How to raise efficiency of furnace?
 

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.

I see what you're saying, I think the problem lies with the thermostat and furnace not being a single unit and not being optimized for each other. Probably has some age on it?

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.

I manually P&G the furnace. 'Stat only goes down to 45 so I shut the system down at the 'stat. So far this winter has generally been between 0 and 32 and I've only needed to Pulse the furnace once or twice per day, just to get it up from whatever it was to 45. This P&G has REALLY cut down the furnace run time! :eek:

I set the blower stat to come on at 120°F.
It sounds like it's working properly aside from the thermostats being set to 90 and coming on.

I took the panel off and saw that it's controlled by a 15amp 120V thermoswitch. I'm not really sure exactly at what temp it comes on but it does come on when the house is around 90*F so I'm guessing that's what it's open/close temp.

I found one that looks very similar but is adjustable:

https://www.grainger.com/product/GRA...n-Switch-6UEE1

I think if I adjust this one to open/close at 70* then it will be closer than what it is now and work much more efficiently at 40*F to 60*F. But first I have to make sure it will fit correctly.

An Arduino that could compare two temperatures and then close a relay would be better of course since I could chose 70*F or more room temp whenever I want without adjusting the fan switch temp.

The non adjustable one should be coming on at around 120°F so it's probably messed up.

Do you think it comes on during the summer due to high 90*F-100*F heat and the pilot light making heat?

Still, wouldn't it work better if the temperature differential were the same? That is, if it's coming on at 120*F when it's heating 70*F air then shouldn't it come on at 90*F it it's heating 40*F air?

Still, I'll go ahead and test it and see if it's working correctly first before modifying the furnace.

The most efficient temperature differential would be where you get the most amount of heat rise in the air leaving the furnace with the same blower speed. Think bsfc. The most temperature rise with the two variables being heat exchanger temp and fuel consumption.

It may turn out that a higher temp in the exchanger is better and uses only a small amount more fuel to get there.

Yes and no. The hotter the fire box and the cooler the exhaust is after it leaves the heat exchanger the more efficient the system is. The problem is the thermo switch in question has no effect that I know of on the fire box temperature. It's purpose is to cool the exhaust and heat the house. Which is the whole idea of getting it to turn on and not just let the flame burn for several minutes sending heat right out the chimney.