Installed a new furnace

In previous posts I’ve mentioned the humidity issues associated with a ventless gas heater. While the exhaust from burning natural gas or propane isn’t too bad bad toxicity wise, it creates a ton of water vapor. Seeing as we have around 2100 gallons of water constantly flowing through our system, the last thing we need is more moisture in the air.

When I was first looking to swap heating sources early December, I called around to a few places for estimates on installing a small furnace. I never received a call back and initially I was irritated at this. I then called a family friend who could possibly do the work and they came over to check it out. As we came up with a plan and discussed the various details of the install it became apparent why no one called me back.

Most commercial and residential install companies have certain brands they service and install. A lot of this is due to that they have the matching ductwork already prefabricated for normal construction and general calculations are known for setting the system up. With this being a 15 sided building with a dome upstairs, this was going to require custom ductwork among other things and the labor cost would be quite high. In addition, the winter time is a pretty busy time for HVAC installers here, so no one really has time to screw with an odd project.

I wasn’t super disappointed to learn this and I’m grateful for my buddy who came out and helped me game plan. I now knew enough of the basics to consider doing the install myself. One of the main advantages in my mind to this project is that permitting is not required which gives me plenty of opportunity to learn and do things that normally would require a licensed installer. That said, everything being done is up to at least residential code to the best of my knowledge.

Preparing for the install

After a lot of game planning and research, there were a few things I learned.

  • A furnace cannot be installed without ductwork. They’re designed to work at a specific duct pressure and the blower will be damaged without minding this.
  • The furnace warranty is voided if it isn’t installed in a residential building, so YOLO.
  • You size your ductwork based on furnace airflow in CFM (cubic feet/minute)
  • It matters how you set up your heating vents in relation to the return (keeps the airflow moving and circulating)
  • A fresh air intake on the return can help create positive pressure in the build which helps keep unfiltered air out (and possibly some of the smaller leaks in the polycarbonate).
  • The furnace needs a dedicated 15amp circuit (I did not have one)
  • A modern condensing furnace does a damn good job dehumidifying the air (yay!)


Normally when a furnace is installed, there are several code manuals that lay out temperature and airflow requirements for a space. This ensures that your spaces are heated evenly and that your vents aren’t too noisy. I don’t really care about any of that because I’m just heating a large open space for the most part, so I used a general chart for determining a relatively close duct sized based on the airflow of the furnace and made sure my vents were similar in surface area of the main trunk in total.

At the beginning of the install I always assumed the furnace would be the hard part of the project and I could just throw together some ductwork from home depot. I was wrong. It seems that most main trunk sections of ducts where smaller ducts branch off of are typically fabricated for the install. This meant that I was going to be banging together sections from flat sheet metal stock. I’ve worked with sheet metal a few times in the distant past, so this seemed doable. I was just going to bend some large sheets into a box and just create a seam with a piece of S-channel.

Here’s my first try at the plenum (the portion that attaches to the furnace). Yes it looks like crap, it’s super flimsy, and probably not close to air tight. I decided to go back into the house and snoop around my furnace and noticed all the nice seams connecting individual panels. After some searching on the internet it seems that a fabrication technique called a pittsburgh seam was the way to go. This not only locks the panels together, but provides some much needed rigidity to the sections.

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I used a 6″ hand seamer from Home Depot and a normal clawed hammer to replicate this technique. It took me a few tries to get it right, but it ended up working pretty well.

Here’s my second try at the plenum. Still not great, but it’ll get the job done. I also created a 90 degree curved section to transition to the main trunk. All that was left to do was to bang out the rest of the trunk including two 45~ish angles, some straight pieces, the return riser and an s-curve to line up between the floor joists.

We then poked two 8″ holes to supply the downstairs and three 8″ holes up under the grow beds to supply the dome. The return is also under the grow beds upstairs. When this isn’t heating, we’re still running the blower to keep the air circulating and a small amount of fresh air coming in.


The furnace was pretty straight forward. I had to slightly rework the existing gas line for the heater’s new position and poked three 2″ holes in the wall to the exterior. Two of these holes were for the intake and output for the furnace burners and the third is the fresh air supply for the return. I ran the condensate trap down through the floor to the sump where the sink drains.


Because I never imagined that I’d be installing a full furnace in the greenhouse, I never planned on having a free circuit in my sub panel. I realized that I had a separate circuit for the upstairs outlets and the general use lights. The lights are led and there’s nothing permanently running plugged in upstairs, it made sense to tie those circuits together to free up a spot for the furnace. We installed a switch box at the furnace for maintenance purposes and connected the furnace and thermostat.

Wrapping up

Including all my research and fabrication time, it took me around 3 weeks working at this after work each day for a few hours to complete the install. I installed a Nest Thermostat E up in the dome which allows me to control the heat from the google home app and it has an included humidity sensor which is really handy. The first thing I immediately noticed was the amount of water pouring out of the condensing trap. It only took 20 minutes or so to take the humidity from 95% to 40%.

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