My tankless water heater is about 6 years old. We use it for radiant heat. I haven't used it much these season, but started too this week. At first the water pump would be on, but the water wasn't circulating. The water pressure gauge was at zero. I filled it up more, and now when it comes on, the water circulates and the pressure is about 15 psi.
But, sometimes when it comes on and the water circulates, the heater doesn't ignite. If I flip the power off and then back on. a gurgle noise will come from the heater and it will turn on sometimes.
It sounds like you have some air trapped in the pipes. This can cause an air lock. Does the system have an air extractor? If so, does it work?
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013