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    • Abstract: SIMULTANEOUS HEATING AND COOLING - THE HVAC BLIGHTSteve P. Doty, PE, CEMA BSTRACTDespite the good intentions of the Energy Codes that restrict unitary HVAC air systems are immune because their design

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Steve P. Doty, PE, CEM
Despite the good intentions of the Energy Codes that restrict unitary HVAC air systems are immune because their design
or prohibit it, simultaneous heating and cooling occurs too provides heating or cooling at any one time, but not both. For
frequently in HVAC systems and represents a large four-pipe hydronic systems and larger HVAC air systems, a
opportunity for energy reduction. Like driving with the brakes prime feature in the design intent is the ability to provide
on but with no smell, it is a silent consumer of energy; so cooling to one zone and heating to another zone at the same
simple that it is often overlooked. In air handlers, the waste of time, thereby accommodating differing comfort needs with
heat-cool overlap can occur within a few feet of each other. In remote central station equipment. While the comfort goal is
systems with primary and terminal level temperature control achieved, the heat-cool flexibility can, without proper control,
the lack of coordination between control settings creates allow higher energy use through unintended overlap. There
similar waste but in a less obvious way. Automatic control are many other examples.
valves ‘told to close’ don’t always comply. Certain automatic
control routines, even with DDC controls, can allow overlap Designing our buildings to use nothing except two-pipe
on a transient or continuous basis. Ironically, the most hydronic systems and single zone HVAC air systems is not the
simplistic HVAC systems are often the most immune. A few answer. Rather, the best approach is to understand and raise
systems require overlap to accomplish a specific function, awareness to the sources of this preventable waste, where it is
such as humidity control, but for many systems the likely to be found, and how to control it. Getting the message
expenditure of energy goes on with no benefit and is an out to building operator trade associations, through utility
obvious energy waste. Using experience from hundreds of white papers, and to automatic controls suppliers are
working HVAC systems, the paper will point out common additional ways. Energy codes and standards, including
causes of unintentional heating and cooling overlap, how to ASHRAE 90.1 already have provisions to prohibit
spot them, how to correct them, and provide tips on simultaneous heating and cooling except for dehumidification.
quantifying the savings. However, with widespread heat-cool overlap observed in so
many buildings constructed under an energy code, there must
INTRODUCTION be a disconnect somewhere between the code phase and the
“Start with the basics,” my friend and mentor told me many operations phase. To be specific, if all the requirements
years ago, “before going after the complicated solutions.” outlined in ASHRAE 90.1 were followed, there would be no
This is that. issue. This is not a criticism of any part of the industry, just a
statement of fact and a missing piece we can serve to fill. As
The subject of heat-cool overlap is not a rare disease. It exists energy professionals, we have a focus on energy and are in the
in system after system. The energy waste is amplified when position to carry this message forward to create savings.
heat and cool ‘buck’ because two units of new energy have Many of the solutions that will be presented are simple and
been spent simply to negate each other; money spent with no low cost, providing immediate results.
value. Savings of 5-20% in HVAC heating and cooling
energy are possible as a result of eradicating unintended The intent of this paper is to raise awareness and equip
simultaneous heating and cooling. Of course there are practicing energy professionals with new tools to locate and
exceptions: Two-pipe hydronic systems and single zone correct causes of unintended heating and cooling overlap.
Additionally, this paper can serve as an educational tool for BUILT-IN ENERGY WASTE
others in the commercial building industry such as design Reference Table 1 and Figure 1. Some central HVAC
engineers, building officials, performance contractors, systems inherently overlap heating and cooling and for
commissioning agents, and building owners and operators. these systems the higher operating cost from energy waste
is literally built-in. Certain measures can be implemented
to reduce the waste, but it is obviously better to begin
ABBREVIATIONS USED IN THIS PAPER with systems that inherently do not overlap. This is the
primary thrust of the energy codes that prohibit heat/cool
AHU Air Handling Unit overlap – to phase out the older style high energy use
ASHRAE American Society of Heating and systems over time, in favor of more efficient systems.
Refrigeration Engineers
Btu British Thermal Units Many HVAC systems are a composite of central
Btuh Btu per hour equipment and terminal unit equipment. Like the central
CC Cooling Coil equipment, certain terminal unit HVAC applications are
COP Coefficient of Performance more prone to heat/cool overlap and are identified
CFM Cubic Feet per Minute separately in Table 2.
CAV Constant Air Volume
DDC Direct Digital Control
degF Degrees Fahrenheit
dT Differential Temperature
eff Efficiency FIGURE 1.
Fa altitude air density correction factor RELATIVE EFFICIENCY OF HVAC
HC Heating Coil SYSTEMS (2)
Hz hertz (frequency)
kBtu Thousands of Btu
kW kilowatt
kWh kilowatt-hours
kW/ton kilowatts per ton
Mbh Thousands of Btu per hour
MIX Mixing Box
OA Outside Air
PI Proportional plus Integral
Qs Heat Load for Air, Btuh, sensible
RA Return Air
SA Supply Air
therm Btu x 100,000
ton 12,000 Btu/hr cooling capacity
VAV Variable Air Volume
VFD Variable Frequency Drive
NOT A NEW ISSUE Simple is Good for Preventing Overlap.
The energy waste from unintentional simultaneous
heating and cooling has been around for a long time. In Some HVAC systems are inherently prone to
the 1980’s one author nicknamed overlapping heating and energy waste, from heating and cooling overlap.
cooling “the uninvited energy consumer” (1).
Note that Multizone, CAV Dual Duct, and CAV
Reheat are the worst.
Page 2
Actually they are both. Often, a major cause of energy
waste from overlap is malfunctioning controls. This was
especially true in the era of pneumatic controls that used
air pressure and calibrated springs, but is observed with
modern DDC controls as well. However, properly
applied and maintained automatic controls are an
excellent watchdog to prevent the waste.
Controller-related issues leading to energy waste from
overlap are shown in Figure 2 and Table 3. While
modern DDC controls have the capacity to prevent
overlapping commands, most do not or could do better.
Some DDC control algorithms that sequence adjacent
heating and cooling equipment emulate a pneumatic
receiver controller. This simple method can work as well
as any other, but depends upon the adjustment of the
actuators sharing the same signal – but a weak spring or a
misadjusted actuator results in overlap. This is especially
true where DDC systems are applied to existing
pneumatic systems. An alternate strategy to sharing a
Controller Sequencing with Shared Signals
common signal is individual control outputs for the
Potential for overlap exists from misadjusted control
sequenced devices. The advantage to this approach is the
reduced dependency upon individual actuator
adjustments. But this too can go awry: If one control
setting is changed without coordination of the
The control system includes controllers and end devices.
Field observations have confirmed that the connection
between “controls” and heat/cool overlap extends beyond
the controllers themselves. Overlap related to the
operation of the control system valves, dampers and
actuators are shown in Table 4.
One very significant contribution advanced DDC controls
can play in reducing heat-cool overlap is the coordination
of upstream and downstream HVAC processes. Too
many systems, pneumatic and DDC alike, have
independent control routines that are oblivious to related
processes, leaving potential savings on the table. To the
extent that the entire HVAC system is considered as one
with an applied focus on optimization, overlap can be
reduced. Examples of this are given throughout this
paper. It is clear that an extra effort at both the control
code level and the actuator level will be worthwhile and Controller Sequencing with Separate Signals
effective at reducing heat-cool overlap. Potential for overlap exists from changing one set
point but not the other, or from PI control.
Some sources are less obvious than others. A few are
noted in Table 5. Consider, for example, a space that is
over-cooled. Whether from excessive VAV box FIGURE 2.
minimum air flows or a simple space control set too low, EFFECT OF CONTROL STYLES ON
if it results in space heaters brought in by the occupants, HEAT/COOL OVERLAP
it’s heating and cooling. Again.
Page 3
Item Sources of Waste Remedy
CAV Reheat For zones calling for heat, the SA must first be heated to Reset supply air temperature from zone demand (best) or from
room temperature before any room heating can occur. OA temperature.
The lower the SA temperature, the higher the reheat
In cooling mode, constant fan energy means constant VAV conversion. This provides fan energy proportional to
heat; at part cooling load the fan heat is disproportionate. cooling load, and also reduces the magnitude of the reheat
penalty, by not allowing heating until the zone air flow is at
VAV Reheat For zones calling for heat, the SA must first be heated to Reset supply air temperature from zone demand (best) or from
room temperature before any room heating can occur. OA temperature.
The lower the SA temperature, the higher the reheat
Multi-Zone Provides automatic heating and cooling in every case Reset hot deck and cold deck temperature from zone demand
(Blending) except maximum cooling or maximum heating. (best) or from OA temperature.
and The two system types are essentially the same, except the Same for Double-Duct hot duct and cold duct temperatures
blending function for the double duct occurs remotely
Double-Duct with blending boxes instead of at the air handler.
Blow-through fan blast upon coil faces creates turbulence Ideally there would be two fans or draw through to prevent this.
and eddy currents that violate the separation of cold and
hot air streams. Temperature measurements have shown A sheet metal divider plate half the height of the coil extending
thermal mixing at the top of the cold coil and the bottom toward the fan will help.
of the hot coil from turbulence.
Perforated baffle well in front of the coils (if there is room) will
even the air flow distribution at the expense of additional
pressure drop.
For Multi-Zone system, constant volume nature requires VAV air handler conversion and split the linkages with
hot deck and cold deck dampers to be linked; while independent control actuators. Allow hot deck damper to open
linked, one opens as the other closed. only after cold deck damper is at minimum position.
For most systems, a single fan is used and the mixed air Where practical, especially larger systems – and most
section is shared by both the warm and cool air streams. applicable to Double-Duct systems:
This forces compromises that limit the benefits of air-side
economizer and supply air reset. For best heating Split the air streams and provide two fans: one for the hot air
efficiency in winter, OA would be at minimum, but for stream / one for the cold air stream. Assign the mixed air
best cooling efficiency the economizer would be active. section to the cooling side and allow air-side economizer to
This usually results in a constant 55 or 60 degree SA optimize cooling savings and also to provide ventilation. The
temperature in winter and a 15-20 degF reheat burden on heating air stream becomes entirely re-circulating, and the
the hot air stream. inherent reheat penalty goes away.
Sometimes it is easier to create a new air handler for the warm
air and blank off part of the Double-Duct casing than to field
Constant Volume Constant pump/fan energy means constant heat; at part VAV / variable flow pumping conversion so flow rate and
Cooling Air Flow cooling load the fan heat is disproportionate. energy transport is load following.
Also In heating systems or heating modes, the heat loss is not a
significant issue since the heat released from motor
Constant Flow horsepower is beneficial heat, delivered at the cost of
Chilled Water electric resistance heating. Savings are then the
Pumping differential between electric resistance heating and other
available heat sources.
Page 4
Item Sources of Waste Remedy
Double-Duct Constant volume nature requires hot duct and cold duct In conjunction with VAV air handler conversion and Hot
Mixing Box blending box dampers to be linked; while linked, one Duct/Cold Duct reset control:
opens as the other closed.
Split the blending box linkages with independent control
actuators for hot and cold ducts. Allow hot duct damper to
open only after cold duct damper is at minimum position.
VAV Box “Heating Single Path VAV systems inherently have some reheat, Ideally, the outside air would be provided independently of the
CFM” Settings due to the ventilation air and the cooling minimum cooling/heating air and the VAV box minimums would be set
setting. to zero. Economics often prohibit the use of this optimal
VAV box “Heating CFM” settings are seldom specified Verify that “Heating CFM” air flow values are no higher than
differently from the cooling minimum by design “Cooling Minimum CFM” values for each VAV box.
engineers, but are sometimes adjusted upwards by
controls suppliers or building operators for various
reasons; always increasing energy use in the process.
Baseboard Heat Baseboard heating with independent thermostat control Sequence the baseboard heat with the terminal unit, as either
Not Sequenced sharing a zone with an air system creates conflicting first or second stage heating, and integrate the baseboard
with Air Terminal control action. Depending upon the settings, one or the controller “call for heating” with air system SA reset routines.
other does too much heating or cooling the other
compensates. Either way it is overlap.
Baseboard Heat When VAV air system zones physically do not match the Re-segment the baseboard heat to match VAV box coverage, or
Zones Crossing baseboard zones (as in after an interior alteration or tenant add/increase reheat coil capacity in the VAV boxes and
VAV Cooling improvement), conflicting control action will result. This eliminate the baseboard system.
Zones happens when one baseboard or fin tube adds heat to
multiple VAV air zones. The portion of the baseboard
linked with the VAV thermostat and within its area of air
distribution works fine, but the section of fin tube in a
foreign VAV air zone is the issue. When it needs heat
and doesn’t get it, there is a comfort complaint; when it
gets heat and didn’t need it, the VAV in that area will
compensate by increasing cooling air flow and energy.
Page 5
Item Sources of Waste Remedy
Boilers running in The boiler (assume hot water for this paper) is not the Turn off the boiler when possible. Whenever the heating water
summer source of overlap, but is the enabler for the other sources flow stops, the overlapping waste from inadvertent heating
of overlap. during cooling season are a mute point.
Chillers or flat The chiller is not the source of overlap, but is the enabler Turn off the chiller or flat plate when possible. Whenever the
plate running in for the other sources of overlap. chilled water flow stops, the overlapping waste from
winter inadvertent cooling during heating season are a mute point.
Some facilities operate a flat plate in winter. While the
chilled water is now produced at a discount rate (chiller
off, auxiliaries on) the negating effect of the heating water
still exists.
Lack of Dead Band As long as the heating and cooling valves are not open at Adjust controls for a 5-degree dead band between sequential
Between the same time, there is no overlap. The dead band heating and cooling equipment, such as between a preheat coil
Sequential measure forms a safety zone between the two to provide and a mixing damper control set point or between a preheat coil
Heat/Cool assurance it won’t happen. Controls drift, controls have and cooling coil.
Equipment operating ranges, etc. so the space in between allows for
these practical realities.
Controlling Applies to air handlers, and also to terminal units with Arrange control software that sequences several components
Methods Can heat / cool capability. Example is preheat, mixing under a master control routine that will keep them
Allow Overlap of damper, and cooling coil all in a row. proportionally spaced apart. This is sometimes referred to as a
Sequential ‘receiver controller’ routine, since it was commonly employed
Heat/Cool Using a shared signal for multiple end devices has the that way using pneumatics.
Equipment advantage of simplicity but depends upon accurate and
repeatable response from the actuators Or
(See Figure 2)
Using independent signals for each end device eliminates Create additional code that allows manual adjustment of only
the dependency on critical actuator adjustments. Each one of the settings, and the others are calculated – maintaining
device has its own setting and control loop. However, if the space (dead band) between them.
the independent settings are changed overlap can be
created without knowing it. Or
With PI control, there can be significant time lag to return Create additional code that allows only one of the sequential
to set point and overlap occurs when near a transition independent controller outputs to be “non-zero” at a time. This
between heating and cooling operations unless a wide approach of mutual exclusive heat/cool control, combined with
dead band is maintained between the settings. transition delays, is an active approach to the problem.
In all cases, extra temperature sensors downstream of each
heating or cooling device allows monitoring and alarms if
heating and cooling occur sequentially
Air Handler and Independent control loops upstream and downstream, but Reset supply air temperature based on zone demand. For VAV
Terminal Unit part of the same duct system, each with their own set air handlers with reheat VAV boxes and for CAV Reheat
Controls Not point, are the source of waste. Each acts without regard systems, the goal is to provide air that is cool enough for any
Coordinated to the other zone needing cooling, but just cool enough and no more –
thereby reducing reheat.
This is very common with VAV Reheat and CAV Reheat
Preheated Air Source of waste is lack of upstream-downstream set point Instead of a fixed set point, adjust controls to reset the
Stream Common coordination. preheated temperature based on demand. Normally, preheating
to Multiple just above freezing (to 45 degF) will eliminate the un-necessary
Systems re-cooling by a mixing damper or cooling coil.
Preheat coil Cooling action of the mixing dampers negates the heating Modify controls to make cooling and heating operations within
operation while energy input just upstream. the air handler mutually exclusive, including economizer
mixing damper is cooling.
past minimum
Page 6
Item Sources of Waste Remedy
Actuator Without sufficient available actuator power, system Actuator and valve/damper assemblies have published close off
Insufficient Close- pressure can force open a control valve while in the ratings. Verify these are well in excess of (e.g. 50% higher
Off Rating closed position. This is especially true of globe valves. than) expected pressures. For two way valves, this is often full
system pressure.
This is a common ‘gotcha’ for variable flow pumping
conversions where, instead of installing new 2-way If unable to hold closed against system pressure, replace the
valves, the bypass leg of the existing 3-way valve is actuator with a properly sized one.
simply capped off. The actuator of the 3-way valve is
normally sized for just diverting the flow and not stopping
it, and consequently the actuator cannot provide proper
close-off. Often this is detected by a whistling at the
Electronic For positive seating, additional force is required once the Replace the actuator if it will not tightly close, making sure the
Actuator Residual device is at its closed position. Pneumatic controls and new one has the residual force feature.
Close-Off Force actuators do this very well: e.g. an 8-13 psi spring range
Feature with a 4psig signal has a residual force applied beyond the
“just closed” point. Good quality electronic actuators
have a mechanism to allow “over-travel” or utilize “force-
sensing” circuitry to create this residual force. Some
electronic actuators do not have this!
Electronic Valve not seated, allowing leak-by internally even while Adjust / replace the actuator
Actuator being commanded fully closed. Usually from improper
Adjustment set-up, or the zero adjustment is improper or has drifted.
When coupled to the valve or damper for the first time,
the technician strokes the device and sets the zero and
span adjustment of the actuator so it knows where full
open and closed is. If care is not taken to adjust for
residual close-off force, the errant adjustment may allow
the leak by for the life of the valve or damper.
This is more of an issue with valves than dampers since
dampers don’t fully close anyway.
Pneumatic Some pneumatic (air-powered) actuators have adjustable With a variable pressure air source or squeeze bulb, slowly
Actuator spring pre-loading. Others are made adjustable through modulate the pressure to the actuator(s) and observe the
Adjustment pilot positioners with their own zero and span opening and closing points. If too close to zero or max pressure
adjustments. If not properly adjusted, valves or dampers at either end, or if overlapping of sequenced devices, adjust the
may not close. spring ranges or replace the springs. Ultimately, there should
be several psi of pressure above and below the travel end points
More commonly, sequenced devices may overlap. and a gap (dead band) between any sequenced devices sharing
the same signal.
Page 7
Item Sources of Waste Remedy
Pneumatic For “hybrid” DDC controls – formerly pneumatic that With the DDC signal at a value to fully close the final device
Transducer now have digital control signals interfaced to existing (valve or damper), the transducer output should be a pressure
Adjustment pneumatic end devices – the transducer is the linking several psi beyond the actual “just closed” point to create the
component between digital and pneumatic technologies. residual force for tight seating. Adjust as required.
These have a zero and span which correspond to the
minimum and maximum travel of the device and When a device is told to be “full closed”, programming to add
minimum and maximum command of the controller. If additional pressure from the transducer (e.g. 0 or full pressure)
improperly set, or if they have drifted, lack of closure is an excellent way to assure maximum available close-off
force or overlap can result. force is applied.
For example: Adjustments that simply align a digital “no
call for heating” command value to the transducer with a
pneumatic output that brings the device to a “just closed”
position has not allowed for residual tight seating and will
very likely leak by.
Leaking Control Conventional globe-style control valves are metal seated. Standard test for valves is to command them closed and verify
Valves or Dampers Debris or wear can damage the seats, preventing full no heating / cooling is occurring downstream through
close-off. temperature measurements, allowing some time for residual
heat to dissipate. The most reliable measurement for air
Large sections of dampers linked together, old dampers, systems is the air temperature rise/drop through the coil. For
and manual dampers used as motorized dampers will not un-insulated piping of sufficient length, this can be detected by
close securely. These are seldom used to regulate heating pipe temperature as well.
quantities, but can admit excessive outside air when
“closed”, creating an overlap condition and undue burden Damper leakage can normally be verified visually and with air
on the heating system. temperature measurements, with the device commanded closed.
It is good practice to attempt to move the damper blades with
your hands, since sometimes they have become free from the
axles and are free wheeling.
If the device won’t move freely or won’t close, and actuator
issues are ruled out, replace the valve or damper.
Leaking Multi- Multi-Zone blending dampers are generally un-accessible Field test by commanding the dampers full closed to heating
Zone or Dual-Duct after installation of ductwork, and it is a fair assumption (full open to cooling). Compare the downstream zone
Blending Dampers that they leak after 10 years of service. temperature to the upstream cold duct / cold deck temperature.
If it is higher, the hot damper is leaking. Repeat for the hot
Dual duct blending terminal boxes usually have access damper test.
doors and the dampers can be visually inspected.
Remedy is usually to replace the dampers. For Multi-Zone this
requires removal of the zone ductwork. For Dual-Duct
systems, replacement of the terminal may be more cost
Page 8
Item Sources of Waste Remedy
Fan heat from During cooling mode any heat from fan or pump transport Lower duct static pressure as much as possible, with the goal of
excessive static equipment is parasitic and adds to cooling load. providing enough, but just enough pressure. Ideally, this is
pressure reset from VAV box demand with the most-open VAV box
damper 90% open indicating “just enough” pressure.
Improper zoning When large single zone equipment is used, a common Ideally, correct the zoning root cause with additional equipment
and addition of mistake is to cross zones. For example if one unit serves and split the ducts by exposure.
duct heaters or both east and west exposures with the thermostat in the
space heaters west zone, it will be over-cooled in the east zone in the Alternately, provide self-compensating diffusers that will close
afternoon. off and reduce the over-cooling air flows.
Often the solution to this is to add duct heaters or space
heaters in the over-cooling areas.
Over-cooling of Without significant supply air reset, it is predictable that Ideally, interior and exterior duct systems will be separate,
VAV interior zones interior zones of a VAV system will be over-cooled. allowing greater tempering of air to interior zones that have no
heating provisions.
Unless building operators quietly adjust interior VAV
boxes to zero minimums and ignore the ventilation Where interior and exterior zones share a common supply air
standards, people will first complain and then will bring temperature, the reset is a compromise between interior zones
in space heaters. that want it warmer and some perimeter zones that need it
cooler. Any reset is better than no reset at all.
Fan heat from During cooling mode any heat from fan or pump transport Lower duct static pressure as much as possible, with the goal of
excessive static equipment is parasitic and adds to cooling load. providing enough, but just enough pressure. Ideally, this is
pressure reset from VAV box demand with the most-open VAV box
damper 90% open indicating “just enough” pressure.
Overlap from For open plan offices with multiple VAV zones of Limit user adjustment to +/- 2 degF. Combined with a 4 or 5
adjacent zones of control, allowing one zone to be set significantly different degree dead band between zone heating and cooling operations,
control than a neighboring zone will create interaction between overlap of this type will be minimized.
zones and waste from overlap.
Notes for Tables 1- 5
1. Safety for people and property, and preventing equipment damage solution. Adding VFDs to standard motors can cause premature
always has higher priority than energy savings. Consider all aspects motor failures. Systems with electric reheat coils may require higher
of any system change. minimums than otherwise desired, to prevent overheating.
2. Reference to resetting

Use: 0.0414