Diverter problem

I have a problem with my bath, specifically the hot water. When I turn on the hot, it will warm up to give hot water, as usual, however after a few seconds the temperature turns tepid. If I turn off the water and leave it for a few seconds, then turn it on again, I get a blast of very hot water for 4-5 seconds, then again it cools off to slightly warm. This is reproducible. Setting it to cold gives cold water. The hot and cold water supply come into a brass fixture with a diverter stem - it is a single tap control for the water. I bought the house recently and have been doing some remodelling of the bathroom. To the best of my recollection, the hot stayed hot previously, however I have not done any work at all on this section of the bathroom and so do not think that I have done anything to cause the problem. So, do I need to replace the diverter, or is there some simpler explanation and fix? Clearly I am not a plumber, but I am reasonably handy. Any advice appreciated, thanks.

A diverter valve changes where the water goes, it has no effect on the temperature. It is used to say change the water from the tub spout to the shower head.

The control valve turns the water on/off and adjusts the temperature mix.

Is this valve a temperature controlled valve? If so, the control may need replacement.

It could be some debris in the line - when there is a flow, it pushes up and blocks the flow of the hot. Turn it off, and a little hot gets out, but then gets blocked again when you readjust it to hot. You may be able to clear this by removing the valve, flushing the line, then reinstalling.

If you have a pressure balance valve and the pressure is significantly different between the two sides, the thing may just be doing its job.

What brand and model is the valve?

Jim,
Thanks for your reply. First, I am not sure of the brand, it is a little hard to see. Second, it seems I used the wrong terms, but in any event, the water flow is even at whatever temperature I am trying to obtain. For your question, Is it a temperature control valve or not? I don't know. I am not a plumber and am a little out of my depth here, but the entire unit appears as follows: hot and cold supply lines come into the side of a brass housing, with a line coming out the top going up to the showerhead, and one going down to the tub. Tub or shower direction is controlled at the tub-fill, so i guess I am talking about a control valve, not a diverter - sorry about using the wrong term. There is what appears to be a valve that is inserted into the front of the brass housing and which comes out past the tile for attachment to the single handle for turning on/off water flow and adjusting the temperature.

Okay, so to remove this and flush the lines, I take it I will need to buy speacial tools? I'd like to try to do it myself, I am not afraid of turning a wrench, but perhaps I should call a plumber so as not to screw it up? Again, thanks for any advice.

water

If the cold water pressure is significantly higher than the hot's, it will overwhelm the hot water and give your symptoms.