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Energy Saver: Furnace, Boiler & Woodstove Combustion
(continued ...) but rather a large, open to atmosphere, tank in the attic or highest point. Hopefully these systems are all gone. The following applies to modern systems with an expansion tank and not an atmospheric tank.
Here are some tricks to make sure that you have all the air out of the system during refill. This is important as air bubbles are insulators and general sources of nuisance in hydronic systems. Of course there should be back-flow prevention device (reduced pressure principle double check), a pressure reducing valve, a shut-off valve and a pressure gage at the fill point of your system. Use a small positive displacement fill pump.
With vents (or unions) at the highest point in the system cracked open, pump your solution in at the bottom of the system. Have someone watch at the vents for liquid and close vents (or unions) as soon as liquid is seen. Stop your fill pump when you get to about 15 PSI for a 2 story house *1. Run the system circulating pump for about an hour. Recheck high vents and all other vents you know about for air by cracking briefly open. If the pressure drops in the system, pump in some more liquid. Repeat until there is no air leaving the vents. Recheck each day for the next week.
On furnaces, the other side of the heat exchanger is the air stream. Hopefully, the air has always been filtered and those filters were changed regularly. Either way, you want to clean the heat exchanger every other year *1. The heat exchanger may be above the fan, so protect the fan or remove it before starting. Here you will use a mild soap solution. Fill a bucket with warm softened water ...