Warm air furnaces: blower speed vs heating ability

[JRHAWK9]

Those of you with warm air furnaces who have provisions to be able to control blower speed based on plenum temps, do you notice any differences in heating ability when you speed up or slow down your blower? Meaning higher blower speeds = higher air volume moving through furnace jacket = lower register temps while lower blower speeds = lower air volume being moved = higher register temps.

I ask because a couple years ago I installed an ICM head pressure (speed) controller on my Kuuma. I added a variable resistor pot inline of the thermistor which allowed me to skew the perceived temp the speed controller saw which allowed me to shift the speed of the blower by a turn of a dial. I immediately noticed better heating when I slowed the blower down. I could only slow it down so much though because I had a sleeve bearing motor and didn't want to drop it too slow at blower shut-off. Recently I installed a ball bearing motor so I could experiment with some really slow blower speeds. What resulted was quite surprising to me. Our house seems to be heating noticeably easier yet with these even slower blower speeds. How slow? Well, the voltage at which I had the old bushing motor cutting off at (~65V) is now the voltage the ball bearing motor is seeing in the middle of a burn with the Kuuma on minimum burn. At the end of the burn the low limit cuts off around the 96°-98° area when the voltage being sent to the motor is around 45V-48V. These voltages were metered using a True RMS meter (actually two different ones), which you need in order to get the correct voltage output of an ICM head pressure controller. These much slower blower speeds also extends the time the blower runs....bigtime. The blower will run from about 10-15 minutes after lighting a fire in a cold furnace till there are hardly any coals left, just mostly ashes. There is NO cycling, none, zip, nada. It turns on at ~125° and off at ~96°-98°, where I have my low limit set.

Now, around the same time I switched to a ball bearing motor I also added a 6" cold air kit to my barometric damper in order to use cold outside air to regulate my draft instead of sending heated inside air up the chimney, but can't see that making -that- much of a difference.

Just curious if what I'm experiencing is normal or if our house is some kind of strange anomaly. It's so much of a difference I'm actually having to adjust how much wood I load and when/if I re-load and even then the house is staying too warm (upper 70's). Granted it's not real cold yet (got down to 25° last night) and the real test will be once it gets to around 0° and below when this house loses a ton of heat.

I'm maybe finding that with warm air furnaces, how the air is being delivered seems to be VERY important to how the house heats. Unless it's just our house, but I guess I find that hard to believe. Before I added the speed controller I played around with different static pressures too. This house loves low air volume/high vent temps.

anyway, just wanted to share what I discovered and if any of you have experimented with blower speeds.

 

[laynes69]

We have 10 foot ceilings both up and downstairs. I've ran slower speeds however we heat better with a faster blower speed. We also get better heat recovery from our furnace with a higher speed.

 

[JRHAWK9]

Interesting, thanks for the input. I'm finding the complete opposite. Maybe it's just our house.

We have 8' basement ceiling but have a 25'+ upstairs ceiling due to having a log cabin style place with loft.

 

[maple1]

Wouldn't you need to be careful not to overheat the air jacket?

And plenum, duct, etc...

 

[sloeffle]

These much slower blower speeds also extends the time the blower runs....bigtime. The blower will run from about 10-15 minutes after lighting a fire in a cold furnace till there are hardly any coals left, just mostly ashes. There is NO cycling, none, zip, nada. It turns on at ~125° and off at ~96°-98°, where I have my low limit set.

Our "main furnace" is a geothermal furnace. It puts out around 100 - 110F warm air when it is running in the first stage. When the installer set it up and was testing it, I asked him why the blower speed was so low. He said the blower speeds are set low so that the furnace runs more and you have less swing in temperature. You also have less cycling.

I also installed the ICM head unit on my Caddy but haven't switched the motor out yet. I personally like it because the furnace isn't off and on, off and on when getting to the end of a burn. Instead the motor slows down and keeps running and keeps the air moving. We have vaulted ceilings also and they might be 12 foot at the top inside of the house. The house does seem to hold its temperature a little better with the ICM unit.

 

[JRHAWK9]

Wouldn't you need to be careful not to overheat the air jacket?

And plenum, duct, etc...

Well, the warmer the plenum temps get the more the blower will speed up, and visa versa. Currently, with the Kuuma set on absolute minimum burn I'll see max plenum/supply temps of ~117°. I don't know what they will rise to once I turn the Kuuma up. My guess is mid to upper 120's with obviously faster blower speeds. I was seeing 120° last winter on maximum burn when the blower was running faster than what it is currently.

I do have a secondary low limit set at a way higher temp wired in with a relay which will send power to another tap in the motor, if plenum temps would ever get way out of hand (failure of the ICM, etc). Also have a high limit to cut power to the computer, which closes off all primary air.

 

[JRHAWK9]

Our "main furnace" is a geothermal furnace. It puts out around 100 - 110F warm air when it is running in the first stage. When the installer set it up and was testing it, I asked him why the blower speed was so low. He said the blower speeds are set low so that the furnace runs more and you have less swing in temperature. You also have less cycling.

I also installed the ICM head unit on my Caddy but haven't switched the motor out yet. I personally like it because the furnace isn't off and on, off and on when getting to the end of a burn. Instead the motor slows down and keeps running and keeps the air moving. We have vaulted ceilings also and they might be 12 foot at the top inside of the house. The house does seem to hold its temperature a little better with the ICM unit.

Are you able to slow down your blower at all (have an adjustable pot or other means to add resistance to the thermistor) and if you do, have you tried slowing it down and do you notice any difference in heating when slowing the blower down? Also, just curious, at what voltage do you have your blower shutting off at? The difference in our house heating this year compared to last year is the result of shifting the blower shut off from ~100° @ ~65V last year vs ~98° @ ~45V this year. Doing so skewed the whole temp range of the supply air up and volume down. The difference in heating the house seems to be pretty darn noticeable so far. Like I said above, the real test will come once we start seeing below 0 temps, as that's when this house tends to loose a buttload of heat.

 

[sloeffle]

Are you able to slow down your blower at all (have an adjustable pot or other means to add resistance to the thermistor) and if you do, have you tried slowing it down and do you notice any difference in heating when slowing the blower down? Also, just curious, at what voltage do you have your blower shutting off at?

Yeah - I have the ICM controller set to the middle of the sleeve bearing setting. My Caddy has a four speed blower and I only run it on speed two ( second to slowest ). So I'm not really running my furnace very fast when it is running at high speed. On any speed higher than two I have to take the back panel of the furnace off for it to get enough return air. The return air on the furnace is tied into the duct work on the house. I read awhile ago ( I'd have to google around to try and find the doc ) that speeding up the blower on your furnace can actually hurt performance because you are creating more resistance.

@brenndatomu hooked me up with the part number for a POT but I never hooked it up. My blower motor seems to work fine without it. I was also afraid that I'd dial the voltage down too far and damage the motor. I really don't have any idea what voltage the motor is pulling at when it is running on slow speed. If I get bored this winter, I'll take a voltage reading and PM it over to you.