Shower (non-electrical) going cold - uses hot water from BAXI combi boiler & manual shower mixer. The pressure guage on th combi boiler keeps dropping and the water needs topping up.

if you are topping up the water in the boiler it means that you have a leak in the system, possibly from the relief valve,find where the pipe is on the outside and put a contaimner under it and check if it collects any water, if it does not then you have a leak elswhere,regards Terry.

Does the pressure drop as a result of drawing hot water ? It may drop at other times too but is it prominent & need replenishing when you've drawn hot water or showered ? If this is yes, then whilst it is still possible you may have a leak somewhere, these are usually fairly slow & would not be exacerbated by drawing hot water. You will most likely find system water is being discharged via the PRV & evident from a discharge pipe on the outside wall. The reason for this would most likely be that the boiler expansion vessel either needs a recharge or the diaphragm has failed & the vessel needs replacing. The boiler won't work once the pressure drops too low & by heating it quickly on a short circuit as when in hot water mode with effectively nowhere for the boiler content to expand into then the PRV simply does its job at 3.5 bar to prevent the boiler rupturing. Once you have topped up the pressure, run a hot tap & watch the pressure gauge until it stops rising, trim the flow so the water gets very hot. If the gauge goes up beyond 2 bar then up to 3 etc then it's a pretty safe bet that the exp vessel is the issue. Hope this helps.

If the pressure drops on your combi, uou have a leak on your central heating pressurised side. Check all the radiator valves and bleed valves, if no obvious signs of water, then inspect under the boiler itself- any water? Then it becomes a little more difficult as you will need a qualified plumber or heating engineer to trace the water leak. The fault is not with your shower, combi's are designed so as they willnot ignite when the pressure drops below 0.8-1.0 bar. Post as a job, get some quotes.

you must a leak in the system somewhere thats why the water pressure keeps dropping

Most probably a issue with the insert on the shower mixer valve. In regards to the boiler, a loss is pressure is not always due to a leak on the CH system. Do you notice it happens more so when the heating is on? This would then point towards a issue with the boilers expansion vessel- would recommend a gas safe registered engineer to investigate.