It is essential that:

1. The primary heat exchanger is clean, particulary if it is a condensing boiler. 
   Symptoms: noise, long warm up period, boiler locks out
2. The diverter valve modulates between heating and hot water and will stick in hard water areas or excess sludge. 
   Symptoms: erratic hot water, lock out, temperature modulates.
3. The waterways of the domestic HW plate heat exchanger are extremely small and block easily. 
   Symptoms: Reduced flow