If they did a decent job of purging the lines, you shouldn't have substantial air in the lines unless there is some problem in the system. I'd consider calling another radiant heat company to see what they say. I have radiant heat in my house and, even though I live in a major metropolitan area, it's hard to find really competent radiant heat folks. The company I currently use guarantees its work for a year. You might want to see if you can find someone else to help you that will stand behind what they do. I agree you should also ask about whether it might help to install an extra air vent and at what location.

Agree with Kudzu - Sounds like potentially two issues really. 1. Do you need an extra air vent? 2. Why is air being introduced into the system?

In order for air to get into the system, it virtually always suggests that water is leaking from the system. You will get small amounts of air in make-up water - and that's what an air bleed is designed to remove. You shouldn't have "a lot of air" in two weeks.

If you had the system open for some reason and had to drain down portions of it, that could explain why you might have to purge it a couple times... but I'd try to get to the bottom of why you have that much air in your system.

You are correct. There was a small section replaced in the summer/early fall. I suppose I'll have to tear up the floor and check it. Do you think the company that did the replacement should be responsible for fixing it?