Temperatures were in the 20’s when this occurred, and the belly in the line is outside – maybe the standing water froze and caused the problem.I’m leaning towards leaving this be for now.

So the whole unit heats up to the desired temperature before the room with the slow radiator.Here are some notes:1. problem radiator is not all that far from the boiler. 

Avoid heat pumps unless your cold temperature doesn't get below 30%.

We also have a very low cost of living (probably because the weather sucks and nobody wants to live here) but if you can tolerate 45 degree shifts in temperature in a 24 hour period you'll love it here!

The one i mainly use is the thermostat to turn the heat on or off, i already programmed it for times and temperature.

In addition to moisture/mold issues, I wanted to add also that if you have any laminate or vinyl plank flooring, the warranty can become useless and flooring damaged if the temperature is not kept at certain levels. 

They have no concept of actually waiting throughout the weekend to have a repair addressed once the temperature goes over a certain limit.We often go over, pretend to fiddle with the unit and can sometimes diagnose the problem as a frozen unit, especially if the box of 12 filters I've left for them at move in have grown dusty with cobwebs still sitting in the closet untouched and the one being used looks like it's grown a nice beard. 

if your competition has a garage and the costs to turn it back into a garage are not too much, then it would be smarter to convert it back, but a cheap way to get back that lost square footage may be a screened-in porch or lanai on the back. depending on how crazy you want to get with it, you may wind up with more sq footage than before and having a nice, temperature controlled outdoor area can be a huge selling point and should increase the value nicely. 

I have lived there on and off for over 3 years and have never seen temperatures exceed 85 degrees, in any part of the country. 

I understand that the electric heat pumps are not efficient at lower (winter) temperatures, which we have a lot of in Philadelphia winters.Are the newer models as bad as some of what I've read online?