Walk the house with every owner in the same room. At each major system, ask two questions: who would call the technician on a cold morning, and who would approve the bill? If the answers differ, or if nobody can answer, the house has a planning problem that no install year can fix.
How the problem shows up
It usually appears as a delay. The furnace makes a new noise, one person hears it and assumes the other has already called, and the service visit happens only after the heat stops. The system was not old in the wrong way. The decision was never assigned to anyone.
Look for three signs. The first is a system nobody can name the age of. The second is paperwork held by someone who is not the person paying for repairs. The third is a planning date that lives in one person's memory and nowhere on paper.
What to check in the house
Open the file for each major system and note where the manuals, receipts, and service notes are kept. If the file is split between two households, a landlord and a tenant, or two co-owners, write down which copy is the working copy and who holds it.
Then compare each planning date with the people who will live with the decision. A system that reaches the end of its typical life during a transition in the household needs a named owner before the transition, not after it.
What to settle before the next season
Ask who may approve a repair, who may approve a replacement, and what happens when the two options are far apart in cost. Ask who receives the technician's written findings. Write the answers on one page and keep that page with the appliance file so the next person can find it.
The planning dates in the lifespan guides are inputs. The person who holds the file turns them into decisions. Start with the gas furnace guide and the heat pump guide, then name one owner for each system.