Selgas
There are a number of questions I would have to ask a client of mine with the symptoms you have described before proceeding with anything else such as servicing the heater such as : did this water heater have this problem before the service people called, did they check and reset the gas operating pressures for both high and low settings, from what you have said they have flushed out the heat exchanger with “vinegar” probably in an endeavour to clear any lime or calcium buildup that may have been in it. Didi they clean out the water flow valve as well – if they did not some of the “muck” may have lodged in the rotor valve and in turn tells the heater’s computer what the inlet flow rate is so the computer can adjust the gas and water rates to provide correct temperatures. If the rotor “sticks” then the computer would try unsucesefully to adjust the flow of water and gas to attain the set temperatures. It is more likely that the fault lies with a fluctuating water flow into the heater from the town supply system but in EVERY case when servicing these units the gas pressure settings MUST be checked and if required adjusted, after all too little gas equates to a lower than set temperature and too much gas could cause the heater to turn itself off and on when a low flow such as a shower is being used otherwise it would self destruct when trying to get rid of all the surplus heat.
My initial thoughts are check the gas pressure settings and the water flow settings to ensure things are what they are suppost to be first.
