I need some thoughtful thoughts about blower speeds.
First the facts: I have a Sears 105,000 BTU / 84,630 BTU natural gas
furnace in a 1,900 sq ft 2 story house.
The thermostat is a Honeywell programmable with "fuzzy logic".
The blower speed is currently set at "Med-Lo" which is about 1,100
cuft/min or 70% of maximum (1,500 cuft/min) volume.
Question: Would the furnace be more efficient at a higher blower speed?
If so, is there a benefit from adding an outdoor temperature sensor to
trigger the higher blower speed when the outdoor temperature falls
below, say 45 degrees F.?
Any thoughts will be appreciated.
Stay with alt.hvac where your comments and wiseass remarks are the norm.
This forum is for helping out not competing with the rest of the gang trying
to show just how dumb you are. BTW, have you ever shown your family the
type of trash you spew out at people?
Blower Range settings: ON 125 and OFF 90
Don't know the actual static pressure, manual gives .2 to .5 range
I'm not looking for trouble, it works well, just ruminating if it could
work better - more efficiently.
the thermostat that calls for heat is best thought of as an on-off
switch where the demand for heat is requested. could the factor of the
house's heat loss might be better handled with a variable speed blower?
remembering the comfort of your home in winter requires a balance of
temperature and humidity and sufficient oxygen from air changes.
if you blow the heat out of the plenum too fast into the house you will
not enjoy even heating of a close temperature differential of one
if you blow it out too slow the house will drop below the desired
temperature. get a digital thermometer with high low memory and monitor
the temperature at the thermostat to see how your range varies, and
determine a course of action to zone your home so no room gets excess
This is Turtle.
What you need to do is get a Service tech out there and set your heat
Rise to about 50 degree F and find out at what speed it should be at.
heat Rise is the difference between the air going into it and the air
Now speaking off the hand and no facts to stand on. Set it up higher
and see how it does but you will have to try to measure the heat rise
to not get it too low. 29 degree F or less would be too low and nothing
over 65 degrees F. differencial or heat rise.
Now speaking off hand here , if you can increase the blower speed it
will get the heat out to the rooms faster and then cut off faster. So
it would help a little to speed the blowers.
Now the out door thermostats is for another post here after you get
Now the fun part, measure blower amp draw where it is at, and at your
desired setting, , calculate run time , at present and new setting, run
your Kwh costs to reflect your increase in kwh and decrease in run time
if any. You just lost in electricity costs. For most of the US Kwh are
more expensive than NG so you are a net looser there. Oh did you
increase efficiency of the Ng side, , im sure not enought to matter or
offset the higher Kwh to run the blower. Say your blower takes 375 watts
and runs 8 hrs a day avg and cost is .125kwh, mine is, your present avg
cost is 9 kwh a day or $33.75 a month.
The heat exchanger will probably be more efficient at the higher airspeed.
...8hx375W = 3 kWh, ie 3x$0.125/kWh = $0.375/day or $11.25/mo.
If you increase the airflow from 1100 to 1500 cfm, the blower may use
(1500/1100)^3x375 = 951 vs 375 watts, according to the fan laws. At
1100/1500x8 = 5.9 hours per day, it might use 5.9x951 = 5.6 kWh/day
or 5.6x0.125x30 = $20.92/mo.
You are obviously overeducated.
The manufacturers specify a temperature rise for each line (and size) of
furnace. Go out of these lines, you void any warranty. Period. These temp
rise numbers are to get the most heat and longest life out of the heat
Get a life.
1) What is your temperature rise? Should be 40 to 60 degrees. If you
increase air flow, your gas efficiency will go up a little as you
remove more heat from the heat exchanger. Your flue temperature will
go down slightly, increasing combustion efficiency slightly. The
electric efficiency will go down a little as you will use more
electricity to move a little more heat. With a lower temperature rise,
your supply air temperature will go down, which may reduce your
comfort. Now your furnace supply air will feel like that from a heat
2) How did you measure the air flow? What instruments did you use?
There are tradeoffs to everything you do.
measure the hot air temp near the furnace.....
stick a thermometer in the duct somehow...
if it is much over 120 deg F after the furnace has reached full temp...
then a faster blower may be a good thing for you.
But if you don't REALLY know how to re-wire it, you can create
make sure the saftey function still works,, if the furnace overheats,
the furnace must turn off and the blower turn on...
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