My thermostatic concealed shower valve only gives either boiling hot water or cold water, there is no inbetween temperature? the cold is fed from mains and the hot from a combi boiler. the plumber has said that he has connected the hot to the cold inlet and the cold to the hot inlet. Would this cause the problems described? the plumber says that it makes no difference

If you have single handle facute one bad cartridge in the system may be acting like a mixing valve and allowing cold water to mix with hot not allowing water to get hot enough .

Probably the mixer internals are getting stuck. Try dismantling and cleaning it up. This video will help you

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

Limit message is when the output water set temperature can not be reached, this is normally when the incoming hot water to the shower unit is not sufficiently hot. Service code 20 is something else inside the unit that is faulty, it could be a number of faults. Try reset Power down unit and re-start, it will either say Wait or Priming, after that it should restart, hopefully it will work. If not, you could have a internal fault with: The mixer valve/motor, the flow control valve or motor, the flow sensor. If you have hot water from other taps, then the mixer valve/motor is the most likely fault. If you only have cold water only from the shower head with limit message it could be the Hot solenoid controlled valve is faulty. Good luck as there are no spares available!

Hi, its probably just the filter to the cold supply needs cleaning; these can get clogged from time to time. Turn your water off, remove unit, unscrew the water pipes which should be the silver flexi type and washout the mesh filters.
Hope this has helped.

A 40 gal. electric water heater will give you aprox. 32 gal. of hot water in one setting, since they do not reheat as quickly as gas. So if you are using a newer 1.5 or 2.0 gpm shower head. You could take a 16 minute shower with straight hot water. If you are running 2 new shower heads at once you could get 8 minutes. If you using the tub spout the times decrease. Some of my customers like to turn up the temerature on the thermostat a little so they can mix more cold and get it to last a little longer. Be very careful doing this.
If your water heater has a lot of calcium built up in it the times will decrease.
With out seeing the heater, you mention it is a mobile home heater so I am guessing that it is bottom fed.
I hope this has helped.

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

norm cover levers off go round cover at four points

Try this before replacing the shower diverter valve.

If this problem is isolated to the shower only, then I would suggest that the shower diverter valve is equipped with an "Anti-scald" safety feature that can not be adjusted, however the temperature regulator/thermostat on the water heater can be adjusted to a lower temperature so as to not activate the the "Anti-scald".

mixing valve must be replaced