a/c stays running

This is a residential central air unit of course with the a coil in the furnace plenum. Air temp blowing out of closest register is 55 deg. The farthest register is reading 58 degrees. I had technicians out and the freon is ok. On a 90 degree day or actually any day above 80 degrees outside, and the digital tstat set at 74 degrees, the furnace and outside fan run all day long and the temp will not come down below 78 degrees. On a 90 degree day with high humidity the temp will not come down past 80-82 inside. The home has all new windows, attic is insulated. I keep thinking the blower is not circulating the air. I need help here and any suggestions are really appreciated.
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what type of house...need more info
a power roof vent.....would help in any application Besides telling you that your system is undersized

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Brick ranch, 1300 sq ft. the a/c unit is 2 ton. There is an attic power vent but not operable at this point. Belt missing and blown in insulation on it completely. not sure of cfm on furnace blower.
Thanx
daytona wrote:

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What part of the country...more info

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Sorry didn't read the entire message. Power roof vents do not have belts. They go at the peak of the roof and have a built in thermostat the brings the direct drive blower motor on when holt (normally above 110 - 120 and then shut it off below 90 - 95 this sucks the hot air out of the attice area

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radiocom wrote:

the a/c unit is 2 ton. 2 ton = 7kW
7000/120 = 58w/m2.
Unit is undersized. Domestic should be around 130w/m2. Either reduce load or put in more capacity.
The lack of grunt is why it runs continually and doesn't catch up.
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The couple things that came to mind were to change the furnace filter. Have your tech check the fan speed on the furnace motor. And use a fan or two indoors to circulate the air.
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Thanx for the responses! The filter is electronic. I pulled it out and was still clean so I left it out a few hours to see if air would move better but it didn't make any difference. Would you suggest a 2.5 or 3 ton unit. I just have a hard time thiniking its the a/c unit when the air out of the registers is 54 to 58 degrees. Thans again for all the helpful info!
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still need the location (what part of the country) different parts ...require different sizes.............!

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radiocom wrote:

The cold supply air indicates the unit is working well. The space staying hot indicates that there is not enough cold air (ie the unit is too small).
Site unseen is difficult to determine. You really need a heat load calculation performed. However, an estimate would be between 3.5 and 4.5 ton. This will provide between 100-140 w/m2.
An undersized unit results in a lower first cost (this is how many contractors stay competative), but is usually more expensive longer term as the compressor has to run longer to overcome the heat load. However, an oversized unit will cause the compressor to have a lot of starts which is also not good. Best recommendation is a heat load by a professional.
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Thanx again for the responses! Sorry I have overllooked the request for the region. It is in Ohio. I will have a tech do a heat load. or get a second opinion. The tech that was out says the 2 ton unit should be fine but as you folks have mentioned, and your sight unseen diagnosis, there is definately something not right.
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wrote:

Leave it in longer. It should come out all slimy and sticky. ;->
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radiocom wrote:

I suspect inadequate airflow & possible ductwork problems.
At those temps & 1300 square footage with 8 foot ceilings it would seem that the system should pull down to around 76 or 77-F, along with 50% Relative Humidity with good air movement you would feel comfortable.
Too Low an airflow could be getting the 55-F Supply Air temp.
I cool over 900 sq. ft. of the first floor area with "a Half-Ton window" unit with Heat Index way up into the 100-F degree areas! Yes, "a Half-Ton window" unit with Heat Index way up into the 100-F degree areas & an old house with many windows & too high an air infiltration rate! http://www.udarrell.com/airconditioner_current_temperature_btuh_charting.html
A Two Ton system working optimally ought to keep it within the "Human Comfort Zone." Read the linked pages & go from there. - udarrell
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Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
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