call plumbers but in the bathroom why not do underfloor heat and eliminate the rad all together.

In our old house, we had a boiler with radiators on the first floor. We did 3 different things with the system and one of these may work for you. First, we gutted the kitchen and we removed the radiator. We found a heating unit that fit under the kitchen sink. It was heated by the boiler and had a fan to blow out the heat. I never ended up using it as the kitchen was warm enough without it. Next we had a unit placed in our attic that heated the upstairs through ceiling vents... In other words, we had 2 heating systems that used the same boiler. Finally, we removed all the radiators in the home and the downstairs was heated through floor vents by the boiler. I should mention that our boiler was 75 years old when we sold the house 2 years ago. I loved hot water heat. The new buyers talked with the heat/air company that we had always used about replacing the boiler. He told them the boiler was efficient and would continue to work much longer than any unit they could find to replace it.