Are there any plumbers here? I am having issues with a low pressure mixing tap on an open vent system. This is an old farmhouse at a remote location and I would like to gather as much information as possible before deciding to call a plumber.

The hot water cylinder is connected to a wetback. There is no tempering valve as an exemption was granted due to low gravity pressure. When trying to heat the room in cold weather, a too hot fire sometimes causes the cylinder to boil and overflow. When this happened recently, I opened the kitchen tap hot water to let steam out and relieve pressure. The caused something in the tap to jam from heat expansion, and only cold water would flow regardless of tap position. In the morning, after everything had cooled, the tap worked normally again.

Unfortunately, the problem occurred a second time, in spite of me being careful not to run the tap. This time, the tap simply stopped flowing in the hot water position. After again letting it cool overnight, I was able to get a reduced flow by turning it off and on. Both hot and cold are working, but the flow rate is about half what it was before and that doesn't seem to be improving. 

Can anyone explain what is happening here? I assume a component in the tap has been distorted by heat expansion and is causing the issue. Is this something that could be repaired? Can the tap be disassembled? Money is an issue, which is why I am asking this. Thanks for any assistance.