Last week woke up and the downstairs main heating zone was not
heating. A second zone feading just 1 room was working and floors 2 &
3 (separate zones) had heat also.
The pipe run actually goes from the furnace, along the wall of the
dining room to the living room where it's baseboard heat, to the
closet, where it's bare insulated pipe, back down the opposite wall of
the living room as baseboard to the addition, where it runs
underground, pops up into a radiator, in the furthest room, then on
the return hits another radiator in the addition, then back
underground to the dining room and another radiator then back to the
furnace.

I know this is a riduclously long run, all told probably atleast 120ft
(living room is 25ft long alone)

Rented the pipe dethawer "Hot Shot 300" and spent the better part of a
day with this puppy clamped to all possible combinations of pipe.
Finally called a plumber who spent 3 hours the next day with the same
machine doing the same thing, before it got to 5 where he stated time
and a half. He suggested we try and get these rooms above 75 degrees
for 48 hours. I've had the further most addition room accidentally up
to104 degrees with a 'jet' LP heater but haven't had the temperature
drop below 55 degrees for 3 days (at night we just leave electric
radiators on).

The circulator is running and I'm able to bleed all radiators, can the
pipes possibly still be frozen? At night it's been down to 5 degrees.
We're on concrete slab, so no basement but I have an electric radiator
sitting on the concrete in the general area between the original house
and the addition where the pipes run underground (in the middle of
some rennovation also).

Any ideas on what else to try would be much appreciated.
thanks
Ian