Measuring the chimney draft on a pellet stove

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Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
I have no idea if that is a design flaw or
what but the ash build up back there is why
people keep saying is your stove clean . It is behind
things and hidden passages that get plugged and
compromise the output and operation of the stove
Well I cut out some thin metal pieces and managed to wedge them in temporarily. I also change the setting for my high feed rate. I think I had it backwards. Plus all the cleaning... Now I'll need to test it out.

To be fair, I've cleaned out that area in back before. Perhaps not perfectly, because it's very hard to reach even with my smallest vacuum attachments. Mfg says to clean it out once/year or every ton of pellets. I'm sure I have cleaned it more often than that, and I was complaining about the heat output back in 2008 when the stove was brand new so ash build up couldn't have been my primary issue at that time.
 

SidecarFlip

Minister of Fire
Feb 7, 2010
5,273
S.E. Michigan
Manufacturers always instruct in owners manuals in general terms. Ash buildup will always depend on the pellets being burned.

About the only thing not in general terms is the distance to combustibles in the install pictures and diagrams and the fact that any stove has to be set on a non combustible surface
 

Clarkbug

Minister of Fire
Dec 20, 2010
1,247
Upstate NY
Well I cut out some thin metal pieces and managed to wedge them in temporarily. I also change the setting for my high feed rate. I think I had it backwards. Plus all the cleaning... Now I'll need to test it out.

To be fair, I've cleaned out that area in back before. Perhaps not perfectly, because it's very hard to reach even with my smallest vacuum attachments. Mfg says to clean it out once/year or every ton of pellets. I'm sure I have cleaned it more often than that, and I was complaining about the heat output back in 2008 when the stove was brand new so ash build up couldn't have been my primary issue at that time.
I hope that makes a difference for you as far as the output.

Maybe email the PelPro folks with that picture to see if there is something funky going on?
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
I hope that makes a difference for you as far as the output.

Maybe email the PelPro folks with that picture to see if there is something funky going on?
I haven't been overly impressed with their responses so far. However, after cleaning those deep recesses of ash, it did impact the vacuum. Rather than being up around .8WC, it dropped closer to .4WC under the same settings. So the ash was restricting the air flow.

My temporary fix for those sections failed during startup. As soon as things started heating up the metal plates gave way to the pressure difference, reinforcing my impression that the exhaust would use those paths rather than traveling over the heating tubes. I need to start over and make something bigger with more surface area to attach. I'd rather drill a hole and put a screw behind the plate, but it is an awkward tight angle/area to work in. Might be worth a try though.
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Well I solved my issues with getting temporary plates to stay put while I tested. I started up the stove, and things went poorly. Those large, unobstructed openings allow a high flow of air between the combustion chamber and the exhaust fan. Once I block them off, the only path is up, around the heating tubes, then back down and out. Just what I want, right?

Well my issue of "too much vacuum", was no longer an issue. The air flow is too restricted, so I'm now getting just barely enough vacuum in the firebox to keep the stove from shutting down during startup. I have to keep the intake nearly shut to manage .2WC. If I open it an extra 1/4", the WC drops to .15 which is below what the vacuum switch is set for.

The space behind the firebox, where I had an ash buildup now has a much higher vacuum than the firebox itself. The steel plate covering the openings to clean out that area is not perfect, so now hot exhaust is being pulled behind the plate directly, as a short cut to the area of higher vacuum. Unfortunately, this means some of the flames which should just be going up to the heating tubes, are now also being pulled into the pellet chute. We all know that's a bad thing. If that gets to hot, my pellets could catch fire before dropping. So I shut it down.

It feels like they made a poorly designed stove, then made the large openings as a cheap/easy way to get around the bad design. It keeps the air flowing and the fire burning bright, while letting your heat flow up the chimney rather than heating your room. No one will be the wiser...
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Hah! Mystery solved...

I went poking around (literally) and there are *no* openings behind, or above the heating tubes. The hot exhaust is not drawn over them at all. They are just positioned at the top of the firebox, so any heat naturally rising up there can be used to heat the tubes. The only way for the exhaust to leave is to be drawn to the side of the heating tubes.

I still think it's a major flaw for the efficiency of this appliance.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
My exhaust is forced over the tubes and then down the back of the stove
The exhaust is then sent up the chimney. There is a removable plate just
under the heat exchanger that forces the heat to the front of the stove and
is pulled over the tubes from front to rear removing the max heat possible
 
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Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
My exhaust is forced over the tubes and then down the back of the stove
The exhaust is then sent up the chimney. There is a removable plate just
under the heat exchanger that forces the heat to the front of the stove and
is pulled over the tubes from front to rear removing the max heat possible
That was my understanding of how it is supposed to work for pellet stoves. This design is just stupid. Not to mention the heat exchange tubes are only exposed to the heat of the firebox for about 6".

I did improve my heat output. By cutting down my metal pieces that were blocking the holes, I still have the bottom 1/2 of the holes covered, forcing the hot exhaust to go up and around at least a couple of the heating tubes. Now my max hot air output was around 235F, compared to 180F the other day. A simple change that would be much better, would be if they had made a thin opening along the top on both sides so the exhaust would go up and around the the full length of the tubes to get out, rather than two large openings at just one end.

My pellet feed seems to be going faster now. So that could also be the reason for more heat. Previously my exhaust temp was around 350F, but yesterday it was solidly up over 400F once the stove got hot.

It sounds like the ultimate solution is really to get a different stove entirely if I want better heat and efficiency. Otherwise I'm spending my money on pellets to heat the outdoors.
 

SidecarFlip

Minister of Fire
Feb 7, 2010
5,273
S.E. Michigan
Otherwise I'm spending my money on pellets to heat the outdoors.
Gotta keep the birds warm.... All biomass stoves shed heat with the exhaust. Fact of operation.
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
I know Harman is recommended as a good brand. Any others? (I'm looking for functionality, ease of maint/cleaning and reliability). Aesthetics aren't important to me. I prefer not to spend a lot of money just to make it pretty.
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Looking at the Harman P Series maint video, it looks like the heat exchanger is setup the same way. The back looks solid, as does the top. So the exhaust is not flowing along the heat exchanger. It's just picking up heat from being at the top of the fire box for 1/2 dozen inches or so.

1635701528321.png
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
I have an Enviro and the exhaust has to go
over the exchanger before exiting the stove.
I like the Enviro easy to clean, easy to use
and they have very few problems. The manuals
are good with flow charts for troubleshooting
and explanations on how to.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Depending on what you want mine is an old EF3 pre-runner to
the Meridian I also like the M55 looks a little like a Harman.
For real heat then it's the Maxx.
The stoves are made in Canada
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Depending on what you want mine is an old EF3 pre-runner to
the Meridian I also like the M55 looks a little like a Harman.
For real heat then it's the Maxx.
The stoves are made in Canada
The Enviro Meridian also looks like it is solid at the back of the heating tubes, although I can't see if there is an opening above the tubes.

1635704501209.png
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
It's a little hard to see, but the EF2 looks similar to the Meridian, with a solid back behind the tubes. Since all these models seem to have a scraper, how does your EF3 work if there is no solid back behind the tubes? How do you clean the tubes once they go behind the firebox?
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
The back of the stove is fluted the exhaust is pushed towards the glass by a removable plate
along the heat exchanger and vented down behind the fluted plate. The fluted rear is removable for yearly cleaning.
The heat does not go over the heat exchanger and then out. It runs along the heat exchanger and then out
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
The back of the stove is fluted the exhaust is pushed towards the glass by a removable plate
along the heat exchanger and vented down behind the fluted plate. The fluted rear is removable for yearly cleaning.
The heat does not go over the heat exchanger and then out. It runs along the heat exchanger and then out
Next time you have it apart for cleaning, could you take a picture or two of the back-top of the firebox where the exchanger goes to?

So far, the Comfortbilt HP22 is looking good. I can't find any pictures or video of that area though.