Wood furnace to duct...

[Shethinksmyflannel'ssexy]

Hey guys I need some advice. I installed this furnace last year and it heats great except the first two ducts, closest to furnace get very little heat, I think the air is blowing by them. I don't have a damper in the main trunk so basically no back pressure, I do get some air going through and coming out my return in our hallway. Do yal have any ideas to connect my stoves hot ducts to the plenum since my chimney and stove are at the end of trunk and I have vent ducts coming off both sides of main trunk. Any help or ideas are appreciated.

 

[steved]

They make a "scoop" that installs inside the duct that directs the air into the outlet you want...I have a couple on my propane furnace up close to the furnace outlet for the same reason.

 

[Shethinksmyflannel'ssexy]

Great I'll have to check in on that, I haven't found a really good way to bring the stove outlets all the way to the plenum of gas furnace

 

[Whitespider]

Three things...

First, if you have heat coming out the cold air return in the hallway you have a back draft problem. You have forced (and heated) air from the wood furnace flowing backwards through the gas furnace, which means air is reversing itself through the gas furnace plenum, heat exchanger, blower, and filter. There's only one fix for that... you need to install a back draft damper in the gas furnace plenum. The cold air return is normally a large duct so any air flowing in reverse direction through it will seriously rob your heating ducts of needed (and important) static pressure. There ain't no shortcut, you need a back draft damper... there ain't no way around it. And, if you use the gas furnace, you'll likely need back draft dampers in the heat ducts coming off the wood furnace also.

Second, "scoops" and diverters may help, but they don't fix the back draft issue (and the resulting loss of static pressure). And, because your wood furnace is at the opposite end of the main trunk, any "scoop" or diverter will have the opposite affect when the gas furnace runs (well... unless you do some fancy engineering which allows the "scoop" or diverter to reverse itself on demand). Fixing the back draft issue, thereby creating more static pressure in the ducting, will likely fix your flow problem at the vents (at least half of it).

Third, I don't see a cold air return going to the wood furnace. A forced air heating system will not, can not, work correctly without return air connected to the blower. I know a lot of guys claim they just leave the basement door open as the "return" air... yeah, that might work (sort'a), but it ain't working near as well as they believe it is. A forced air heating system works on static pressure, both positive and negative. When the blower runs it creates a high pressure zone on the output side, and a low pressure zone on the input side... air will naturally flow from the high pressure zone to the low pressure zone. This natural flow increases the output from the heating vents over what the blower can move just on it's own (i.e., the blower doesn't work as hard, while actually moving more air). With the blower sucking room air from an open basement, it flat cannot create the proper low pressure zone... this is likely the "other half" of your flow problem at the vents.

Think of your forced air heating system as a closed-loop circuit... the rooms you are heating are part of the closed-loop. Each room needs access to the high pressure zone (a heating vent) and the low pressure zone (the cold air return). If your home has a single return air vent in the hallway, closing the door on any room will remove (or at least immensely reduce) it's access to the low pressure zone... closing the door takes it out of the "loop" (so to speak). Removing access to the low pressure zone will greatly reduce air flow coming from the high pressure zone because that air is looking for the path of least resistance from high pressure to low pressure.

Fix your back draft problem and connect return air to your blower and I'm bettin' dollars-to-donuts your problems disappear... and your wood furnace heats your home a ton better than it does now. I think you'll find the difference mildly surprising.
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[Whitespider]

Do yal have any ideas to connect my stoves hot ducts to the plenum since my chimney and stove are at the end of trunk and I have vent ducts coming off both sides of main trunk. Any help or ideas are appreciated.

Sorry, I ran short of time this morning and couldn't address this question.
Running ducting the full length of the basement, from your wood furnace to you gas furnace plenum, would be self defeating. That would be a lot of metal surface to radiate heat during the trip across the basement... meaning you'll lose some significant amount of heat before the warm air even enters the heat ducting. Not to mention the increased area would also affect static pressure... and also, it wouldn't bypass the need for back draft dampers (likely it would make the back draft problem worse).

You could build a plenum for the top of your wood furnace and tie it into the main trunk of your ducting... much like a stand-alone installation, rather than an add-on installation. But you'll still need the back draft dampers... there flat ain't any way around that... period. When I installed my furnace I cut one large rectangle hole in the top, built a plenum, and tied it into the gas furnace plenum with a single 14 inch duct. That increased the area for air flow from 100 inches (for two 8 inch pipes) to 153 inches (for one 14 inch pipe), and reduced air-to-pipe friction at the same time. I used two 8-inch, and one 6-inch flex ducting for return air equaling 130 inches of area, but it was pretty much a short, straight down run... a longer return with a horizontal run and I'd have gone with at least 150 inches of return area. Finally I installed a back draft damper above the heat exchanger in the gas furnace, and another one at the outlet side of the blower where it attaches to the wood furnace. Forced air, from either appliance, can only move one direction, into the main trunk and out the vents... even if both are running at the same time (but I haven't turned on the gas furnace since installing the wood furnace).

 

[Shethinksmyflannel'ssexy]

Thanks spider I have been thinking about it all day and talked to one of our engineers about it too. Do you recommend spring loaded dampers or the electrical dampers. I just want them to work in all comditions, wood heat and closing off gas furnace, then If my fire burns down, my gas furnace kick on and wood stove damper close and how will it work with a/c? What if the dampers fail is home still safe?

 

[Whitespider]

If possible, I'd personally stay away from electrically controlled dampers (I just don't trust tiny electric motors)... but you don't haf'ta use spring loaded either, there are some that open/close by static pressure and/or gravity alone.

I fabricated my own dampers. The one mounted above the gas furnace heat exchanger opens by static pressure from the gas furnace blower, closes by gravity (static pressure from the wood furnace blower seals it tighter). You could mount it above the A/C evaporator and accomplish the same with no effect on A/C performance. I installed the damper for the wood furnace where forced air enters it from the blower. It's nothing more than a door that swings open when the wood furnace blower starts, and falls closed when the blower stops (static pressure from the gas furnace just seals it tighter).

I've seen 8-inch round back draft dampers that will fit your wood furnace heat outlets at places like Menards, Home Depot, and such... but they seem a little pricey to me. And the bigger, square ones I've seen for furnace plenums are flat-azz way to pricey for my blood. I mean... c'mon... it just needs to be a flapper that allows one-way air flow. I built mine with a few pieces of scrap sheet steel and aluminum I had laying around... used a dollars worth of rivets, and maybe a couple hours of my time.
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[Shethinksmyflannel'ssexy]

Great that's what I was thinking too by static pressure dampers you mean the blower blows it open and when it stops the flap falls back shut? You wouldn't happen to have a pic of your dampers do you? Do you use your front blower and thermostat much, mine seemed to do ok but I ended up not using it much

 

[fordf150]

I can testify to the difference a properly ducted furnace can make on a house. My old house had 8 6" registers and 2 8" returns in a 7 room house. I redid all the ducting to include a 6" return in every room plus an 8" at the bottom of the stairway.Going off memory on the number of ducts but you get the idea. Thought I lived in a different house. Furnace ran half as much(half the heat bill too) with the added benefit of a more even temperature in the whole house. Well worth a weekends work and couple hundred dollars in materials that it took to complete

 

[Shethinksmyflannel'ssexy]

Fordf150 can't beat those trucks by the way, I love mine. My stove heats well and does a good job for whole house but I like for things to work as best as they can and I think more returns would definetly help since I only have the one central return vent

 

[brenndatomu]

Fordf150 can't beat those trucks by the way,

Especially with the right tires on them, huh WS?!

Powered back draft dampers can be bought spring loaded and set to spring to open or closed (open or close on power fail) If you can use static or gravity dampers, well, that's probably the best... K.I.S.S.

 

[fordf150]

77 half ton with a stock( or more accurately what should have been stock IMO) 300 6cyl. 456 gears spool(detroit locker didnt hold up) in the rear and traklok up front. 35" procomp xterrains. Goes really good when you young and dumb and don't realize how useless it will be after doing all that to it.



Lots of good information on hvac if you look for it but unless you have a really nice hvac company locally you are going to be making guesses at some of the sizing and flow characteristics that are needed for optimum performance. I couldn't even get someone to tell me what size my main trunk line should be. Lots of reading online and some educated guesses and I ended with a system that is great but could probably be improved on. They aren't near as nice at sharing knowledge as the chainsaw community.

 

[brenndatomu]

Hey f150, just FYI, for anyone near a Menards, they will lay out your duct system for you. My co-worker just did that, he said they have some program that they use. He has yet to fire his system up, so he has no idea how well it's gonna work yet, but it's gotta be better than guessing.

 

[Whitespider]

You need bias ply tires on those F150's before they become a real truck

...by static pressure dampers you mean the blower blows it open and when it stops the flap falls back shut?

Yes, exactly...
But mine didn't need to be "optimal", the gas furnace is a backup system... honestly, it hasn't run for three or four years now, except for a yearly test run and a couple real cold nights before I installed the latest wood furnace last fall. If my wood furnace was a "supplemental" heating system, rather than "the" heating system, I'd have likely put a bit more thought into design.

Yeah, we use the "front" (forced draft) blower and thermostat... but it doesn't run a lot this time of year. During milder weather it's more about how much, with what, and when you load the thing to keep the house comfortable. It runs more during mid-winter, and makes for fast temperature recoveries. I used a programmable thermostat, so it warms up nicely in the morning, drops back down during midday, warms up a little in the evening, and cools down during sleeping hours. I'm also a bit of a tinkerer; I changed a couple settings and added another sensor so the draft blower doesn't run continuously under demand... it cycles on/off according to firebox temperature (that way it doesn't eat near as much fuel). I rewired the thing for 230-volts and used a large three speed furnace blower for heat circulation... it automatically changes speeds depending on heating demand. To keep the draft blower from running on a cold firebox I wired the thermostat (on the wall upstairs) through a snap switch on the furnace... when the firebox goes cold it cuts power to the thermostat. I added a bypass circuit on a timer switch so I can use the draft blower for start-up on a cold, dead box... I can set that for up to an hour if I want. Finally I put an on/off toggle switch on the draft blower so, if it is running, I can conveniently shut it off during a reload or such.

Here's some pictures of the damper I built for the gas furnace...
First I removed the plenum, took measurements, and made a plan... fabrication...

Some final fitting and riveting...

"Flapper" installed with blower off... and blower on...

Reassembly of the plenum... and installation of the 14-inch tie-in duct...

And then I cut the rectangle hole in the top of the wood furnace air jacket, built the plenum for it, and final connection.

If I was to re-do it I'd have made a double or triple "flapper" rather than a single... the single is just a bit heavy.
It works fine as a backup... but if the gas furnace was used often (I even have the gas shut off) I'd want two or three lighter "flappers".

I don't have any pictures of the damper installed on the wood furnace... but it's a real simple affair.
It's just a small aluminum door mounted on the blower housing... it swings up and open when the blower starts, falls back down when it stops.
Both dampers are seated closed "tighter" by static pressure from whichever blower is running.

If you'd like to see more pictures of the total installation here's a link to my "album" where you can scroll through it step-by-step.
http://www.arboristsite.com/community/gallery/photos/263/
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[fordf150]

You need bias ply tires on those F150's before they become a real truck

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Bias tractions are some of the best off road tires if you can get them. Around here they are impossible to find


From the sounds of it you just need to hire whitespider to come set your heating system up.

 

[Shethinksmyflannel'ssexy]

Great when I get off I'll have to check that out do you think a flapper would work ok in this straight section in the pic or does it need to be right above heat exchanger?

 

[Whitespider]

...do you think a flapper would work ok in this straight section in the pic or does it need to be right above heat exchanger?

It can be anywhere between the blower and the first heating duct down stream from it... but probably better if it's somewhere between the heat exchanger and first duct.
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[Shethinksmyflannel'ssexy]

Great album spider, I am thinking of using this type of damper for wood furnace and gas furnace, what you think?

 

[Whitespider]

...thinking of using this type of damper for wood furnace and gas furnace, what you think?

I don't see any reason why those won't work... provided you can get them the correct size.
I'd probably remove the screening... no need for it, and no sense restricting/disrupting air flow any more than necessary.
Make it easy on yourself... install them so you have convenient access for cleaning, maintenance and such.
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[Shethinksmyflannel'ssexy]

I will have to make them to get correct size I think but that's not a big deal. I want use the screens either thanks for all your help.