Tips for energy-efficient HVAC design

As energy costs continue to rise, your customers need to take advantage of energy-saving opportunities in HVAC systems. How much energy does heating and cooling use? On average, HVAC energy consumption in commercial buildings accounts for around 51% of energy costs.

HVAC efficiency involves minimising energy input. To accomplish this, your design can optimise ventilation, zone control, heat recovery and operations.

Let’s say you’re planning energy efficient HVACs systems for commercial buildings. The first step is to consider where the loads originate from:

• Building envelope
• Lighting
• Occupancy
• Equipment being used
• Ventilation

You can’t just design heating and ventilation for energy efficiency alone. You must also consider air quality, which you can learn about in Designing HVACs: What should you know about indoor air quality?

Ventilation load depends on either the number of occupants or the mechanisms used to control particulate matter (PM). One strategy for energy conservation is using the ASHRAE mechanical room-ventilation rate. This is done through demand-control ventilation as set out in ASHRAE Standard 62.1, but the climate of where the HVAC will be installed plays a key role.

Using available environmental resources is fundamental to energy-efficient HVAC systems. You should also look at technologies that not only improve occupant’s comfort and indoor air quality, but also reduce energy use.

Improve HVAC efficiency with zone control

If you’re designing for a facility that requires high ventilation rates, most of HVAC energy use is reheat. This is the heating that takes place after cooling the supply air, which is done to avoid overcooling an area. Many HVAC systems consume excessive amounts of energy for reheat by as much as 50% more than is necessary. How do you minimise reheat?

Efficient heating and cooling systems are incorporating zone control to almost eliminate reheat. Zone control allows the end user to control the temperature in each room. Unoccupied rooms don’t have to be heated or cooled at the same temperatures as occupied areas. Controlling how much heating and cooling is done at the zone level almost eliminates reheat and reduces cooling energy.

This energy-saving heating and cooling system is not cheap. However, zone control just might be the most cost-effective HVAC systems. They pay for themselves in the long run, reducing energy costs by up to 25%.

HVAC energy conservation: heat recovery

Heat recovery in efficient HVAC systems typically exchange heat between discharged air within a room and air introduced from outdoors. The system might exchange only sensible heat or both sensible and latent heat. This type of heat-recovery system preheats and/or pre-cools ventilation air, which reduces the system load. This is typically accomplished by putting the two airstreams through a heat exchanger.

A sensible heat exchange can look like this:

This psychrometric process illustrates an increase in the incoming air’s sensible heat that could occur in a:

• Cross-flow plate heat exchanger;
• Regenerative thermal wheel; or a
• Run-around coil

This is a basic sensible heating or cooling process, depending on the opposing airstreams’ temperatures.

• The heat exchanger sensible heat effectiveness: εS = ṁO(θB – θO)/ṁR(θR – θO), where ṁO and ṁR are the respective air mass flow rates of air at outdoor temperature θO
• Room temperature: θR
• Temperature of outdoor air after going through the heat exchanger: θB

If the temperature of the incoming air, θO, is below the dew-point temperature of the extracted air, θRdp, condensation in the airstream coming from the occupied space will occur, providing increased heat exchange.

HVAC energy-saving technologies

The best energy-efficient HVAC systems rely on technologies you should consider before you begin your design. You can help your customers save energy heating and cooling with these methods:

Technology	What it does	Why it’s energy efficient
Demand controlled ventilation	Combines economisers to maintain desired air levels with carbon dioxide sensors	Avoids over ventilating, which wastes energy
Dedicated outdoor air systems	Improves humidity control by conditioning outdoor air apart from indoor return air	Removing moisture from external air reduces load on the cooling system
Displacement ventilation	Uses a low-velocity stream of fresh, cool air at floor level, displacing stagnant air near ceiling	Improves indoor air quality while saving energy
Electronically commutated motors	Lowers electrical heating and cooling consumption, helping to maintain proper air flow through the system	Saves energy by varying HVAC blower speed as needed
Zero-degree heat pumps	Transfers heat from outside air to an indoors – even when temperatures fall to 0°F	Moves heat instead of creating it
Energy recovery ventilation (ERV)	Exchanges energy contained in exhausted air and uses it to treat incoming outdoor ventilation air	Saves energy by reducing the HVAC’s needs, yet maintains comfortable air quality levels
Desiccant dehumidification	Removes moisture from the air with a desiccant and evaporates it with heat	Removing humidity and precooling air reduces air conditioning load, saving energy
Duct sealing	Aerosol systems seal leaks and should be used as needed during maintenance	Prevents leakage to avoid energy waste
Microchannel heat exchangers (MCHE)	At least one fluid, typically refrigerant or water, flows through tubes or enclosed channels. At the same time air flows cross-current through connected fins	Extremely efficient, but involves high initial cost
Thermal energy storage	Cools by extracting heat from ice or chilled water during peak hours	Electricity is used during off-peak hours, which reduces costs
Chilled beam cooling	Uses cool water in coils that exchange heat with room air	Moves heat around a building efficiently and reduces energy use
Variable speed drives (VSD)	Adjusts the speed of motors based on the HVAC load requirements and operation schedule	Significantly reduces energy consumption
Variable flow volume (VFV)	Large-scale, ductless system uses multiple compressors, providing part-load performance and zoned temperature control	Works at the needed rate, which enables substantial energy savings at load conditions

What else should you know before designing an HVAC? Be sure to also read about:

Designing for easy maintenance

Vibration isolation

 

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