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Old 01-22-2008, 09:14 AM   #1
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Tempstar EVA 4800 fan speed settings


Concerned over the time it was taking to cool and heat our newly constructed home (actually now 3 years old), and seeing what seemed like higher than appropriate electric bills, I started investigating whether or not the air handler was set properly.

I contacted the manufacturer to get the original air handler doc's (not provided by the builder) and I obtained the original blue prints where I found the required sizing of the HVAC system which totaled up to ~1800 cfm.

The air handler doc's had fan curves for setting the proper dip switches for cooling and heating independent of each other. First thing I noticed was the max fan speed setting capability at lowest static pressure was right at 1800 cfm, ie all other settings of the dip switches would produce less flow. The current setting of those switches was only for about 900-1000 CFM for cooling (medium speed dip switch setting) and only about 600 cfm for heating (min speed dip switch setting).

Not knowing much about this stuff, my laymen interpretation is that the fan is set too slow for cooling, and way too slow for heating, given the CFM requirement of 1800 cfm.

Now, I've read that for cooling I should be at medium to high speed and for heating low to medium speed, but if I do that I won't come close to meeting the homes needs, CFM wise (~1800).

My second conclusion is that the air handler may be undersized since I can only get to the 1800 cfm rating by running the fan at the maximum of its capability and this will only occur if the static pressure is low. That high a fan speed and won't I raise a concern for blowing condensation off the coils and away from the drain pan ?

thanks for any help you might provide.


Last edited by crsanders; 01-22-2008 at 09:18 AM.
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Old 01-22-2008, 04:58 PM   #2
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Tempstar EVA 4800 fan speed settings


Just because the air handler can put out 1800 cfm doesn't mean that is what is required for your house and the manufacturer would have no idea what your installation is. The only way to find out what amount of CFM's is required for your house is by doing a proper heat loss/gain and then do a matching duct design. Your builder should have had to submit a copy of the heat loss/ gain to teh building department for your house, you may want to start there. What size condensing unit is matched to that air handler? 1800 cfm is between 4 and 5 tons of a/c. How many sq ft is your home?

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Old 01-22-2008, 05:27 PM   #3
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Tempstar EVA 4800 fan speed settings


Thanks for the reply, I must not have been clear or you misunderstood my point.

The builder's blueprints are where I found the calculated need of ~1800 cfm. That just so happened to also be the absolute max the air handler was capable of putting out. So that's why made the two conclusions that 1) the current dip switch settings were not delivering the flow the builder said was required and 2) that I have an air handler that is really sized too small ... ie shouldn't it have been sized to deliver the builders required flow at something closer to medium speed rather than max ?

And the compressor side is sized for 3.5 ton and the part of the house that is cooled by that unit is 2700 sqft.

Thanks again
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Old 01-22-2008, 06:06 PM   #4
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Tempstar EVA 4800 fan speed settings


If I get what your saying, you have a 3.5 ton system serving 2700 sq ft? What type of construction? Rule of thumb for standard construction is 600 sq ft per ton and 400 cfm per ton, so something doesnt add up.
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Old 01-22-2008, 06:54 PM   #5
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Tempstar EVA 4800 fan speed settings


Correct, 3.5 ton 14 SEER serving 2700 sqft, cement block construction, located in South Florida.

Sounds like I may have more than just an undersized air handler, if your rule of thumb is right.

Thanks, I think it's time to invite an expert out to evaluate the overall sizing and settings.
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Old 01-22-2008, 09:08 PM   #6
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Tempstar EVA 4800 fan speed settings


Excellent idea. Just get someone good, that will be honest.
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Old 01-25-2008, 08:58 AM   #7
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Tempstar EVA 4800 fan speed settings


That's the hard part but fortunately I know a contractor I trust and I have a brother with a good bit of knowledge to bounce info off of for a reality check.

But before I head down that path, I really was just looking for confirmation of my understanding on how this works. Please let me know if the following statements are generally correct:
1) The builder performs calculations to establish the tonnage required for our home. The hope is that the methodology and assumptions used adequately provide capacity for normal and worst case conditions throughout the year. Opinions vary greatly on how best to do this.
2) On the prints I see a CFM value for each line and a total sum value. This total CFM represents the required flow setting to be used when setting up the air handler using the flow curves provided by the manufacturer.
3) Given the total CFM required, the air handler should be selected/sized to provide this flowrate with a fan speed of medium to high for cooling and low to medium for heating. Picking an airhandler that only delivers this total CFM when speed is set to maximum is both inefficient and raises the potential concern for inadequate time to cool the passing air and concerns for blowing the condensation to undesireably locations.

My plan is to:
1) obtain the builders tonnage/load calculations from the building department (their required in Flrodida to deliver their calc's when the submit plans, permits, etc). From this I can get a better opinon on assumptions, methodology, and then assess whether or not the 3.5 ton is ok or not.
2) if item 1) is ok, then I will turn to the calculations of CFM to see if they are ok or not.
3) if item 2) is ok, then I really need to think about the air handler sized only to deliver this CFM at max speed and likewise why it is currently set to only deliver medium speed for cooling and min speed for heating.

thanks to anyone who can help me improve my understanding of how this all works.

Last edited by crsanders; 01-25-2008 at 11:18 AM.
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Old 01-26-2008, 08:50 AM   #8
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Tempstar EVA 4800 fan speed settings


Get someone independent to do a loss/gain calculation and duct design. Then compare to what you have. 3.5 tons of a/c can't really be pushed to more than 1450 cfm or the velocity will push condenstaion off the coil, causing other issues and won't dehumidifiy, which I would think is the bigger issue in your location.

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