Say what no water feeder??? I sure hope it at least has a low water cutoff on it.If not stop right there and offer one or the other infact offer both.
It sure sound like you only have water on the boiler in not in the distribution system..as in no pressure.
I too would have to agree with you if the boiler pressure
was the same as the domestic pressure then the relief valve should open.
It sure sound like to me you have a boiler that has no pressure on it.
Dryfiring a boiler is bad news.I would make it top pritory
to get back over there with a pressure/temp guage and see
what going on.
Lets us know what you find.