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Hi my shower runs hot then cold then hot,cold etc etc,the other taps seem to surge a bit,with the pump, but the temp seems constant, could this be a lack of pressure,or a mixer problem,? not something I`ve come across before, any thoughts?

Most of the mixing valves for domestic residential use are pressure balancing to prevent scaulding temps. They have a shuttle spool that will shut off flow if a pressure imbalance occurs..........don't know what type of mixing valve your using, but it sounds like that is what's happening.

Ray

Todd,

What Ray said...

And since you're surging on your taps, that's an indication that your pressure tank is either, bad, not the correct pressure, or too small. A lot of boats just have a small plastic pressure tank, which is OK for sinks but not showers with scald protection mixers.

You'll possibly need to replace the pressure tank, (sometimes called an expansion or accumulator tank), or replace the mixer, or both. Also, the water pump itself can play a big factor in the equation. You need a pump that supplies good pressure and flow rate to handle the rigors of a shower head. Most modern shower mixers require at least a constant minimum water pressure of 30 psi and a minimum flow rate of 3 gpm.

Some water pumps are designed with an internal bypass valve and do not require a pressure tank. As a mater of fact, a pressure tank will interfere with the internal bypass valve. Flojet and Shurflo are a couple of examples.

So we'd need to know the specific pump, mixer, and pressure tank you have, to go further...

going to replace the pressure tank and check the pump ratings ,will let you know the outcome
Regards Todd