Over-Conditioned: The Super-Sized HVAC Problem


Over-Conditioned: The Super-Sized HVAC Problem

Technology, it seems, has given us the opportunity to defy nature’s impact on our lives in all but the most extreme conditions. This is particularly true within the confines of our houses and other buildings. In our homes, the words “climate control” no longer suggest the nefarious plan of a volcano-dwelling, Bond villain, but rather an expected level of modern comfort. Our HVAC systems are powerful enough to swing our homes from sauna to tundra regardless of the conditions. We can lay around in our favorite shorts in the middle of January and slip into a comfy sweater in August. But, what is the cost of this “comfort”? How much control do we really need?

Water Heating Dilemma

One of the challenges of building energy efficient homes in the US is the idea that the occupant needs god-like control over their interior environment. This often means over-sizing a system to the point where it can massively overpower the influence of the most rare and extreme weather conditions. As one might expects, this sort of sizing has several major drawbacks:

New Mechanicals for 100K

This is probably the least surprising drawback of an over-sized HVAC system, but it is a particularly glaring problem in homes where serious investment has been made in insulation and air sealing. All that cash that was dumped into sealing up the house should pay back immediately by reducing the HVAC load and thus the size and cost of the HVAC system. Over-sizing your heat and air-conditioning systems simply negates that savings.

Huge gains have been made in HVAC efficiency over the years, but many of those gains are negated by over-sizing. Efficiency ratings on the best HVAC systems are high because when sized correctly there is little cycling loss. This means that the system isn’t constantly turning on and off which uses a lot of energy. HVAC systems are much more efficient when running for longer cycles, a condition that won’t occur if your system is over-sized. This may seem counter-intuitive but if your neighbors’ AC is running less than yours, they are probably spending more on electricity (unless of course they simply don’t mind sweating).

Short cycle times also hurt an HVAC system’s ability to regulate humidity which can lead to condensation and mold, especially in particularly damp climates. It is much better to have your AC running constantly at an efficient level than to have it blasting for a few minutes and shutting down, particularly if it is helping regulate humidity.

That cycling issue rears its head again when it comes to the comfort of occupants. Even a well insulated home tends to lose it’s heat at exterior walls, windows and doors. With a centrally located thermostat the quick blasts of heat that warm the center of the house may come too late or too infrequently to help the extremities. A short cycling HVAC system is like wearing a vest when the weather calls for a coat, everything outside the core is uncomfortable.

There are a variety of other reasons people might give to disparage the over-sized HVAC system including increased noise and the physical size of the thing, but I think you get the basic idea . . . over-sizing is bad. So, why is it done? If we know it is less efficient, less comfortable and more expensive, why is it such an ongoing problem?

The answer lies in the first two paragraphs of this post. People want control. Regardless of the outside conditions, there is this idea that one’s chosen house temperature is an inalienable right. If we want it to be 78 degrees in our home all the time, we expect out HVAC systems to make it so. A right-sized system might not be so compliant, particularly on those days of rare extremes.

Does this mean the occupants are destined for discomfort? Of course not. The vast majority of days temperature control will be as flexible as you could want, but on those aberrant extremes there may be some limitations imposed. One might need to settle for 68 and a long sleeve shirt on a sub-zero day in Philadelphia. In Maine, one might need to turn on a fan and wear a pair of shorts when the temp tops 90. Generally, a right-sized system expects one to behave (and dress) slightly more seasonally, but the overall effect will actually be more consistent throughout the home and thus, more comfortable.

So, what is your ideal indoor temperature? How much fluctuation would you be willing to allow the seasons to impose on you? Did I miss anything important about HVAC sizing?

Give us your thoughts, opinions and expertise in the comments.