stove top temps, Fisher insert

[spoon059]

I have a 1981 Fisher insert, have had it for about 5 or 6 years. Grew up with a 1979 Fisher that my dad has had installed in two houses now. I've got a stainless steel 8" liner all the way up my chimney and the full surround panels on the front of the stove. Because of that, I can't measure my flue temps 18" above the deck like I would want to do. I use an IR gun to measure temps of the stove top to give myself an idea of what is happening. I try to keep my stove top at 450 degrees.

We have had some colder than normal temps in the DC area, down to around 0 overnight. I've managed to keep the house relatively warm throughout, sometimes dipping down to 67, but usually able to keep the house around 71 or so, which is my goal. I will admit that at night we load up the stove and close down the vents to keep the fire going longer, but we get it up to 450 before we go to bed and it will still be in the high 200's or low 300's the next morning.

With these colder temps and even worse wind chills, I've noticed some pieces of creosote in the snow in the back yard. I went and looked at my chimney cap (standard cap with mesh to keep animals out) and noticed that I have an excessive buildup of what appears to be creosote on the mesh. I've never noticed anything like this before and I'm not sure what I am doing wrong.

I would think 450 degrees on the stove top are sufficient temps for burning. My wood is mostly oak or maple that has been stacked for about 3 or 4 years. I'm curious if just colder than normal temps and high winds are causing my exhaust to cool off faster, leading to more creosote buildup, or does it sound like I have another issue?

I appreciate the help. Tonight is supposed to be the last of the bitter temps, we should be seeing temps above freezing during the day and into the high teens at night now... more normal temps for my area.

 

[coaly]

You didn't mention if the liner is insulated. That makes a big difference keeping the gasses hotter to the top. (above 250* exiting with smoke present prevents formation in the flue)

The water vapor that normally rises out of the stack hits cold enough air like the exhaust coming out of your car. It condenses and allows smoke particles to stick.
If you have a masonry chimney you can put more heat in to it daily by burning 10 or 15 minutes with doors open and screen in place. (suggested in manual)
Screen blockage in bitter cold temps with wind is common since no matter how hot the gasses are exiting, cold air blowing across the screen is going to be cold enough to condense any water vapor on it.

A baffle in the firebox reduces smoke particles, reducing the particles that cause the formation. You then run the flue damper open more than without baffle, since the baffle slows the rising exhaust which is what the flue damper does.

 

[WV burner]

Coaly is spot on(as always) I had the same problems with my fisher insert. I added baffle (see pic) and got a new cap. My liner is insulated with vermiculite. It was bad before with temps like we had this week. It appears a lot cleaner from the ground. I hope to get on the roof to double check tomorrow.

 

[spoon059]

Thanks Coaly and WV! I believe my liner is insulated with a metal cap on the top of the masonry chimney, but I'm not 100%. I'll double check this spring when I get up on the roof again.

I have seen the mention of the baffle, but I've resisted thus far because I love the huge firebox and ability to load up with wood overnight. If I had a baffle I would have less smoke particles, but would I get higher temps or longer burn times? I'm really hesitant to modify in a way that reduces my interior volume, I LOVE being able to load up and burn good through the night... especially this last 2 weeks of unusually cold weather for our area!

Again, I appreciate you both giving me timely responses

 

[coaly]

When out, put a load of wood in so you know how much you're loading. Make a cardboard baffle and see if it fits on top of your wood load or how much you would have to remove to make the baffle fit. That will give you an idea if it encroaches on your wood load. It doesn't decrease the loading capacity of the single door stoves.

You'll get higher temps on the top that extends out of your hearth and it takes the high temperature spikes out giving you a more controllable burn. A masonry chimney without an insulated liner can benefit from that extra heat going into the mass, but a metal or insulated chimney wastes that intense heat lost up the flue.

 

[WV burner]

I thought the same things and was hesitant but it has served me well. The front of the stove seems hotter also. There is never any creosote on the inside of my doors now. Good thing is it is cheap to do and if you dont like it you just take it out. I have also found my coals durin night burns seem to stay hotter longer.

 

[spoon059]

WV, what did you use for the plate? Is that a piece of stone or is it a piece of steel?

 

[coaly]

I wondered that too, it looks strangely thick!

You want to use a steel plate 5/16 thick for the width of an Insert. The thickness of firebrick takes up space and you want the plate to get hot enough to ignite particles that come into contact with it. Think of a heat range of a spark plug getting hotter with less mass. For that reason a thinner plate would be better, but the width of the Insert requires 5/16 to prevent warping and sag in time. 5/16 has stayed straight in mine since 1985. It is pictured in the baffle thread and looks like new today. Over fired very rarely, but it was my sole heat source for many years.

You may be tempted to raise it and keep it flat, but lower it in the back and raise the front to angle the plate. A horizontal plate adds much more resistance, (too much for some chimneys causing smoke roll in opening doors) and causes flame tips to dissipate too much heat called flame impingement. This lowers the burn zone temp causing more particulate.
His notches cut at the ends prevent stagnation in the corners. Cut them out about 2" X 3".
This design was proven through Fisher research and development to reduce smoke from 60 grams per kg burned down to 6 grams per kg burned.
It wasn't enough to pass the stricter EPA regulations that came into effect over time which shut down Fisher as well as many other manufacturers in 1988.

 

[WV burner]

That's 5/16 or 3/8" steel it just looks thick in the pic. I first used it without notches last year but added them this year and it seemed to help with smoke roll. I only have 6" liner ( existing when I bought house) which is not enough so I open the doors slow. Last year I also tried to make things similar to the factory heat shields,but direct the air to top of stove. That was a mistake as it lowered my flue temp and was a creosote disaster! I removed a buck stove insert to install the fisher and there is no comparison as to heat this Fisher produces!