I'm in Michigan and we run them in inside walls or up thru the floor but not inside an exterior wall.

I too am in DFW so know how unusual this persistent, deep cold weather is--and now my backyard has 1 inch of new snow to top it off...
It could be that what froze this time is the water coming IN to the tankless--and that is why you get no flow--no new water came in to fill it since you left it running...

if the ice is in the cold water feeding the tankless then you still could get water at your cold water faucets in other locations in the house--and I assume your cold water is not stopped from what you said...

Adding insulation to pipes inside that exterior wall would not be done through taking off the bricks--
they would remove the tankless from the wall and open up the sheetrock--and if the freeze IS in that water line and it splits it--then the sheetrock will have to come off anyway

You better get a plumber out to open up the wall behind the tankless and see if they can thaw the line
OR you need to keep that hot water faucet turned on and hope that the thaw will happen w/o bursting a pipe

If you can't get plumber out to find/fix the problem before the thaw Saturday then you better go get "key" to turn off your water at the street meter Friday and try to drain the lines to minimize the damage if the pipe bursts...

that can be hard to do because those street valves can stick or freeze up themselves--so be careful

These heaters are outside and not freezing because of their freeze protection mode (use energy though) and the heat from units are protecting some of the wrapped pipes but not out far enough?

Concentrated heat tape installed up to wall is the best bet.

They both defrosted with no problem today, I'm thinking we need hear tape installations as well...as soon as weather permits. Thanks guys!

Deb

Not to feed/encourage the spammer, but I missed this on the first go-around. Our Noritz is outside, and the exterior pipe is insulated, plus I have a self-regulating heat tape that is under the insulation. I plug it in in the fall and unplug it after the last freeze in the spring. Not uber efficient, but I didn't want to go to the expense/hassle of putting a thermostat on the tape.

The pipes are copper, and I suspect conductive enough that the heat tape takes care of the couple of feet that are inside the exterior wall and adjacent to the part that's heated.