Blower Fans Speeds

Problems related to residential installations.

Blower Fans Speeds

Postby JTRFLASH » Wed Feb 06, 2013 11:14 pm

Hello, I am new to the board.

I have a new Luxaire Gas furnace, I recently received the manuals.

As I have done before I wanted to verify the blower speed settings for each function.

To my surprise I see something is not normal or standard as I know it and I would appreciate confirmation.

I have learned that for the COOL function the highest blower speed should be used and for the HEAT function a MID to LOW speed setting. With this furnace it is set the opposite, it is brand new 4 onths old.

If there is a standard colour code, the COOL terminal is connected to the BLUE wire (MID speed) and the HEAT terminal is connected to the black wire (HI SPEED), totally opposite to what the manufacture shows in the wiring diagram.

Has something changed?, should not the COOL be on the highest speed setting? If so you would think there was manual intervention by the indstaller as this is not the normal setting. But why change it. I believe it is going to be very difficult to get cold air through the house in the summer months.

I do see an option that a jumper can be connected between the HEAT and COOL terminal on the motherboard and then connect the black wire which would give me HI SPEED for both functions.

Any suggestions appreciated.

Best Regards,
Jim Taylor
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- Blower Fans Speeds

Postby Freon » Thu Feb 07, 2013 10:44 am

I will assume you have a generic PSC motor with various motor taps that determine blower speed and air flow.

Blower speed is a function of two design factors of your heating and cooling system. First, what can your duct system support. Based on your duct work you can determine the air flow (CFM) that it can handle. The second, what is the allowed temperature rise (heating) that your furnace is designed for.

Two other things to think about. As air temperature increases the volume the air occupies increases.

In your case find the design air temperature rise for your furnace. Then adjust the blower speed, based on the CFM chart in the installation manual, to fit your current duct system. Now you need to measure the actual temperature rise across the heat exchanger and make sure that rise is within the manufacturer's design temperature rise. If you find the temperature rise greater than the design temperature, you'll need to increase the blower speed. Although increasing the speed is not the best choice because of increased static pressure, you have to make sure the temperature rise is within the furnace's design temperature or the heat exchanger will possibly fail prematurely.

For cooling, since colder air occupies less volume, you can set the blower one speed higher than the duct system would dictate. If you're still not getting adequate air flow in the far registers, you can try increasing the blower speed again but you're once again operating less efficiently. I would check your duct system for different ways to increase air flow to problem rooms other than increasing blower speed.
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- Blower Fans Speeds

Postby JTRFLASH » Thu Feb 07, 2013 11:15 am

Thanks Freon for your input.

I understand now reading the install guide about temperature rise and looking at the difference.

The thing is the HEAT terminal is already connected to the highest fan speed, and since that is not the normal default setting as per the manufacture I would like to think that the installer made an informed choice (temp rise calculation) to run the fan at the highest fan speed for HEAT. Meaning upon his calcualtion that at the medium fan speed the temperature rise exceeded the manufatcures specifications.

I have limited experience but in the furnaces I have had over the years the HEAT was always set to a medium fan speed and COOL to high speed. So knowing this why wouldn't the installer have least put a jumper between the HEAT and COOL terminal, knowing that COOL setting needs the highest fan speed setting possible. As I see it now he has sacrificed one for the other.

I guess I will have to do the temperature rise test myself, but I fear there will be an issue. When the furnace is running I cannot hold my hand on the pleum for more that a few seconds, very hot and that is at the highest fan setting, perhaps that is normal as the MAX temp is about 165F as per the manual. I think moving the HEAT to the medium fan will give me an exceeds in the temperture rise calculation.


Thanks again for your help.

Regards,
Jim
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- Blower Fans Speeds

Postby Freon » Thu Feb 07, 2013 11:58 am

The installer may have been forced to use the high speed for heating due to temperature rise considerations. He may have used a lower speed for AC based on the duct design and the duct system's inability to handle air flow that would be greater than what he has set it at. This assessment is only a guess.

Drill a small hole (one eighth to one quarter of an inch diameter) in the plenum immediately after the furnace. Then get thermometer so you can get accurate measurements. A digital oven meat thermometer should work. Subtract the actual house air temperature shown on the house thermostat from the reading you get for the plenum temperature. Now you have good data.

It seems from your anecdotal description that the furnace does need the high speed to maintain the correct temperature rise, but this is just supposition. You do need to take measurements. I would also see what the blower chart says is the CFM for the highest motor speed at 0.1" H2O static pressure. You may need to do some simple duct work to increase the air flow. Count all the supply (air out) registers in the house. I will assume a 6" duct feeds each register meaning each register is designed to deliver approximately 110 cfm of air flow. Compare the blower data at high speed, 0.1" static pressure with what you calculate for your duct system's ability to handle air flow (number of supply registers X 110 cfm/register). Then compare the blower data for the AC speed the installer used and you will have an answer to why the lower cooling speed.
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- Blower Fans Speeds

Postby JTRFLASH » Thu Feb 07, 2013 3:09 pm

Hi Freon, yes that makes sense that in this case he was forced to use it. It is interesting though as I am in a new community, all 120 homes so far have this same furnace, so you would think this setting of blower speed has been known, preset, for about 5 years. I am wondering if this is a mistake or they way it should be, I just have to do the temp rise test and see what it is.

Good idea on using the digital oven meat thermeter. I did have a good calibrated one for this purpose but it has gone missing.

The temperture rise for my furnace is in the 19C to 36C range according to the manual and I guess if the differencial is higher (above 36C) at a lower blower speed then that is basically not in specification and it should put it back to the higher speed, but I will take the present rise before I make any changes on blower speed.

I will also check and count the registers, I believe my ducts are mostly 5" and some 4", it is a bungalow. Off the top of my head I believe the CFM rating is 1200. Could you tell me the CFM for 5" and 4" ducts please?

So I now have a project for the weekend and I will report back.

Jim
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- Blower Fans Speeds

Postby JTRFLASH » Thu Feb 07, 2013 8:39 pm

Hi Freon, here are my results.

My temperture in the plenum after 5 mins of the furnace running was 55C, my thermostat temp was 22C, the difference is 33C and my specs for rise are from 19C to 36C. So the temperture rise is in specification for the speed of the blower. I think you would agree that if I reduced the blower speed the temperture rise would most likely be exceeded, above 33C as only 3C to go to be above the limit.

I also found on the furnace door plate, confirmed once again for my 80k input furnace, the HEAT terminal should be on the HI fan setting and COOL terminal on the MED-HI fan.

There ya have it. Thanks for your help. Now to adjust the vents in each room. Oh the furnace is rated at 1200 CFM.

Jim
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- Blower Fans Speeds

Postby Freon » Fri Feb 08, 2013 10:50 am

I agree, leave the blower on high speed for heating. You may want to use high for cooling too. Be careful not to restrict air flow from the supply registers since you are close to the upper limit of the allowable heat rise. Also check your returns since those are usually undersized. You might want to observe the furnace after the 5 minutes to make sure the burners are not cycling on and off due to the high limit safety.
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- Blower Fans Speeds

Postby JTRFLASH » Fri Feb 08, 2013 11:12 am

Thanks Freon, now that I know I can jumper the HEAT and COOL terminals together I could run the COOL at HI speed also.

I have left the temp probe in place so at anytime I can check the temp, but in the spec it says the high limit is 75C! Wow

What I may adjust is the time the blower runs on after flame shut off, it looks like the temp is about 33C when it does, I think there is still lots of heat there that could be used without casuing a cooling affect.

The default setting is 120 seconds (defualt), the next and final step is 180 seconds, so another 60 seconds should make too much of a difference.

So for the way the furnace is set up, should I basically go around and just fully open all of the ducts? From my understanding in this case you wouldn't want to restrict the output as I believe you are saying you could exceed the temperature rise. Is there an engineered method of balancing? I am trying to balance the rooms, with the basement, etc., seems a daunting task.

Thanks again for all of your help it is great to have someone to confirm your understanding, especially when I am not in the business of furnace installation and maintenance.

Best regards,
Jim
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- Blower Fans Speeds

Postby JTRFLASH » Fri Feb 08, 2013 2:48 pm

Hi Freon, I happen to pass the furnace and check the plenum temp and it was 58C, so that is right on the top of the rise speficiation.

Now I am not sure what to do, the manual says if the rise is too hi increase the blower speed, well I have no where to go.\

Also the maximum output temp specified is 73.5C, so I assume that the furnace did not cycle to becuase of a hi temp as the max is 73.5, or is it?

Could 36C be the recycle shut down.

As I asked earlier should and open every single vent to the MAX? My theory to let more air out to drop the rise temp? Not sure if that makes any sense.

It seems the blower is not sized right for the furnace.

Jim
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- Blower Fans Speeds

Postby JTRFLASH » Fri Feb 08, 2013 2:49 pm

Where are there stars in the text where I have put words?
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