In a hot-water heating system, a pump circulates water at about 180°F through pipes and radiators. If the pressure gets too high, a relief valve lets water escape. This may indicate a lack of air in the expansion tank
Drain a hot-water boiler only for repairs or in the case of a prolonged power outage when the pipes might freeze. To drain, turn off the power and water supplies and let the water cool (there’s a temperature indicator on the boiler). Attach a hose to the drain valve. Lead the hose to a drain or collect the water in buckets. Once water is flowing, open the air vents on top-floor radiators.
To prevent freeze-ups, an antifreeze compound can be added to the system. This may involve changes in the water supply to the boiler and should be done only by your serviceman.
Refill the boiler by closing the drain valve and turning on the water supply. When you hear water in the top floor radiators, close the air vents. Later, bleed air from all valves
In a steam system, the boiler turns water to steam, which then circulates through radiators, condenses to water, and runs back to the boiler. If the expansion tank pressure gets too high, a safety valve lets off steam. Check the valve periodically by lifting the handle. If it won’t lift, replace the valve. Drain a steam boiler only for a long absence; do it as you would a hot-water boiler except do not touch the radiator valves.
A glass gauge indicates the boiler water level; the water should be at least halfway up the gauge when the boiler is off. Unless you have an automatic refill valve, check the gauge weekly (more often in very cold weather) and refill the boiler by turning on the supply valve until the water in the glass reaches the proper level.
To prevent sediment buildup, once a month or whenever the water looks dirty, open the drain valve on the low water cutoff and drain a pail of water. Be careful; the water will be hot. Refill the boiler to the correct level. If the water remains dirty, add a boiler-cleaning compound. Remove the safety valve and pour in the compound following the package directions.
If a steam-heat system cycles on and off frequently but you’re not getting heat, the boiler may need filling. If the water level is correct, an accumulation of sediment maybe causing a condition called surging. To relieve it, add an antisurge compound.