What causes server room AC units to freeze? Skipping maintenance!

It's getting warmer here in Oregon. We peaked at 98F last week, which must seem like nothing compared to other folks in 120F+ climates, but when you're used to it be 70F and overcast, it reminds you why you have AC both at home, in your car, and in your server room. Our warehouse Data Center at work is equipped with two 5 Ton AC units and this weekend, the primary unit iced up. After the unit shut itself down, it did what a block of ice does best in a 90F room: Melt. We don't have a raised floor, so water ended up under the first three racks, storage area, and in the adjoining office space on the other side of the wall. This was only a minor disaster- we could have lost UPS's, PDU's, and server gear. But what caused it?

Dirty, dirty filters.

I'll be honest here, I'm solely to blame on this one. I could spin a story on how my boss never responded to my inquiry about getting maintenance contracts on the units, or how the staff at that facility are supposed to handle building/equipment maintenance, but at the end of the day- it was on me. When I got a $1.1k quote for 1 year of filter changes/inspections, I laughed and told myself I can easily handle this, no need to bother the staff. The filters were only $200 and there's a local supplier down the street. No problem, right? That was... 8 months ago. As it turns out, the filters I forgot to change were the problem. The restriction caused by the dirty filters wasn't allowing enough air to pass through the super cold coils. Add some humidity and bam. Ice block. After a trip to a local  supplier and a quick visit from the HVAC tech to verify everything was ok- it's back up to running at capacity.

So don't skimp or forget about maintenance! And don't worry, spare filters are stocked and a calendar reminder (or two) has been put in place. 

*EDIT*
Sadly the unit froze up again last night, even worse. While we wait on a diagnosis from the HVAC company, I've gone ahead and ordered moisture sensors.

Marc