How to add on a wood furnace
The Daka is very simular to "Hotblast" furnace I have, and it will have the same drawbacks as well. The design is horrible and inefficient and smokey. The furnace works best with a tall chimney with a strong draft, shorter chimneys with a weak draft will cause smoke to come out the air inlet on the loading door. I ended up sealing close the combustion air inlet in the loading door and just using the one in the ash pan door. The furnace like all old furnace designs will tend to over heat your house and when you cut back on the combustion air to reduce the size of the fire, the fire gets very smokey of course and soots up your chimney. I solved that problem by installing a 1 ½ inch pipe through the front of the fire box to add post combustion air at the edge of the smoke shelf. Now I can dial down my fire without smoking up the neighborhood and my chimney stays much cleaner.
On hooking up the furnace, the easiest way of doing it, is to place the wood furnace next to your gas furnace, with foot or more space in-between to prevent over heating the gas furnace from the radiant heat that the wood furnace will give off. Cut a hole into the plenum, the vent piece that connects your gas furnace to the vents above it, and install a sheet metal piece that blocks off the top of the plenum from the duct system, then using the hole you cut, run a duct to the bottom of the wood furnace on the side towards the gas furnace. You should end up with a large rectangular duck running down between the two furnaces connecting the blocked off plenum of the gas furnace to the bottom of the wood furnace. The Wood furnace should be underneath the largest duct in the duct system, cut a large square hole in the top of the wood furnace covering and in the duct above and connect with a sheet metal plenum. The idea here is to put the wood furnace in series with your gas furnace, with the connecting ducts being large enough not to restrict the air flow. This is the simplest and best way to add on a wood furnace. The control system is simply to wire your gas furnace fan to a fan control switch installed in the wood furnace plenum. If you wish to be able to run the furnace without electricity, install a bypass duct that runs from the cold air return duct to wood furnace with a slide in damper which is only removed when you are operating the wood furnace without electric power. When the side in damper is removed, the air from the cold air return is directly connected to the bottom of your wood furnace and with a large plenum you can probably heat your house pretty well without the fan to move the air.
The above is the best way of hooking up a hot air wood furnace, but, it does require that you are rather good at sheet metal duct work. They do sell prefab duct parts and you can make a large plenum by putting two or more smaller ducts together as one. Otherwise the quick and dirty way to add on one of the cheap wood furnaces is to use 6 inch round duct and just connect the hot air from the wood furnace into the duct work, and tie the fans of both furnaces together electrically so they both run when either one runs. (stupid, stupid, stupid) That is how they will tell you to do it, and it can work, but wastes power and has big problems if one fan has much stronger 'push' than the other. If your gas furnace fan has higher out put air pressure than the fan on your wood furnace, it will push the hot air backwards right out of the fan housing on the wood furnace and you would end up heating the basement instead of the upstairs. Which is why the series hook up is the best, it takes more work to set up, but avoids lots of problem and doesn't waste power running two fans.
I also will recommend that you shop around on the internet for a better wood furnace, DAKA is cheap junk and will waste a lot of wood. A better furnace will give you the same heat for half the wood and will not be smokey. The only two draw backs are higher cost and they need electricity to run. Check out Yukon's "Wood Jack" add on wood furnace. "Woodchuck" is nice too and burns coal also. Always look for furnaces that have "post combustion air" or don't buy one.
Sincerely Yours; Wm Scott Anderson