Adding a new furnace in conjuction with existing electric - how to separate the air supplies?

[dean]

Hello all,

Simple question here (I think), but I'm adding a second furnace (wood, providing hot forced air) to run with my current electric (hot forced air, similar thing). New furnace will sit next to existing one.

If both of them feed into the air ducting close to each other, do I need to separate the one so that it doesn't blow through the other unnecessarily? What's the typical way of doing this? I had a quick look over the installation instructions and there is nothing about adding it as a backup or using the existing one as a backup. They both have their own blowers.

A few hints would be very appreciated.

Dean

[Pop]

Heat activated damper to keep wood heat from going into the electric furnace. Feed heat with Y, not a T, connection. Not too close to each other. Get an experienced heat man in.

[Chris Lewis]

According to Pop :

I'd be _very_ leary of paralleling furnaces together, especially if one or more of them is a combustion-based furnace (ie: wood).

The slightest malfunction, and one furnace may blow combustion products directly into the ductwork.

Yes, you're going to need a very experienced installer. And check local building codes/get the result inspected.

[Greg O]

One way, gravity dampers in each supply duct. Gravity closes them, the force from the blower opens them. Either that, or motorized dampers that are interlocked somehow to each furnace so the one with the open damper runs and the other will not. Trickier with a wood burner, perhaps a heat activated switch for the wood side. Either way you will need some help from an experienced HVAC company. Greg

[dean]

Ok thanks all!

Take a look at this link:

The hot air is just fed straight into the existing system. I wonder what the problem is doing it that way? Is air being forced back through the electric heater? Or is a baffle inplied there.

Does anyone know if there are regulations preventing both the furnaces from heating at the same time? I am only asking because my friend is installing this for me and he does this all day every day for a living, but in commercial environments, so I want him to know AMAP before starting on this.

Thanks all,

Dean

[Greg O]

The picture on the web site in not very clear in reference to the ducts, but it is best to damper the supply or return to each furnace when you use the existing return and supply ducts.

As far as regulations for using both furnaces at the same time I don't know of any, but your duct work is designed for a certain CFM. Two furnaces, twice the CFM, and you are looking for trouble!

I do HVAC for a living too. We have installed electric air handlers, paralleled with gas furnaces. The only way I know to ensure proper air flow is to put in control dampers in the return, or supply duct, one for each furnace. I have seen other attempts to parallel two furnaces without dampers, and they did not work well. Greg

[Goedjn]

Why parallel, instead of in series? Set up the supply air from the wood-furnace to feed the returns into to other furnace, with a thermostat in the duct, and set up that thermostat in series with the house thermostat(s) so that it only goes on if the air feeding the conventional furnace is below its designed output temperature.

That way the conventional furnace comes on whenever the house wants heat, and the wood furnace isn't providing it. You'll need a booster fan somewhere, since you're trying to drive air through two heat exchangers all the time.

>

[Greg O]

You answered your question with your last sentence, parallel install, no extra restriction through the duct work. I would never install them in series, unless the equipment was designed to do so. It is pretty simple to add controls to run dampers if you use powered dampers. Gravity dampers will do the job just as well with no controls, if the situation allows. If the main source of heat is wood, just set the gas furnace's stat down to 60-65 degrees and it will pick up if and when the wood fire dies. Greg

[Stormin Mormon]

What did the other furnace do for heat? Oil, propane, and NG would be combustion, too......

[dean]

It was electric heat. We do have a fireplace in the basement, so I am installing a SS liner up to the roof and will exhaust the wood furnace through that.

[Chris Lewis]

According to Stormin Mormon :

Exactly.

It's not so much the flues themselves, it's what the furnace does if it gets reverse flow thru the heating duct.