You do not need to know how a water heater works to run the service call. You need a written symptom, a named person who lets the technician in, and a clear answer on what happens after the truck leaves. The repair is the middle of the job. The family owns both ends.
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.
Before you call
Write down what you are seeing in the words you would use with a stranger. Which tap runs cold, or does none of them run hot? Did it stop all at once, or fade over several days? Is there water on the floor near the base of the unit, and where exactly? Note any sound, any smell, or any indicator light on the unit. Find the make, model, and serial information on the label on the side of the unit and photograph it, so the company does not have to ask your parent to go look.
Find the shutoff valve for the water supply to the unit and the switch or breaker that controls its power, and write down where each one is. Do not open the tank, remove panels, or adjust gas controls yourself. If you smell gas, leave the house and call the gas utility from outside. Before the call, your job is to gather facts, not to fix anything.
Book the visit with three questions
Ask whether the visit is a diagnosis with a separate repair quote, or one visit that includes the repair. Ask whether the technician will give you a written estimate before any work starts, and whether it lists parts, labor, removal and disposal of the old unit, and any permit the job needs. Ask whether the company is licensed for this work in your city or county, and how they confirm it. Put two companies' answers side by side in the same order. The shape of the estimate matters more than the first number you hear on the phone.
Ask for an arrival window and a call before the technician leaves for the house. Write the window on the calendar with one owner, not as 'sometime Tuesday.'
On the day of the visit
Name who lets the technician in and who is in the house with your parent during the visit. If you cannot be there, the person at the door should know what the technician is allowed to do and what is still yours to decide. Keep your parent out of the work area and away from the tank and the water while work is underway. If the hot water is out for the household, plan the bath or the handwashing for that day before the technician arrives, not during the visit.
Before any work starts, ask the technician to explain what they found, in plain words, and to show you the part being replaced. Ask what the repair will still leave unresolved. A repair on an older unit can fix the current symptom and leave the next one open. Ask how long they expect the repair to hold, and what would make them recommend replacement instead. Decide before the visit what you will do if the answer is replace. If you wait until the technician is in the basement with a yes or no question, you will say yes to whatever is fastest.
Before the truck leaves
Run the hot tap at the sink, and the tub or shower, long enough to confirm the water is steady. Look at the base of the unit and the floor under it for water. Check the power or gas indicator the technician described. Ask for the invoice with the work written out, the warranty terms in writing, the company's phone number for future calls, and where the paper will be kept. Put the invoice in the same folder as the household backup plan, so the next person who needs it does not have to search.
Ask the technician what maintenance the unit needs and how often. Turn that into a calendar item with a named owner. A maintenance task with no owner is the same as no maintenance.
The next morning
Test the hot tap again before the house wakes up, and look at the floor again. If anything has changed, call the company using the number on the invoice and describe what you saw, in the same order you wrote it down before the first call. Keep the written symptom and the invoice together. The second call goes faster when the first one left a paper trail.
Who owns the next call
Name one person in the family who makes the next call, and write their name on the same sheet as the company number. If that person is you, say so out loud to whoever else is in the family, so the job does not wait for someone else to notice a problem. Handing off this job is part of the plan, not a failure of it.
If the hot water is part of your parent's daily care, and the schedule now depends on you being home for every repair visit, take the caregiver side of that to the Local Help for Caregivers in St. Louis page if you are in that area. You do not have to carry the house, the calls, and the care alone.