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High efficiency gas boilers use electronic computer type pc boards which do not break down where often sensors, flame rod sensor will have to replaced. A high efficiency gas boiler is going to be the norm where at heating, plumbing show today everyone was telling me they are soon going to phase cast iron type of gas boilers soon as the new efficiency requirements will be mandatory. If want to avoid a lot of electronics but willing to take a drop in efficiency then the Slant Fin Victory with sealed combustion will be close to 90 but a bit below. A lot depends on the amount of Btu where made need a few boilers to meet the requirement where a Tekmar electronic control will allow precise control of multiple boilers. It maybe worth considering installing a indirect hot water tank as the standby loss is less then one degree per hour where the boilers provide the heat for the indirect hot water tank. Check out You Tube about gas boilers and indirect hot water tanks. Slant Fin has website as well. Slant fin has very good reputation for service along with not breaking down very often. --147.194.xxx.xxx

For my own house as well as my rentals, I prefer to use a gas fired 80-90% boiler with a SS indirect water heater. I find that combining these with an automatic flue damper, outside weather sensors, good insulation and air sealing give me the best long term bang for my buck. --72.93.xxx.xxx

Let's see you want a flame with no safety other than a pilot flame thermocouple, right? Rebuild what you have because anything with a combustion fan has a circuit board, I suggest a single stage but when you get above 150k the energy drop off get ugly so the 2 stage (not variable or modulating) is likely gonna be your replacement, add LED light to each device so you can monitor the different gizmos coming on, after a short time you will understand the steps of ignition and full/low heat. --76.188.xxx.xx

Cast iron with a power vent which has sealed combustion where combustion is drawn from outside the efficiency will be close to high efficiency gas boiler. A indirect hot water tank does not lose 7 to 12 degrees per hour in stand by mode which is often less then one degree with a indirect hot water tank. Two or more gas boilers staged are more efficient then one larger boiler as only on the extreme cold days and nights all the boilers are required. Often after operating temperature is reached one boiler stays on then the others shut down. Insulating all the heating, hot water pipes in the boiler area puts more heat into the building where the efficiency of the entire heating is increased. Cleaning all the fins on copper finned radiator then installing reflective tape puts out heat at lower temperature. Every time the temperature is dropped then amount of natural gas goes down. --147.194.xxx.xxx