Suggestions for controlling high chimney draft?

[Brokk]

I have a PelPro 120 that has been performing poorly since I got it 10 years ago. Yesterday someone volunteered to come take a look with a fresh set of eyes. In general everything looked good, but when I mentioned I was at the max chimney height allowed for this stove (33'), he decided to check the temp of the stove pipe going into the chimney. It was hot. Way hotter than he expected. A quick measurement was around 150 degrees on a 4" double wall pipe. This is with the air intake nearly closed. My heat is going up the chimney rather than heating the room. Looking online, I can't find a damper for a 4" double walled pipe. A barometric damper sounds like a perfect solution, except they aren't intended for positive pressure vents like we have on pellet stoves.

While it is possible to get something installed at the top of the chimney, I would highly prefer something that could be adjusted easily, since wind and outdoor temperatures can so drastically impact the natural draft.

What are my options for fixing this problem?

 

[brenndatomu]

I don't see why most any key damper wouldn't work...single wall and double wall are the same ID for the most part (vary a bit by brand/model) but dampers don't fit tight inside anyways, so...

 

[Brokk]

I don't see why most any key damper wouldn't work...single wall and double wall are the same ID for the most part (vary a bit by brand/model) but dampers don't fit tight inside anyways, so...

Are you suggesting I buy a single wall pipe with a damper to install in between my double wall pipe sections? I believe I have Duravent and they have this twist-lock thing going on that I don't think typical single wall pipes have.

Or... are you suggesting I modify an existing double wall Duravent pipe to install a damper inside it?

 

[brenndatomu]

Or... are you suggesting I modify an existing double wall Duravent pipe to install a damper inside it?

Yes, the install is the same either way, just drill a 1/4" hole (or shaft size) for the damper shaft and then pop the pipe apart to install it...doublewall just has two layers to go though that are roughly 1/4" gap between them...just makes the spring a bit tighter, which they need anyways generally...the spring is often a little loose for my taste with singlewall.

 

[railfanron]

Installing a damper designed for a negative draft chimney is going to work in a positive draft chimney? How do you keep the hole from leaking?

 

[brenndatomu]

I'm guessing that with a 30'+ tall chimney that it actually is pulling a negative pressure on the stove pipe...that's why the temps are so high...that, or there is an air adjustment on the combustion side of things that needs to be cut way down...

 

[Brokk]

Installing a damper designed for a negative draft chimney is going to work in a positive draft chimney? How do you keep the hole from leaking?

I'm guessing before the draft kicks in, that you will get small puffs of smoke coming out of any opening while the exhaust fan is trying to push it. Reading up on furnaces with draft controls, it's possible to install a draft inducer, which is basically just an exhaust fan mounted just before the wall entry to the chimney. This would prevent those early puffs of smoke by creating a vacuum on the far side of the barometric damper. Seems like there are a variety of ways to approach the issue. None of them are simple, easy or recommended by the pellet stove mfg.

 

[Brokk]

I'm guessing that with a 30'+ tall chimney that it actually is pulling a negative pressure on the stove pipe...that's why the temps are so high...that, or there is an air adjustment on the combustion side of things that needs to be cut way down...

There is an air intake damper. I have it set to mostly closed. We played with it the other day when testing. If you open it all the way, pellets begin leaping out like jumping beans, even before they light, the flames reach for the top of the stove and whatever fuel was in the grate is burned up or leapt out in about 20 seconds. That's with the fuel feed on high. So yeah... air flow problem. Getting those high pipe temps is happening with the air inlet damper nearly closed.

 

[bholler]

There is an air intake damper. I have it set to mostly closed. We played with it the other day when testing. If you open it all the way, pellets begin leaping out like jumping beans, even before they light, the flames reach for the top of the stove and whatever fuel was in the grate is burned up or leapt out in about 20 seconds. That's with the fuel feed on high. So yeah... air flow problem. Getting those high pipe temps is happening with the air inlet damper nearly closed.

Have you actually measured the draft and followed the instructions in the manual for adjusting it?

 

[Brokk]

Have you actually measured the draft and followed the instructions in the manual for adjusting it?

I have followed all of the instructions for setting the draft and tuning the flame. Until this week I haven't even heard of a draft meter before. I've been shopping online the last couple days looking for one. The magnehelic differential pressure gauge looks perfect, but it's a bit pricey. Most others I'm seeing to measure vacuums are done at such a higher scale (up to 30" WC), that looking for .05" would be near impossible to see. There is one manometer that measures with a red liquid that also looks interesting, but I'm unclear how it's supposed to be setup or how well it works for this purpose. I'd like something I could permanently hook up, so I could just glance over at it to determine if I need to adjust the damper.

 

[bholler]

I have followed all of the instructions for setting the draft and tuning the flame. Until this week I haven't even heard of a draft meter before. I've been shopping online the last couple days looking for one. The magnehelic differential pressure gauge looks perfect, but it's a bit pricey. Most others I'm seeing to measure vacuums are done at such a higher scale (up to 30" WC), that looking for .05" would be near impossible to see. There is one manometer that measures with a red liquid that also looks interesting, but I'm unclear how it's supposed to be setup or how well it works for this purpose. I'd like something I could permanently hook up, so I could just glance over at it to determine if I need to adjust the damper.

Any draft gauge will work

 

[Brokk]

Any draft gauge will work

Some of the ones I found were digital, so you have to turn them on to take a measurement. They are also handheld, so not as easy to just mount somewhere. Also batteries... I'd rather just have something that is physical in nature that always shows the current state of things.

 

[bholler]

Some of the ones I found were digital, so you have to turn them on to take a measurement. They are also handheld, so not as easy to just mount somewhere. Also batteries... I'd rather just have something that is physical in nature that always shows the current state of things.

There are many many options. The only thing you need to really watch for is the sensitivity

 

[brenndatomu]

Some of the ones I found were digital, so you have to turn them on to take a measurement. They are also handheld, so not as easy to just mount somewhere. Also batteries... I'd rather just have something that is physical in nature that always shows the current state of things.

Many of us wood furnace guys use the Dwyer mark ll model 25...cheap and simple...can buy used/NOS in eBay for $25-40 often times.

 

[Brokk]

Many of us wood furnace guys use the Dwyer mark ll model 25...cheap and simple...can buy used/NOS in eBay for $25-40 often times.

That's exactly what I ordered. It came in and I went to install it. Two questions:
1) Do I really need to use all 8' of tubes it came with? My pellet stove is in an alcove so the best spot is relatively close to the pipes (18" or so).
2) Since they advise not to leave it hooked up all the time, what are the recommended solutions for being able to connect/disconnect easily, while not leaving a hole for smoke to come out of, plus keeping in mind the heat limits. I read about someone putting a metal tube into the pipe so that it's not directly expose to the heat from the exhaust. If so, what type, size, length, etc...?

 

[brenndatomu]

That's exactly what I ordered. It came in and I went to install it. Two questions:
1) Do I really need to use all 8' of tubes it came with? My pellet stove is in an alcove so the best spot is relatively close to the pipes (18" or so).
2) Since they advise not to leave it hooked up all the time, what are the recommended solutions for being able to connect/disconnect easily, while not leaving a hole for smoke to come out of, plus keeping in mind the heat limits. I read about someone putting a metal tube into the pipe so that it's not directly expose to the heat from the exhaust. If so, what type, size, length, etc...?

You can shorten the tube (singular) as needed...no need for both in this application, just pull them apart...and you will want to hook to the right side (high) port, this will give you a larger range to work with.
Most people just get a section of copper tubing or automotive brake line to hook to the pipe...and leave it installed all the time...I just bent my copper tubing into a large S shape and leave it hanging in the hole in the pipe (which is about the same size as the tubing OD)...some people get fancy and thread in compression couplings/etc...you can if you want, but for my furnace in the basement, the S shaped tubing is good enough....oh, and my copper tubing was somewhere 18-24" long before I bent it into the S shape...honestly if you come off the pipe 8-10-12" with metal tubing that would probably be enough to keep the rubber hose cool.

 

[brenndatomu]

Since you have a (potentially) pressurized flue, I think I would drill a hole and thread a 1/8" NPT compression coupling into the pipe...then you can slide your metal tubing into that...then you can put another compression coupling on the other end with a barbed fitting for the hose to attach to...my 2 cents...I'm sure there are lots of good ways to do it.

 

[Brokk]

Ideally I would like something I could seal off, so a fitting sounds good. I like the brake line idea as well. I have a SS tube from one around here that was extra that might work.

 

[Brokk]

It took some machining and a die to match the threads on one side, but this seems to work well. I like being able to close it off. I don't have a manual damper installed yet. It won't arrive for another 10 days, but I decided to test it out since I was curious about my draft.

It's not bad IMO. After I got the pellet stove up and running at max burn rate, I gave the chimney 15 min to warm up after it was fully started. The WC for the draft was only -.055. I'm not sure if you should subtract the exhaust fan positive pressure which would bring it to -.9 WC which is not as good, but not as bad as I expected. Of course it's only 53F outside, so it's only going to get worse as it gets colder.

Despite the draft not being as bad as I predicted, the double wall exhaust pipe was still pretty hot. Laying a meat thermometer on it gave a temperature of 163F. Since it's a less than ideal way to measure surface temperatures, I'm assuming it's actually much hotter than that. The temperature of the hot air coming out of the exchange tubes was just under 130F.

Any other ideas about why the exhaust pipe is so hot?

P.S. Since I'm still waiting to install the damper, I have not sealed up all the stove pipe as air tight as I plan to, so that could impact the draft measurement somewhat.