There's More to Comfort Than Just Picking a Furnace or an Air Conditioner

There's a lot of hype about heating and cooling systems these days, and for good reason; they're an expensive part of your home to buy, to maintain and to power. There's also a lot of talk about smart thermostats and smart vents and things that you can add on top of what you've got, but you're much better off if you get it right in the first place. There are a few parts of your home that are complicated and contradictory — it simply helps to know more about how they work. For this series, I'm going look at the options and weigh the available choices.

The key point to start with is that you're not buying a heating, ventilating and air conditioning system; you're buying comfort. Engineer Robert Bean notes that the definition of thermal comfort is “a condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.” Basically, it’s all in your head — and in your skin, where there are about 165,000 thermal sensors connected to your brain.

Those sensors don't just sense the temperature of the air around you; they mostly feel the heat loss or heat gain to the building around you. It’s all about the Mean Radiant Temperature (MRT). Bean notes that comfort is much more than equipment and thermostats:

No matter what you read in sales literature, you simply cannot buy thermal comfort — you can only buy combinations of buildings and HVAC systems, which if selected and coordinated properly can create the necessary conditions for your body to perceive thermal comfort.

It's a critical starting point in any discussion of heating and cooling. Bean goes so far as to claim that the building codes should be rewritten.

I say, if building codes dropped the reference to controlling air temperatures and switched the requirements to controlling mean radiant temperature, building performance specifications would have to change overnight.

Bad buildings have high MRT in summer and low MRT in winter; they have drafts and uncomfortable floors, hot spots and cold spots that no thermostat is smart enough to deal with. Here's a great video that proves what and how we feel has little to do with actual temperature.

So the first thing we have to do to ensure comfort, before we even think about the heating and cooling systems, is how we build or fix the envelope of our homes to get the interior surface of the wall as close to the temperature of our skin as possible, to minimize heat loss from our skin sensors to the walls. That means lots of insulation and good quality windows used sparingly (because windows are never as good as a wall.)

Elrond Burrell, an architect in Britain, gives more detail about the components that go into a good building envelope. This is the essence of his mantra:

• Insulation, lots of it but it varies according to where you live;
• Glazing, good quality windows, triple glazed in the North;
• Shading, to take into account the power of the sun to overheat our homes;
• Airtightness, so that we are not throwing all that energy away through cracks and holes and not getting drafts from them, and finally
• Ventilation in a controlled and calculated manner so that we are getting fresh air and circulation all year round.

Elrond is actually describing what is known as a Passivhaus or Passive House design, meeting a very tough energy consumption standard developed in Germany but now being implemented around the world. These houses are so well designed, insulated and detailed that they barely need any heating at all; the cliché is that you can heat them with a hair dryer. In many climate regions air conditioning is superfluous. But every one of Elrond's points apply to any house one might build today. Some have suggested that it is enough to build what they call a Pretty Good House.

Elrond, like Robert Bean, gets to the gist of it:

There is no point in a building being energy efficient if it isn’t also comfortable and suitable for people to occupy and use. Shelter, and therefore comfort, is the primary function and purpose of a building.

It's not about getting a geothermal heat pump or a Nest thermostat, it's the whole package.

A typical example of code requirements.(Image: U.S. Department of Energy)

So let's start here: figure out what your basic needs are and we'll go through the steps of explaining how buildings work, what the building science is, what's hype and what's real. Everyone in America lives in a zone where the conditions are pretty well-known and information is available; there are the minimum requirements across the country. But these are the minimums — it's a place to start, but you want to do more. You can then calculate the heat loss and heat gain and figure out how big a heating and cooling system you need. It isn’t really a matter of choice but of physics.