The whole deal for a basement wood furnace

I don't recall all of the discussion, but I think someone pointed out you might have trouble in a power outage. Tundra keeps making heat, but there's nowhere for it to float away in the above picture.

Also, the series installation may be trouble; look at page 49 of the manual, you'll see a similar picture (wood furnace and LP furnace are reversed), with a big X through it saying it's not approved.

Your house, your decision, just thought I'd bring it up, I know it's easy to forget all the things to keep in mind.

If you are stuck on running them in series perhaps you could put a normal open damper between the tundra supply and the main duct and tie it into line power so it would only open when the power fails for a heat dump.

The issue with tying in to the cold air side of the main furnace is that all the hot air is trapped in the event of a power loss...Tundra overheats.
If you want to do it this way then You need a emergency heat dump door like Yukon sells (http://www.yukon-eagle.com/FURNACEACCESSORIES/SAFETYHEATDUMP/tabid/200/Default.aspx) which will mean that you will have to install a plenum on the Tundra, or I suppose you could put a large "power closed/spring open" damper on the Tundra plenum to dump heat to the basement during power failure.
I would think you will need a 24vac controlled relay to control the damper. 24v side to the main furnace thermostat, 120v side to the damper. Unless the dampers run on 24v...then maybe it could just be wired direct to the 'stat. Or maybe some dampers have relays built in, I dunno. Maybe someone more familiar with powered dampers will chime in

roofyroo said:

Page 49 Diagram 7.5.3 clearly shows the "other" furnace feeding into the Tundra as not permitted, not the other way around. We're not planning on running both at the same time.

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Well my point is that a series installation has risks, regardless of which furnace comes first. I would bet SBI would be even more adverse to the liability of their product dumping hot air into the other furnace than their aversion shown in 7.5.3.

Risks such as overheating components in your electric furnace; your in-laws watch your house and run both furnaces; the next owners of your house run both and sue you after it burns down; etc. I admit I don't know that any of this will happen and I certainly hope it doesn't. I'm just fairly conservative about risks that can easily be avoided upfront. Your house and decision; not mine to judge.

DoubleB said:

Well my point is that a series installation has risks, regardless of which furnace comes first. I would bet SBI would be even more adverse to the liability of their product dumping hot air into the other furnace than their aversion shown in 7.5.3.

Risks such as overheating components in your electric furnace; your in-laws watch your house and run both furnaces; the next owners of your house run both and sue you after it burns down; etc. I admit I don't know that any of this will happen and I certainly hope it doesn't. I'm just fairly conservative about risks that can easily be avoided upfront. Your house and decision; not mine to judge.

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I agree with DoubleB and brenndatomu, I would not run them in series unless I cleared it with SBI first, get there recommendations and be sure to include some kind of heat dump if they ok the setup.

edit: Hey look, my 100th post! not a new member any more.

If the furnace was meant for series, it would clearly be listed in the manual. I have not seen a single wood furnace that allowed a series install with a restriction in the top of the jacket (2-8" ducts). The Caddy can be series, but only if the plenum of the woodfurnace leads directly into the main ducting, not the other furnace. In the case of a Caddy, the central furnace would push all the heat in one direction. If a power outage occured, there's a large plenum opening for the heat to escape the jacket.

Also the same reason almost all furnace manufacturers won't allow for a down flow configuration. The first power outage without proper flow will allow for a heat trap to form along with dangerous temperatures. If a furnace does overheat in an outage, I wouldn't want to suck that superheated air into a location with a blower, a circuit board, etc.

I'll also remark that I sympathize with your situation. So many options, each with pros and cons and limitations and requirements. It can be kind of daunting. Most of us have been there. You've already invested a lot of effort and money, you've done lots of good work already that you'll be very satisfied down the road to look at your accomplishments and know that you don't have a weak link that limits the robustness, performance, or safety of your system.

I started planning my install 12 months ago, lots of decisions and work along the way, and I'm still not quite done, but I much prefer knowing I have avoided or addressed known potential problems. I think others report feeling the same way.

roofyroo said:

I'm struggling with understanding the exact "how" the back draft damper will work. Does it tie into the thermostat of the electric furnace? Do I need to buy a control box?

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Good question. You can buy off-the-shelf backdraft dampers, or have one custom made. Either way, they are passive and mechanical (no control box or connection to a thermostat). They have one or more flaps that blow open when air blows through the correct direction, but if air tries to blow backwards, the air pushes the flaps closed against a stop. If you know what a check valve is for fluid, it's the exact same idea. Your post #54 is the right idea, wherein the backdraft damper on the left would allow the electric furnace to blow air out, but not allow the wood furnace to blow air back into the electric furnace; and wherein the backdraft dampers on the right would allow the Tundra to blow air out, but not allow the electric furnace to blow air back into the Tundra.

Another option, such as Smoke Signals and Brenndatomu described, does involve an electrified damper. JRHAWK depicts one example quite well in his post #71. In my opinion, I like his system because in the event of a power outage, a spring in the electrified damper opens it, so that there is no chance that a backdraft damper, even if lightly balanced, would in any way limit the quantity of hot air naturally flowing out of his wood furnace. Let us (or him) know if you have any questions about that.

Another option, which I took for my house, was just to make a simple duct system for the Tundra that is not connected to the LP furnace. No backdraft dampers there either. For what it's worth, the manual says Canada requires a separate duct system. Evidently the US doesn't, so I'm not arguing, but at least I'm covered if my house moves to Canada.

IF the tundra will be running most of the time and you will be around if the electric needs to run (like if you were out of wood) I would go with the above set up with the exception of I would use manual dampers in the two 8" pipes coming from the tundra to the main trunk line. Then just close the manual dampers when you need to run the electric furnace.

roofyroo said:

What would be a good interim damper setup

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If I recall what you're doing now (cardboard in the elec furnace filter?), that is probably fine to limp through the rest of the spring. Just as long as you don't run the electric furnace without properly restoring it and unplugging the Tundra. I like to open breakers to make sure someone (like myself) doesn't inadvertently turn something on.

roofyroo said:

Can I make a manual rectangular damper for the time being? They don't look like much.

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If you're handy, as you seem to be, then I'm sure that you can. If your current setup is temporarily ok, then you can have the spring and summer to make sure the damper is well thought out and fabricated without having to rush.

I have another idea that I'll try to post later if I get some time.

I have my system dampered and wired in such a way I have full back-flow protection of both furnaces as well as gravity flow from wood furnace in case of power failure. In addition both furnaces can run at the same time or separately and still be protected from backflow. I have a gravity damper in the plenum of my LP furnace, a spring-open powered damper in the duct connecting my wood furnace to the main existing duct. I am using a relay wired like -THIS-. I'm using the EAC (electronic air cleaner) circuit on the LP furnace for one trigger and the snap disk on the wood furnace as the other one. Everything works great. I have my LP furnace set to 65° and have a resettable -MINUTE COUNTER- wired to it to let me know if/how long it runs. I then log any minutes in a spreadsheet daily along with weight of my daily loadings.

Personally, I don't like the manual damper idea, it's just asking for trouble if you ever forget to open them when using the wood furnace. That's the beauty of how I have mine setup. It's totally fool-proof and completely automatic. I don't even have to change anything going from heating to cooling season.

roofyroo said:

Fully automatic would be awesome - but maybe later in the year once we have more funds.

How about the manual damper inline with the backdraft damper as a "just incase"?

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Please forgive me if you have already answered this but how do you plan to use the Tundra? 1- As your primary heat 2- in conjunction with your electric furnace or 3- as backup to your electric?

Here is another idea for you, 2 variations of the same. The advantage that you could put the backdraft dampers in your rectangular duct (instead of above your electric furnace). The disadvantage that you would have to make two backdraft dampers for your rectangular ducts (instead of just one).

First picture can be set up like JRHAWK did. Motorized damper can be had from amazon, ebay, even Home Depot for $100. Second picture uses the backdraft dampers you already purchased over the Tundra, which many others do.

Disclaimer: I am not a real HVAC guy.