total. Methane flue gas containing more available energy to recover than propane or fuel oil relatively less. The condensed water is corrosive due to dissolved carbon dioxide and sulfur oxides from the flue and must be neutralized before disposal. Condensing boilers have a higher seasonal efficiency, typically 84% to 92%, than non-condensing boilers typically 70% to 75%. The seasonal efficiency is an overall efficiency of the boiler over the entire heating season as opposed to the combustion efficiency which is the boiler's efficiency when actively fired, which excludes standing losses. The higher seasonal efficiency is partly because the lower boiler temperature used to condense the flue gas reduces standing losses during the off cycle. The lower boiler temperature precludes a condensing steam boiler and requires lower radiator temperatures in water systems. The higher efficiency of operating in the condensing region isn't always available. To produce satisfactory domestic hot water frequently requires boiler water temperature higher than allows effective condensing on the heat exchanger surface. During cold weather the building's radiator surface area is usually not large enough to deliver enough heat at low boiler temperatures so the boiler's control raises the boiler temperature as required to meet heating demand. These two factors account for most of the variability of the efficiency gains experienced at different installations. Maintenance An intensive schedule of maintenance is needed to keep a high pressure railway steam boiler in safe condition. Daily inspection The tube plates, the fusible plug and the heads of the firebox stays should be checked for leaks. The correct operation of the boiler fittings, especially the water gauges and water feed mechanisms, should be confirmed. Steam pressure should be raised to the level at which the safety valves lift and compared with the indication of the pressure gauge. Washout The working life of a locomotive boiler is considerably extended if it is spared from a constant cycle of cooling and heating. Historically, a locomotive would be kept “in steam” continuously for a period of about eight to ten days, and then allowed to cool sufficiently for a hot-water boiler washout. The schedule for express engines was based on mileage. Today's preserved locomotives are not usually kept continuously in steam and the recommended washout interval is now fifteen to thirty days, but anything up to 180 days is possible. The process starts with a “blowdown” while some pressure remains in the boiler, then