Disadvantages of two furnaces in series?

Page 2 of 2  


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 engaged.
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? :)
-Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

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.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 the firebox.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

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 rather straightforward. If you want a less complicated system, then I suggest you either forget about it, or make that hole bigger. :)
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 situation?
-Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

The heat pump air won't harm a thing, it's in the summer when you're running the AC that you don't want the evaporator upstream. The wood furnace would quickly rust out. HTH.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
RP wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 enough capacity. You can only do it with a boiler that can shut the fuel supply off on high limit.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.