There is no heat in one room which has a water heater radiator.

If I understand correctly... you are not getting heat from a forced hot water radiator... right?

If you look on the radiator there should be a coin or key vent... either way, need to remove the AIR from the radiator by opening that vent at top until water comes out for 30 seconds or so.

You may need to purchase a RADIATOR KEY (or use needlenose pliers) and loosen counterclockwise until you hear air... then as stated, keep open until water comes spitting out (have cup handy to collect excess water).

Note here, that sometimes the radiator vents have been painted over or damaged so they don't work. If this is the case, may need to stick needle or strip paint from painted vent... or may actually need to replace the entire vent... and that involves shutting /draining heat zone or entire boiler system to do that!! So, just hope that is not the case (if it is though, let us know).

Also, if no water comes out of OPEN VENT, could be that the angle valve shutoff going into the unit is defective... rare, but does occur.

Hopefully I understood this... let me know if I didn't!1

Try what I suggested and then get back to me. Good luck... MARK

.