It may depend on use but we found servicing the Aqua-Hot every other year works just fine. At first, I was having it done every year for the first couple of years. Then, when I saw the nozzle that the tech removed from the unit I thought it was the new one.
So now, we have moved to a two year cycle and all has worked well. The tech will clean out the burner, check the boiler fluid, change out the nozzle, blow out the exhaust system and replace the fuel filter. Ours virtually has no smoke when operating, just a slight hum.
I was able to give it a good test recently since we went through Colorado and the lows dipped into the 40's. The system worked perfectly. All the heat registers were blowing hot air which was telling me that the boiler fluid was being pumped to the right areas and all the pumps and fans were working, etc. Our unit is 7 yrs. old and been 99% trouble free.
The only problem we have had was not the fault of the Aqua-Hot. I had a kitchen hose split which caused water to run down the hose and drip water onto the Aqua-Hot electronic board shorting it out. So, the board had to be replaced.
Smooth sailing every since.
MM.