hot water on demand system

I have had trouble with my hot water on demand system since moving in to a new home 2 years ago. Made many calls on the one year builder warranty. A plumber would come out ( a day or two later) and it would be a temporary fix. The hot water goes totally off. You can imagine getting in a shower and it goes frigid. Then you spend the next half hour in the basement or ouside trying to fix it. I have learned how to remove the lint from the trap by removing the fan, how to check for ice build up outside, and how to restart the system. One or the other has worked until this winter. With extended -20 and colder weather the hot water shut off totally. I called the manufacturer, it has a 15 yr. parts waranty. They said its condensation freezing in the line inside the house somewhere. Nothing to do with their system. I have to call a plumber to remedy. In the meantime they said to remove the unit cover so the system could suck air from the house rather than the line outside. This worked initially, but shut off again in a couple days. Then when I tried to restart it, it sounded like a train and started shaking, but started up. Since then the weather is better, the cover is back on and it works again. However, it is harder to get a tub full of hot water. The hot is on full blast and it fluctuates from cold to hot. This unit has really been an inconvenience. Question 1: I heard of someone that had some kind of a box installed on the ouside of the house to prevent the pipes freezing up inside. Does anyone know what this is, and how it's installed and if it will even fix the problem? Question 2: Should the builder have some responsibility since the problem was never fixed.