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bbowens

Inadequate CFM

19 posts in this topic

    Hi, I have a 40-year old Fraser-Johnston central furnace/ac unit with a 1/5 horsepower blower. I've checked with the current mfg (York), and they indicated that this is an underpowered blower, and 1/2 hp would be more appropriate for this size hvac unit. I've gone through some sizing estimates, and it appears that the size is correct for this house (2ton ac, 80k btu furnace).

    The airflow through the vents is minimal, even though it's cold, and the AC is running continuously for 8-16 hours a day; my thermostat is set at 78, but the indoor temps are getting up to 85. Very inefficient and expensive!! I had the system checked out a couple years ago, and the guy said that the unit is operating correctly but the blower is just underpowered.

    So, my question is: can I just replace the blower with one that produces more CFM? or will I end up with other problems as a result such as electrical or ducting? The York company said that this unit sounds like it was designed to be a furnace only (based on the blower size being 1/5 hp), and that it probably was adapted to handle the AC, which makes me wonder further about consequences of increasing the blower outpout.

thx,

brad

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Need appliance parts? Call 877-803-7957 now!

Replace the whole system with something more modern and efficient. What do your ducts look like? Were they properly sized for A/C? If you want total comfort you may have to go with an ECM motor drive unit. They are so nice.

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well, sure-- except that the whole system will surely cost many more dollars than the blower. I'm sure the duct work is poor, too. Still, my question remains about upsizine the blower...

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[user=11178]bbowens[/user] wrote:

well, sure-- except that the whole system will surely cost many more dollars than the blower.

Of course it will.

 I'm sure the duct work is poor, too. Still, my question remains about upsizine the blower...

Don't waste your money.

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What size house are we talking here?  For 2 ton A/C it's going to have to be awfully small. 

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are you saying that the blower is going to have to be small because of the 2 ton ac size? The house is about 1000 sq ft, and the blower size I'm looking at is 1/2 hp.

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No.  I'm saying that for the middle of the summer, with only 2 tons of A/C to work with, your living space better be small.  We still don't know a lot about your heat load, i.e. insulation, # of windows, exposure to afternoon sun, geographic location, etc., but even under the best of conditions your system is already operating beyond the general rule of thumb of 1 ton/400 sq. ft.  Remember, that rough formula is based on the range of heat months during the year.  You just happen to pick the hottest time of the year to register a complaint.  If I could magically change the A/C capacity of a house for the months of July and August, we'd be closer to 1 ton/300-350 sq. ft. 

I'm not saying a little more air on the evap wouldn't slightly help the matter, if the blower was undersized, but 2 tons is 2 tons.  Unfortunately physics won't let us create another ton of A/C by adding more air flow. 

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ok, here's some more background on the situation. I am a renter, and my landlord has indicated that she will not fix it if it's not "broken." So, I'm willing to spend a $200 of my own money as long as it won't break the system (ie, too much power draw, too much airflow, etc). I already converted the furnace pilot to electric for $50, and that's worked out terrifically. So, primarily, I'd like to know what risks I may be taking by upsizing the blower.

As for sizing, as mentioned, I did talk with the York Co, and they confirmed that 2 ton is appropriate here. I also checked this website and got the same confirmation (for Zone 2):

http://www.acdirect.com/systemsize.php

atty, I appreciate your comments; they are appropriate. But...I'm just trying to make a bad situation a little more manageable here. If it absolutely won't work (the more powerful blower), then fine, I'll pass, but if there's a 50/50 chance it will, then I'm willing to move forward.

Any more comments will be greatly appreciated!

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Without casting aspersions on York's chart, which assumes, I can tell you, a highly energy efficient structure, let's do a little test to see if you have any "lost" capacity in the evap.  Measure the temp at the return duct, and then measure the temp at one of the outflow vents.  If you're 18-22 deg. difference, then I'm afraid your evap is doing all it can.  If you come out on the high side, i.e. 22+, then a bigger fan might buy you some more cool.  I fear, however, that you are not going to see $200 worth of difference.

As for the landlord, a bigger motor is not going to cause any damage, assuming you don't exceed the current rating for the fan relay.  In short, if you could snag a motor somewhere for about $50, then what the hell, but for me, $200 is a bit more than I would want to spend on that roll of the dice. 

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Put the dough toward a nice, portable window unit. When it's leavin' time, unplug it and take it with you.

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[user=5479]atty[/user] wrote:

Without casting aspersions on York's chart,

well, we can't point fingers at York on this-- the chart is from ACDirect.com...

Anyway, the intake is showing 76, and the output is 66. Maybe I can find just a motor for $50, that would be great. Would a motor upgrade be ok with the same blower/vane thingy? I was thinking that the vane blades might be different on a more powerful unit.

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[user=9503]AccApp[/user] wrote:

Put the dough toward a nice, portable window unit. When it's leavin' time, unplug it and take it with you.
unfortunately, these windows are the aluminum frame type; I can't see how I'd get a window unit into one of them. They also are part of the problem, because they provide little insulation.

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If you only have a 10 degree delta, I'm afraid you're a long way from solving your problems with a bigger blower.  Sorry

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for some reason, later in the day, around 6pm, the diff. was 17 (80/63). In this situation, what size AC would you expect? 2.5T? or even bigger? Also, what temp's would you expect to see coming out of the vents?

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18 to 22 degrees lower than what is going in.  For 1000 sq. ft, you need 2.5 ton minimum.  I don't know who drew that chart, but I would love to set him in Florida on a July afternoon for about an hour or so, then ask him what he thinks about his 1.5 ton per 600 to 900 sq. ft. 

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If your looking for a good price on a new fan motor to try this would be the place to look: http://www.usamfg.net/

All you servicers would do well to take a look at this place also. If they happen to have what you need you won't believe the prices, go to the website or call them and have them send you a catalog.

Fan motors that whole sale for over $100 for $10-$25. The parts are all factory new, just closeouts and buyouts of places going out of business is the reason for such low prices.

Wide varity of products also, bath and kitchen faucets that retail for $100's for $25 and less, older 10,11 and 12 Seer Split System Condensing Units, (I don't do HVAC so don't know what normal prices are for these type of items, here's some examples: York 12seer 1-1/2 ton 600cfm $645.00, York 12Seer 5 ton 2000cfm $1228.00). Compressors (LG Elec. QA075CDA 5,250btu. $5.00, Panasonic 2R10B3R126G 6,545btu R22 $14.00), HVAC pocket calculator converts temp to pressure for all gas types $13.00, Some G.E and Whirlpool coldcontrols $3.00 - $5.00, Whirlpool Elec Dryer GCEM2990LA0 $150.00, All run/start capacitors less then $5.00 most in the $2.00 range, Honeywell heating gas valves VR800A1384 $21.00 - VR800C1240 $13.00, Misc. V845Axxxx, V845Bxxxx, V845Pxxxx $5.00 - $8.00.

Hope this is of use to ya........

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Thanks, Willie.  Prices do look good, for sure. 

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so, I went to look for information on the motor, and it's a 3 speed. It appears to be set on #2, so could getting a little improvement be as simple as setting it to #3?

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The key word is "little"....but yes. 

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