Author Topic: Furnace Thermostat  (Read 1477 times)

YttriumNitrate

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Furnace Thermostat
« on: February 12, 2017, 03:55:01 PM »
About six months ago I moved into a new-to-me house with a Honeywell Chronotherm III furnace thermostat (https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-0690.pdf ) that touts its energy efficiency by using numerous heating cycles to raise the temperature as compared to standard thermostats that just turn the furnace on when the temp is below a first threshold and turn the furnace off when the house temperature exceeds a second threshold. Based on my experience so far, because of all the furnace cycling, for a good chunk of the time the furnace is blowing air at less than maximum temp (i.e., the air is cool till the ducts warm up, and then after the gas shuts off the blower continues for a while). It would seem that while this may save on gas, it will use more electricity to run the blower for longer periods of time.

So, my question is what is more efficient in terms of natural gas and electricity, longer heating cycles with a ~2 degree swing in temps, or multiple cycles to keep the house temp dialed in at a set temperature?

Goldy

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Re: Furnace Thermostat
« Reply #1 on: February 12, 2017, 05:10:24 PM »
I also moved into a new house about 6 months ago and my furnace does the same exact thing.  When I'm home and it's running to keep a stable temp I'll turn it down a degree just so it stops that for a while.

Syonyk

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Re: Furnace Thermostat
« Reply #2 on: February 12, 2017, 05:46:08 PM »
The blower motor doesn't use much power.

What it should be doing is running the burner a bit before the motor fires up, then running the blower after the burner shuts down until the ducts are blowing almost room temperature air.

YttriumNitrate

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Re: Furnace Thermostat
« Reply #3 on: February 12, 2017, 06:02:39 PM »
The blower motor doesn't use much power.

What it should be doing is running the burner a bit before the motor fires up, then running the blower after the burner shuts down until the ducts are blowing almost room temperature air.
Yes, the furnace does that. The general process is: 1. Ignite burners, 2. wait ~30 seconds, 3. start blower, 4. run for a bit, 5. extinguish burners, 6. wait about a minute, 7. turn off blower.

I suppose for maximum efficiency one has to consider the cost of natural gas vs. electricity for whether short cycles or long cycles (the length of step 4) are the most cost efficient.

Syonyk

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Re: Furnace Thermostat
« Reply #4 on: February 12, 2017, 06:11:27 PM »
Well... you can find the power rating on the blower motor, your furnace rated efficiency, and the cost of your power and gas. Do the math!

soccerluvof4

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Re: Furnace Thermostat
« Reply #5 on: February 13, 2017, 04:41:24 AM »
I know several people that have installed the Nest thermostat and say its saved them a ton. I was surprised on how it allows to kick on and off so much but they all claim they save hundreds on their bills once it figure out your patterns.