Furnace temperature rise versus blower speed


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Old 03-08-14, 11:43 AM
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Furnace temperature rise versus blower speed

My 1974 Lennox G12Q5 is rated for 165kbtu in.

I recently replaced the old 5-speed blower motor with a new 3-speed. In both cases, the blower was/is connected for the lowest speed on HEAT.

The temperature rise stated on the nameplate is 70-100F.

I just ran it hellbent for leather from 63 to 75 on the thermostat. The differential peaked at just over 130F.

Iím thinking that I should increase the blower speed to Ďmedium.í In fact, Iím going to do just that, and re-run the test. Meanwhile, Iím interested in any comments!

Paul in East Troy WI
 
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Old 03-08-14, 12:13 PM
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Sounds like a good move. Where were you measuing the temps?
 
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Old 03-08-14, 01:09 PM
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Interesting dataÖ

For the curious among you, I drilled a pair of 7/32 holes. One is in the cold duct, just about the humidifier. (Yes, when they installed the humidifier, there was no decent place in the hot side, so they installed it in the cold side. My best guess is that it does approximately nothing.) The other hole is in what I would call a plenum, above the furnace, where all the ducts distribute. Through each of those holes I passed an airstream thermocouple, and connected them both to a Fluke 52 K/J Thermometer. Itís a two-input device, and has a differential mode. (Prior to the test, the differential with the probes on the bench was 0.6 degrees F.) I slipped a third thermocouple through an existing hole, connecting it to an Extech multimeter, for confirmation. I also swapped the airstream probes a couple of times to assure that the readings were consistent and accurate.

I switched the blower to its medium speed, and re-ran the temperature rise test. I was really surprised to see the temperature differential stabilize at about 126-127F; I thought it would be lower. Surprised, and disappointed. I pulled the filter, to see how much its restriction of the airflow influenced the differential. I saw no meaningful change. I must say that I did not Ďsenseí much difference in the airspeed/blower speed (compared to LOW) Ė but that is a very subjective measurement.

Then I went to the thermostat and set the blower to ON, forcing high speed on the blower. The differential ticked right donít to 89, then rebounded to 90.

I wish I had a tach handy; Iíd like to know the actual speed of the blower.

I think that Iíll check the voltage to the blower during the heat cycle. Itís not out of the question that a failing relay contact (the one that selects the blower speed for heat and cool) is restricting the voltage and current thatís being delivered. But for that to happen, I would have to be dumping a lot of power in that relay. The wasted power would be given up as heat, and I would not expect the relay to last long. In truth, I donít expect to find anything there.

So: If it were yours, what would you do?

I know that it isnít Ďnormal,í but unless someone here convinces me otherwise, I intend to hook both the heating and cooling to the high-speed winding of the fan motor. Iíll be curious to see how we experience the system with higher air flow. But I think it makes a LOT of sense to get the temperature rise down into the range the manufacturer intended.

Thanks in advance for all comments, wisdom, and advice,

Paul in East Troy
 
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Old 03-08-14, 03:37 PM
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I should not have trusted the virtually insignificant change in differential temperature when I switched from low speed to medium.

The data I collected was;

Speed Differential

LO 131 F
MED 127 F
HI 89 F

Whatís wrong with this picture?

I started out with cooling on HI blower, heating on LOW.

Heating differential (temperature rise) was too great, so I switched MEDIUM and LO speed wires. So I should have then had cooling on HI, heating on MED. The differential was still (way) too high Ė and when I switched the fan control to ON, it jumped right down into the desired range. Has anybody figured it out yet?

What I REALLY did was switch the HI and MED wires. So when I thought I was taking differential temperature measurements with the fan running at MEDIUM speed, it was really still on LOW. Then, when I hit the fan ON switch, the speed jumped up Ė not to HI (as I thought), but to MED. And the differential temperature was perfect.

Measure twice, cut once.

So: I will leave it with cooling on the HIGH winding, heating on MEDIUM, and all will be well. At least I didnít do something REALLY smooth, like connecting the neutral to a speed tapÖ

It's a case of allís well that end well.

Iím so pleased to see the unit operating within its intended parameters!

Iím really grateful that there are people out there who are willing to share their experience, their wisdom, and their patience, to support and guide a guy like me. The internet is cool, but people like you (all) are MAGNIFICENT!

Thanks again,

Happy Paul in East Troy WI
 
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Old 03-08-14, 03:50 PM
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I will leave it with cooling on the HIGH winding, heating on MEDIUM
That's how most furnaces are set up.
 
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Old 03-08-14, 05:28 PM
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Paul, I must congratulate you on your knowledge of motors, relays, & electrical theory in general. As a friend of mine used to say: "Ya done good, Bunky."

Many (most?) of the folks we get here are completely lost when it comes to anything other than an exact swap of electrical components.

BTW, as far as fan speeds go, I'd be willing to wager a fair sum that 99% of the furnaces out there have never had a temperature split checked in the heat mode.
 
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Old 03-11-14, 02:08 PM
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More airflow actually makes the furnace more efficient, but too much can cause condensation to form.

With a split of 89F on high, I would have wired it so heating and cooling use high speed.

------------
One importing thing:

The temperature rise isn't a reliable way to see if a furnace is getting proper airflow until the fuel input is verified by clocking the meter.
 
 

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