Joe - evap coil expert!!

Joe or Charlie,
I've got a problem on one of our older buildings. It was built with a Modutrol zoned system controlling a hot deck/cold deck. The 40 year old remote condensing unit on the roof is finally giving up. It's the semi that sounds like a "bucket of bolts" and it keeps tripping out on high head temp. I Have two HVAC contractors working on quoting the repair. This is a 20 ton unit but Carrier only makes R22 remote condensers starting at 25 HP and up. The one HVAC contractor says it's fine to run the 25 on the old 20 evap coil. The other, wants to convert over to the 411A with a new remote unit but says a new evap coil is needed due to the 411 running at higher pressures!! The biggest issue is this unit serves our corporate offices and man, can those guys complain!! A new coil is going to be a big expense!! I already went through a custom built steam coil last fall!!!

Now, I'm leaning towards just a compressor. But, I'm not to sure about the controls in this damn thing. The old mechanical timers were replaced with a solid state and a time delay was added to by pass the low pressure switch during pump down. Man, I don't need those corporate guys yelling at me every 10 minutes!!!

Pete,

It sucks but there is no way i would put a nice new condenser on that old evaporator. I agree switching to 410a would be good for future. it does run at higher psi. R-22 runs at about 70 psi and around a 40 degree coil as to a 410a system runs around 115-120 to get a 40 degree coil. Temp*press. chart will be exact. also i would get rid of the old modutrol, automated DDC would be the nice way to go. If you do decide on just changing the cond. make sure they flush the line set good with r-11 and nitrogen.

Well, after talking with two of my HVAC companies and finding out that a complete remote condensor change and a new custom sized coil would approach 35K both have recommended and quoted the easy fix. That's just putting in a new 20 ton compressor that still runs R22!! This spring to get it going, we did plenty of other repairs (and $$$!) to that remote condensor. Two of the three condensor fans and motors were replaced, the "mother" board updated, contactors, and compressor head gasket! Those OEM 480 volt condensor fan motors are not cheap!

Joe,
Thanks for the input. The replacement 20 ton unit and the crane to get the old one out and the new one in is on it's way Friday. I love it when the damn things are in the middle of a 200' wide building!! Those long reaches kill you with the size of crane needed!! BTW, I went with a good known rebuilder. The OEM Carrier compressor was almost 4K more!!!! (the project is almost 9K, so the 4K is a big difference!!)

After all is said and done, we'll see if they thing still trips out. All the opinions say if it does, get rid of the new "board/controls and replace it with some old time, old fashion mechanical's. They worked for many years and still do to this day!!