The heater is usually one of the most expensive pieces of equipment on your pad, so you'll want to make sure you're doing everything you can to take care of it. Water chemisty is very important with heaters. The heat exchanger on the inside is made of copper and is suseptible to corrosion when the chemistry is off. Low pH is very acidic and can eat through that exchanger causing it to leak. If this happens you'll have to replace the entire exchanger and thats about 90% of the cost of a new heater. Therefore, make sure pH always stays within 7.4-7.8. Its best to bring a water sample in to have us test it to make sure all the levels are balanced. 

Mice also love the warmth of heaters and can chew through wires or build nests inside. Look for evidence of mice near the heaters and ??????