This one has me stumped...
Our house is only 3 years old, and so far we've not any plumbing problems.
Today I noticed that the shower in our spare washroom (very rarely used) was not working properly. When you turn on the cold water its OK, but as you turn the knob to the hot the water pressure starts decreasing. When the knob is turned to 'full hot' there is no water coming from the faucet. So it looks like only the water from the cold pipe is working. I tested the washroom sink, and the hot water seems to be flowing fine. What could be causing this?
Thank you in advance..

I'm by far a pro but I would guess it could be the "scald control". We just installed a new faucet with the control. If it is set too low, you will get very little hot water. With ours you just take the handle off and change the setting.

Jaimie

The previous poster has a good point in checking the scald setting. The problem may also be from the fact that it is "very rarely used". My friend had the same problem-house about four years old-on a rarely use shower valve. Even though they have city soft water, the valve just seemed to clog up and wouldn't flow hot water. He had tried various cleaners and nothing worked & had to replace the valve unit. Maybe Speedball has some good insight to this problem :)

Thanks for the feedback, I'll check the scad setting. We've never changed d any settings and it use to work fine before, so I'm somewhat doubtful it's that. But can't hurt to check.
How big of a project is it to replace the valve unit?

Jamie gave you a excellent answer. However in case you haven't got a anti-skald feature on your valve let me give you another option.
Water heaters build up minerals in the bottom of the tank. If the heater isn't flushed out on a regular basis,( more about that later) then the mineral can build up and get out into the system. Once out in the system it gets sucked up into the first fixture making a hot water draw blocking the hot water inlet ports in the valves cartridge. To remedy this shut the water off to the valve and check the cartridge for trash in the ports. Before you reassemble the valve turn on the supplies and flush them out. Now back to your heater.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Let me show you how. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear.
Good luck, Tom

Housei35 yrs old. When you turn hot water in shower it starts, then a few minutes later it shuts down to a trickle. Water heater is just 2 year old. Rest o house gets hot water. Husband changed the washer, but it still does the same thing. Starts fine & then down to trickle of water.