Disadvantages of two furnaces in series?

Page 1 of 2  
Hello all,
I'm about to duct in a wood furnace in series to an electric one (both forced air).
This is what several pros have told me is the best way.
But surely the one that's on at any moment will have to push or pull all the air through the fan blower of the other one? Would that not constrict the air way incredibly?
-Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

Put the electric furnace ahead (upstream) of the wood furnace and remove the fan assembly from the wood furnace. You will have to rewire the system correctly in order to make it workable. I would recommend that you have a pro make this conversion for you, and preferably one who has done this before.
BTW, the wood furnace should offer little restriction to airflow. If necessary you can bump up the blower speed.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Richard,
I think I see what you mean: remove the wood furnace blower completely, and wire the electric furnace's blower to be used instead. Correct?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
They should not be put in series. They should be put in parallel, with automatic dampers in both the supply and return ducts, and a 2 stage heating thermostat.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bob,
I think I am beginning to favor the parallel method. I searched for the last hour or so for "Automatic Damper" and the like, but I'm finding it hard to see anything to get a pic of what it looks like. Could you give me a brief description of how it works so that I am not in the dark? This is a motorized damper?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
How will you be using this system? Will you be using wood in the daytime and electric at night, or will you be using wood both day and night, and only using electric when you go away for more than a day?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Wood full tme (except on vacation) but would like the electric to kick in if its enabled and if the wood runs low, for example if I'm too tired to fill up:)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If you were leaving one off for a long period of time, then you could probably get away with manual dampers. If not, then you'll probably want automatic dampers. There are several types on the market. Some can be wired to the thermostat, and some can be wired to the equipment. You'll have to see which one works best for you. Check out 'Metal-Fab' and 'Zoning'.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
What I am looking for is a mechanical damper, not a powered one. I could put into the cold air return so that it can only suck, and not blow out. Put one just before each furnace. Not sure if there is such a thing, but I think I have seen similar ones on roofs that have louvred slats that open when the fan is on, and close automatically when the fan is off. If its made properly and balanced, it would not offer much resistance.
Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

Dean, you should probably think twice about taking the advice of this person, who is obviously not a professional in the field of HVAC. You're going to burn down your fucking house if you continue with your present plan.
http://www.canren.gc.ca/prod_serv/index.asp?CaId 3&PgId`6
"[...] The add-on is placed beside the existing furnace, and special ducts are installed to connect the two units. The air passes through the original furnace, then through the add-on and into the ducts to be distributed throughout the house. Note that ONLY AN EXPERIENCED PROFESSIONAL should install an add-on."
Scroll down the page to near the bottom for a drawing that depicts the proper installation.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I've seen such a thing but don't know the brand name. Heater goes on, the damper opens. heater goes off, it closes. Sounds like a better method than in series.

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

Nevertheless, it isn't. On systems with a backup unit, otherwise called a redundant system, such as on several facilities that we maintain, the damper is the way to go. But that is because we have two complete systems, including evaporator coils in each, one of which is an emergency backup system. The idea is that conditioned air can continue to be supplied in the event of any sort of failure of either of these units. If they had a common blower feeding them, then a failure of that blower motor or its control circuit would leave the building with no air conditioning at all. The second unit is called a redundant system. OTOH, in the case of the wood stove the idea behind it usually isn't redundancy, but rather it only provides the customer the option of using different energy sources. You wouldn't for instance want to configure a typical heat pump/gas dual-fuel system in parallel as you described for the wood furnace/electric heat above, though you certainly could if you wished to. It isn't however economical to do so. Same with the wood heat/electric system. Typically the evaporator coil, if cooling is supplied as well, is located directly on top of the wood stove with a length of duct between them. In that length of duct (plenum) are located a typical insertion fan/limit to operate the common blower when wood heat is provided, and a line-voltage stat's element which is used to lock out the electric heat when the plenum temp rises above preset value to prevent overheating in the event that both units are producing heat. Burning wood cannot be cycled off with a stat. The Longwood furnace comes close, by using a t-stat controlled damper door, but still no cigar.
Economy may take a back seat to redundancy in your area in all cases, but it doesn't in my region of the world. Redundant systems and dual-fuel systems have completely different end goals in mind. There are wood burning systems on the market however that are designed to be coupled to an existing central air system, that are already configured controlwise, i.e. that require no field engineering of safeties etc., simply follow their pre-engineered instructions.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks for the info, that was very useful. Let me just describe exactly what I have:
1. Existing heat pump/electric heater and air conditioner system, with 1HP blower fan. 2. Wood furnace with no air con, but with 3/4 HP blower. It has a plenum temperature fan/limit control, which controls and air opening in the front door for combustion. Separately the thermostat controls the combustion.
If its in series, I would have to either remove one fan, or make sure they both come on together, as I think there would be too much bottleneck to push air through a turned-off fan.
Its one of these:
www.charmaster.com
(The wood only system charmaster wood controls).
Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

Correct. You would remove the wood furnace's blower completely from the wood furnace, wire the heat pump blower's selected speed tap through the fan side of the existing fan/limit switch on the wood furnace, in parallel to the heat pump's fan relay contacts, or optionally run a 24v circuit from the heat pump through the fan side of the fan/limit that will call for the indoor blower. The main difference in these two methods is that wiring the motor directly through the fan/limit will allow heating with wood even in the event that the heat pump control circuit shorts, or becomes inoperative for any reason, thus allowing a bit of redundancy in the system that won't be available if you run the 24v circuit through the fan/limit.
Unless the heat pump is configured to run on the same speed during all modes, then a relay will be required in order to lock out any other speeds that might be energized by the heat pump's controls. IOW, you don't want two speeds of the blower to be energized simultaneously since that would let its magic smoke out of it. The door damper on your wood furnace will continue to operate on its own t-stat and the heat pump on its own t-stat. As long as you have the additional high-temp limit or temperature control installed that I spoke of earlier, then it won't matter if both units call for heat simultaneously, though it is possible to install yet another relay that will close the door damper upon a call for electric heat. The limit side of the fan/limit switch will however automatically close the door damper, so this relay would only serve as an optional safety.
Being a heat pump, you'll need a second high-temp limit besides the one above, that will need to be set somewhere around 90 to prevent the compressor from running when there is wood still burning or smoldering in the wood furnace. This is to prevent compressor damage. Since the heat pump won't be able to run with even the slightest heat being emitted from the wood, even after the door damper has closed, the house temp will drop and the heat pump stat will eventually call for resistance heat. As a result the heat pump won't run in heat pump mode at any time when you are also burning wood. Just something to keep in mind. As you might have gathered I have installed systems like this one. Yours will be a triple-fuel system, which is quite simple compared to the quintuple-fuel system that I put together summer before last :)
Richard Perry

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

Let me back up a minute to say that if your evaporator coil is upstream from the wood furnace, which it likely will be, then you won't need the additional 90 limit that I spoke of, and your heat pump will run normally unless locked out by the ~150 limit (that *is* required).
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Richard,
Thanks very much for all the very detailed descriptions! I never thought about some of the issues you mentioned. You brought up a particularly important point, about the heat pump not coming on and me paying for resistive heat. If I used that as a back-up on my wood furnace, it would probably end up being more expensive than just leaving the heat pump to work. Makes running on wood alone now more attractive.
Your point that I could run the electric upstream of the wood furnace is a good one, but when I look at the blower hole in the wood furnace it is very small, maybe 4x10", so if I take out the blower, that constriction is still going to be there unless I butcher the thing, which I don't plan on doing (the wood furnace has all kinds of directional fins to direct the heat around the firebox and heat exchanger so I don't want to mess with that.
Supposing then that I do stillI want them in parallel, and rely only on wood on a regular basis. I want the wood to be utmost efficiency, considering the effort it takes to collect the wood! So, I have a main switch near the furnaces, to select one or the other, but not both. Safe? Then I put in a one-way valve at the cold air return, on both of the furnaces. No need to play with the plenums, the air can't get through for this reason. I use two separate t-stats, but since we have a safety interlock, only one will ever be doing any controlling.
How does that sound? As you can see, I am working this out now so I get it right, and I want to know all the good options.
Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

You can do whatever you want. Parallel will work, it just isn't the best way. If you're serious about this installation then get a pro out to look over the situation and to design it for you. I can't see it from here. Good luck with it.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
My fundamental concern is the constriction going through either furnace, when the blower is removed. I keep reading that even just a 90 degree bend cuts the air flow down by 20%, so what is pushing through a constriction and whole second furnace going to do? Please keep advising! I am only researching now, and I will get a pro in to install it but I want to make sure I know what he's doing and that he's doing it right.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
dean wrote:

If the restriction cannot be overcome without interrupting even airflow over the firebox, then another option is to run a bypass around the wood furnace. This will require some pressure balancing, thus a bypass damper will be required as well. The advantage would be an ability to control static pressure and a simplification of control/safety issues.
Unless you want to wait for the wood fire to be completely snuffed out before switching on the heat pump, then you won't want to go with the parallel install. What will happen with a parallel install is that if you switch immediately to heat pump after burning wood, then the wood furnace will heat up and its fan/limit will call for it's blower, and with the changeover damper closed off to the wood furnace that air will have nowhere to go. If OTOH, you disable the wood furnace's blower on a call for heat pump then the fire box will collect heat and become very hot, possible hot enough to present a hazard. You'd either have to allow the damper to switch back and forth, short cycling both units until the wood cools down, or simply wait until it cools down before bringing in the heat pump. Neither of these options would be acceptable to me, in my own home.
Based upon your descriptions I'd probably go with a series installation 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. I simple temperature control could be installed to lock out resistance heat based upon discharge air temp. If your evaporator coil is located downstream of the wood furnace, which again it probably won't be, then the controls will be a bit trickier. It isn't a good idea to have the evap coil upstream, since this will cause condensation within the wood furnace in cooling season. This is the reason that the coil is always placed downstream on a gas furnace, i.e. to prevent rusting out of the heat exchangers. Most heat pump air handlers, OTOH, have the evap coil within them rather than sitting on top of the air handler, so it's difficult to relocate to a point after the wood furnace. So it seems your practical options are to either: 1) Go ahead with the parallel install and put up with the changeover issues. 2) Go with the bypass damper on a series install, remove the coil from your air handler and place a boxed coil downstream from the wood furnace. This will require some form of compressor lockout to prevent the compressor from running when the discharge air from the wood stove is above 90-100deg. 3) Place the evaporator coil in the bypass ducting.
HTH.
Richard Perry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Do you live anywhere near NJ? :)
Ok thanks for that further discussion, I need to chew over this some more, off work time. I very much appreciate your time and effort.
Dean
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

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.