Adjusting fan speed on direct drive blower


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Old 12-27-09, 05:43 AM
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Adjusting fan speed on direct drive blower

I have a Oneida/Royal LOB oil fired furnace with a direct drive blower. At some point, an AC tech bumped the blower speed to high, because there wasnt enough air flow. Now, Im thinking it was set for heating speed, which is supposed to be different than cooling speed (of course)

Id like to set it back to a lower heating speed so the furnace will run longer, more efficient, more comfortable (I hate forced hot air! but I dont have a choice)

Looking at the schematic Im not 100% positive what wire needs to be connected to the "speed lead" for the blower. The blower has 3 speed settings, using a red/blue/black lead. I see a yellow coming off the fan control going to a main junction box. From that box a yellow, black and white come into the blower junction box. White to white, obviously. Schematic shows Black to Blue on blower motor, and Yellow to Black on Blower motor.

My question is......which lead needs to be changed to get to a lower fan speed? Or, exactly which lead CONTROLS the fan speed.......is it the yellow from the Fan/Limit control?

If this is clear as mud, I apologize. Ill try to get a pic/scan of the schematic if necessary.

Thanks in advance.

-Chris
 
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Old 12-27-09, 06:57 AM
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Speed kills

is this a newer furnace?if so,you do not just change the spped without checking the nameplate on the furnace,and it must be within the design temperature rise.it should say=temp rise 35-65 degrees.you need to check the temp and the return duct,and the temp in the supply duct,not close,and see if it is within that range.example=return temp is 70 degrees,supply temp is 110 degrees.110-70=40,you are within the furnace specs and ok.if you lower the speed and you get above the designe temp rise,you can damage the heat exchanger,and possibly cause the furnace to trip out on high limit.he may have bumped it because the temp rise was on the high end,so you need to move more air as to not damage the furnace
 
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Old 12-27-09, 08:48 AM
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Yes I understand the temp rise values. Right now they are within spec, but actually towards the low end of the range, so slowing down the blower would put it more in the middle.

And since I can reliably check, I will do so once I figure out how to slow it down.

It was definitely an A/C tech that sped it up, he was worried the coil would freeze because of lack of air flow. Which makes sense, everything Ive read points at high speed is for A/C, medium or low is for heat. I dont mind changing it manually if it makes the heating season feel more comfortable in my house.

-Chris
 
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Old 12-27-09, 06:07 PM
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Post the diagram.. Not sure what you got.

After the changes been made, I want you to take a temp reading to be sure you are with in the specs on the furnace.
 
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Old 12-28-09, 10:28 AM
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See attached.

I have a multimeter with a kprobe t-stat to measure the return and supply air. There are access holes in the plenum on each side specifically for doing so, so I should be able to get pretty accurate readings.

-Chris

 
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Old 12-28-09, 01:45 PM
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The yellow wire is power for both heating and cooling.

So what ever the yellow wire is on, that's the speed you are going to get. If you change the yellow wire down to a slower speed, the A/C is going to get the slower speed.

So if the yellow wire is on Black, you are getting a higher blower speed.

You can change it down to a slower speed, when comes A/C season, you may want to take a reading on the temp drop to see if you are with in range. (and temp rise in heating)
 
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Old 12-28-09, 03:08 PM
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Thats what I thought it was. last night I experimented with the other leads (since it was already on black, I tried red and blue).

The nameplate on the furnace shows 3 different firing rates, based on nozzle size. .5, .65, .75

The allowable rise for .5 and .65 nozzles is 35 to 65 degrees F, for the .75 nozzle 70 to 100 degrees F.

On any speed but high, the temp rise was somewhere in the 80s to 90s. I currently have a .65 nozzle installed, so this rise is unacceptable according to the nameplate, at least for that nozzle size. Cant figure out why the rise was so much different on medium speed........low i could understand, but medium not so much. And I dont think there is that much of a difference between speeds. CFM wise I think the range is between 900 or 1000 on low and 1200 at high speed

I reset it back to high speed. My question is, if a temp rise of 70 to 100 is acceptable on a .75 nozzle, why would it be different with a smaller nozzle?

The house was definitely more comfortable as I was running my tests. The slower speed really help even out hot/cold spots. But alas, if its going to damage something, I cant use it that way.

Another question that came up.....there is a black wire that goes INTO the blower junction box (you can see it on the schematic, it goes from the main j-box into the blower j-box, and is separate from the black lead to the blower motor)....any idea what that lead is?

Thanks again.

-Chris
 
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Old 12-29-09, 06:09 AM
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Originally Posted by chrisexv6 View Post
I reset it back to high speed. My question is, if a temp rise of 70 to 100 is acceptable on a .75 nozzle, why would it be different with a smaller nozzle?
I'm not sure why that is...

The slower speed really help even out hot/cold spots. But alas, if its going to damage something, I cant use it that way.
No, the burner may trip off on limit.

Is the air filter clean/new? How does the blower cage look? Are all the vents open, and/or not blocked?

Another question that came up.....there is a black wire that goes INTO the blower junction box ...any idea what that lead is?
Just a dead wire. Not hooked up to anything... Not sure why they did that.. You said the tech had to bump the speed up for cooling, and how knows how many years it was running at the slower speed before, all was fine.
 
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Old 12-29-09, 06:16 AM
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The cage looks good. No obvious large stuff, but maybe some "caked on" dust on the fins of the blower. I might think about taking it out and cleaning it at some point. Not sure how much of a difference a little dust/dirt could make, but I suppose it might be something that could make more of a difference than meets the eye. All vents are fully open and not blocked. There are a couple balancing dampers in the second floor ducts, but only on 3 supply vents (the first 3 in the line.......they were getting all of the air and the other end of the line was being starved....the manual dampers were added right at the trunk where the flex lines come off to lessen the amount of air to the first supplies, and in turn boost the amount of air towards the end of the line)

I was watching the high limit switch as well and the pointer never got close to the high temp cutoff. I stopped the experiment once the temp rise got to 80/90. It might have actually worked in that case (like you said, who knows how long it was working at a slower speed). Ive seen/heard the furnace go off on high limit, but the only time its happened is if Ive really cranked up the temp (for testing) and the furnace had to stay running for a very long time (i.e. bumping the t-stat up to 76/77 when the house was at 69)

The filter is pretty much new, and its not a pleated or anything, just a standard rock catcher blue fiber filter.
 
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Old 12-29-09, 06:31 AM
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I wouldn't worry about the blower if it don't look that bad.

Little bit not going to stop the air flow that bad.
 
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Old 12-29-09, 06:40 AM
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OK.

It gets cleaned yearly with our maintenance, they just dont pull the whole thing out and clean the fins so whatever they can reach with a vacuum gets taken out.

Too bad I didnt have the 4 speed motor, med high might have been some help.
 
 

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