razl,the pumps now come with a built in check valve at the pump outlet.Sometimes even when the tubing has a constant downhill drop it will still hold water.The check valve is mainly for rising tubing applications where the water would tend to drain back at the pump.If you dont have much lift you can try to remove the valve and that should keep the tubing from creating a vacuum provided it can drain.

I can't quite picture your setup, but could you just put a heat tape on the condensation tube? They're usually thermostatic, so they wouldn't run if they weren't needed.

If you were to introduce air as you mentioned, maybe a T adapter, and a vertical column from that, with elbows so it ends up facing down like a typical air intake, you'd reduce the risk of water running where it shouldn't, but I'm thinking you need the air intake closer to the outside, whereas it sounds like you're proposing putting the air intake right near the pump - it has to be at or past the highest point.

That water is under pressure so be carefull punching any holes are installing tees.

The verticle height is about 8 ft, so removing or disabling the check-valve is probably not going to work.

pjb999, if your in my basement, the tubing runs parallel in between 2 joists. In a most houses here, the tubing would penetrate the plywood siding to the exterior (just like most outdoor faucets are installed. However, my kitchen sink area is like a bay window, where the floor extends past the foundation, about 2 feet. Because of this, once the tubing is past the foundation wall, it is routing down so it penetrates the plywood straight down. From the outside, I can get on my back right under the bay window area and look straight up and see the tubing.

Maybe I can try making a hole about a 1/2" below the point the tubing meets the outside plywood, and see if that helps with the draining. I was thinking of a T, but I'm not sure if I understand how you are proposing. From the sound of it, I'd end up with 2 tubes holding water. Unless you are suggesting I have the tubing from the second T drain back into the pump basin?

Since your drain is just below the kitchen sink, a trap primer could be the simplest solution. This is a device normally used in commercial applications. It attaches to supply water. It provides a constant drip to your drain line. If you do not want it to run all year, install a ball valve ahead of it, which has a 1/4 turn lever handle. Total cost of parts: about $50-$80.

Not that anyone would do this ;-)

Here is a link that might be useful:

Now you got me thinking! I guess I could just attach a second dishwasher drain connector (#2 in the photo). This would eliminate any freezing issues.

Probably violates any number of codes....

Simply elegant! I don't think backflow would be a problem, so no need for a check valve.

This subject came up again...here.
http://hvac-talk.com/vbb/showthread.php?t=132570
HTH
Chris

Here is a link that might be useful: HVAC-talk diagram