Wood stove inspection passed – and thoughts on duct work

So this weekend I finished up a few things (wall stud installation, trimming one spot in the roof that didn’t quite meet the 2″ clearance) in preparation for my first/final inspection on the wood furnace installation. The inspection was pretty quick and simple. I think it helped that I was quite knowledgeable about all the specs, clearances, requirements, etc so the inspector did not have to ask a lot of questions. I knew the things he’d look for so I pointed them out to him in order. I passed with flying colors and have my first burn going right now. I disconnected the ducts since the first firing is pretty stinky for maybe 30-60 minutes while the paint cures. I also had my manometer hooked up to the chimney so I could check the draft, a good thing since while the load was burning at full load I was in excess of 0.1″ water column, which exceeds the manufacturers limits. Only with the manometer was I able to tell that and see that I needed to adjust the barometric damper to get it in spec. I have this pressure tip, though I’m not POSITIVE it is the right one. It was pretty hard to get the tubing over the end of this tip, though I was able to get it on. I like the fact it is held on magnetically so I can easily leave it hooked up. So far running the furnace wide open it heated it up 4 degrees from 68 to 72 degrees in maybe half an hour or so. I have it throttled down now and the temp is slowly dropping. The fan is still kicking on every 5-10 minutes or so; it isn’t like once you turn it down it won’t turn on any more so the temp just slowly drops back down. For reference, the blower came on at 1:48 and back off again at 1:54 and was blowing around 98-100 degrees from the vents (when at full fire it is more like 118-120 from the vent). The heat kicked back on again at 2:01.

So my thoughts about duct. Well, my plan changed considerably from the beginning of this project to the end in many aspects. The duct work was one of the more annoying though. I was able to ultimately figure out what sizes I needed (I could probably do a whole post on that, but don’t hesitate to email me if you have questions and I can share what I can). For reference, 10″ diameter (or 8×10 rectangular) is about the right size if you have 10-25′ of duct before the first takeoff to handle 500 cubic feet per minute (cfm) After that I dropped it down to 6″ round (and about 250 cfm went to the first vent then 250 cfm goes to the last two vents and is split between the two of them. So whats the big deal?

Well, firstly my floor joists are 16″ on center and only 12″ tall. Well, with a 10″ round I can still meet my 1″ clearance to combustibles no problem. Except there is no way for me to split it off and hit a vent above it without the whole duct being SIGNIFICANTLY below where the ceiling would be if I ever finish the drywall. This thing would have to be hanging down probably 6-8″ and that would look terrible. I found some special order “saddle T’s ” that would take 6″ off the 10″ duct with minimal height, but it was still too much and would require at least 2-4″ more than I had. So, I took all the 10″ round off that branch and started scrambling. I found a local sheet metal shop (that primarily does duct I believe) and they were willing to work with me since I knew what I needed at that point. I needed 11′ of 8×10 rectangular, a transition are to 7×10, and then end caps. The reason for these dimensions: My floor vent hung down 5″ from the floor and I wanted to contain it all within the joists; better if it was 1″ ABOVE the bottom so that I could drywall at some point and maintain my clearances. I ended up cutting down that vent by an inch so it only hung down 4″. The 7″ was decided so that the end of the run was big enough I could put a 6″ takeoff on there (the tabs on the inside wouldn’t have been able to be folded if it was only 6″, otherwise the extra inch of clearance would have made life easier). The custom fitting ran me around $20-25 which I thought was pretty reasonable, but the BIG deal is that the duct work only cost me $12 per 4′ and the clips were about 8 cents each (compared to about a dollar at the store for fittings and about $24 for 4′ of duct from the store!). So with those savings I ended up getting the custom fitting for just a couple bucks over if I had been able to use everything from the store.

So the rectangular duct was SO easy to work with, it goes together easily (one hint, the S clips put one on each section and then while the two sections are separated by an inch or so angle them and start them on the other piece. Now just slide together and they clip in solidly easily) and it is lower profile than the round for the same amount of cross section. Not only does it go together easily, takeoffs are low profile and you can easily put in place with a jig saw and a drill (or the fancy hole cutter from the store, but I had 3 to cut and didn’t want to spend $30 on that special tool). The round duct? Well that brings me to point #2. That stuff is a HUGE pain to work with. It doesn’t actually come round so they can stack it more densely, so when you clip it together its oblong. THe crimps are not deep, so when you are trying to fit it together in place in the ceiling it does NOT want to go together! If the crimps were deeper it would be SO much easier, but they are not. What a pain. The T’s that are available aren’t exactly low profile, so even in 6″ I was barely able to keep everything within the 12″ depth I had available; my first takeoff there I had to again cut it down so it would be lower profile.

Point #3: Talk to the person who is going to inspect your work. The first person seemed to b making stuff up as they went as far as some rules and that threw off some of my plans which made the final connections to the furnace quite a bit more difficult. A week or two later I was ready for inspection and the person changed. The new guy was SO much more laid back. Now, I was ready for nit-picking to the specs in my manual, everything was done either exactly or in excess. However, I was NOT ready for little by-lines or footnotes in the mechanical code which I didn’t have available to me. Make an inspector mad, or just get one that wants to be a pain and I’m sure they can fail just about anything for some reason or another. The second guy while I was talking to him ahead of time is willing to work with a person and if there is anything minor found he was willing to wait for it to be corrected then and there rather than failing you and making you pay for an additional inspection. That is a big part of his personality, but it may have helped that I tried to grease the skids a bit by asking if there was anything I could do ahead of time to make sure the inspection was as easy to do as possible.

In all, this was not terribly difficult, but there was a decent amount of work. Had this not been a wood FURNACE, rather a stove, the install would have been EASY. The chimney was probably only 5-10 hours of work and was pretty straight forward. It was the duct work and the figuring / engineering to try to get a good system that was the most work. On the engineering side there are not many helpful examples that I could find online meant for the homeowner (or even the professional for that matter!). On the install side working with duct without all the proper tools makes it more difficult, its a pain anyway, and the stuff is SHARP. I had quite a few cuts on my hands when all was said and down (the sliding 90’s at the store have some sharp edges on the inside where they didn’t seam it properly and I cut my finger tips pretty good on those).

Overall I’ll be pretty happy when the project is wrapped up (the inspection is hardly the end! I can fire it up, but I still need to clean up, drywall/enclose the chimney, and clean up the house) but I am happy to have it to a point I could quit on it if I had to. I have an engineering background which helped make sense of some of the HVAC charts I found. Without that I think my advice for MOST people would be to at least work with an HVAC person to help lay out the design. Menards had free duct design service but I IMAGINE that they won’t be able to effectively take into account the height of the joists and the protrusion of the takeoff boots. Maybe they can, if so that would help a lot. But having and HVAC person come to your house and LOOK at what there is to work with would help. You may be able to find a person locally that would come take a look at and design it for you for $100-200 and that would likely be money well spent. I have $60 just in 10″ duct that I likely can’t return because it has holes in it from the self tapping screws that held it together. If you poorly design your duct and it doesn’t distribute the heat well that will cost you in the long run. That said, it is a manageable project and you can likely luck into an OKAY system by winging it. I think MOST people would likely pay an HVAC person to do all the duct work including installation. It would likely cost you an extra $400-600 but many people would be well-served going that route on the duct portion.

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