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Maintaining constant pressure
Our building has 4 HB Smith Mdl 19A cast iron sectional boilers set up for steam. In summer we run one boiler to make DHW with a heating coil in a 3500 gal storage tank. This year we added a solar-thermal system to pre-heat incoming water goiing into the tank. On a sunny day the solar panels will provide between 90% and 100% of the energy needed to maintain DHW temp. Our HVAC contractor says even though the storage tank may not be calling for heat from the boiler (it has a motorized steam valve controlling the coil), we still have to maintain a constant 4-6 pounds of steam pressure in the boiler to prevent the cast iron sections from cracking under expansion/contraction. From 9 am to 5 pm we use very little hot water, and from 11 pm to 5 am we use zero hot water - but we must run the boiler 24 hours according to the contractor. Would like a second opinion on the need to maintain constant pressure with this type boiler, and or any suggestions on how to save fuel. Many thanks.