40 foot run from wood add on furnace.

Hi, actually two questions about a possible add on stove.

1. I would have to run about 40 feet of 18" x 18" duct from my add on wood stove before it connects to the central furnace main duct line. Then from there the heat would be distributed to each room from the central duct line. All in all, it would be about 60 to 80 feet of ducting before the heat reached each room.

Is that too long? Too much heat loss?


2. I have a couple of room that are heated via coil ducts, there is also duct tape and foil tape a various junctures. Now I understand that you should only have metal duct for add on wood stoves. In this case the heat would travel over 40 feet before it ever reached any coil ducting. Is this a safe operation?


Thanks, looking forward to spending time here.

I would recommend someone who deals in hvac. An 18x18 trunk is pretty big, larger isn't better. The ductwork would need to be sized correctly to keep velocity up to the main trunk, possibly stepping down the ductwork before it met the main trunk. Why so far away from the central furnace, and what model of furnace are you looking at? Could you post some pics, or describe the setup a little more? Insulation would help cut duct loss and being a large run, you will want to make sure things are sealed correctly. Our main supply trunk is a little over 40' but things work well and are sized accordingly.

I'm looking at the energy king 385. I have to set the furnace so far from the central furnace because the way my basement is setup, there's just no room to set it near the central furnace, and then of course no where practical to have the flue. So it will have to be on the other side of the basement.

I have an havoc guy coming out. My concern was the heat buildup in case of power outage, and the coil ducting that was over 40 feet away. Not sure if that was sufficiently safe or not.

You could always get a boiler instead and use a water to air exchanger in your existing duct work. It may be more $$$ but you wouldn't have the safety concerns.

I had a wood furnace with a long run. Duct work was in attic, but insulated. One problem was that towards the end of ductwork run , air flow is greatly reduced. Could close off other vents to help out, but that leaves some areas with little heat. The other problem was that the air that exited ducts was not very warm. This made far end of house cold. You can get booster fans for the ducts.

Really need to get an HVAC guy do a good sizing design. I used to be in the Tin Knockn' business. Installed house sized systems to large commercial jobs. It may work, but my first guess it will not distribute the heat without a lot of loss, due to low velocity or fan not big enough to push the air correctly.

Now, for the duct tape and foil tape on your old system? Needs to be tightened up and sealed with duct sealant. Flex or Coil as you call it, is bad for air flow. Usually the rule of thumb was no more than 6 inches in length. Basically use it to do an easy hook up between hard duct work and registers. And it needs to be connected with zip ties, no duct tape.

Ok had the havoc guy out. BTW, I'm thinking of all options including a fireplace upstairs..

Anyway, back to the wood furnace. Havoc guy said it would be expensive to run so much steel and heat loss might be great.

My second thought, what if I just run the wood furnace directly to 4 or 5 vents that are in the floor of my most used rooms? Each vent would need a backdraft installed since they are tied into the central furnace but the run would be shorter. The first 2 vents would be 15-20 feet from the furnace. It wouldn't heat the entire home but would at least heat the living area where we spend most of our time.

My concern, as I stated was heat buildup. For some reason this really bothers me. I read the cautions about clearances, but when someone posts that they've seen water droplets sizzle on their plenum, that really bothers me. Ami being too cautious here? If I tie a wood furnace into just 5 floor vents it seems there would be a huge amount of heat into those vents?

Had a neighbour who did this sort of thing, when he lit it this fall the wood furnace building caught fire the fan kept running and filled his house with smoke the damage was huge $.