can anyone give me the pros and cons of using the a/c unit as make-up air unit. i thought that instead of throwing warm and humid air into the space from the mua i might as well passed the air first to the a/c coil.
(interlocking with the exhaust fans is not a concern).
I had a customer insist I do that to a 5 ton Carrier RTU with an economiser last year, for a factory plant office. They needed cool only.
Installed a remote bulb stat in outside air to lockout compressor below 58 deg ambient, and set up econo to drive full open whenever cool was called for(not modulating to maintain 55).Also made some control wiring changes. So on call for cool, fan/econo on and full open, above 58 outside air compressor enabled. It still works for them although I dont approve of this and let them know that unit was not designed for this application.
Constant filter changing and Im sure the life of unit is not long.
The biggerst difference between the two is blower cfm. On a rooftop, the blower is designed to drive enough air across the coil to achieve 18-20F temp drop. On a mau, the temp drop is more like 30-35F. The fan is slower, so the air stays in contact with the coil longer. The evap coil is also larger for more de-humidification. In your case, the rtu will not cool when it is humid and or hot outside.
100 CFM per 1 ton of A/C, if you use damper or econo, is about all you can expect to bring into the building.
Although it is code to install some sort of outside air on RTU's it is not the best way to limit your self to conditioned outside air.
Use the unconditioned make up to satisfy your exhaust. If there is not enough make up air, add with conditioned air to balance your building effect.
The more RTU make up air used the harder it is on the filters and TD. You also have to leave the RTU fan locked in the run position during store operation.
UCP added a stat to shut down the compressor per climate conditions. That was a very smart thing to do for his customer. I have seen simpler solutions by adding a 1609-101 to the suction line of the compressor and setting the cycle temp at 20* off with a 7* differential. This too will shut down the compressor per climate conditions but will also add a compressor safety shut down caused by any flood back or superheat drop.
Jeff gave you an uppercut with his de-humidification factor. It is a double edged sword. Just try not to slide down the wrong side of the blade.