Any thoughts other then clogged drains why my roof a/c evaporator freezes up? After about 20 minutes the air flow starts to slow down because the evaporator turns into an ice cube. I'm going to make sure the drain tube is clear but was wondering what else would cause this?
You a/c evaporator needs to be cleaned. Not the outside coil but the inside coil. You have to remove the sheet metal covering from the top of the unit to access the coil. You can get a spray coil cleaner from an RV supply. It probably has a black / dark gray mold, etc clogging the air flow. After the coil is cleaned it won't freeze up any more.
you might be low on freon
you can tap the line and add a fitting. A tech did this last year, but mine is starting to do the same. I'll upgrade to a 15,000 btu
Not an expert on roof air but automotive units have a temp sensor in the fins of the evaporator that controls the "thermostatic expansion valve." As the temp drops, the sensor signals the valve to cut back the amount of refrigerant allowed into the evaporator. This should keep the air cold but not allow ice to build up. This is especially important in high humidity environments as the evaporator can ice up in just a few minutes.
Pierce
Here is a link to a PDF describing the function of a TEV along with troubleshooting: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Fwww.danfoss.com%2FNR%2Frdonlyres%2FCA5E4257-4F6B-4F41-A268-8CB4FE6874DD%2F0%2FPFA00A102.pdf&ei=cb9lUJHYJoW9yQHD9IB4&usg=AFQjCNH3ATgAwEqBe8olJW3oGHpxZZ6qGA&sig2=I01mUdFQvts94qYTUyumnA (http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Fwww.danfoss.com%2FNR%2Frdonlyres%2FCA5E4257-4F6B-4F41-A268-8CB4FE6874DD%2F0%2FPFA00A102.pdf&ei=cb9lUJHYJoW9yQHD9IB4&usg=AFQjCNH3ATgAwEqBe8olJW3oGHpxZZ6qGA&sig2=I01mUdFQvts94qYTUyumnA)
Pierce
I have used this stuff it's meant for coil cleaning. Gary B. used it as well and he had good luck with it:
Air Conditioner Coil Foaming Cleaner-AC-920 at The Home Depot (http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=100599017&storeId=10051&langId=-1&catalogId=10053&ci_src=14110944&ci_sku=100599017&cm_mmc=shopping-_-googlebase-_-D29X-_-100599017#.UGXCoa6cbtk)
Keith
I recently cleaned both coach ac's and both ac's on my house with that same stuff. Works well and makes the ac smell good.