Is there a big difference between cast iron boiler manufacturers?
There can be differences between the casting design. Most cast iron boilers are pin type boilers but some manufacturers use three pass cast iron boilers. The three pass boilers are more efficient as the flue gasses are in the boiler longer. Some cast iron boilers may have internal baffles which make them a bit more efficient due to controlling the path the water flows. There are a lot of European boilers being sold today and the American boilers are just as efficient. Just compare apples to apples. If it is a three pass European boiler you cannot compare it to a pin type American made boiler. Three pass boilers made abroad and in the USA are tenths of a point apart. For instance the Burnham MPO oil boiler has a higher AFUE rating than almost all other cast iron boilers on the market. They also have built in boiler protection against flue gas condensation. Compare efficiencies and warranties.
