and ~500 flue temp (~1 hour in after cold start with ~7 splits, ~35 F outside)
  • Photo #2 shows the stove at peak, firing at ~650 stovetop and ~650/700 flue temp (~45 minutes in after warm start with ~8 splits, ~35 F outside)
  • Photo #3 shows the flue offset connector box to the liner. I had a Rutland 600 degree high temp silicone sealing the liner to the block off plate, but that got melted off the liner, and I realize I probably should've used a 2000F cement.
  • Photo #4 shows the inside of the flue offset. The flue thermometer sits right above the top of this picture where the offset connects to the liner.

Because this wood stove is my first, I don't have any sense of normal operating conditions for comparison. I get the sense that our draft is very strong because the fire is typically very energetic, almost as if the flames are getting pulled up out of the stove towards the chimney. I can leave the bypass closed and air control off for a reload, and any smoke will still get pulled up the chimney. When the stove is loaded up and firing, it has a loud, continuous whistle (that goes away if I push down on the bypass). Even when not in use, though, the stove has a high-pitched whistle on a windy day. Finally, it eats up wood faster than what others have shared online is typical for them, especially for an EPA stove.

My questions/concerns in order of priority are:

1. Given the setup I outlined above and placement of flue thermometer, what is a safe operating flue temperature?
   • I understand that ~250-450 F is ideal for a thermometer placed ~18" above the stove on single wall stove pipe.
   • My thermometer reads well past those limits, but it's also on a screwhead ~4" above the stove and very close to the flue offset connector box. I'm wondering if it's picking up temps from the stove and misleading me to think the flue gases are at an unsafe temperature (>1K F continuous).
   • I also don't know if the same guidelines for a single wall stove pipe apply to an insert's stainless steel liner
   • At peak, my flue thermometer on the outside of the liner will read ~500-650 (maybe even 700) continuous for an hour or more. It can reach that temperature quickly (~20-30 minutes in), and it typically levels off over time to match the stove's temps. For example, an hour in, and the stove could be cruising at 600-650 F, and the flue would read the same. As the fire calms, the stove will stay in the 400-600ish range, and the flue temps will go down to ~300-400. Although sometimes the flue will run ~50 degrees hotter than the stovetop temperatures continuously.
2. Am I losing excessive heat up my chimney because of strong draft?
   • Our house is ~4K sq. ft., leaky, not insulated well, and runs cold. This issue is mostly separate from the wood stove and is due to many old windows with broken seals, lack of insulation, and aging/warped siding.
   • The stove itself sits in a Great Room with ~22 ft. ceilings and lots of windows, so I'm sure there's inevitable heat loss. If I load up ~7-8 splits into the stove and start it with a Super Cedar firestarter (with outside temps in ~30-40F), it is firing strong secondaries within ~10-15 minutes, and I can expect to get ~2-3 good hours of burn out of that load with meaningful heat, plus maybe another ~1-2 hours of some heat before everything's burned down and it's ready for a reload. ~3-5 hours per load feels low relative to others I've seen loading in the morning and at night, with maybe a third in the middle of the day.
   • Beyond the safety question of high flue temps in #1, I'm wondering if the stove is heating outside, not inside. We might start at 60 degrees inside and get to 63 degrees at its peak. It's a noticeable improvement, and the heat from the wood is nice. Given how warm other homes nearby us with wood stoves get, though, it doesn't feel like we're getting the full benefit of the wood we're burning. Maybe that's just how it is with a big, leaky house?
3. How do I get rid of the annoying whistle when the stove runs or when it's windy outside?
   • I called Travis Industries / Lopi, and they said the whistle has happened for other customers with strong draft. They suggested wrapping gasket around the bypass right at the point it meets the stove (where there's a little hole/gap). I did that, and that maybe improved it, but it didn't get rid of the sound like they said it would.

I went back to the installers and explained everything I've described here. They basically said I should be thankful for a strong draft, and there's not much more that can be done. Maybe they're right, but I'd really appreciate additional input and confirmation from experts. I'm primarily worried about safety and then efficiency.

If the draft is too strong, the only solutions I've read about are i) lowering chimney, or ii) installing a key damper. I could potentially lower the chimney ~1-2' and stay within the 3-2-10 code, but I don't know if that small amount would make much of a difference. My installers were opposed to a key damper, and I'm not even sure if there's space to fit one around the flue offset box and block off plate, although maybe that's what's needed to control the draft and settle the burn/get rid of the whistle.

I appreciate any advice and insights!

 

[stoveliker]

Fantastic description - that's a good way to get good advice. Kudos too for studying and understanding the requirements.

My initial thoughts are:

Yes, I think flue temps are too high, you're loosing heat up the flue, and I think that's due to too high draft.
A key damper should be looked at (it's tough with an insert but there are folks doing that, and it seems it shoudl be possible at the location of your third picture), and a few feet off the chimney will help a bit as well (keeping at the 2-3-10 rule).

 

[mellow]

You have an offset box so that is helping to restrict your draft, it would be worse without that. Installing a key damper is not possible with that setup, no way to adjust it.

Maybe you can adjust your incoming air via a magnet over the air supply?

I would go outside to the back of your fireplace and take an IR gun to your chimney like the example below:

Finally got around to insulating my fireplace

Mellow - How did you get the Roxul to stay in place on the top block plate? I have my 4" pipe coming down through what is left of my old damper with Roxul stuffed into the remaining cavity - nothing on the side walls.

www.hearth.com

 

[mellow]

Also, if you have a large layout that 2 cubic foot insert is going to have a hard time heating it, you mention 4k feet which is double what that stove is rated for.

I looked at the user manual for that stove but it does not list where the incoming air comes from for the Primary and Secondary, you can use a lit incense stick to track this down as the stove is not listed as having an Outside Air Kit.

 

[joop]

can i ask how much do you close primary air?

 

[evergreenvt]

Thanks for the responses so far. A few important clarifications:

1. It's helpful to know a key damper could help. Mixed feedback, though, on whether one could actually fit into my setup.
2. I have to shut the air control and bypass quickly once the fire's going to keep it under control (typically ~10-20 minutes in, I shut down completely). Sometimes I even have to lower the air control or shut the bypass just to get a fire going because the draft can put the beginnings of a fire out. The air control throughout my burns is 100% shut off, and the photos/numbers I reference are with everything (bypass, air control) shut down completely. The peak today was a 700F reading on the flue thermometer (4 in. above stovetop) and 650F on the stovetop ~45 minutes into a 8-split E/W burn, with the door shut at ~10 min and air control/bypass shut at ~5 min. after the door. This peak lasted for ~30-45 min, and gradually the fire cooled down until it was embers by ~3 hours in.
3. I should've mentioned it, but this wood stove is secondary heat for the home. There is a forced hot air / oil-fired furnace that is primary, and the wood stove mostly heats the main floor and upstairs, especially on cold days. The area the stove can affect is more like 1.5-2K sq. ft. The expectation isn't for the stove to heat all 4K sq. ft. on its own. Typically, heating is minimal overnight, and I'll have the furnace get the house to ~60 F in the morning (normally ~3-4 gallons of oil on a Vermont winter day). Then I'll fire up the stove to maintain ~63 F throughout the day. Ideally, we'd get to ~67/68F to be comfortable (and we do if it's sunny out), but that's not possible now unless we run the furnace and the stove concurrently.
4. The chimney is exterior on a framed corner of the house. I'll see what the outside looks like with my IR gun, but since I've noticed less snow / snow melt on that particular corner with the wood stove, my hunch is that there's definitely heat loss through the masonry hearth and chimney.

 

[EbS-P]

I have a thermal couple probe on the exterior of my liner 4”
Above my F400 stove 15’ of insulated liner. I have my alarm set at 650. Low cruise is 525. Stove top 475

I measure flue temps at the appliance adapter where I have a damper installed on my 1800i insert with 23’ on insulated 6”. The alarm there is set at 1050 F. Low cruise is 750. Stove top 550.

since you have access to the offset clean out I would be tempted to add a piece of steel that cover some (start small like less than 20%) of the outlet opening of the stove into the offset box.

The whistling is probably coming from the primary air intake I’m not familiar with your stove but it could possibly be reshaped to lessen the whistling.

All this said these are modifications I am hesitant to recommend until all other possibilities have been exhausted. When you star messing with air intake and exhausts you are really change how the stove burns a must insure you are no creating one problem (creosote) to solve another.

 

[joop]

do dollar test on door make sure it's sealed properly,if it is then maybe a magnet on air intake like they do on jotuls

 

[begreen]

Taming draft on inserts with a tall liner can be challenging. As a test, if the chimney cap is accessible, remove the cap and replace it with a 6" x 5" reducer. If that works, then maybe make it permanent with a stainless reducer and a 5" chimney cap. Sometimes this is all that is needed to tame a strong draft.

A more extreme fix would be to modify an offset box with a sliding damper that could partially close over the flue outlet. I have never done this, so it's just a spit-ball idea.

 

[joop]

or test the door with a dollar bill,or try magnet .cheap fix first then work your way up

 

[evergreenvt]

Hi all, thanks so much for the advice, and apologies for my delayed response. I've been in contact with my installer to investigate and implement some potential fixes. Thankfully, we seem to be through the worst of winter, so I have some time to get this Evergreen Insert sorted before the cold really returns.

First, a few updates and observations:

• I checked all sides of the insert door with smoke from matches, and it didn't draw in the smoke. The air control did a bit when it was fully shut down, but I was told that's normal.
• I doubt it affects everyone's advice, but my chimney is actually ~29 feet. I was previously measuring the liner itself (from top of stove), and it's really ~28-29' if you go from the bottom of the firebox (with the flue offset and a chimney offset "reducing" its effective length).
• I noticed during a burn the other day that - at peak when the fire is really roaring (stove and flue temps at ~650/700) - there's still a lot of smoke coming out of the chimney. I can visibly see the secondaries firing, and I can also look outside and see a lot of smoke out the chimney. As the fire slows down to a steady burn in the ~550/600 temperature range, the emissions disappear like they should when secondaries are firing. I don't know if that indicates a strong draft or issue with the stove itself, but I thought it was worth mentioning.
• My installer is very skeptical the draft is too strong based on what I've shared with them. They're sending a service team out to start and observe a fire.

Here are the suggestions so far as I understand them in order of ease of implementation:

1. Reduce primary/secondary air supply with magnets
   • The Evergreen Insert manual doesn't specify where the secondaries are I don't think, so I'd have to locate them with smoke/incense or call Travis Industries.
   • If any Evergreen owner who is familiar can advise, that'd be appreciated.
2. Add 5" stainless reducer and chimney cap
3. Reduce chimney height by 12-18 inches
   • I currently exceed the 3-2-10 rule by ~18 inches. I could modify the chimney and liner to reduce the total chimney height.
   • I am skeptical that a foot or so will make a significant difference. I'd be curious if anyone's made this adjustment and successfully mitigated a strong draft problem.
   • Is ~12-18 inches enough chimney height reduction to make a difference in draft?
4. Restrict draft with a damper
   • Option 1: key damper in liner (potentially can fit)
   • Option 2: sliding steel piece in offset box to partially cover stove's flue outlet, starting at ~10-20% and slowly working up if needed
5. Insulate behind hearth
   • This is unrelated to the strong draft, but could improve heat loss.
   • Since the fireplace hearth has exterior framing, I'd have to improve insulation as part of a broader re-siding effort. I assume proper insulation between the hearth and exterior framing would be sufficient to prevent heat loss.
   • I would guess there's nothing between the hearth and siding right now.

 

[EbS-P]

Seems like you just need to measure draft. It’s not hard. Manometer needs 0.01” water column or less resolution. There are several threads on how to set and take that measurement.

At this point I would stop guessing and rely more on data.