and a motorized bypass damper.
In the series configuration the heat generated by the wood stove after
it's door damper has closed won't present a problem, and there would be
no short cycling unless perhaps your backup resistance heat were also
Do you still mean to use the blower in the electric furnace, which is
upstream from the wood? The motorized bypass damper that you mentioned
- what is that bypassing? Sorry I'm confused about this point. Do you
have a telephone number and 10 free minutes? :)
The bypass is just a duct that runs parallel to the wood furnace. The
main air stream is split as it leaves the fan coil, some of that air is
fed through the wood furnace and the rest is fed through a duct that
skirts around (bypasses) the wood furnace. These air streams are tied
back together on the other side of the wood furnace. It's actually a
series-parallel configuration. Yes, we're still talking about removing
the blower from the wood furnace.
Just think about a stream that splits in two and then rejoins
downstream. Now put your wood heater in one branch of that stream and
your evaporator coil in the other, with your air handler located
upstream of this split.
Ok, but that doesn't help push air through the wood furnace. In fact it
would reduce the air going through the wood furnace, some of it would
bypass the wood furnace (we are relying on the electric furnace's
blower). I think its quite important to have maximum air flow around
According to your description the air handler has a bigger blower motor
than the wood furnace has. I would think that the air handler blower
should be capable of pushing sufficient air through the wood furnace.
In fact, some of it would have to be bypassed around the wood furnace in
order to prevent too much airflow through the wood furnace. At the
same time this bypassing of air will reduce the static pressure in the
plenum, allowing the blower to run more efficiently, and it will reduce
noise at the return as well. The purpose of the bypass damper is to
control the volume of air through the wood furnace or alternately
through the evap coil, as necessary. The damper will allow air to bypass
the wood furnace when the heat pump is energized, and will allow air to
bypass the heat pump evaporator coil when the wood furnace is heating.
You won't have a call for both units simultaneously. An interlock
circuit will prevent both from calling at the same time.
When the fire dies down in the wood stove and it is no longer supplying
sufficient heat to the space, and when you aren't there to add more wood
or perhaps don't want to add more wood, then the heat pump stat will
energize the compressor, modulate the bypass damper open to the evap
coil, and at the same time the door damper will be closed on the wood
furnace. A small portion of the airflow will still be allowed to flow
through the wood furnace, just enough to prevent it from overheating and
from rising enough to lock out the heat pump again. I forgot to mention
that you'd have a temperature controller on the wood furnace that would
lock out the heat pump at a given temp. It is the drop in temp of the
wood furnace that would cause this same control to allow the heat pump
to take over. Now where was I?
Once the fire is completely out the damper modulates full open to the
evap coil, allowing full required airflow through it. There are several
other ways to control airflow beside this one. One is to have a second
damper, another is to place the coil downstream and have a normal series
installation. The bypass system is definitely more complicated than the
normal series installation, but it would provide an ability to supply
the correct airflow to both the coil and to the wood furnace as
necessary. Normally the wood furnace doesn't offer enough restriction
to pose a problem. The only reason that I suggested the bypass in your
case is that you had some reservations about getting all the air through
the wood furnace without it producing an excessive restriction to
airflow. Normally this isn't a problem, and the series installation is
If you want a less complicated system, then I suggest you either forget
about it, or make that hole bigger. :)
Right:) I am convinced. Once again - thank you!
So, in the normal series installation (assuming 1HP is enough), would I
still need a lock-out to stop the heat pump running when the wood
furnace is hot? I've read sometimes that you have 2 thermostats, and
set the electric 5 degrees below the wood. However, I assume that
someone can fiddle with these, so I either get the interlock, or a
dedicated single thermostat designed for this type of system.
What would happen if the heat pump came on and fed warm air into the
wood furnace jacket? Would it not just turn down the door damper
because it doesn't need so much fuel? Would this be a hazardous
I was telling someone about this idea, That I would love to do something
like this in my next house I have custom built. I was ask why not have a
wood fuel boiler that used water circulated to a coil as a second unit
on the heat pump, then would just need to have the circulator pump shut
off when temp was not high enough and the heat pump take over.
Is this doable?
I think back to coal fueled boilers supplying steam to radiators, Just
the radiator would be in line with the heat pump.
Probably not unless you had some other type of hydronic heat system
connected to the wood boiler. When wood boilers get too hot, a circulator
has to come on to dissipate the heat, and a hydronic coil wouldn't have
You can only do it with a boiler that can shut the fuel supply off on high
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