Are you a homeowner or building manager?
Find a Contractor »

The Ideal HVAC System for Multifamily Buildings: Designing for Health, Comfort, and Efficiency

Posted on:

The primary considerations for any HVAC system design should consist of health, comfort and efficiency – probably in that order of importance.

If we achieve efficiency at the expense of health that’s unacceptable.

Likewise, providing for health in the absence of comfort is equally undesirable.

Surprisingly, this happens much more frequently than you might think.

The good news is that designing mechanical systems for multi-family housing doesn’t have to be a juggling act. We have the ability and the technology to attain health, efficiency and comfort simultaneously without sacrificing any of the three.

HVAC systems designed for humid and mixed-humid climates that do not account for moisture control along with properly delivered ventilation are the engineering equivalent of Russian Roulette. You might get lucky some percentage of the time but eventually the odds come calling. Don’t take my word for it, just ask a colleague who has had the unpleasant experience of a mold lawsuit.

Controlling moisture inside apartments is a crucial component of durability. Mold spores are naturally occurring and ever-present. In order for mold to grow they just need the right temperature, a food source and moisture. Spores cannot be completely eliminated. Temperature is determined by the resident. Building materials, apartment furnishings, clothes, food, or anything organic is a potential food source for mold. Moisture is the single element of the above equation that can be adequately controlled to prevent mold inside apartments.

Health, comfort and efficiency should be the mutual goals of every designer, contractor, building owner, and occupant. If we agree on this point then the rest should be easy, right? In the words of Lee Corso, ESPN college football analyst, “Not so fast, my friend.”

Ever try to juggle?

Maintaining comfort along with providing healthy ventilation while at the same time being energy-efficient is mostly tantamount to a juggling act. Focusing on one of the balls often means taking your eye off the others.

Typical HVAC Systems

Consider that most HVAC systems installed in multi-family buildings are single-stage conventional split heat pumps and air-conditioners. Why is that?

Most multifamily developers do not select mechanical systems because they provide better comfort, deliver appropriate ventilation or achieve optimum efficiency. Most developers select systems based on upfront cost.

You may be asking, “Why don’t single-stage conventional systems work?” They do… but only sometimes. It doesn’t exactly require a planetary alignment, but it’s not that far off either.

In order for single-stage conventional equipment to properly accomplish essential HVAC system goals, the system must first be sized properly, meaning the HVAC system cannot be too small or too large for the space. If the unit is too small it cannot maintain comfort (sensible load). If the unit is too large, it cannot maintain appropriate humidity (latent load). The latter scenario occurs frequently with smaller sized apartments and condos since conventional systems smaller than 18,000 BTUs (1.5 tons) are not currently available.

According to John Straube, Ph.D, P. Eng, “The lack of humidity control provided by normal HVAC systems can be a major concern in climates with humid summer weather (e.g., Minneapolis, Miami). Only a random amount of dehumidification is provided if and when the cooling operates for long enough to allow water to collect, build up on coils, and then drain down and away. This process takes about 10-15 minutes of continuous cooling operation before dehumidification begins. Hence a separate dehumidification device is required for the perfect HVAC system.”

Conventional HVAC systems struggle to control part-load humidity during the shoulder seasons when the temperature is not warm enough for the cooling system to run extended periods and remove sufficient moisture. While the air conditioner may be sized properly for cooling during peak summer loads, it becomes “oversized” in the spring or fall when the outside temperature drops and takes the sensible cooling load down along with it. The latent load remains unchanged as moisture is not dependent upon temperature (sensible load).

Building Performance Programs
The energy efficiency of a building is a major factor of building performance. There are several building performance programs now that stipulate design criteria including tighter building envelopes, lower air changes and higher efficiency HVAC equipment along with mechanical ventilation.

We go to great lengths to seal building envelopes and measure for effectiveness with blower door and duct blaster testing. Together, these measures are extremely effective at reducing cooling loads and saving energy in large part by shortening air conditioner run cycles during the cooling season. Latent loads (moisture) typically are not adequately addressed as a result of more efficient sensible cooling.
When this occurs, the resident of the apartment is faced with one of two choices. First, they might elect to “overcool” the space to satisfy moisture removal. Alternatively, the occupant may select a more comfortable temperature setting which results in moisture buildup.

Oversizing air conditioners is not a problem from the standpoint of sensible loads or cooling. In fact, an oversized air conditioner will cool an apartment very quickly and cycle off once the thermostat setpoint has been reached. Oversizing negatively impacts the latent capacity of the air conditioner – its ability to dehumidify.

This is precisely the scenario that plays itself out in many senior housing apartments. As we age our bodies become naturally become more sensitive to cold. Medications including blood thinners exacerbate this condition. When combined with limited incomes and smaller apartments using oversized air conditioners and mechanical ventilation pulling in humid outside air, we are a lot closer to The Perfect Mold Storm than The Perfect HVAC System.

The question this than begs, are we truly optimizing for energy-efficiency when we rely on “overcooling” as a strategy to satisfy moisture loads inside apartments?
More efficient, higher SEER (seasonal energy efficiency ratio) HVAC systems have become a staple of performance programs. Most, if not all, green building programs now require higher SEER HVAC systems to certify for their building programs. Certain types of higher SEER systems can work against moisture control in multifamily systems.

HVAC SEER ratings are certified by the Air Conditioning Heating and Refrigeration Institute (AHRI) . It is quite common to match a larger air handler and/or evaporator coil with a smaller condensing unit to achieve a higher SEER rating. In apartments, this frequently appears on mechanical schedules as a 2-ton air handler matched to a 1.5-ton condensing unit.

The larger evaporator coil will result in the air conditioner reaching the desired setpoint on the thermostat quicker meaning less run time. The larger the evaporator coil, the more water it will hold. Since the larger coil typically does not get as cold as a smaller coil would, less moisture is removed. Additionally, systems that employ a fan delay causing the air handler to continue to operate after the condensing unit has cycled off can increase the unit rating by up to .5 SEER. However, field tests have shown this can also result in an increase in relative humidity (RH) inside dwellings by up to 10% as moisture remaining on the evaporator coil becomes reintroduced into the dwelling.

Low-load or part-load conditions may persist for months at a time during the spring and fall, especially in mixed-humidity climates. Think about a day in the fall when its 70 degrees and raining. In this instance, an air conditioner may not be needed to cool the apartment but there will still be a latent load from internal moisture generation and possibly mechanical ventilation.

A common approach to HVAC system design aimed at handling part-load humidity is to use a multi-stage condensing unit matched with a variable speed air handler. A system with a two-stage compressor as well as a variable-speed air handler can drop down to first stage at roughly two-thirds total capacity when cooling loads are lower.

These systems require a thermostat with a built-in humidistat to measure relative humidity (RH). When the control recognizes elevated RH, it will slow the fan-speed on the air-handler so that the evaporator coil gets colder and removes more moisture. This strategy is overcooling to dehumidify and can result in comfort issues. These systems are common in single-family, owner-occupied applications.

Installing this type of system in a small apartment or condo can have disastrous results. One such scenario involving a one-bedroom, 700 square foot apartment in climate zone 4 using a two-stage heat pump with variable speed air handler was documented in a case study available at
At last, conventional mechanical split HVAC systems work but only IF:
1. They are SIZED properly
2. They are USED as intended

Occupant Generated Moisture
House as a system speaks to a dynamic, ever-changing system of which the resident is a key component. The three ongoing processes continuously under way in an apartment are heat flows, air flows and moisture flows. These flows have a significant impact on energy efficiency, occupant comfort, indoor air quality and the durability of an apartment. If the flows are well managed, the dwelling will perform well.

If not, significant problems can occur. It is important to note that tenants are at the controls and largely determine this performance.

Cooking, showering, cleaning and even respiration introduce moisture into our dwellings. Using a rule of thumb, we can estimate the volume of typical moisture generation through normal daily activities.
One person adds about a quarter pound of moisture from breathing plus an additional quarter pound of moisture from activities to a home per hour. Using that estimate, four occupants would add approximately two pounds of moisture to a home per hour. A pound of moisture is roughly the equivalent of a pint. Assuming the four occupants were present and active in the apartment 12 hours per day that would equate to approximately 24 pints of occupant-generated moisture every 24 hours.
Of course, this is only an estimate. In fact, occupant-generated moisture varies greatly between apartments. The number of tenants residing in the apartment, referred to as tenant density, is a key factor determining moisture generation. The frequency and style of cooking also plays a large role. For example, someone who frequently boils pasta would introduce considerably more moisture in the form of water vapor into an apartment than someone who eats packaged salads, etc.

A recent Department of Energy study concluded, “The relationship with occupancy suggests that interior moisture generation is not only a significant source of moisture; it is highly variable from day to day.”

Exhaust-Only Ventilation Sucks

Do you recall why most developers choose conventional mechanical splits for apartments and condominiums? If you said, “because they are the least expensive option,” you would likely be correct.

No surprise here, so it also goes with ventilation systems. Exhaust-only ventilation is commonly used in apartments and condos because it’s cheap and because it used to work.

According to Joseph Lstiburek, Ph.D., P.Eng., Fellow ASHRAE, “… it does not work in tight building enclosures and certainly not in new houses, apartments, townhouses and row houses. Basically, in nothing new that we are building.”

One of the fundamental challenges of designing ventilation systems for multifamily buildings lies in the nature of the structure itself. Are multifamily buildings a single commercial structure or several separate residential structures sharing the same building? For good reason and the sake of brevity, let’s assume it’s the latter.

Stack effect driven airflows in tall buildings compromise smoke control and fire safety, adversely affect indoor air quality and comfort as well as increase operating costs for space conditioning
energy. By compartmentalizing individual units, we can better control stack-driven interior airflows.

However, there are practical limits to compartmentalization which is why supply-only ventilation is not an option for apartments and condos. Using supply-only ventilation can push contaminants and even moisture through wall assemblies into adjoining units resulting in odor and contaminant issues.

Exhaust-only ventilation also leads to significant depressurization in apartments constructed to meet the 2015 International Building Code.6 According to Joe Lstiburek, we only have one option when it comes to a ventilation system that works in everything everywhere – balanced ventilation. “So, distributed balanced ventilation systems work in everything everywhere. But, and a big but, in small units and small houses we are going to need supplemental dehumidification….”6

In February of 2018, ASHRAE proposed Addendum o to Standard 62.2-2016, Ventilation and Acceptable Indoor Air Quality in Residential Buildings. This proposed addendum would require that new multifamily dwelling units have balanced ventilation. This would address concerns that unbalanced ventilation performs poorly in multifamily units.

So how do we best provide comfort and healthy ventilation in an energy efficient manner? The ideal HVAC system for apartments and condos should include provisions to not only heat and cool, but also ventilate and filter while maintaining acceptable humidity.

Almost all apartments and many condos are used as rental units and therefore not owner-occupied. This is an important distinction that is often glanced over. As such, shouldn’t we design mechanical systems with this in mind? My ideal HVAC system for apartments and condos would take this into account.

As a result of improved technology, we can now design HVAC systems for rental-units that not only provide health, comfort and efficiency but also include some assurance to owners and operators that the HVAC system is designed in such a way to have a fail-safe provision against “operator-error.” For instance, when the resident decides to turn the air conditioner off, this system would continue to protect against humidity-related microbial issues along with accelerated aging of building materials.

How do we do that? First, we can keep with using single-stage split air conditioners and heat pumps which will keep equipment costs down for developers. Next, add unit-level balanced ventilation to minimize pressure imbalances throughout apartments.

Ultra-Aire MD-33 In-Wall Dehumidifier

Lastly, we need a stand-alone dehumidifier capable of detecting elevated humidity and running independently of the air conditioner only when required to remove excess moisture. This unit should be hard-wired with concealed controls and of course be EnergyStar rated for optimum efficiency. Concealing unit controls prevents the tenant from turning the unit on or off which ensures moisture control remains in the hands of property management.

It’s not the heat, it’s the humidity…

Have you ever heard of The Apparent Temperature Effect? Apparent temperature is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin
cooling for various combinations of ambient temperature and humidity.

Note that 75 degrees at 50% RH feels like 75 degrees whereas 75 degrees at 70% RH feels like 77 degrees.

Several recent case studies have indicated that many residents will increase the cooling setpoint on the thermostat when RH can be maintained at a consistent and comfortable level. When this happens, the air conditioner runs less, and energy is conserved.

During periods where required conditioning consists of moisture removal as opposed to cooling, a small capacity, high-efficiency dehumidifier will consume a fraction of the energy required by even the smallest, most efficient conventional split-system air conditioner. As a result, the resident experiences optimum comfort and the building owner maintains a sustainable asset without the fear of mold. Health, comfort, efficiency.

Now we’re juggling.

Ron Revia

Posted In: Building Performance

Looking for an ACCA QA Accredited Contractor?

Are you a homeowner or building manager?


join now

PLUS It's Risk Free!