Internet connected computer controlling a dumb fan. Inefficiencies of the modern gas forced-air furnace

Chris Brooker
March 14, 2013

I recently moved into a new house. Yay! I know. We needed more space for our son to play and grow up. We found a place, signed the lease and 2 weeks ago moved in. This house however, is the first time that we’ve had to pay our own utilities. Electricity, Gas and Water. Now our consumption directly affects our wallets.



One of the first things I did when we moved in is replace the old, busted and (to me) much hated mercury switch thermostat. Heating and cooler costs typically account for 50% of your utility bills and using a programmable thermostat improves the efficiency greatly. I replaced it with something I’ve wanted for a while but had no use for, the Nest Learning thermostat. If you haven’t heard of it or seen it, it’s an internet connected thermostat that learns your behaviour and automatically adjusts the temperature based on your patterns instead of you having to program it. It also allows you to control it remotely on the web or from you iPhone, iPad, Android phone, etc. It’s amazing and awesome, but that’s not the point of this post. You can check it out here.

Now I basically have this internet connected (awesome), computer controlling my heating and cooling. It’s super smart and knows when I’m home and when I’m not but it’s attached to one of the simplest, dumbest pieces of hardware in the house the gas forced-air furnace. I’m not too sure how many of you know how the furnace works or how many of you have opened up the panels and tinkered inside so let me briefly explain it. I’m only talking about your typical residential Gas Forced-air furnace.

When the furnace is instructed to go on it goes through a 4 step cycle. 1) A small inducer motor spins up and makes sure there is negative pressure venting the combustion byproducts to the outside. 2) The gas is turned on and ignited, heating up the heat exchanger. 3) The main blower fan starts up and blows the hot air through the ducts. 4) The gas stops, the inducer motor stops, when the heat exchanger cools down, the blower stops. The repeats every time the thermostat requests heat. This is a little simplified as there are a number of safety checkes that get done. But essentially that’s it, a fan blows air through a hot box and then through some tubes and into your house.


The biggest inefficiency is with the furnace itself. I’m new to this home and more than likely have things arranged differently than the previous tenants  plus lots of work has been done and things have changed. Now the vents aren’t balanced, some rooms are hot, some are cold. The super smart thermostat only reads the temperature in 1 room, so that room is the right temp but all the others aren’t. I go down to the furnace room and manually adjust louvres on the vent trunks to adjust where the warm air goes, in hopes of balancing the heat a little. This is ludicrous! Most of the time 80% of the rooms are empty.

The problem is the blower speed is constant, on or off. If I adjust the louvres, that pressure doesn’t just go away, it get redirected somewhere else. If the room is empty, it doesn’t need to be heated to the same temp as rooms that people are in. A truly smart system would adjust everything, based on where people are, how many, etc automatically.

Ideally, there would be a wireless temp sensor in each room, and an independent, discrete, heat source in every room that could adjust to the constant needs of the household. Lower the temp in empty rooms and raise the temp in rooms with people. There are sort of system that exist like this today with zoned heating, but they’re mostly in commercial spaces or very large homes. I hope in the more energy efficient future, these types of super smart HVAC systems find their way into more residential home.

You could even just have more but smaller heat exchanges, 1 on each main vent trunk line and a blower for each. At least then you could have a little more control and use way less gas.

Closing vents and otherwise redirecting the air does not improve the efficiency of the system, it in fact makes it less efficient.

And don’t get me started on how gas meters get read and estimated, that’s a whole other rant.

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