A house you use part of the year does not run its systems on the calendar most catalogs assume. The clock that matters is the run clock: how many months the heat or cooling actually runs, and how many months the house sits with the thermostat set back or off.
Count the months before you count the years
Start with a plain list. Write down the months the house is occupied, the months it is empty, and the months it is held at a set temperature with nobody inside. Those three lists are the fit. A system that runs through most of the occupied months wears on a different timetable than one that runs all year in a house kept warm for guests.
Then add the install year and the planning date from the matching system guide. The boiler page shows how an install year becomes a planning date, and the central air conditioner page does the same for cooling. Treat the planning date as a starting point, then move it with the run pattern you wrote down. A house that stays warm through the empty months has run its heating longer than the calendar suggests, so check the heating pattern before you assume the planning date holds.
Check the empty months on purpose
Empty months are where the house surprises you. Ask whether the heat is set back, turned off, or held above a set temperature. Ask whether the cooling is off or running to keep humidity down. Ask whether the water heater is left on or set to a vacation mode, if it has one. Each answer changes the wear on the equipment, and each answer changes what you should log when you return.
Match the service visit to the run pattern
Schedule the service visit for the weeks before the system goes back into heavy use, not during the months the house is closed. For heating, that means the weeks before the house needs warmth again. For cooling, it means the weeks before the first hot stretch. Put the visit on the same list as the occupancy months so the two stay together.
When you book a visit, ask what the technician checks, whether the equipment records run hours, and whether the idle months will be noted in the service record. A written record that shows the idle months helps the next person read the planning date correctly.
What to bring to the decision
Bring the occupancy list, the install year for each system, the planning date from its guide, and a note on how the house is held when nobody is inside. With those in hand, you can decide whether a replacement belongs in the next planning window or the window after that.