What do I need to know about BTU ratings?

Many people are confused about the BTUs required to heat a home because oil and gas furnaces do not run steady hour after hour.  If they did, you’d spend $20,000/year to heat a home.  As it is, some people are spending over $4000/year to heat their homes.  This is because they cycle on and off.  Your BTU needs are delivered intermittently.  If your home is poorly insulated, or if for some reason you let it cool down, your oil and gas furnace would then run steady.  Generally, a furnace rated for 100,000 BTU/hour runs from 1/4 to 1/3 of an hour on the coldest of MN days, delivering about 30 to 40,000 BTUs/hour to maintain a comfortable heat level for your home, i.e., 70 degrees F.  Most people feel cool in their homes in between the on and off cycles of the oil and gas furnaces.  Therefore, many people prefer wood heat because it’s continuous.

Now, heating a home with our Kuuma Vapor-Fire furnaces is really special because they deliver very even continuous heat hour after hour.  They don’t smolder the wood and blast the wood (the two extremes) like other wood furnaces.  Instead, its unique automatic draft system avoids these extremes with a continuous front-to-back burn pattern that allows the user to adjust the burn rate, which in turn varies the BTU output.  In mild weather, you use a low setting for 15-25,000 BTUs/hour.  in moderate weather, a medium setting for 25-45,000 BTUs/hour. in cold weather, a high setting for 45-60,000 BTUs / hour.  It adjusts simply by turning the knob on the computer from low to high and anywhere in between.  Most people never go above the medium setting all winter long.

Remember that all wood has the same # of BTU’s pound for pound.  The heavier, more dense wood (oak, hickory, maple) has more available BTUs/piece than the lighter, less dense wood, such as poplar and pine, only because each piece weighs more.  Every pound of wood that is 100% dry has approximately 8,600 BTUs/pound.  Say, for instance, your home needed 100,000 BTUs/hour to heat, and it would take 11.6 pounds/hour of 100% dry wood to heat it.  Now, if your wood has a 20% moisture content, each pound of wood has only 6,880 BTUs in it (it’s 8600 x .80).  Then, if your wood furnace is only 60% overall efficient, your BTUs would only be 4,128 BTU’s/pound (that’s 6,880x.60).  This would result in a ridiculous amount of wood, 24.2 pounds/hr., to get the needed 100,000 BTU’s/hr.  In a 24-hour period, that would mean 576 pounds of wood required for 100,000 BTUs/hr. after hr.  This would result in probably more than 20 cords of wood to heat your home for one winter.  So it makes more sense to say that most homes in the coldest of weather only use 30-40,000 BTUs/hour and probably use 7-10 cords of wood/season to heat their homes.  With our Kuuma Vapor-Fire furnaces that are 85% overall efficient and 99% combustion efficient, most people use only 3-5 cords of wood to heat their home/season, and that’s what makes our furnace very special.  We also have a clean burn of less than 1 gr—emissions/hour to top it off.

What do I need to know about the Kuuma Vapor-Fire 100’s EPA Phase 2 Certification?

The Kuuma Vapor-Fire 100 is EPA Phase 2 certified.

What sets us apart is that we chose not to pursue the EPA NSPS Phase 1, which only certified a manufacturer from May 15th, 2017, until May 15th, 2020.  Instead, we chose to go immediately after the 2020 EPA Phase 2 emission standard (much harder!), meaning that each certified test burn had to result in emissions below 0.15 pounds of particulate matter per million BTU of heat generated.  Our Kuuma Vapor-Fire 100 came in 40% below this stringent standard.

The road to certification was not easy, and we were the Guinea Pigs for forced-air furnace testing and certification.  It was a paperwork nightmare and a red tape jungle.  It took from October 2016 until October 2017 before we finally received our certificate.

The Kuuma Vapor-Fire 100 currently holds two records.  It is the cleanest-burning and most efficient cordwood furnace ever tested.

Does the Kuuma Vapor-Fire 100 have an automatic damper?

Yes, a 24-volt computer controls a 24-volt (step) motor for your primary intake air used in combustion.

Does the automatic damper operate by a remote thermostat?

No, the adjustment is right on the computer itself.  You turn the knob to adjust (mild weather = lower, cold weather = higher).  It makes it simple to operate.

How do I replace a computer or damper that isn't operating correctly?

All of our electronic components carry a 1-year factory warranty.  Fortunately, the majority can be repaired and updated to the latest versions for $245 + your shipping cost to our repair technician.  This process includes us shipping you a loaner to use until the repair is complete, and a return label for you to ship it back.  Otherwise, you could purchase the latest versions of our computers for $685 or a damper for $625.

Can you install it alongside a backup furnace using the same ducting?

Yes, this is called commoning the two furnaces.  Today, we always connect the furnaces in parallel so that one furnace can run, or the other furnace can run, so both can run together.  You will need to install 2 one-direction ducting flaps.  Preferably, a simple check flap; for more information, please see the operator’s manual (Commoning Two Furnaces Video).

How to make a simple check flap (PDF)

Does the Kuuma Vapor-Fire 100 have a secondary chamber for the wood gases to burn?

Yes, right in and around the top of the fire chamber, gasification occurs in a ceramic-lined area with secondary air present.

What type of blower system is used to deliver heat?

It has a two-speed blower that is controlled by a remote thermostat.  The blower system is activated by the low limit button thermostat that adjusts 105-130 degrees F. for the on temperature.  Once it activates, your remote thermostat will determine if it needs high speed (calling for more heat) or low speed (room temp is satisfied.) The high speed is approximately 1500 CFM, and the low speed is approximately 500 CFM, depending on the system backpressure.  Therefore, the computer setting and the 2-speed blower help you to maintain your desired level of heat.

Is the firebox brick-lined?

Yes, it’s not only brick-lined right to the ceiling but ceramic-lined behind the brick.  All the internal parts are stainless steel and replaceable (ceiling, back, brick holders & straps, front).  It’s a lifetime furnace design.

Where should I place my Kuuma Vapor-Fire? 

I always tell people that placement is first based on a good chimney connection and second on connecting to the ducting.

Can your chimney provide the proper draft needed for all modern furnaces?

• Don’t use a restrictive chimney cap.  We recommend a chimney hat with no screen, mesh, or restrictions.  This is the one we use, and it works great!  Famco (Wind Directional Chimney Cap)
• You should only use one 90-degree bend at most, and that needs to be directly behind the furnace to go straight up the chimney.
• Use 45-degree bends.  Make every effort to avoid horizontal runs.  Hot air doesn’t want to run horizontally.
• The Vapor-Fire uses a 6″ diameter stove pipe.  You can connect to a chimney of equal or larger size, but nothing with a smaller diameter.
• If your chimney is taller than 25′ in a windy area, you may need an additional barometric damper to control the chimney draft.
• External chimneys (especially unlined), either masonry or stainless steel, may experience draft issues with all modern furnaces.  Please ensure your chimney can provide the necessary draft.

What length of stove pipe do you allow from the furnace to the chimney?

Up to 12' of stove pipe can be run from the furnace to the chimney as long as it always angles upward towards the thimble (where the stove pipe attaches to the chimney). It is best to place an adjustable angle directly into the smoke collar on the back of the furnace and then angle up to where you attach to the chimney.

What size plenum is needed?

The opening for the bonnet is 2’x2′ & 2’x2′ for the return air.  The bonnet height should be approximately 2 feet.  Your heat output duct should be at least 180-200 square inches to prevent excessive backpressure.  (Plenum Sizing Video)

The plenum isn’t included but can be purchased locally.  For example, here is one at Menards (click here).

What size filter does it take?

It takes two 14x24x1 filters, which are included with the purchase.  When you need to replace it, we use Filtrete MPR 300 MERV 5.

How do I operate the Barometric Damper for Kuuma Vapor-Fire Furnaces?

Our owner’s manual says you should have a draft between .03 and .06.  These are generic numbers based on the average house.  In reality, the chimney of a two-story home that is 30-35 feet tall in most cases will generate substantially more draft than a chimney in a ranch-style house.  A good starting point on a two-story home would be to set your barometric damper on #3.  If you have a one-story, ranch-style house, start on #4 or #5.  If you get a high temp alert (steady tone from computer control), reduce the number on your barometric damper by 1.  On the other hand, if you consistently get a huge coal bed at the end of the burn, then increase the number on your barometric damper by 1.

Will the furnace heat during power outages?

Yes, when the power stops, the blower and computer both shut down.  The computer closes to its pilot air opening, causing the fire to die down slowly.  You should open all heat ducts, remove the air filters, and rely on gravity airflow then.  The front lift-off hood can also be removed easily to allow additional heat to escape from the air jacket during power outages.  A backup generator or a 24-volt ac source would be handy.

Power outages should not be an issue with this furnace.  If you are not home – just leave it alone.  but if you are home – we suggest removing the front hood, as this will allow more heat to be released.  If you think power will be out for more than a few hours, you can simply put a standard grounded plug on the power line going to the furnace.  If the power goes out, simply unplug the cord and attach it to an extension cord.  You can then run this to even a very small generator, and you will have your full heating capacity.  However, I would not do this unless you expect the power to be out for many hours or days.  This is not mandatory, just an option to have full heat.

Can the furnace be used off the grid?

Yes, you would need an alternative power source (solar/wind) to produce 120 volts up to 6 amps when operating at high speed.  However, most of the time, it would be less using low speed, only running at 2.6 amps.

Can you run off-grid without using the blower?

Yes, in this case, remove the entire blower assembly to allow better gravity feed through the furnace. You can run the electronic controls using a 24-volt battery source (flashlight batteries), which only consumes millivolts.

Can this furnace be put in an attached garage or shed?

Yes, many people do that, but it is required that you hook your cold air return to the cold air return in your house. You can’t draw cold air from the garage for fear of carbon monoxide, and all your ducting and plenum need to be insulated.

What is a fresh air inlet, and do you need one?

If you have a very tightly sealed home or experience smoke getting into your home, you likely need to add a fresh air inlet into your furnace room.  It’s common that when running the laundry dryer, range hood, or bathroom exhaust fans in a very tight home, you can actually reverse the draft in your chimney.  The solution to this is adding a fresh air inlet.  When done correctly, this will allow adequate combustion air for your wood-burning appliance without flooding your furnace room full of cold air.  As you see in the diagram, we suggest putting a vent through the sill plate of your home and using a dryer vent cap on the outside; inside the furnace room, run a simple 4″ flexible duct from this dryer vent cap down to about 6″ above the floor.  Place a Sonotube (often used for concrete deck piers) around the duct tube.  Cold air does not like to rise, and the Sonotube will prevent the cold air from flooding the furnace room.  The furnace will draw only as much air as it needs to burn effectively.  In many states, a fresh air inlet is code for new homes.  It costs about $20 to make one of these.

Fresh Air Inlet (PDF)

See blog post

What length of wood is best, and how large diameter should they be before they should be split?

The best length is 20″.  If they’re smaller pieces (4″ or less in diameter), can leave them round or un-split, but if they’re larger, they need to be split so they will dry better (5-8″ in diameter).

(Firewood Basics Video)

Safety Features?

It has an insulated air jacket for 6″ clearances.  Low flue temperatures and complete combustion add to safety.  The 24-volt computer is set to keep the fire from under burning (causing creosote) or overburning (causing fire chamber damage).  The 24-volt computer has an alarm that goes off if the furnace ever overheats (ash pan or main door left ajar).  It has a high limit plenum sensor that causes the blower high speed to activate regardless of your remote thermostat and will also shut the computer off if the temperature continues to rise to 250 degrees F.

What size home will it heat?

It will heat approximately 3,500 square feet if the home is properly insulated.

How does the burning of firewood compare to other fuel costs?

“I have a 3200 square foot home and usually buy 5 cords of birch and maple wood a year.  The cost is $75 – $100 per logger’s cord, and I cut and split it myself.  My total cost is about $500 per year or less, approximately 1/4 to 1/3 the cost of using another fuel such as gas, oil, or electricity.  The heat is much more constant using wood, that’s why I like it.  Our backup heat is electric baseboard.” says Lamppa Manufacturing Owner Daryl Lamppa

What is the white exhaust you see coming from your chimney?

When the temperature is 32 degrees F. or less, the exhaust you see is actually water vapor condensing to form steam.  The two byproducts of complete combustion are water vapor and carbon dioxide.  If the outside temperature is above 32 degrees F., no visible exhaust occurs when complete combustion occurs.

How often do I clean the furnace?

Once a year, I use a rake that comes with the furnace to scrape out the heat exchanger.  Then, I proceed to vacuum through the clean-out cover.  Finally, I take the pipes off the back and blow them out outside.  I’ve run this furnace for 28 years and never clean out the chimney; as a matter of fact, I don’t even own a chimney brush (NO SMOKE=NO CREOSOTE).

Installation and Operation Tips:

Selling factory direct across the country, we’ve worked with pretty much every type of individual setup that you can imagine.  The original Kuuma Vapor-Fire was created 40+ years ago, so we’ve fine-tuned how to operate it optimally in the real world, not a test lab.  If this was sold through a dealer, you would easily have to pay an additional $2,000.  (Installation Videos 2021) 

First Recommendation:

Call us directly and speak with Dale.  It’s much easier to explain individual setups or questions over the phone or in person than via e-mail.  If desired, we can use Facetime for specifics as well.  We want to get it installed optimally.

General Recommendations:

• Troubleshooting (Blog Post).
• Use seasoned cordwood (cut, split, and seasoned for one year.  (18-28% moisture is ideal).
• Recommend 20-21″ cordwood cut in larger chunks 6-8″ diameter.  6″ rounds are ok; 8″ rounds should be given a single split.
• You can burn almost any kind of firewood, including Tamarack/Larch, Hedge, furs, Pines, etc.
• Normally you should be able to get a 12+ hour burn on one load of cordwood.
• Install your furnace on cement blocks.  It makes loading much easier as you age and want to keep burning wood.
• It’s best to install the furnace in the basement or an attached garage.
• If you have long, warm air duct runs, please advise us because we may want to step up to a larger blower motor to ensure adequate heat transfer.
• Make sure your chimney can provide the proper draft
  • Don’t use a restrictive chimney cap.  We recommend a chimney hat with no screen, mesh, or restrictions.  This is the one we use, and it works great!  Famco (Wind Directional Chimney Cap)
  • At most, you should only use one 90 degrees bend, and that needs to be directly behind the furnace to go straight up the chimney.
  • Use 45 degrees bend.  Make every effort to avoid horizontal runs.  Hot air doesn’t want to run horizontally.
  • The Vapor-Fire uses a 6″ diameter stove pipe.  You can connect to a chimney equal or larger size but nothing with a smaller diameter.
  • If your chimney is taller than 25′ in a windy area, you could need an additional barometric damper to control the chimney draft.
  • External chimneys, either masonry or stainless steel, may experience draft issues with all modern furnaces.  It is your responsibility to ensure your chimney can provide the necessary draft.