It is 99 degrees here the inside temp in my home is 75. This is as cool as it will get. What would be a good range of pressure for my system? I am running 70 on the low side and 250 on the High. Also the home is around 1900 sq. ft. brick R25 in the attic with storm windows. What is an est. tonnage for my home? I think what I have is 2.5 t.

It's been a long time since I did the cooling end of things, so I don't remember the pressure side of things, but

1. If you're cooling to 24 degrees below outside temp then you're doing good.
2. 2.5 ton for 1900 sq ft is good tonnage for your condenser... assuming you've got the correctly sized inside unit.

You may know this, but just in case you don't: If you don't have a license to handle R-22 don't get caught hooking up those gauges to your system. The fine is HUGE. $10,000 I think.

We've got a couple good hvac folk here on the boards... I hope one of them will come along to pipe in on this too.

1900 Sq. Ft. would be undersized using a 2.5 ton unit. I wouldn't look to put anything less than a 4 ton unit in a brick house that size. Pressures look a tad low. I would expect to see 275 and 80 on a real hot day depending on the indoor temp and humidity... Is this a 2 story house or are you including the basement in your 1900 figure? If you're including the basement, then a 2.5 ton unit is right...

Holy cow, 4 ton for 1900 sq ft! Wow. I find it hard to believe, but fallen: listen to rickdb1: he knows his stuff.

On average, you will get 400 Sq. Ft. per ton. This is just a general rule of thumb, depending on the structure, windows, insulation, ceiling height. Etc... Here's a little calculator just for estimating in your area... Pretty in depth:

http://www.consumerreports.org/cro/appliances/air-conditioners/reports/sizing-worksheet/index.htm

Thanks Rick!

A good way to see if your AC is doing what it was designed to do is to make a check of the temperature drop as the air goes across the evaporator coil. Check the temperature of the air entering the return air dict nearest to the air handler, then check the air in the plenum above the evaporator coil. This should be between 18 to 20 degrees F. If it is higher than that you need to get more air across the evap coil, if it is lower then you need to get less air across the coil or possibly you have a loss of freon. The pressures that you quoted were correct only if all other things were working right, such as a clean filter, the fan motor speed is right, the registers in the rooms are balanced, the coil is clean and no ice has formed on the evap coil.

Do a manual J to really find out about A/C size. It is the only acceptable way for a professional to tell. Depending on your location house construction 600 sq per ton is the average. To large a unit will not allow proper run time and you will not remove the moisture in the air which in turn will make you feel cold and clammy. You will also spend more money for the unit size and the electric it takes to power an improper size unit.