Both showers have little to no cold water

[Brendan20211]

Both showers/baths have either very hot or very cold or no cold water at all. Problem fixed itself for a couple months then came back yesterday. The pressure drops for a few seconds with waves of water from shower head. Hot water heater only 9 months old. All other faucets work normally.
Hot water heater capacity is 48 US Gallon + expansion tank
Please advise
Thank you kindly

 

[Aquarius]

Is the cold fed from mains pressure or is this via the tank as well?

 

[Brendan20211]

I'm not sure the hot water tank has a hot and cold side. How can I find out?

 

[Aquarius]

You could (if you have one) turn the incoming main stopcock, and run the cold taps in the bathroom, if they stop, likely to be mains fed, if they don’t, then likely tank fed.

 

[Brendan20211]

You could (if you have one) turn the incoming main stopcock, and run the cold taps in the bathroom, if they stop, likely to be mains fed, if they don’t, then likely tank fed.

The cold taps stop when I close the main valve

 

[Aquarius]

What could be happening, is the cold is pushing the hot out of the way, as the supplies are unbalanced.

 

[Brendan20211]

What could be happening, is the cold is pushing the hot out of the way, as the supplies are unbalanced.

How would I further troubleshoot?

Thanks

 

[Aquarius]

How would I further troubleshoot?

Thanks

You may need to have check valves fitted, or equalise the pressures.