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Originally Posted by Isaac Zackary
I'm thinking of possibly ecomodding my furnace.
The way I understand that my furnace works is that the house thermostat kicks on and it starts burning natural gas. There's some sort of thermostat inside the furnace that heats up and after a certain temperature, apparently around 90*F it turns on the fan which blows air through the heat exchanger and throughout the house.
I guess the idea is that if your house is around 70*F it would be a waste of electricity to turn on the fan if the heat exchanger is still close to 70*F since it wouldn't really be heating the house. There has to be a temperature difference for it to really work efficiently.
I see one big flaw with this design. Or maybe two. For one, I don't have A/C so on a really hot day the furnace fan will kick on for no reason. I guess it reaches that 90*F threshold and turns on. Well that's not the main problem because I can turn off the whole unit during he summer months. The major flaw is that if I have the thermostat set low, like 40*F, it takes a long time for the fan to turn on. And after that it turns on and off several times as the house warms up.
That's an inefficient waste of fuel. If the fan should kick on at 90*F heat exchanger temperature in a 70*F room then the fan should kick on at a 60*F heat exchanger temperature in a 40*F room for the same efficiency instead of waiting until the heat exchanger reaches 90*F in such a cold house.
Well the first thing I need to do is to make sure this isn't just some malfunction. Maybe it's not supposed to wait that long in a cold house. But I'm pretty sure the thing is working fine and like it should. So the next step is to figure out how to build a device that compares house temperature and heat exchanger temperature and kicks on the fan when it's at an efficient difference between the two.
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Pretty much all gas heaters/furnaces will have this problem. The main thing is: you're not using them as designed. They're made to heat a 65 degree house to 70, not a 40 degree house.
They're all like that with a single heat tube sensor point. I have the same problem with my screenhouse heaters. My screenhouse has a winter heating setpoint of 30F (allowed to freeze, but not hard freeze). The gas heaters are activated by a central thermostat, but they end up kicking off and on as the air blowing over the heat tubes is too cold for them to maintain the designed heat tube temp (probably 90 in those too?).
They struggle, but it's what I've got.
If your furnace is controlled by a simple thermistor, you could probably rig something up with a two thermistors (one for the heat tube temp, one for the ambient temp) and an Arduino. Just have the Arduino measure the difference between the outside air and the tube and kick the fan in with one of its outputs if the differential is greater than say, 10-20 degrees between the two.