Hot water taps and showers pressure very low cold taps ok

If it is hot in the kitchen, then once the water has reached the shower it should be hot. The leads me to believe that the issue is with your shower mixer unit. Nearly all are thermostatically controlled.

Or, if the heater is downstairs and the bathroom is upstairs, you don't have enough hot water pressure. You need a good plumber with a pressure tester (called a manometer).

first id check shower head most has a screen in them and do get clogged from time to time since the water flows good from faucets

it probably has an anti-scald control in the shower line that lets cold water in to prevent burns
If so that unit has a wax bulb that sometimes fails and the default fail setting is cool water
have a plumber check it out

1) If possible, it is Best to solve pressure problem before installing new shower.
http://waterheatertimer.org/Low-hot-water-pressure.html

2) If the cold water has good pressure, then Electric shower could fix problem ... but shower would have to be connected to the cold water line and not the hot.

3) Seems the expense of adding a electric shower would be greater than having plumber fix low hot water pressure.
Assuming you have access to plumbing pipes, and this is not apartment building.

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You might have clog in water lines.
If shower is only place where low pressure exists, then clog is probably at shower valve.
Disassemble valve and see if clog will flush out.
From there, you have to keep moving back along cold water line looking for pipe filled with sediment or elbow that has obstruction.

If pressure problem is bad at every cold tap, but doesn't affect Hot water, then clog is inside Cold line somewhere after water heater connects to cold line.
Buy a pressure gauge that connects to water hose.
Buy adapter that also fits pipe thread.
For example Brady BTG 100.
Test outdoor spigot for baseline pressure.
Open cold tap inside house and see how much pressure drops.
Close cold line and open hot tap and see how much pressure drops.

Next, move pressure gauge along cold water line.
Take apart pipes and inspect for obstruction at elbow or pipes filled with sediment.
Look for any place galvanized pipe connects to copper pipe, since this will be likely place for obstruction because of bi-metal corrosion.

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I had the EXACT same problem (installed my tankless last winter and worked fine, now this summer it fluctuates, have to turn the shower completely off and then on again to get any hot water and then it only lasts for a couple of minutes!) Argh - it is frustrating!

Here's what I found: the demand for hot water is lower during the summer (hot) months because the "cold" water coming in is already quite warm. You don't need as much hot water to make the shower temp comfortable. SO, my tankless was shutting off due to low flow demand. I've removed the flow restrictor from my shower head and it helped alot but didn't completely solve the prob. Next step is what has been recommended to me by several people but is much more expensive - change the pressure valve on your shower to a temperature valve. Or just wait til winter and it should work ok again :)

The thermostatic mixing spool cartridge may be worn out. The low water pressure may indicate sediment in the cartridge as well. This can be easily removed by hand tools

I have a triton opal shower.

I turn shower on low pressure and cold water. then no low pressure and piping hot water for a very short time then low pressure and cold water.

I've changed the solenoid and the little black plastic ball.

Help !

Many states building codes require a pressure regulator and expansion tank for hot water heater installation. Check the pressure regulator if you have one as this would only affect the hot water pressure. Adjust or replace. Rap the expansion tank like knocking hard on a door. Hollow sound is good. Dull thud means it needs to be replaced as it is filled with water.

mixing valve must be replaced