Does a leak in radiator valve cause low heat?

Hot water heat is typically a closed system, and the pressure gage is used to determine if the system is full of water. Loss of water in a closed system will create air gaps in the water flow, and the boiler is not capable of heating air. Low water level arriving back to the boiler will trigger the high-temp shut off switch, and the water will not become hot.

Add water to the system and check the gage. To add water, open slightly the system drain valve (should be located in a pipe at the top of the system) and fill the system thru the inlet valve with a hose from a faucet until the spray from the open drain valve is solid. Close the drain valve system, and then close the inlet valve. Close the faucet valve and remove the hose.