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Q: The main floors of my three-story townhouse get heat and air conditioning from a Trane natural gas furnace with an outside air conditioner unit. The basement apartment has a Lennox heat pump. I want to arrange a maintenance plan for these systems, but prices vary greatly, as do the services included. I've seen everything from a 10-minute check of the gauge to longer cleaning of inside bits. How do I select a good company, and what maintenance tasks should they do? Should they clean the filter? Change the humidifier pad?

Mount Pleasant, Va.

A: You're not alone in experiencing the frustration of trying to figure out what a maintenance contract should include. More than a decade ago, the Air Conditioning Contractors of America, a trade association, recognized that heating, ventilating and air conditioning contractors were using many different approaches for inspecting and maintaining equipment. As the association says on its website, "There was no way to determine if the many types of 'seasonal tune-ups', 'clean and checks', and 'maintenance services' performed on HVAC equipment were equivalent." So the association developed checklists, including the minimum tasks that should be done during a maintenance check.

One of those guides is spot-on for your needs, because it covers maintenance of equipment in one- and two-family dwellings of three stories or less. You can download it at the association's website, acca.org/industry/quality. This document includes a checklist for inspecting and adjusting gas furnaces on pages 7 to 9; one for your main air conditioner (including the evaporator coils inside and the condensing unit outside) on page 14; and one for the basement heat pump (listed as air-to-air heat pump condenser) on page 16. Just skimming these sections makes it clear that a company that merely checks gauges isn't doing an adequate job. The company should do a whole list of things, including checking that electrical connections are tight and inspecting the heat exchanger for signs of corrosion, cracks or other problems.

The association's website also includes a "find a contractor" service that allows you to search by zip code for members near you. Whether you confine your interviews to companies listed there or consult with others, you can use the association's checklists to ensure that the services a company offers include the items that the industry considers necessary. You're unlikely to find the same wording, however. Heating and air conditioning companies usually start with the industry guidelines and then tweak them to "make it into your own" list of service points, said Ryan Bramble, operations manager for Aire Serv Heating and Air Conditioning in Front Royal, Va. (855-259-2280; aireserv.com), which is listed on the association's website and serves your community.

In some cases, the industry guidance is vague. For example, regarding the two issues you raise — whether the service company should clean the filter and change the humidifier pad — the checklists say only to inspect and clean or change if necessary. One reason for that: Equipment varies. In these cases, it pays to read what the manufacturer of your equipment recommends. Lennox, for example, recommends replacing one-inch pleated filters on its air conditioners once a month (a homeowner task), but says that a pro should replace all other filter sizes at an annual maintenance visit.

You can learn other useful things by reading the manufacturers' maintenance guidelines. For a natural gas furnace, the Trane website offers tips for an annual maintenance check that homeowners can do, along with advice about when a pro should be brought in. If family finances are tight, it might be tempting to skip a professional maintenance contract and just call in help if something troubling turns up, especially because many of the steps seem simple. You do a visual inspection, vacuum out the furnace cabinet, check the belt for fraying and loose tension (Can you push it down more than ½ inch?) and change the furnace filter.

However, there is a difference between how a homeowner can check things and what a pro can accomplish, Bramble said. A homeowner can check whether flames are mostly yellow or orange, rather than blue — a sign that the gas isn't burning completely, which not only wastes money but also increases the risk of carbon monoxide's getting into living spaces. But a pro with the right equipment and an appropriate to-do list will be able to determine oxygen and carbon monoxide levels, flue temperature and other details, allowing the furnace to be adjusted for optimum performance.

Whether you need a maintenance contract depends partly on your personal cost-benefit calculations. Some companies promise priority service and reduced rates for service calls to customers who have maintenance contracts. There is also the peace of mind that comes from having someone with an experienced eye do the inspection. On the other hand, if you hire someone who does little beyond changing the furnace filter, you are wasting money. Other than following instructions on the package about which side should face away from the fan, this task is a no-brainer. No homeowner needs a pro to do that.