Kuuma Vapor Fire 100 – Transition Season Operations

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burtman4

Member
Oct 16, 2014
5
Maryland
Looking for some tips or advice for fine tuning my Kuuma Vapor Fire 100 during the transitional seasons. I installed the wood furnace in January 2018, and it has outperformed my expectations so far and I am extremely happy with its performance. I am still learning how to fine tune things in the transitional seasons. Some background. I have a 1900 sq. foot rancher with a full basement, so I am heating approximately 3800 sq feet. The furnace is attached to a 23’ (approx.) masonry interior chimney with a 6” stainless Rockford liner that was installed when I put the furnace in. I am also located in Maryland near Annapolis. For my alternate heat, I have a 4 ton heat pump.

When the days are in the low 40’s at day in mid 30’s at night, I am struggling to keep it from making the house too hot at night. I keep the thermostat at 72, and prefer that temp at all times. During the day when it is in the 40’s, I can feed 3-6(ish) logs at time and slow things down when things are too hot. I like to load around 9pm night. If I fill ½ full or more, I end up with the house being 74-75 degrees about 1-2 am in the morning and the wife and I can’t sleep with it that hot. If I load ¼ to ½ I sometimes end up having to reload at 3-4am. I want to be able to load around 9am and at least go to 5 am or so and keep the house at 71 or above. I can do it, but it seems I have to load just exact right amount to do this. My damper is set between 5-6, and probably a little closer to 6, and I have the heat output on the furnace on the lowest setting. I burn mix hardwoods, oak, hickory, locust and some cherry. In the transitional seasons I do mix some popular in there when I have some, which helps.

Looking for suggestions, or past experience on what other users are doing. I am thinking of adjusting my damper down to 3-4 during these times, but wanted some feedback first. When it gets down to the mid 30’s at night I notice the in-efficiency in my heat pump, so want to use wood furnace. In the 40’s or above, I am fine with the heat pump as I don’t notice it on my electric bill that much. When it’s colder than this, the wood furnace does a great job at keeping the house at a steady 72 at all times.

Looking for any advice or suggestions here from other users. The furnace is great, and I have zero complaints about it’s performance.
 
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brenndatomu

Minister of Fire
Aug 21, 2013
6,556
NE Ohio
Lower your draft to -0.04".
Keep gathering poplar, soft maple (one of my favorites) box elder (in mine now) basswood, pine, or whatever else kinds of low BTU woods you have in your area to help with the low heat load times of the year.
And for the shoulder season I have found that making the firebox "smaller" helps. I put a row of firebrick on each side (mine are 2" thick) to make so it takes less wood to make it "half full"...burns better when its stacked high rather than short n wide.
You can also take the baffles out of the heat exchanger until real winter sets in too.
 
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JRHAWK9

Minister of Fire
Jan 8, 2014
1,728
Wisconsin Dells, WI
Just load the correct amount of fuel (wood) for your heat load. Judging loadings by volume of wood can be tricky when dealing with varying wood species though. I load by weight, so 25lbs of pine will give me approximately the same heat as 25lbs of oak. Why can't you load whatever it is at night and if the fire dies out, so be it. Letting it die out at 3am shouldn't be a big deal during the more milder temps of the shoulder seasons. The house temp will drift down some heading into day break and maybe by the time you get up you can then re-light/load again if needed. If you are trying to keep a constant fire going in these more milder temps, good luck. On low it consumes ~3-5 lbs/hr....so ~20 - 30KBTU's/hr gross. Delivered BTU's will be less (vs loading on coals) if lighting a fire from scratch, due to inefficiencies of a cold firebox and ductwork. So doing a cold start (or two) a day will work to your advantage in milder temps, even though you will be "wasting" more BTU's. If your house does not lose the BTU's you are supplying, you will overheat the house. You just need to better match your BTU output to what your house is losing for BTU's. Wood furnaces are not the best when being used during shoulder seasons. They take some good "fuel management" in order to match BTU's being supplied to house heat loss.

For example, I started a fire about 7pm the night of 12/4 loaded 20lbs with the house at 67° IIRC. Before bed at 10pm I loaded 35lbs on a nice coal bed. The fire died out and Kuuma blower shut off at 6am, according to my monitor. When I got up at 8am it was 18° outside, it was 68° in the house though, so at some point during the night the house temp peaked and once the fire died out the house temp started drifting back down. When I got up I started a fire again and did another small 25ish lb load because I knew it was going to be warming up into the low 30's and I didn't need much wood.


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burtman4

Member
Oct 16, 2014
5
Maryland
Thanks for the great input. Just lowered my damper down to .04. I like the weight idea during the shoulder seasons, will see what I can do here to setup a way to measure. That will help me get the right amount in the evening, as I do get it right sometimes now. Also, I have been trying not to let it go out for a cold start, not sure why, but you right, it won't hurt and that will help. The temperatures this week are in that zone so I will start here and see how I do.
 

JRHAWK9

Minister of Fire
Jan 8, 2014
1,728
Wisconsin Dells, WI
Thanks for the great input. Just lowered my damper down to .04. I like the weight idea during the shoulder seasons, will see what I can do here to setup a way to measure. That will help me get the right amount in the evening, as I do get it right sometimes now. Also, I have been trying not to let it go out for a cold start, not sure why, but you right, it won't hurt and that will help. The temperatures this week are in that zone so I will start here and see how I do.

If I may go one more step into "geek-oid", you may want to keep track of your daily loadings and HDD's. After awhile you will probably see a pattern develop that will allow you to get a pretty good idea of how much wood you will need for the day just based upon forecasted hi-low temps. ==c

I know what you mean about not wanting it to go out, I too, used to do that and I'd have house temps in the upper 70's. I know for me in the beginning it was a novelty thing....I just liked having a fire going. Loading on coals also makes for a cleaner burn and keeps the flyash down some in the heat exchanger. So the downside of having to constantly do a re-light is you may have to clean your heat exchanger one extra time. At least it's what I have noticed....it seemed worse when I was burning solely pine and doing two re-lights every day. I typically only clean it twice a heating season. Once at the very beginning before I even start burning and once sometime when outside temps allow me to sometime in January/February.
 

Highbeam

Minister of Fire
Dec 28, 2006
19,304
Mt. Rainier Foothills, WA
It’s the same thing in the stove world. I live in a pretty moderate climate. My stove company is famous for supplying a one year’s supply of matches when you buy a stove. You open the book and it’s one match!

Most of the year, full time burning too much heat for us so I start a new fire most days. Make it painless. Prechop a buttload of kindling and acquire a finger tip ignited propane torch. Get good at it.

Sure, in the dead of winter you might need full time fire. Maybe even full time high output fire but if your wood burner is capable of providing enough heat during that cold snap then it may be too big for the warm times. These wood burning heaters don’t have that much of a range. So in the warm times, let the fire go out.

The way some people fight this is by throwing in a handful of sticks every hour to “win” and keep a low fire running 24/7. I’d rather let the house temp swing a little.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,556
NE Ohio
That's why they need to get that VF200 back in production...the 100 is too big for many houses! I think we will see this type of question a lot here for a while just due to the fact that some people are buying what's available, rather than what they really need.
 

burtman4

Member
Oct 16, 2014
5
Maryland
I was thinking about keeping notes on load vs. temp as I tweak things to help. Good feedback. Going to load it up less tonight with damper at .04 and take notes, and not worry if it goes out. Can tweak from there. Will load on on my kindling too this week.

When I put the furnace in, I did a complete HVAC redo. I had oil forced air heat, an oil hot water heater and an older air conditioner. I put in a nice 2 stage compressor Trane Heat Pump, a heat pump hot water heater and the VF100 in. I keep good data on my electric use before and after the big switch. My plan was to be able to pay for the whole thing with the savings in fuel oil and maintenance over 5 years and hoping to keep my electric bill the same. I am close to on track for this. My electric use came down during the summer due to the higher efficiency heat pump, and went up slightly in the shoulder seasons and winter, and was close to net neutral overall. When it's in the 40's I don't notice a big difference in electric with the heat pump so I just run that. Once it gets in the 30's I then want to run the furnace as I see the KWh numbers go up on the electric.

The VF100 is sized great for me once it gets out of the shoulder temperatures and in the low 30's or below. When it get's down in the teens or lower with a good NW wind, I have the VF turned up, though the house is always at 72 degrees and I have never had an issue keeping the house warm. During a normal winter day, I also like only having to load it twice a day.
 
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Highbeam

Minister of Fire
Dec 28, 2006
19,304
Mt. Rainier Foothills, WA
Sounds like a good fit. Just need to suffer through the shoulder season with some small interior temperature swings.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,556
NE Ohio
Another thing you can try is to cut some 16" wood for light heat days...that way you can still stack enough wood in to make it burn right, but not last so long...