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
A few hints would be very appreciated.
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.
: Hello all,
: Simple question here (I think), but I'm adding a second furnace
: providing hot forced air) to run with my current electric (hot
: air, similar thing). New furnace will sit next to existing one.
: If both of them feed into the air ducting close to each other,
: need to separate the one so that it doesn't blow through the
: unnecessarily? What's the typical way of doing this? I had a
: over the installation instructions and there is nothing about
: as a backup or using the existing one as a backup. They both
: own blowers.
: A few hints would be very appreciated.
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.
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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.
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.
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.
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.
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.
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