I had the gas company over on Saturday to flush my forced hot water lines because I thought that I had an air lock. The service repairman told me that my boiler was "out of code" because it lacked an automatic fill and backwash preventer. He told me that installing one would forever prevent any air from entering the system. My system was installed in 1965.
Is the automatic fill device new to code? Will it prevent air locks? My inclination would be to have one installed. Opinions?
I cannot tell you if the automatic feed valve is new to code, however I can tell you that the backflow (not backwash) preventer is relatively new, as my oil fired boiler did not have one when it was installed in 1959, and most of the boilers in my neighborhood also lack backflow preventers.
The purpose of an automatic feed valve is to maintain the correct pressure within the boiler. My boiler operates at approximately 14 psi, which is pretty typical. My house water pressure is about 45 psi, so you can see that it is necessary to reduce pressure between the house and the boiler. The automatic feed valve reduces the pressure to 14 psi, and maintains water in the boiler if water is drained for any reason (leak, drain boiler).
The purpose of a backflow preventer is to eliminate the possibility of boiler water backing up into the potable water system. This could happen if the pressure in the potable water line dropped (example, you drain the system, your well fails, city water pressure drops etc.). Without a backflow preventer on the automatic fill line, if the pressure in the boiler exceeds the pressure in the potable water fill line, you will get boiler water into your potable water line, which you could then drink once pressure is restored to the potable water line. This would be a bad idea, hence the backflow preventer.
Neither the backflow preventer nor the automatic feed valve removes air from your system. So far as I understand, the only way to remove air from the system is with an air bleed valve, which can either be placed on each radiator (a coin valve), or you can install a whole boiler air bleed system, which typically goes on a pipe out of the boiler. Air can get into the system in a variety of ways, none of which are cured by either the backflow preventer or auto fill valve. You still need air bleeds.
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