A.Doug Mossbrook, president of Eagle Mountain HVAC, responds: Thermal mass can be a double-edged sword. With too little mass, the boiler runs more often, and there is an increased likelihood of indoor temperature swings. With too much mass, the system can be slow in responding to sudden outdoor temperature changes, resulting in spikes and valleys in the indoor temperature.

In the case of high-mass systems, there are ways to reduce this effect. Outdoor temperature controls, which are common in Europe, can be used in what is called a "constant flow" system. With this method, also called "flat lining," the temperature of the fluid is modulated based on the outdoor air temperature. If the heat loss...

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