Could someone look over my duct system plan for me? I'm buying the materials tommorow.

[lugoismad]

I have a wood furnace that I'm currently running duct work for.

Could you take a look at these pictures, and tell me if you think I'm doing it ok?

This is a basic layout of my house, first and second stories. My house is 1400sqft.


First Floor

Second Floor

The red are forced air ducts, the blue are return air ducts.

I'd like someone to look at my ducting system with me, and tell me if I'm going about it right.

The first floor, atleast all but the utility room, has a 3 foot crawl space under it. That is where all the ducting is being run. In the garage I am installing a plenum, and each down stairs vent has a six inch flex-duct ran to it and then comes out of a trap door into the garage. I believe this is called an "octopus" system, much like how coal furnaces were setup. I chose the route because I live by myself, and I wanted the option of being able to control the flow of air to each room individually. Also, because I am inexperienced with running duct work, and after speaking to a professional, was told this was the easiest way to not screw it up. Running a single core line and branching off of it is much more of a science.

The forced air that goes to the utility and bathrooms are being ran through the wall from the attached garage and tie into the system out there at the forced air plenum.

The first floor cold air return lines each will have a 8" flex duct ran to it, and again, to an insulated plenum in the garage, which then attaches to the furnaces blower.

As for the upstairs, it is about 1/2 the size of the downstairs. Its being fed by a 4x12" square duct that I ran up the inside of an interior wall. That then lets off into 2 6" flex ducts, one of which runs to either room. The return line is a seperate 4x12" square duct, and terminates in the upstairs bedroom. All of the upstairs ducting is contained within the attic space to the left of the bedroom. Both of the 4x12" ducts let out into the crawl space and attach to 8" flex pipe which is then ran to one of my two plenums.


Do you have any advice on anything I should add to this duct system?

 

[Redox]

At first glance, it seems you are on the right track. It is hard to tell without seeing heat loss calculations, but some of the rooms might be coming up a little short on air. I'm not a big fan of the individual flex ducts, but they are easy to install. I'm not sure how good an idea it is to put the furnace in the garage as there is a possibility of circulating CO to the living space. I think code may require a fire damper in the wall to the house, and lots of little fire dampers would be expensive. The return looks too small by about 25% or so and I would consider a large low return (in the utility room?) as it really isn't worth breaking the return up into smaller ducts. The cold air will get back to the furnace one way or the other with a big central return.

What kind of fuel will you be using and are you considering central AC? Most gas furnaces can be turned on their sides and would fit in the crawl space, thus avoiding all the losses in the garage. Horizontal oil furnace are available, but more expensive.

Chris

 

[lugoismad]

i'd install dampers where the main trunk connects to the insulated flex tubes so to balance the heat flow to different areas.

Thats exactly what I plan on doing, that way I can turn down the air to individual rooms. For example, I plan on keeping my computer equipment in the spare bedroom, it won't need heated during the winter, but will need extra cooling in the summer.

 

[lugoismad]

At first glance, it seems you are on the right track. It is hard to tell without seeing heat loss calculations, but some of the rooms might be coming up a little short on air. I'm not a big fan of the individual flex ducts, but they are easy to install. I'm not sure how good an idea it is to put the furnace in the garage as there is a possibility of circulating CO to the living space. I think code may require a fire damper in the wall to the house, and lots of little fire dampers would be expensive. The return looks too small by about 25% or so and I would consider a large low return (in the utility room?) as it really isn't worth breaking the return up into smaller ducts. The cold air will get back to the furnace one way or the other with a big central return.

What kind of fuel will you be using and are you considering central AC? Most gas furnaces can be turned on their sides and would fit in the crawl space, thus avoiding all the losses in the garage. Horizontal oil furnace are available, but more expensive.

Chris

Its a wood burning furnace. Yes, I know its against code in some areas to have them in your garage. Not in my area. Its already been inspected by my local fire chief and my insurance company gave it the all clear.

Everything will be sealed tight as a ducks butt, so no air from the garage will get into the system. The reason I went with the many smaller returns is because AC will be added to this ducting system, and I wanted to draw return air from several areas throughout the house. Had I just been using it for heat, I would have just put one large return in the utility room.

My house has good insulation and brand new windows, so I calculated low on heat loss.

Yes, I am wanting to install a 3 ton AC unit before next summer as well.

The furnace puts out 2 8" ducts off the top of it, so I think 3 8" ducts for the return should feed it just fine.

 

[lugoismad]

I went and purchased all my ducting just now. At the advice of the guy at the supply store, I decided to add one additional return to the 2nd story.

 

[Redox]

Not to throw a wrench in the works, but A/C is a different matter. From what I can see, you only have supply for about 2 tons and return for 1-1/2 tons. An 8" duct is only good for about 200 CFM or about 1/2 ton. 6" lines are about 1/2 this number. Adding that second return upstairs isn't going to help this much because you are limited by the 4x12 duct which is only big enough for 175 CFM (less than 1/2 ton). However, the house probably only needs about 1-1/2 to 2 tons if my rough seat of the pants engineering is correct. Don't fall into the trap of oversizing the A/C as it will not work any better than a properly sized unit. Oversized A/C just slugs the compressor and shortens its life as well as kill the efficiency.

I have expressed my concerns about using a modern evaporator with a plastic drain pan on top of a wood burning furnace, but I haven't heard any horror stories yet...

Chris

 

[lugoismad]

Not to throw a wrench in the works, but A/C is a different matter. From what I can see, you only have supply for about 2 tons and return for 1-1/2 tons. An 8" duct is only good for about 200 CFM or about 1/2 ton. 6" lines are about 1/2 this number. Adding that second return upstairs isn't going to help this much because you are limited by the 4x12 duct which is only big enough for 175 CFM (less than 1/2 ton). However, the house probably only needs about 1-1/2 to 2 tons if my rough seat of the pants engineering is correct. Don't fall into the trap of oversizing the A/C as it will not work any better than a properly sized unit. Oversized A/C just slugs the compressor and shortens its life as well as kill the efficiency.

I have expressed my concerns about using a modern evaporator with a plastic drain pan on top of a wood burning furnace, but I haven't heard any horror stories yet...

Chris

Good news actually.

Its a bit complicated to explain, but I figured out how to run a 12" duct from the garage (Where my air handler will be) to the second story of my house, straight through, in only about 10 feet. Basically, my garage was an addition upon an addition, and from the garage I have access to the roof of my house where the garage kinda hangs over it a bit. So a cut a hole in the roof right there, where its still inside the garage, and I can pass a 12" duct right through to the attic on the 2nd floor. Off of that I'm going to T to 2 8" ducts, for a return in each room upstairs. This will help out ALOT in the summer keeping the hot air out of the 2nd story.

And, on the advice of the guy I bought everything from, I'm putting 3 8" returns in the first floor. He said you want almost 1.5x as much return as you have forced air, especially for an AC.

I used an online calculator and figured out that I need a 2.5 ton, according to the age of my house, depth of insulation in the attics, lack of insulation in the floor (nothing below the subfloor in the crawl space)

 

[lugoismad]

I have expressed my concerns about using a modern evaporator with a plastic drain pan on top of a wood burning furnace, but I haven't heard any horror stories yet...

Chris

Wait wait wait, your saying I can use an AC air handler INLINE with my furnace?

I wasn't even going to attempt that, because your always going to get a little bit of smoke sneaking by the firebox that could gum it up.
Plus, in the summer time If I have the furnace in line, all the cool air from the house will go

house -> gigantic metal box of a wood furnace -> air handler -> back to house.

This means I'd be cooling a giant metal box in my garage with all of the moderately cool return air that my furnace would be sucking. This seems like a giant waste of effeciency.
I had planned on using a seperate AC blower with valves on the duct work, so I could have only the furnace of AC on at once.

 

[Redox]

I had planned on using a seperate AC blower with valves on the duct work, so I could have only the furnace of AC on at once.

No, THAT will work fine, as long as you have good dampers between the furnace and A/C. You really don't want to put any cold air back through the stove as it will turn it into a rustbucket very quickly. My concern is over putting an A coil on top of the furnace as most conventional furnaces do. A regular furnace will top out about 180F or less before the high limit trips to save the day. I think most wood furnaces are capable of exceeding this number very quickly if the blower were to quit. The coil would melt and possibly catch fire or tox you out with fumes.

I have done a lot of formal heat load/loss calculations in my day and have developed a few rough rules of thumb on equipment sizing. I am assuming that you have a similar climate to MD and all my approximations are coming up at 1-1/2 to 2 tons. Since I haven't seen the house, I can't say for sure, but I think even 2-1/2 will be oversized. A thorough heat load calculation is tedious, but well worth it in the long run. I have seen too many units that are oversized based on the assumption that more is better and these homes have issues. I have also seen units that were undersized and do just fine. Most contractors tend to err on the high side to be sure, but I have downsized units and made people happier.

Hope some of this is helpful.

Chris

 

[mtfallsmikey]

Wood furnace in a garage....any gas/oil/combustable liquids stored in there? Or park your car in there?

 

[lugoismad]

Wood furnace in a garage....any gas/oil/combustable liquids stored in there? Or park your car in there?

Its a wood burning furnace. Yes, I know its against code in some areas to have them in your garage. Not in my area. Its already been inspected by my local fire chief and my insurance company gave it the all clear.

I do not plan on parking my car in there, and I have an unattached shed to keep my flammables in.