Boilers

Because a non-condensing boiler is damaged by the two things cold return water causes. Sustained low return keeps flue gas condensing inside a boiler that was never built to handle it, which is corrosion. And return water arriving far colder than the idle boiler is thermal shock, which is what cracks cast iron. Manufacturers publish both limits separately: a minimum return temperature, and a maximum difference between return and the boiler's own temperature. They are different rules for different failures, and they are routinely quoted as if they were one.

Yes. Short cycling drives condensation in the vent and on the heat exchanger, swings system temperature, and wears out the components that are most expensive to replace. At least one major manufacturer puts a hard number on it and says to keep the boiler at six cycles per hour or fewer. Constant cycling is usually a symptom rather than the disease: an oversized boiler against a small load, a control problem, or too little system volume. Fixing the cause is what stops it, and adding buffer volume is sometimes the honest answer.

No. Federal minimum efficiency standards apply to equipment being manufactured, not to the boiler already in your building, and there is no requirement to replace working equipment. It is also worth knowing that a condensing boiler only delivers its rated efficiency if your return water is cold enough for it to actually condense. Dropped into a system designed around high supply and return temperatures, a condensing boiler often will not condense and will not pay back what the brochure implies. The distribution system decides that, not the boiler.