I HAVE A BOSCH GWH10-2 G HOT WATER BOILER. THERE IS GOOD WATER PRESSURE ON THE COLD TAP BUT WHEN IT COMES OUT THE SHOWER HEAD ITS VERY LOW PRESSURE. WHAT CAN I DO

Can you get the manual online? You have the Manufacturer info so you probably have the model number too. I found a manual on Manuals.com for my boiler so they may have yours too which will hopefully have some information on filling boiler although I realize each installation is somewhat different but that's why it's difficult to answer you question. Usually there is a manual pressure valve that regulates the water pressure in the boiler which is usually pretty low...like less than 20 lbs. The manual should tell you the pressure as well. But this pressure valve has a flip handle that comes off the cold water line to the boiler to do just what you want to do...refill it with water. When refilling the boiler (if you find the valve, you may have to purge the air out of the lines which there is a whole other process for. But you shouldn't allow cold water into a hot boiler...it could crack it when the cold water hits the hot cast iron. So make sure the boiler is cool when filling it up.
Now go try to find that manual!

Were is the water boiler located.

check hot water pressure valve

Oh some of these there was a small black heatexchanger just for hot water they some times block up . or it could be a blockage in the water section where it flows in .I know Valliant have a very good tech line and fitters who do house calls .

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.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

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 :)

Not sure but check the inlet water at the heater, the filter may be clog up a little that restrict the flow. Did u use the heater at all during the winter? If not, the heat exchanger may be clogged with built up in there. U need to descale it. Good luck. Chinhcu

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 !

mixing valve must be replaced