OAK for pellet insert on interior chimney with three flues

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New Member
Jan 27, 2024
I have a ranch with basement, and an interior brick chimney that has three flues, all lined with stainless liners. One flue has the oil boiler in the basement. The second flue is waiting for me to install a wood stove as backup heat, also in the basement. The third flue is for a Harman P35i insert in a fireplace on the main floor.

I had the insert professionally installed by a Harman dealer in 2017 and have been having trouble with smoke for years. After getting tired of the original installer not being able to fix the problem, I'm taking matters more into my own hands. I've done a lot of things over the years, with differing amounts of success. My next plan is to install an OAK.

The interior chimney makes it tricky. I know I can do the intake at the top of the chimney if there's enough vertical separation between the intake and all three outlets. I suspect that would mean extending all three flue exits vertically upward by the amount required by code, and having the intake live at the top of the masonry. While possible, this option is less than appealing to me.

What other options am I overlooking? Is it feasible to make a different path to the outside? Does anyone put the intake through the wall of the chimney, above roofline, but below the cap? Do people punch holes downward into the basement and then out a wall? The complexity of the three-flue chimney gives me pause.
I have no experience with you unique set up but I would ask, do you know where the smoke is coming from? Why do you think an OAK would solve that? How old is you house? Is it really that tight? Has it worked correctly in the past? If yes, what changed? I think you circumstance makes it difficult for us to help without some additional info. I would suggest that you fire up the stove at night with the lights out and use a flashlight to identify where the smoke is coming from. I doubt an oak would resolve a smoke issue. Good luck.

Oh, just thought. Smoke=leak in stack or bad seals.

Another thought, could the smoke/exhaust be coming out of one flue and being drawn down another. Again, perhaps not enough detail to even speculate.
House is 1960, and reasonably tight.

I have looked extensively for where the smoke might be coming from, but because it's an insert there are a lot of places I can't look while it's running. It has never worked properly. I wasn't at home when the installation happened (fall 2017, I think), but the installers were cleaning up when I got home. It was running, and it was smoky in the house when I stepped through the door on day one. We went through a lot of back and forth with the installers and they never came up with anything. As far as I can tell, they never did a draft test, and never adjusted the draft adjustment.

During the first season, our CO alarm went off several times. We got the installers back and they never found anything they could point to. They eventually threw up their hands and replaced the stove with their floor model (slightly different version of the same model, 2013 mfg date). The CO alarms stopped, but the smoke never went away.

We gradually learned how to reduce the amount of smoke, by only ever using constant burn and above about midway on the knob; by doing a full cleanout (including pulling the stove and brushing the flue) about once a month, and a couple other things. It helps, but not enough to make the problem really go away.

Most recently, I realized they'd never done the draft test, and did it myself with a Magnehelic gauge I borrowed from work. Turns out that not only was the draft test port plug bolt loose, but the low draft reading was about 2x high. I turned it back to where it was supposed to be. I think I can come up with reasons that the low speed draft being wrong could cause it to smoke, but I haven't fully convinced myself yet.

I can imagine pulling smoke from the pellet flue down through the oil flue or the not-yet-used wood stove flue, but it's an awfully long path to get back upstairs. When the pellet stove is smoking, you can smell it coming out of the distribution blower vents. I strongly suspect it's pulling smoke from inside the surround. This could be at the docking gasket, anywhere up the liner run, or somewhere else local to the stove itself.

I think one of my next testing steps is to measure the negative pressure in the surround space when the stove is running. The combustion blower is getting its makeup air from that space, and if the negative pressure there is "high" (meaning a large negative number) compared to the draft test number, it's easy to imagine that the distribution blower is stealing air from the combustion blower, perhaps by pulling it past the docking gasket. If that's the case, and OAK would remove the negative pressure in the surround space and allow the combustion blower to push the exhaust up the chimney without the distribution blower stealing it partway out.