Originally published in Vol. 109, No. 4 of the Energy Engineering, Journal of the Association of the Energy Engineers
The focus of this article is the performance of a specific type of heating, ventilation, and air conditioning (HVAC) system known as Variable Volume & Temperature (VVT) operating in cold climates. Information was gathered about system performance in an 8250 combined degree day climate at approximately 44°N latitude in the Eastern United States. The observations are based on the experience gained from installing new systems, retrofitting controls on existing systems, system commissioning, and responding to client distress calls of both newly installed and existing VVT systems from 1987 to the present. The results indicate VVT systems are most often favored for low first costs but lead to significant issues with comfort, excess operational expense, and tenant retention problems.
Variable Volume & Temperature (VVT) is a zone damper terminal air unit system typically used to create zoning when paired with packaged rooftop air handling units (RTU). The classic deployment is the small to mid-sized low-rise commercial office building. The strategy is to create multiple zones downstream of a single rooftop unit with a bypass damper between the supply and return ducts at the RTU to relieve excess duct static pressure when zones are throttled back or closed. A common system is schematically depicted in Figure 1.
The intent is for each zone to be able to control its own temperature to the desired setpoint of the occupant by modulating a damper installed in the ductwork. This system is different than a Variable Air Volume (VAV) terminal in that airflow in VVT is neither a measured nor directly controlled at the terminal unit. Supplemental heating at each zone is not provided.
Each zone has a wall thermostat which controls its respective zone damper but also communicates with a master unit that polls all zone thermostats for their heating or cooling requests. Based on the greatest need or result of “the vote”, either heating or cooling is energized at the RTU. Since the RTU can only provide either heating OR cooling at any given moment, the zones calling for the opposite of the current condition are forced closed. When some zones close, the duct static pressure increases and is sensed by a duct static pressure transmitter which in turn modulates the bypass damper to relieve the pressure and maintain the duct static pressure setpoint. After the calling zones are satisfied, the unit will switch over to the opposite mode to satisfy the remaining zones. The previously open dampers will close and the bypass damper will continue to modulate to maintain the duct static pressure setpoint.
In most of the many applications observed by the author, the VVT system has been proposed as a “value engineered” solution after other higher value systems such a VAV with reheat have been rejected due to high first cost. Many times the systems are offered by contractors or contractor/equipment vendor teams as “design/build” projects. Usually these systems of lower cost and value are accepted by decision makers who do not fully understand the consequences of their decisions and who at the same time, are not coached to lower their expectations regarding the level of comfort or energy performance.
A series of comfort and energy performance observations can be made when reviewing an operating VVT system over the seasons in a cold climate, particularly during the “shoulder” or in-between seasons when the loads are light and may vary significantly from exposure-to-exposure or from morning to afternoon.
More zones cost more money, therefore attempts to reduce first cost invariably leads to fewer zones, resulting in less granular control. When the zoning is configured to serve multiple floors, problems will be detected relating to the difference in the thermal conductance of the building envelope on the various floors. Especially noticeable is the summertime condition where the heat gain through the roof creates a significantly different thermal environment on the top floor when compared with the lower floors, making comfort control difficult or impossible. Portable electric space heaters operating on the lower floors have been found because the occupants are cold due to the system staying in cooling for extended periods to satisfy the upstairs zones.
In a building with significant exterior glass on a cold but sunny day when the sun is low in the sky, the southern exposure will require cooling while heating will be needed on other exposures. Since the rooftop unit can only supply heating or cooling, not both at once, some zones will be performing inadequately. Southeast and southwest corner offices can be particularly difficult locations in which to maintain comfort.
Cooling Coil Icing and Heating Shutdown
Another commonly observed situation is when only a small percentage of the zones are calling for either heating or cooling and a large percentage of air is bypassed from the supply air to the return air duct. If the RTU happens to be calling for cooling, the bypassed air often gets too cold and results in icing of the direct expansion cooling coil which in turn reduces airflow through the coil, furthering the problem. If the RTU happens to be calling for heating, the bypassed air gets hotter and hotter and often times trips the discharge air high limit thermostat shutting down the heat altogether, a particularly bad problem to have if the outdoor air temperature is below zero and no one is aware of the situation for an extended period such as overnight or over a weekend.
Return Air Spilling
Often VVT systems can experience a phenomenon known as return air spilling. This condition will occur when the outdoor air is appropriate for economizer cooling, the majority of the zones are calling for cooling, but some zones are calling for heating. The resultant high discharge air static pressure causes the bypass damper to open with the intent of re-circulating the air back around to the fan. However, when the outside air damper is open or mostly open, the return air damper is mostly closed, eliminating much, if not all of the return path to the fan. In this case, cold economizer bypass air has nowhere to go except back down the return air duct to the nearest return air grill and spill back out into the space. This condition will usually be exacerbated by exhaust fans operating in the building. Numerous complaints of being too cold have been expressed by occupants who happen to be located under or near these return grilles.
Low Radiant Temperature in Winter
Radiant heat transfer is usually the least thought of, yet most important comfort factor when applying VVT in cold climates. Physical objects such as building components, furnishings, and people absorb or release heat in relation to the other objects that are in line of sight. They do this object-to-object heating by emitting long-wave electromagnetic radiation without heating the air in between. A very good example is that the earth and the people on it are heated by the sun from a staggering distance when in line of sight. But, place a cloud in the way, put up an umbrella, or rotate the face of the earth away from the sun and the heating slows or stops. When a person is placed near a cold surface such as an exterior office wall or north facing sheet of glass, their body heat will be radiated directly to the cold surface making them uncomfortable over time. All-air VVT systems are convection systems and rate poorly at heating objects to a level which overcomes this experience.
Since the decision for the rooftop unit to provide heating or cooling is predicated on polling the zones, an undesirable situation can occur when a particular zone does not satisfy its zone thermostat, swings the vote, and keeps the unit in either heating or cooling for a prolonged amount of time. This often occurs when one zone is driving the cooling load such as an IT closet or if a zone is undersized. During this “stuck” condition a significant number of the zones will not be getting their desired request.
Because some of the zone dampers will be closed at times, perhaps for a while, there will be times when adequate ventilation is not being delivered to some zones and other zones are over-ventilated.
VVT systems are not efficient energy performers. Systems run with iced coils using electricity with little or no beneficial work being done. The units switch back and forth from heating to cooling often several times per hour swinging space temperatures. Spaces are often over ventilated using more energy than necessary. Static pressure control is via bypass damper, not variable frequency drive (VFD), so there is little or no opportunity for fan energy savings during the times when the full volume of air is not required. Electric space heaters used by occupants to overcome comfort problems add to the building’s energy consumption and their use should be anticipated in cold climate all-air VVT systems.
This system type, while simple to understand in concept, can be difficult and expensive to maintain. One of the reasons is that the systems are often found to be switching frequently from heating to cooling and back, causing many starts and stops which can shorten the equipment life. Another observation made over the years is that these systems have a high comfort complaint factor, resulting in frequent service calls, which costs real money. Even the best service technicians, try as they might cannot solve some of the inherent dilemmas of VVT system operation. Typical “fixes” attempted include changing balance damper positions, wiring in simpler thermostats, blocking off diffusers, and removing damper actuators. After a while, the owner or tenant sees the service company as the problem and not the system, developing an attitude of “throw the bums out”. Once this happens there is usually a parade of “bums”, generally good service companies and techs that are faced with a system that can’t really be fixed. Ongoing service call expense to do what is essentially babysitting the system piles up over time. Over the life of the system this expense may well amount to difference between the VVT system and a higher first cost system.
“Fixes” as mentioned above sometimes solve a problem for the moment. Sometimes it works for a while because you are deep into the heating or cooling season and more zones act similarly. Things seem to settle down for a while and the customer starts to feel better about the system operation. About the time things seem settled, the seasons change, the building load changes, and conditions turn uncomfortable again. “I thought you guys had this fixed” is a familiar refrain. Regularly, the frustration with VVT systems becomes emotional and escalates to a very high level, up to and including tenant loss and legal action.
Attempting Repairs with Individual Measures
Many readers of this article will offer ideas on how to fix the situation (it’s in our DNA!). So, here is a list of things I have tried, thought about, or heard from others. Included for each item is a response regarding the measure if implemented by itself.
One manufacturer’s website now offers VVT controls whose description is as follows:
“Buildings with diverse loading conditions can be supported by controlling reheat (single duct only) or supplemental heat. The VVT zone controller can support two-position hot water, modulating hot water, 3-stage electric heat, or combination baseboard and ducted heat.” 1 This seems to contradict implicit language from another document that says “Before VVT a precise level of comfort in all building zones or areas was impossible when using a single zone constant volume HVAC unit without energy wasting reheat.”2
For VVT systems that have already been installed a comprehensive plan that will address both the comfort and energy efficiency concerns mentioned above will require that the system be fundamentally changed from an all-air system to a hybrid air/hydronic variable air volume system. Additionally, a fully programmable Direct Digital Control (DDC) system which integrates zone control and RTU control will be needed. The addition of a variable frequency drive to the fan motor will allow the bypass duct to be eliminated and will resolve return air spilling. The static pressure sensor will now control the fan speed to maintain duct static pressure. Adding capacity control to the compressor will eliminate coil icing during part-load cooling and periods of reduced airflow. Ventilation efficiency can be improved by implementing demand controlled ventilation using a CO2 sensor (or multiple sensors) and taking control of the RTU economizer with the DDC system. This will also allow for better economizer cooling control. To overcome the problem of being unable to simultaneously heat and cool different zones at the same time, reheat coils need to be added to each zone with any exterior exposure. Baseboard heating could be added to the coldest zones to raise the mean radiant temperature, improving comfort performance during the most extreme outdoor air conditions. The reheat coils and baseboard radiation could be electric or hot water. Either choice will require infrastructure upgrades, electrical distribution for the former and a boiler/pumping system for the latter.
For projects where the system design has not yet been chosen, the best solution in a cold climate is to simply choose another system design that is capable of delivering both heating and cooling to diverse zones at the same time while effectively and efficiently providing ventilation. Such systems include:
The incremental cost to upgrade at the time of design will be recouped over time in more energy efficient operation, a reduced number of service calls, improved worker productivity, higher tenant retention, and increased asset value.
The allure of low first costs is powerful but the additional expenses associated with service calls, pre-mature equipment failures, and energy inefficiency must be considered in a life-cycle cost analysis when comparing VVT to other systems types. In cold climates satisfactory comfort is impossible to deliver to all occupants at all times with an all-air VVT system with on/off heating and cooling. In leased space this can lead to tenant loss and lower asset value of the building.
Significantly improving an already installed VVT system in a cold climate is possible and requires a comprehensive solution that fundamentally changes the VVT system into something else and carries not only the expense of the improvements, but also the additional expense of doing a project of this magnitude in an occupied building. Improvements at the design stage will lead to higher first cost but those costs will buy more energy efficient operation, better comfort, fewer service calls, improved tenant retention, higher asset value, and a measure of good will.
Client dissatisfaction with VVT systems is palpable and its negativity harms the reputation of those involved and the industry as a whole. As professionals, the architectural, engineering, and contracting communities should be doing the difficult but important work of helping clients understand what they are buying, the true costs of ownership, and what their comfort and energy use expectations can be when they make one of the most important investments in their building.
3 Arthur C. Clarke, “Profiles of the Future”, 1961 (Clarke’s third law)
About the Author
Randy Mead, C.E.M, CBCP, CMVP, LEEP AP has 27 years of experience in assessing and improving building performance with 25 years in the building controls and automation industry with Control Technologies, Inc. Randy is a lifetime member of the AEE, is an ASHRAE member, a member of the BCA, and is a member of the Vermont Technical College Architectural Building & Engineering Technology Advisory Board. Randy can be reached at email@example.com.