He built his own furnace and now he has problems

I built my own wood furnace and I have a question concerning the chimney I need for proper function.  The furnace I built typically exhausts the smoke between 250 and 300  degrees F.  Although the smoke normally travels through a system of heat exchangers, for tending the fire there is a by-pass baffle that lets the smoke directly enter the flue pipe above and behind the feed door.  The  flue pipe is 8" diameter and the door is 14" x 21", resulting in a ratio of  6:1 for door area to flue area.

There are no restriction or bends that  the smoke must make to enter the flue with the by-pass open.  I currently have the stove connected  by 2 feet of 8 inch pipe and two 90 degree elbows to a masonry chimney that has an  8" square flue. From inlet to outlet the chimney is about 20' tall.  The chimney is plenty high enough  above the roof line, and there are no hills or trees to the west of the chimney.  The house is an old leaky farmhouse, so there is no problem  with negative pressures inside the house. Most of the chimney is inside  the house.  I looked up the chimney with the aid of a mirror and there is no blockage.

The stove burns fine, but I can not tend the fire without a great deal of smoke entering the basement.  In fact, I can't open the door more than a few inches before smoke enters the house.  There is less than .02" of water column vacuum in the chimney.  I measured the temperature of the smoke exiting the chimney and it was 125 degrees F.

Would an 8 inch insulated stainless steel chimney take care of my problem?  I know that a round flue draws better than a square flue of the same area,  and by not cooling the smoke so much in the chimney,  I'm sure to get  more draw.  I don't want to spend the money on a new chimney if it won't  solve my smoking problem.  I was hoping you could make some suggestions.



I think the problem is that your door is too large, you have too many restrictions in the venting system, and your temperatures are too low.  With a smaller door, say 12 x 12 (which is all that is really necessary), you might get away with two flue pipe elbows and another 90 degree change of direction at the chimney base. Alternatively, you might get away with a big door if you had a perfectly straight system from the flue outlet up.  But the combination you have is guaranteed to spill smoke when the door is opened.  Also, with draft of only 0.02" and a flue gas temperature of only 125 F almost any system would spill.

Here are some suggestions, starting with the cheapest and easiest things to try:

  • try starting your fires top down so you don't have to mess with the fire so much at start up.
  • try to get your flue gas temperature up higher; good systems need at least 200 F and 0.05 draft to limit spillage; you probably need double that considering your other problems.
  • replace the two 90 degree flue pipe elbows with two 45 degree elbows to eliminate the horizontal run and reduce restriction
  • since you built this thing yourself, you could modify the door to reduce its size (a lot)
  • your 8 x 8 tile chimney is probably only 6 1/4 x 6 1/4 inside which is very marginal for such a large door opening; you could consider having a sweep or installer remove the clay liner and install a 7" stainless steel liner



How to heat a large L-shaped bungalow

There was a brief mention of Tarm boilers as being the only boiler/furnace to match the efficiency and emissions of wood stoves. Is this still the case, and if so, how does one find more information on this brand? Also, do you know anything about Charmaster? They claim to have a high-efficiency furnace design that burns the hot gases. The problem I have with all these manufacturer's claims is that they don't give specifications for what high-efficiency means, so I don't know how to compare it to a good stove. I'm also concerned about the emissions and again, no one publishes figures for their products.

By the way, the reason we ruled out a stove is that our house is a 1 story on an L-shaped plan and is 2500 sq. feet, with a lot of hallways and doors, so there doesn't seem to be a good place to put a stove. Furthermore, David thinks a furnace in the basement will help dry out the large unfinished basement (which I hope someday to use for yoga classes) and keep the floors warm on the main level. The house already has 2 electric furnaces with heatpumps (noisy, inefficient things, don't know why anyone would put them into a house in central Ohio); we would connect a furnace to the existing ductwork and also pre-heat our hot water which is also quite expensive and inefficient.

Any other suggestions for heating this space efficiently will be greatly appreciated. Janis

Hi Janis,

You wrote:

They claim to have a high-efficiency furnace design that burns the hot gases. The problem I have with all these manufacturer's claims is that they don't give specifications for what high-efficiency means, so I don't know how to compare it to a good stove.

Exactly, which is why we think all wood burning equipment, including fireplaces and furnaces, should be regulated under the EPA certification program. If you check the promotional literature put out by manufacturers of EPA certified stoves and fireplaces you will see very little hype about combustion systems with made-up names. They concentrate on other features that are a lot harder get away with lying about. Fortunately, there are now several wood furnaces that have EPA emissions certification.

I've heard of Charmaster, but I don't think they have an EPA certified product at this point.



Combination wood/gas furnace??

My wife and I are building a new 2000 sq. ft. home this spring.  We are thinking about installing a dual fuel heating system.  We want thermostatically controlled forced air heat.  This system will use LPG (Propane) as well as wood.  We would not want a pellet stove as the home is on forty acres of hardwoods that need thinning badly.  We figured we could cut and split our own firewood and use the LPG when necessary only.   Is there such a system out there?

Marty & Suzanne

Hi Marty and Suzanne,

I don't know of a combination wood/gas furnace.  In fact, in Canada such combinations have been deemed incompatible by safety code committees.  Even if you select separate furnaces and have them sharing the same ductwork (which I think is the only viable option) do not attempt to vent them through the same chimney.  Gas and wood venting requirements are not compatible at all.

Are you aware that a well-constructed 2000 sq. ft. house can be heated effectively with a single wood stove or heat producing fireplace?  These are much more efficient than most furnaces, cleaner burning and you get the bonus of a spectacular fire to watch all winter long.



Wood burning to heat several rooms

I am looking for a fireplace design that will send hot air to several rooms as well as have a nice living room view of the fire burning in a typical fireplace. I have seen something like this on THIS OLD HOUSE or one of Bob Villa's programs. Can you lead me to such a product? Thanks!

There are only a few fireplaces like this: Security BIS II, RSF series of fireplaces, Regency WarmHearth, Fireplace Xtrordinare, Vermont Castings WinterWarm are about it. Check with the specialty fireplace shops in your area. They will carry one or more of these.



Do central systems get a bum rap?

I know this website is about woodstoves and fireplaces but I think central wood heating is getting a bum rap from some of the writers.  I have a Tarm boiler and it is very efficient and very clean ( 1 gram emissions per hour ).  Not all central heaters are dirty and wasteful.


Hi Ross,

I agree completely with your assessment of our site and with the frustration I sense regarding the treatment of central heating systems.  The fact is that I started my wood burning career in a wood furnace factory and was a true believer in central heating for several years.  You are quite right that not all central heaters are low-tech and that the Tarm is a fine example of what is possible.  The Tarm is not EPA certified but there are now some forced-air wood furnaces and outdoor boilers that are emissions certified.

The other problem is that the installation of central heating systems is complex and they have electric control systems that must be installed right or the unit will overheat and be hazardous. Some people might be able to self-install them, but I think they would be the rare exception. Most experienced wood heat retailers don't sell or install central heaters.

We don't particularly want to be seen as promoting just stoves and efficient fireplaces.  Our mandate is to promote responsible wood burning, not just a couple of classes of appliance.


Ross responds:

John, you are not the only one frustrated.  I'm a large animal veterinarian in northeastern Wisconsin.  I spend my day working with farmers - one quarter of whom still heat with wood.  When I tell them about my Tarm I get one of three responses.

  • Hey, that sounds good.  Where is the nearest dealer?
  • Hey, that sounds good.  When my current furnace wears out I'll look into that.  Where is the nearest dealer?
  • If I knew about them earlier I wouldn't have bought that XXX!!! outdoor wood boiler. Where is the nearest dealer?

I have to tell them, of course, that there is no nearest dealer and they would have to order them from the distributor and then find a qualified installer.  At that point their interest usually ends.  Boilers are expensive units and with past low sales. Central furnace and boiler manufacturers seem unable to maintain a dealer network.  Now that fuel prices are up more units could be sold but they don't have the network in place to do so. Most people have never heard about central heating so never consider getting one. Everyone has heard of or seen an outdoor wood boiler so that is what ends up getting bought. This is a Catch 22.