Plumbing – Shutoff in shower very tight all of the sudden

From your comment, it sounds like the root issue is the lower water pressure from your hot supply. With lower water pressure, when you only slightly open the valve, there might not be enough pressure and/or flow for the water to overcome the other forces acting against it (gravity, restriction from valve, etc.). Similar to say a garden hose - with low pressure, it might not leak at all, but open the valve up all the way and it starts leaking at certain places (except in this case, the leak is the flow you want).

The first thing to check is that the shutoff for your hot water supply is fully open - at the sink, at the tank and anywhere else along the way where there might be a shutoff. If all of the shutoffs are fully open then it could be due to incorrectly sized pipes, mineral build up (depends on type of pipe). Check to see if the pressure is low at any other facuets - this will give you a good idea if its a problem with the plumbing or just that particular fixture.

It sounds like when you serviced your boiler that some how or another some debris got into your water lines. It could actually just be some buildup that was already in the pipes that got broken free when the other work was going on. In any case this particulate has gotten into the inner workings of your shower faucet.

You should be able to shut off the water supply and then disassemble the shower faucet so that you can get it cleaned out inside. There is a possibility that the rubber seals inside the faucet got torn and thus would need replacement as well. It is hard to offer specific advice on how to disassemble the faucet without knowing what make and model that it is. My experience is that most of them become pretty obvious how to take them apart once the knob and face plate are removed.