Why You Have to Design For Usage Scenarios

I hate my furnace.

I really, really, can’t stand it. In fact, I don’t think I’ve ever disliked anything as much as my furnace. Ever. What is it about my furnace that evokes such strong emotion and contempt for something that simply pushes air around my home?

Functionally, the furnace does everything it is suppose to:

  • When heat is requested from my thermostat, it fires up the natural gas burners, warms up and pushes warm air through my ducts
  • When cooler air is requested from my thermostat, it doesn’t fire up the natural gas burners and pushes the cool air from the central air conditioner through my ducts

All of these interactions have taken place with the thermostat. There is only one interaction that I actually have with the furnace, and they blew it. Big time.

Changing the filters.

While on paper this sounds incredibly easy, this is how engineering only to requirements instead of also engineering to usage scenario cause this type of frustration.

Easy to Change Furnace Filter DesignFirst, let’s talk about good furnace design. This was the type of furnace I had in my previous home. By the way, that’s not me in the hot-pink sweater. Had I known how good I had it with my old furnace, I might have taken a few pictures of this process. But I didn’t so I borrowed this one. Let’s review. No tools required, no disassembly of the furnace, you don’t even have to turn it off. You can “hot-plug” the filters. When you start to pull the filter out, it automatically switches off. It won’t turn back on until you install another filter. This also prevents people from running their furnace without a filter, which my previous owner did for his entire 8 year occupancy. That’s a whole different post. With this design, you can replace an old filter with a brand new one in less than 10 seconds.
Now let’s talk about the Frankenstein that is my current furnace. For starters, my furnace requires two filters, not just one. So my “filter budget” has just doubled. Let’s talk about that. Unlike the nice 90 degree angle filter in the good design above, both of mine sit at 45 degree angles to the air flow to create a “V” inside the plenum. Yup, I said plenum, not furnace. That’s the yellow part at the top of the furnace picture to the left. It’s where the air is brought into the furnace. It’s also where there are no access points to the filters except from the bottom up. Remember, there are two filters that need to placed in there. If you had x-ray vision, and I’m not saying that you don’t, you could look through the plenum in this picture and see the two filters sitting in a “V” position.

The picture on your right is looking up to one of the filters. This is where the fun begins. First off, you’ve got the return duct right there in the middle. Since that wasn’t enough pain to put me through, the brilliant minds behind this beast of a creation decide to mount the electronics on the right-hand side and the furnace and the pièce de résistance, the furnace motor behind the return duct.

Now that we’ve talked about all obstacles, let’s walk through my misery.

  1. Remove the first filter. I’m not a big guy and I’ve got long, lanky arms. However, even with my long reach and fairly thin arms, I need to reach through that little gap on the left-hand side of the return duct, over the furnace motor, and try to pull down the first filter. To do this, I need to lift it up, pull the bottom of the filter towards the small gap between the left-hand side of the furnace wall and the motor. The gap is about the same size as the thickness of the filter.
  2. Remove the second filter. This one is more difficult because it’s on the other side of the furnace. The side I can’t get to because of the electronics box. Similar process through. Reach long, lanky arm through the little gap on the left-hand side of the furnace, over the motor, across the bar that holds the bottom of the filters in place, carefully maneuver filter through small gap on left-hand side of furnace wall and motor.
  3. Install first new filter. This is just the reverse of the first two instructions. Only it’s worse. Removing the old furnace filters I can be careless and rip them out, cut them on sides of the furnace motor, or better yet all the little nail/screw endings that marry the furnace outlet to the plenum. I can’t do this when I’m installing them.

Needless to say, after 5 or 6 fantastic, rust-filled, scratches all over my arms and hands, and an average of 15 minutes of my life spent fighting with this horrible furnace, I’m ready to strangle a kitten. Nothing puts me in such a pissy mood than that one, and only, interaction I have with my furnace.

This post was brought to you by the letter “D”, the number “7”, and how not to design a furnace.

About the author

Terry Blanchard

I'm the Vice President of Engineering at Readdle, ex-Apple, design evangelist, drummer, and gadget junkie extraordinaire.

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