Somebody set the thermostat the last time anyone was in the house. Maybe it was you, on a Tuesday, when your parent said it was cold. Maybe it was your parent, who turned it down in the summer and never turned it back. Nobody wrote it down, so the number on the wall is now a guess.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Name the setpoint
Write down the number the house should hold during the day and the number it should hold at night, in whichever unit the display uses. Then ask whoever coordinates your parent's care whether there is a temperature range they want kept, and write that down too. Do not set the number from how warm the kitchen feels to you. Thermostats are often mounted in a hallway or near a sunny window, so the reading on the wall may not match the chair where your parent actually sits. Put a simple room thermometer where that chair is and compare the two readings for one week before you trust either one.
Find out who can change it
Check how the thermostat is controlled, because each setup has a different owner.
A plain thermostat with buttons can be changed by anyone who stands in front of it. That is the problem to solve. Ask whether a locking cover fits the model, or whether the upper and lower limits can be set so the number cannot drift far.
A keypad-locked model is controlled by whoever holds the code. Write the code in the same place as your other household codes, not on a sticky note on the cover, and give it to the second person in the rota.
A connected model with an app is controlled by the account holder. If the account is in your name, your parent cannot change it without you. If it is in your parent's name, you may not be able to see or change it when you need to. Decide which one it will be, and if the app allows a second user, add the other person now, while everyone is calm.
Check the battery and the service line
Some thermostats run on batteries. A low battery can blank the display or make the setting stop responding, and nobody notices until the house is the wrong temperature. Look at the back plate once, note the battery type on the same page as the setpoint, and write down the date the battery was last changed. Then find the name and phone number of the company that services the heating and cooling. Put that on the page with the setpoint. If there is a maintenance agreement, read whether the thermostat is included, since some agreements cover only the equipment and leave the controls out.
Set the rule for an empty or cold house
If the house will be empty for a day or more in cold weather, decide the lowest setting anyone will allow before anyone leaves, and name the person who checks the house while you are away. Water lines can freeze in a cold house, and that repair is much more expensive than the heat you saved. Write the minimum on the same page.
Keep one page where people look
Keep a single sheet on the fridge or in the kitchen drawer with four lines: the day setpoint, the night setpoint, who holds the code or login, and the service company's number. Review it when the schedule changes, when a helper starts, when the service company visits, or when anyone changes a setting on purpose. If the sheet and the wall disagree, the sheet wins until someone has checked the wall.
If you are in the St. Louis area and need help working out who to call about heating or about in-home support, the local help page for caregivers lists where to start.
A thermostat that three people can change and nobody owns will drift. Give it one name on the sheet. When the number is wrong, you will know whose job it is to fix it, and so will everyone else.