Got heater working on Chrismas day

The contractors decided not to work on Christmas Day. (They originally said they did not mind continuing the work. But it was perfectly fine. It's Christmas after all.)

I checked the heater again, some of them seemed finally heat up after one night. But that was not right. At first we suspected the new pipe was somehow clogged and causing water to flow the wrong way, even though still felt strange about the assumption. Finally, my bf went look at the hydraulic system, and read some vague descriptions on it, then suggested we might just not have enough water inside the pipes for the compressor to reach enough pressure to heat up the system. So we opened the incoming water valve briefly. The heater immediately started the 'heating sound' and the water pressure meter went up too. But we did not know what the correct pressure should be, so we just shut off the water value quickly and waited. After a while, it seemed the pipes were flowing hot water as they should be. I called that an end for now.