Interesting air flow test result

[pgmr]

Well, interesting to me at least.

Since getting our high efficiency furnace/hp ~ 4 years ago, I have been running the fan on low pretty much all the time when the windows are closed. It does seem to keep the dust levels down and the air less stagnant. Was also using it to distribute warm, wood stove heated air throughout our 2200 ft² ranch.

Today I ran a test to see what would happen to temps in the far reaches of the house if the furnace fan was shut off and the blower on the stove was left to do it's thing. Wood stove is installed in a fireplace in the center of the house and bedrooms are down two hallways with two 90° turns - master is ~ 33' from the stove. When test started, inside temp at the thermostat (right outside MBR door) was 69°F and outside was 35°F. Stove has been burning for several days, so inside temp had pretty well stabilized. Several hours later, inside had climbed to 71°F while outside had dropped to 32°F. It is still at 71°F and is now 25°F outside.

There are definite convection loops going in the house. I held up a Kleenex by two corners at each doorway and each time it was being moved off plumb away from the stove.

Think I might have to keep that furnace fan off when heating with wood.

 

[fbelec]

depending on how much you pay per kilowatt hour for electric you'll see a big drop in your bill. if you were paying 10 cents per kilowatt hour running the blower 24/7 you would save 36 dollars. that is at 500 watts of motor. most motors i've seen run more than that but i'm just using round numbers.

 

[ckarotka]

I too have a ranch about that size only mine is almost a horseshoe shape. I run the blower once every 1.5hrs for ten min or so, give or take I don't actually time it, just to blow some hot air off the ceiling. I didn't see any big improvement after I wired the switch to run blower only that first year like I thought I would. I use two small stoves to heat, one at each 90* angle of the horseshoe. One stove is a newer Quad EPA by the front door, the other is an the old buck in my sig. in a masonary fp. Do to my work sched I have multiple days in a row off, during those times the Quad can actually do all the heating because of the 22/7 burning (small fb). On the days I work (12hr or 24hr shift) 12hr when I get home I light them both. On my 24hr shift the furnace is used 100% because my wife (works mon-fri 8hr days) is not that comfortable alone with both stoves going and she forgets about them=overfire (only once no damage and once was enough for her and me) With those long shifts I only work 10-11 days a month. Thinking about a PE t-4 now for the front for a longer burn, selling the buck and moving the quad to the masonry fp for those really cold nights or a quick warm up.

Thought about one big stove T-5 but it would take too long to get heat back to the corners and the kids would be cold if we got home from a function late and the stoves were cold.

 

[pgmr]

depending on how much you pay per kilowatt hour for electric you'll see a big drop in your bill. if you were paying 10 cents per kilowatt hour running the blower 24/7 you would save 36 dollars. that is at 500 watts of motor. most motors i've seen run more than that but i'm just using round numbers.

That's quite true for a conventional PSC (Permanent Split Capacitor) motor. Our furnace uses an ECM (Electronically Commutated Motor also called DC by some) blower, which only draws ~95 watts on low speed. See this pdf for a pretty good review of ECM technology.

I'm going to not use the furnace fan for a while when stove is going to see if the test result is valid over a longer time frame. My guess is that the air moving through the hvac ducts is losing both some volume (to leaks) and temperature (to conduction) in the non-conditioned attic and crawl space and causing the average temperature to fall a bit.

 

[woodgeek]

I would agree completely with what pgmr just said--elec consumption really depends on blower speed and the motor--newer systems with ECM variable speed motors should not be lumped in with the old systems.

I would add it also matters if your ducting is in the conditioned space or not, and how well insulated it is.

As in all things--a little experimenting (like the OP) is the best approach.