Harmon MKII Backdrafting

doctorclean · By: **doctorclean** On: Sat Oct 27, 2012 6:50 pm

Hello Gentlemen,

I hope you can give me some advice on my Harmon MKII setup. I purchased this house a year ago, I have been burning sub-bituminous coal from Healy in Alaska. The problem I have is that the stove backdrafts when the wind blows from the side of the house that the stove is mounted in. I did install a downdraft preventer that is shaped like an H. It doesn't move in the wind but is supposed to create a vacuum in the stove pipe, This addition did not help. I believe my next course of action should be to make the stack taller according to what I have read here. One Thing I am curious about is the size of the pipe. The hole on the back of the stove is a 6" the previous owner has chosen to immediately adapt the stack size up to 8" for the rest of stack. Should I remove all of the 8" pipe and replace it with 6"(expensive). I am concerned about Carbon Monoxide poisoning. We do have 3 CO detecters in the house. I will post pics of the downdraft preventer soon. I do have a manual damper and no manometer at this time.

freetown fred · By: **freetown fred** On: Sat Oct 27, 2012 8:04 pm

DC, that chimney height looks good in the pix-(sometimes deceiving) as long as you are a couple foot above that higest roof line, you're good. NOW, just an old farmers thoughts--lose the 8" pipe, sorry buddy, expensive or not, I'm pretty sure that will be your fix.Fancy cap or not, that 8" is a lot of pipe--I have a hand fired & have only an MPD--no mano, no baro--I think on these more complex set-ups they might be a great idea. My draft test consists of lighting a piece of paper & sticking it near my exit vent--if it draws it's good. I too am on top of a hill with no trees around the house as wind break--I'm a firm beleiver in the KISS concept. SAFETY is the rule--with 3 CO alarms, you're set--I do new batteries every year, needed or not. Nice place by the way.

McGiever · By: **McGiever** On: Sat Oct 27, 2012 8:35 pm

Looking at that roof pitch...I'd say you need more height.

And that 8" pipe is just fine for your sub-bit Alaskan Coal.

doctorclean · By: **doctorclean** On: Sat Oct 27, 2012 8:49 pm

Here is a pic of the new top

freetown fred · By: **freetown fred** On: Sat Oct 27, 2012 10:57 pm

Looks like it should work in theory. Hopin it does.

Rob R. · By: **Rob R.** On: Sat Oct 27, 2012 11:03 pm

Any chance the backdraft occurs when you are running the clothes dryer or something else competing for air with the stove?

doctorclean · By: **doctorclean** On: Sun Oct 28, 2012 12:10 am

The backdraft only occurs when the wind is blowing from the west.Bathroom fans being on do not change the draft problem. The new cap also did not help.

doctorclean · By: **doctorclean** On: Sun Oct 28, 2012 12:42 am

Here are some more pics from various angles

Freddy · By: **Freddy** On: Sun Oct 28, 2012 6:07 am

Beautiful place!

You say it happens when the wind blows from the right? Usually this type of thing happens when it blows from the direction that I have pictured. Perhaps the shed roof makes your geometry different than most. Nonetheless, a couple of comments.... one, they say you should have a minimum of 10 feet from the chimney top horizontally to the roof. It looks like you might be a bit close and more height would help. Also, I do think a 6" pipe might help. Logic says a 6" pipe would have to have more flu gas speed and it seems that could only be a good thing. It would certainly be a weekend killer, but with some luck it wouldn't cost you a penny. Find out how much a new 6" would be. Then put an add on Craigs list (internet) and try to sell your 8" for that price. If it sells, voila, you got a 6" for nothing.

Again...beautiful place! If you want a different solution to your problem, just trade your place for mine. <grin>

Rob R. · By: **Rob R.** On: Sun Oct 28, 2012 7:14 am

Most of the buys that burn bituminous coal recommend 8"...it tolerates soot better. I think the chimney needs to be higher.

Most people would say that the chimney on my house is too large and won't draft properly, but it is about 32' high and works fine.

freetown fred · By: **freetown fred** On: Sun Oct 28, 2012 7:30 am

So much for theory--I'm probably just an old fool, but when something is designed to accomodate a 6" pipe, I gotta kinda figure that the designers aren't idiots no matter what the rocket scientists think, and think about it--what options do you have left. Freddy's advice on the pipe venture sure looks good in my book.

blrman07 · By: **blrman07** On: Sun Oct 28, 2012 7:34 am

Much quicker solution. Install a motorized draft inducer. picture from ebay You can get them on the internet for between $100 and $150. One is currently on ebay for $90.

Rob R. · By: **Rob R.** On: Sun Oct 28, 2012 7:37 am

Sub-bit coal is a different beast, I expect it would clog that draft inducer up in no time.

freetown fred · By: **freetown fred** On: Sun Oct 28, 2012 7:43 am

What Rob R. said

SteveZee · By: **SteveZee** On: Sun Oct 28, 2012 8:08 am

I'm with Rob on this one. I'd stick with the 8" pipe (for the moment) and extend it at least a section (4') first off. That's the easiest and most practical thing to try. If it works, your off to the races. If not, then you may have to spend the bucks and convert to 6". I understand what that (wily) old farmer Fred is saying about design, but in the past many stoves were designed with a standard 5" or 6" ( some furnaces up to 8") pipe outlets that exhausted into larger masonry chimneys well before the advent of matching sized stainless chimney.
Is a matching flue to the size of the stove outlet better? Probably it is, but you have 8" which isn't out of the question larger, and putting an extention on to test, is the easiest way to find out if it solves the problem. JMHO.

Harmon MKII Backdrafting

Harmon MKII Backdrafting

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