All About HVAC

Choosing heating, cooling, and ventilation systems can be complicated. Here’s an overview of common options and things you need to consider to make smart selections.

Synopsis: Contributing writer Scott Gibson explains the basics of selecting HVAC systems by using the ACCA Manual J, Manual S, and Manual D software for calculating your heating and cooling loads, choosing right-sized equipment, and integrating ductwork as needed. Included in the article is a snapshot of heat-pump systems, three common cooling strategies, and ventilation options.


When it comes to heating and cooling equipment, which is more important: lower initial cost or lower operating costs? Should the equipment be all-electric, or is it OK in the era of climate change to buy appliances that burn fossil fuels? Is it safe to invest in new technology, or will that turn out to be an expensive mistake? There are many nuanced questions to answer when planning a new HVAC system or looking to upgrade existing components. Having a strong sense of your needs and the available products is a good place to start.

Energy and design consultants working with computer models can offer precise estimates of heating and cooling needs and the type of equipment—if not the manufacturer and model number—best suited to meet them. When the goal is to certify a new or renovated house under a rating program such as Passive House, that level of detail is a must. Energy consumption right down to the kilowatt-hour is a make-or-break question for certification.

Whether or not a computer-armed consultant is in the cards, having a grasp of heating and cooling basics is a good idea for anyone building a new house or renovating an old one. Why bother? You won’t be spending money unnecessarily. You’ll at least know the right questions to ask the HVAC contractor. Plus, you’ll be investing in equipment that makes sense for your climate and house—not some hypothetical average.

With that in mind, consider the fundamentals of HVAC design. These are the standard industry planning tools developed by the Air Conditioning Contractors of America (ACCA): Manual J, Manual S, and Manual D. These ACCA manuals are software programs available from a number of companies and used primarily by HVAC professionals. All of them are important and the foundation for good design.

Start by calculating your needs

Manual J is the first step in planning a system. Its purpose is to calculate how much heat your house will lose on the coldest night of the year, and how much cooling you will need on the hottest day of the year. These are the heating and cooling loads, independent of what heating and cooling equipment a homeowner may choose. Loads are determined on a room-to-room basis so a contractor knows how much heating and cooling you’ll need to stay comfortable. Some of the most repeated advice from experts is that a Manual J calculation is essential to getting a heating and cooling system that works properly. Heating and cooling contractors develop their own rules of thumb over time. That’s understandable. If you’ve installed 500 systems and they all seem to work well enough, you’d probably be just as confident in recommending one. The issue is that little extra a contractor may want to add to the capacity of a furnace or air-­conditioning unit simply to guarantee they never get a call in the middle of the night from a homeowner complaining the house is too hot or too cold.