Right-Sizing Can Be Just Plain Wrong

The problem of sizing HVAC equipment can be a particularly tortuous one for many people. It should involve some sort of a loads calculation to determine the amount of heating and cooling necessary over each hour of the year, although in homes it may often be done using rules of thumb that might have been based on such calculations in the distant past1. A key part of such a loads calculation is specifying the activities that will take place and desired indoor environmental conditions.

Sizing it would seem then is not so hard. One can find the worst hour of the year, add a safety factor and you have the size of the equipment need. There are, unfortunately, a few problems with that approach. One problem is cost; the larger the capacity of the equipment, the more it costs2. Oddly, no one wants to pay more than they have to for a basic function; so there is always the pressure to push down first costs.

On the other side, contractors want to avoid callbacks and avoid a customer with comfort complaints. The issue is normally not a problem for heating3; so let’s focus on cooling. This inability to meet the load in a home (resulting in occupants being too hot) is often due to poor installation: poor refrigerant charge, leaky ducts and poorly insulated ducts in hostile environments like 140 F attics lead to not enough cooling delivered to the home and occupants. There is also the issue of meeting expectations. A system sized to perfectly meet the design condition load will not meet the load when weather is more extreme than that assumed in sizing calculations. So for a few days a year the home will be too hot in the summer (or too cool in the winter.) This is a trigger for callbacks and the bane of a contracting business. So contractors also need to be in the business of managing customer expectations, or at least discuss the options and tradeoffs: i.e., this correctly sized system might save you energy – but you may not be comfortable a couple of days a year. Finally, there is the issue of what indoor temperature to use when sizing. Different people have difference expectations for indoor temperature. So, the right size for one occupant might be under or oversized for others. This is much more critical for cooling equipment because the smaller indoor-outdoor temperatures mean that a few degrees change in preference for the occupants has a proportionally larger effect on sizing.

We also have the issue of part-load efficiency. Sizing to meet peak load means that virtually all the time we would be running at less than full load. Typical equipment runs less efficiently at these part loads than at full load. That is a waste of energy (and money). Depending on the equipment, part-load operation can negatively impact the moisture removal of traditional cooling equipment, as well.

Finally, utilities do not like this kind of sizing either. The utility must make its plans for generation, transmission and distribution based on the aggregate connected service to its customers. Larger equipment means higher potential capacities that must be planned for. But if the usage is low, the utilities have a hard time making their bottom line. This is especially true if the maximum load is only for a small numbers of days per year4.

All of these factors have led to a desire to “right size” the heating and particularly cooling equipment that gets installed. The notion is that it would be better if the equipment were smaller (i.e “right”), but it turns out there are a lot of myths about right-sizing that are not actually true.

1 A classic rule of thumb used to be 1 ton of cooling per 500 sq. ft. As buildings have gotten more energy efficient, however, thumbs have gotten smaller and more climate sensitive. 2 And if you put ducts outside the conditioned space, the size of things alone would shrink your thumbs. 3 To be a little more precise, it’s not usually a problem for combustion equipment or electric resistance which is easier to add capacity too, but those things with coils can be problematic. 4 This is the so called “Load from Hell” to a utility planner. Such planners like capacity limited situations because load diversity does not help them when on the worst day of the year, everyone’s air conditioning is on full blast. It is said that right-sizing is better, but for whom is it better? It is surely better for the builder/contractor; it cuts his cost of construction. It is better for the utility because the peak load it has to worry about is capped by right-sized capacity. But, is it better for the actual user—the occupant who is in the space? Often not. The so-called “oversized” units are the only ones that can provide what the users want.

Simply put, right-sizing means that there will be times when the load is not met; temperatures will be outside the desired band, for example. Consider a typical example of right-sizing a home. Here the size of the equipment will be determined ostensibly by the 99th percentile of the steady state load. That does not sound so bad but it might mean there are 88 hours5 per year where the occupants will be uncomfortable unless they take some other action such as reducing clothing levels6 or turning on ceiling fans.

Even that is an understatement because most residential load programs for sizing consider steady-state loads. Few occupants operate their home by leaving the thermostat at a single value all the time. Set-back, set-up, and off periods are quite common operating strategies. They save energy by reducing operation of the system, but extra capacity is required to recover from such periods. A “right-sized” system is not sized to provide this recovery in a reasonable time and will lead to more hours of occupant discomfort7.

Recovery is not the only reason the system may need extra capacity. Systems are often sized for typical occupancy and activities, but there are easily foreseeable situations where that is not the case. Special events happen in all buildings and it might be unacceptable, for example to be sweating at Grandpa’s birthday party in July.

It is said that the part-load efficiency and moisture control benefits of right-sizing are so significant that the penalties to occupant comfort and flexibility are warranted. This myth is often repeated, but bears some further examination.

It is generally true that most single-speed equipment work best at full load and loses performance at part load. This fact is the basis for saying that right-sizing improves efficiency. While true, the largest savings comes not from part –load efficiency improvements, but from insufficient capacity to meet load. That is, from occupant discomfort.

There are a variety of ways to improve the part load performance without limiting capacity. Multiple smaller units can be staged. In a residential environment this can be done by using two smaller systems rather than one large one. Another approach is to use two-stage or variable capacity units, many of which are on the market today that allow systems to run continuously for a larger fraction of the year at lower loads while retaining the higher capacity required to meet requirements for hot days, recovery from setback and occupant directed loads. Intentional oversizing of such system cans even improve the efficiency of the overall system.

Another myth is that systems running at part load take significantly less moisture out of the air. All things being equal, more capacity means more moisture removal, just over a shorter time. But not all things are equal. Cooling systems dehumidify less effectively during the first minute or two of each cycle, as the coil goes from being warm to being cold. Since an oversized system cycles on for shorter periods, latent performance degradation at start up will have a greater impact on the oversized system. It is also true that a larger system can experience more re-evaporation of the condensate, depending on the design, operations and maintenance of the system. After each cycle, moisture remains on the coil and in the drain pan. Oversized systems will have bigger coils and drain pans with more moisture available for re-evaporation between cycles. This is made worse, of course, if the drain pain does not drain well or the AHU is an attic

5 These are not 88 continuous hours but maybe spread over many days in the peak season—maybe all the days in the peak season. 6 Woohoo! 7 The amazingly backward sounding recommendation to overcome this, is to run the system all the time— even during times when no one is home—so that it can meet comfort better during occupied periods. or other hot environment. If there is any flow of air through the duct system from thermosiphon effects, then this moisture is brought back into the home at the end of a cycle. The impact of this evaporation of end of cycle moisture also depends a lot on how hot the system gets at the end of a cycle. Well insulated ducts in a crawlspace will tend to stay cool and have little thermosiphon effect as the cold air in the ducts stays in the ducts. However, ducts in a hot attic present a worst case for setting up thermosiphon flows and allowing the air in the ducts, the coil and the drain pan to get hot. This increases the ability of the air to hold moisture (and therefore transport it into the house) and evaporation rates into the air.

These issues are minimized with proper drain pan installation, good duct insulation and good duct and coil/pan placement. The biggest problem, however, is fan overrun. If the air handler shuts off when the compressor shuts off, there will be slow re-evaporation of condensate and the coils will stay wet for a long time. If the fan continues to run, moisture on the coil and in the pan will evaporate and raise indoor humidity.8 Minimizing fan overrun is a good way to have better humidity control for all sizes of system. Overall, there are many factors that contribute to part load moisture removal that are at least as significant, and often much more so, than the system size.

One final nail in the coffin of right-sizing as a panacea for moisture control is that high indoor humidity conditions in homes tend to occur in shoulder seasons where there is long time between cycles and cycles are short because loads very low. This does tend to reduce the ability to dehumidify9 – if the on time is so short that the coils do not get cold enough to condense moisture, and long off times lead to much more likelihood of water evaporation from pans and coils. At first blush this might look like a rationale for right- sizing to lengthen on-times and minimize the moisture available for evaporation. The real issue in shoulder seasons, however, is that even a right-sized system does not operate enough to remove a significant amount of moisture10 because it simply operates for too few hours of the day.

A popular misconception is that sizing affects system off times, but this is correlation not causality. A high performance building will have longer off times because there is less load, and a high performance building will also require tend to be sized smaller because there is less load, but it is not the size of the system that is affecting the off time. The off-time is only a function of the building, the load and the thermostat properties.

So, the idea of right sizing is a myth. The principle outcome of right-sizing is loss of thermal comfort. The mythical disadvantages of right-sizing can all be dealt with by proper design, operation and equipment selection. So for much of the time “right-sizing” is just plain wrong.

8 Essentially you have just turned your evaporator coil into a swamp cooler. An excellent idea for the desert southwest, but a really bad one if humidity control is an issue. Which could explain the poor sales of evaporative cooling in Florida. 9 Another thing to consider is the degree of potential humidity control improvement given typical oversizing. Is an extra 0.5 to 1 ton really going to change the humidity control aspects that much? We are only talking about a 15 -25% change in capacity – so the most we could expect to change humidity removal by is on a similar range and at part load this is not much moisture removal. 10 In homes with high moisture in shoulder seasons you need a dedicated humidifier, not a right-sized air conditioner. The whole issue of residential humidity control is full of myths and ghost stories, but that is for another day.