High-Efficiency Commercial HVAC

23%of this guide, read. The rest of it is below.

1. 02 The mechanism Part-load reality: your system almost never runs at design conditions

   Commercial HVAC systems are sized to meet design heating and cooling conditions: the hottest day of the summer and the coldest day of the winter. Those conditions historically occur only 1% to 2.5% of the time. Your HVAC system is therefore intentionally oversized for at least 97.5% to 99% of the time it is running. On top of that, most equipment is further oversized to handle pick-up loads and to provide a safety factor.

   How often an HVAC system actually runs at full load

   2.5%

   Time at design conditions, at most

   97.5%

   Time the system is oversized, at least

   50%

   Of rated capacity, or less, in normal running

   Design-day conditions are rare, so the system spends almost all of its hours oversized and running well below rated capacity.

   Most heating and cooling equipment only reaches its rated peak efficiency when it is fully loaded, meaning operating near maximum output. Most HVAC systems run at 50% or less of rated capacity. So the full-load efficiency printed on the spec sheet is largely irrelevant to how that equipment performs in your building. Part-load performance is the critical consideration for HVAC sizing.

   The controls decide what happens during those part-load hours. Their job is to make the HVAC system perform properly, reliably and efficiently during the conditions that occur 97.5% to 99% of the time. The controls are the mechanism by which you actually capture the efficiency you paid for in the equipment.

   03 What it does to you The oversizing trap: higher bills and clammy spaces

   When the part-load reality is ignored at the design stage, the building gets an oversized HVAC system. Greatly oversized equipment operates less efficiently and costs more than properly sized equipment. An oversized cooling system short-cycles: it comes on, cools the space quickly and shuts off before it has run long enough to dehumidify the air. The result is spaces that are cool but clammy.

   How stacked design assumptions become an operating cost

   Assuming a simultaneous worst case for occupancy, lighting, shading devices and weather, then applying the highest safety factors on top, is unreasonable and expensive. The safety factors and pick-up load allowances in the ASHRAE energy standard are an upper limit. They are a ceiling, and a designer should not add more margin on top of them. Sizing should start from a baseline built on reasonable assumptions, and hour-by-hour computer simulation, using tools such as the DOE simulation programs, can anticipate how building design and operation produce peak loads. As that analysis reduces uncertainty, oversizing factors can be reduced or applied to a more realistic baseline.

   1. 1 Ask the designer or contractor what baseline assumptions they used for occupancy, lighting and weather at the same time.
   2. 2 Ask whether they ran an hour-by-hour simulation or stacked worst-case assumptions and applied safety factors on top.
   3. 3 Ask whether the ASHRAE safety factors and pick-up load allowances were treated as a ceiling or as a starting point for more margin.
   2
2. 03 What it does to you The oversizing trap: higher bills and clammy spaces

   When the part-load reality is ignored at the design stage, the building gets an oversized HVAC system. Greatly oversized equipment operates less efficiently and costs more than properly sized equipment. An oversized cooling system short-cycles: it comes on, cools the space quickly and shuts off before it has run long enough to dehumidify the air. The result is spaces that are cool but clammy.

   How stacked design assumptions become an operating cost

   Assuming a simultaneous worst case for occupancy, lighting, shading devices and weather, then applying the highest safety factors on top, is unreasonable and expensive. The safety factors and pick-up load allowances in the ASHRAE energy standard are an upper limit. They are a ceiling, and a designer should not add more margin on top of them. Sizing should start from a baseline built on reasonable assumptions, and hour-by-hour computer simulation, using tools such as the DOE simulation programs, can anticipate how building design and operation produce peak loads. As that analysis reduces uncertainty, oversizing factors can be reduced or applied to a more realistic baseline.

   1. 1 Ask the designer or contractor what baseline assumptions they used for occupancy, lighting and weather at the same time.
   2. 2 Ask whether they ran an hour-by-hour simulation or stacked worst-case assumptions and applied safety factors on top.
   3. 3 Ask whether the ASHRAE safety factors and pick-up load allowances were treated as a ceiling or as a starting point for more margin.
   04 The trap that costs money Buying boilers and chillers on the full-load number
   3
3. 04 The trap that costs money Buying boilers and chillers on the full-load number

   On the heating side, the conventional boilers you most often find in existing buildings (firetube steel, watertube steel and cast iron) run at combustion efficiencies of 78% to 86%. A condensing boiler extracts so much heat from the flue gases that the moisture in them condenses, and typically fired on natural gas it runs at 95% to 96%. Condensing boilers also run more efficiently than non-condensing boilers at part load, which is where heating plants spend most of their hours.

   Combustion efficiency, conventional versus condensing boilers

   If you are procuring commercial boilers, ENERGY STAR certified models require a thermal efficiency of at least 94% and a turndown ratio of at least 5 :1, and they use 14% less energy than a standard model. That turndown lets the boiler run down to 20% of rated input before it cycles off. Most modulating-flame boilers turn down to 25%, and some reach 10%. Condensing boilers come in 0.3 MMBtu/hr to 2 MMBtu/hr units and can be connected in modular installations, so schools, municipalities and manufacturers can stage capacity instead of running one large unit at a low fraction of its rating.

   On the cooling side, chiller efficiency is rated in kW per ton, and lower is better: the chiller delivers the same cooling while drawing fewer kilowatts. Chillers are where the efficiency numbers vary most, and where operators are most often misled by spec sheets.

   Chiller efficiency by type, worst and best of each range

   A small air-cooled chiller at 1.6 kW/ton uses more than 3 x the electricity per ton of cooling of a best-in-class water-cooled chiller at 0.5 kW/ton. If you are replacing chillers and water-cooled is an option, the efficiency case is substantial. Even for best-in-class water-cooled units, part-load performance must also be examined. Buy the best full-load number and find out the unit runs poorly at partial load, and you lose a large portion of the savings you expected.

   What the spec sheet implies	What happens in the building
   The best full-load efficiency number is the best purchase.	Equipment that wins at full load often loses badly at part load, where it runs almost continuously.
   A larger unit is the safer choice.	Greatly oversized equipment runs less efficiently, costs more and can leave spaces cool but clammy.
   Efficient equipment delivers its savings on its own.	The controls are the mechanism that captures the efficiency you paid for during part-load hours.
   A newly installed system works as designed.	Commercial HVAC systems do not always work as expected after installation, from design faults or equipment and controls connected or installed wrong.

   Compressor type also bears on maintenance and uptime. Centrifugal compressors are used in chillers of 100 tons to 7,000 tons and are the most efficient of the large-capacity chillers. Scroll compressors require less maintenance than reciprocating compressors and are available in water chillers of 20 tons to 500 tons.

   4
4. 05 Your leverage Controls, recovery, commissioning and what to ask

   Because an HVAC system spends most of its hours at part load, the controls decide how much of the equipment's efficiency you actually collect. On the chiller side, variable speed drives match motor output to the load and let the chiller cycle off at a lower fraction of capacity than a constant-speed unit. Water temperature reset raises the chilled water temperature as demand falls. Integrated chiller plant controls optimize the chillers, cooling towers, fans and pumps together, which is substantially more effective than optimizing each piece alone and also harder to implement.

   Chiller

   Variable speed drives

   Match motor output to chiller load and cycle off at a lower fraction of capacity than constant-speed chillers.

   Chiller

   Water temperature reset

   Raises chilled water temperature as cooling demand drops, so the chiller runs more efficiently than at a fixed setpoint.

   Plant

   Integrated plant controls

   Minimize the combined energy cost of chillers, cooling towers, fans and pumps as one system. Most useful in larger facilities with multiple chillers.

   Building

   DDC replacing pneumatic

   Repeatable, reliable, monitored from a central workstation, and lower maintenance than pneumatic controls. Less labor and fewer service calls for a stretched team.

   Air distribution is where reheat waste hides in many existing buildings. A constant air volume system serving several zones cools the supply air for the zone with the highest demand, then overcools the others or reheats the air at the terminal units. A variable air volume system varies airflow to each zone at a constant supply temperature, saving fan energy and using less reheat. Fan-powered VAV terminal units hold zone airflow steady while the central fan throttles down to the minimum needed for ventilation. VAV can now serve areas with as little as 6 tons of cooling load, and variable speed fans are the preferred way to control air volume.

   Carbon dioxide based ventilation control reads carbon dioxide in the return air and opens the outside air damper only as far as needed to hold target levels. Carbon dioxide does not account for contaminants released by carpets, furniture and other building materials, so a minimum outside air quantity must be maintained even when spaces are unoccupied. If your facility has indoor air quality requirements, as hospitals, schools and office buildings do, design that minimum in from the start.

   If your facility runs a cooling tower, a waterside economizer is worth investigating. A heat exchanger and controls between the cooling tower loop and the chilled water loop let cold tower water cool the chilled water directly, without running the chiller, when the outdoor wet-bulb temperature is low. Cooling towers themselves run more efficiently with variable-speed or multiple-speed fans, wet-bulb reset of the cooling water temperature, and variable frequency drives on fans and pumps.

   Heat recovery technology	Efficiency, low end	Efficiency, high end
   Plate heat exchanger	60%	75%
   Glycol loop, including pump energy	50%	70%
   Heat pipe heat exchanger	Not stated	80%
   Desiccant wheel, heat and moisture	Not stated	85%

   Heat recovery is one of the most frequently missed opportunities in commercial HVAC, and for facilities pulling high outside air around the clock, such as hospitals, recovery on the exhaust stream cuts energy cost without reconfiguring the system. Electric heat recovery chillers receive up to 50% of the heat they reject, and gas-fired engine-driven chillers retrieve 20% to 50%. For manufacturers with simultaneous demand for cooling and process heat, packaged cogeneration of 60 kW to 600 kW is widely available. On the power bill, off-peak ice storage runs chillers at night to cool the building in the afternoon, and an energy management system can shed non-critical HVAC load during short peaks before they inflate your demand charge.

   Commissioning is the step most owners skip, and skipping it is where a large fraction of expected savings disappears. Commercial HVAC systems do not always work as expected after installation. Commissioning tests the system under all aspects of operation, finds and corrects problems, and makes sure your operations and maintenance staff are trained on how the systems function. ASHRAE's commissioning guideline recommends a comprehensive protocol for HVAC, and Total Building Commissioning extends it to every building system. The HVAC decision also belongs in context with envelope and lighting: if you replaced lighting with LEDs, you cut fixture heat and cooling load, and an HVAC design that ignores that pays for capacity it does not need. A building using 40% less energy than code will often have a different basic system type, not only smaller components.

   5
5. Decision matrix

   When a high-efficiency HVAC project is worth acting on

   ✓ Worth acting on

   • You are facing a boiler or chiller replacement and can compare part-load ratings and turndown
   • Your building still runs pneumatic controls or a constant air volume system with reheat
   • You run a cooling tower and have cooling load through the cold months, with no waterside economizer
   • You pull high outside air around the clock, as a hospital does, with no heat recovery on the exhaust
   • Occupants complain of cool but clammy spaces, which points to oversized cooling

   ✗ Not yet, or not this way

   • The proposal is built on full-load nameplate efficiency with no part-load data
   • The design load stacks worst-case assumptions with maximum safety factors on top
   • The budget leaves no room for commissioning before you accept the system
   • The HVAC decision is being made apart from your lighting and envelope plans
   • You have no bill baseline to measure savings against after the project
   Questions for your morning huddle

   1. What is our current HVAC energy cost as a percentage of our total electricity and gas spend, and do we have a Btu-per-square-foot benchmark to compare our current performance against?
   2. If we are evaluating new or replacement chiller or boiler equipment, are we looking at part-load efficiency ratings and turndown ratios, or are we deciding on nameplate full-load efficiency alone?
   3. How was the design load for our existing or proposed HVAC system calculated: did the engineer run an hour-by-hour simulation, or stack simultaneous worst-case assumptions for occupancy, lighting, shading and weather and then apply maximum safety factors on top?
   4. Has our HVAC system been formally commissioned, and do our operations and maintenance staff have documentation and training on how the controls are actually supposed to function?
   The one thing to remember

   Selecting HVAC equipment by full-load nameplate efficiency alone is an expensive mistake, because the system spends 97.5% to 99% of its hours below the conditions it was designed for.

   Before your next boiler or chiller decision, ask for part-load efficiency ratings and turndown ratios, ask how the design load was calculated, and commission the system before you accept it from the contractor.

   The Energy Decision Blueprint

   Know if the numbers actually pencil out before you sign anything.

   A written second opinion on the project in front of you, whether that is a rate change, new equipment, or a renewable installation.

   1. 01A short call, to figure out quickly whether we can actually be helpful. If we can't, we'll say so on the spot.
   2. 02We pull the data, your bills, your rate structure, vendor proposals, project specs.
   3. 03You get the verdict in writing: whether the payback will materialize, and the opportunities or risks nobody has raised.

   Get a Blueprint at blueprint.tac-nrg.com Free for Indiana-based operations spending five figures or more a month on electricity. No obligation. You keep the write-up either way.
   6
6. The one thing to remember

   Selecting HVAC equipment by full-load nameplate efficiency alone is an expensive mistake, because the system spends 97.5% to 99% of its hours below the conditions it was designed for.

   Before your next boiler or chiller decision, ask for part-load efficiency ratings and turndown ratios, ask how the design load was calculated, and commission the system before you accept it from the contractor.

   The Energy Decision Blueprint

   Know if the numbers actually pencil out before you sign anything.

   A written second opinion on the project in front of you, whether that is a rate change, new equipment, or a renewable installation.

   1. 01A short call, to figure out quickly whether we can actually be helpful. If we can't, we'll say so on the spot.
   2. 02We pull the data, your bills, your rate structure, vendor proposals, project specs.
   3. 03You get the verdict in writing: whether the payback will materialize, and the opportunities or risks nobody has raised.

   Get a Blueprint at blueprint.tac-nrg.com Free for Indiana-based operations spending five figures or more a month on electricity. No obligation. You keep the write-up either way.
   Questions operators ask

   how much of a commercial building's energy goes to hvac

   HVAC systems account for 39% of the energy used in commercial buildings in the United States, which is roughly 4 of every $10 spent on electricity and gas in the building. That makes heating, ventilating and air conditioning the largest single energy line item in most commercial buildings and the first place to look for cost reduction.

   how much can high efficiency hvac save and what is the payback

   High-performance HVAC equipment alone can cut HVAC energy and cost by 10% to 40%. Paired with whole-building design and extended comfort strategies, savings reach 40% to 70%. A 30% reduction typically carries a simple payback of 3 years to 5 years, and accepting a 7 years payback pushes savings to roughly 40%. Those results depend on sizing for part-load operation and commissioning the system.

   how do i know if my commercial hvac system is oversized

   Commercial HVAC is sized for design conditions that occur only 1% to 2.5% of the time, so most systems run at 50% or less of rated capacity. The clearest symptom of oversized cooling is short-cycling: the unit cools the space quickly and shuts off before it dehumidifies, leaving rooms cool but clammy. Ask how the design load was calculated and whether safety factors were stacked on top of worst-case assumptions.

   condensing boiler vs standard boiler efficiency

   Conventional firetube steel, watertube steel and cast iron boilers run at combustion efficiencies of 78% to 86%. Condensing boilers, typically gas-fired, run at 95% to 96% and also perform better at part load. ENERGY STAR certified commercial boilers require at least 94% thermal efficiency and a 5 :1 turndown, and use 14% less energy than a standard model.

   7

7. how do i know if my commercial hvac system is oversized

   Commercial HVAC is sized for design conditions that occur only 1% to 2.5% of the time, so most systems run at 50% or less of rated capacity. The clearest symptom of oversized cooling is short-cycling: the unit cools the space quickly and shuts off before it dehumidifies, leaving rooms cool but clammy. Ask how the design load was calculated and whether safety factors were stacked on top of worst-case assumptions.

   condensing boiler vs standard boiler efficiency

   Conventional firetube steel, watertube steel and cast iron boilers run at combustion efficiencies of 78% to 86%. Condensing boilers, typically gas-fired, run at 95% to 96% and also perform better at part load. ENERGY STAR certified commercial boilers require at least 94% thermal efficiency and a 5 :1 turndown, and use 14% less energy than a standard model.

   what kw per ton should a water cooled chiller hit

   Standard water-cooled electric chillers run at 0.8 kW/ton to 0.7 kW/ton, and high-efficiency water-cooled chillers reach 0.6 kW/ton to 0.5 kW/ton. Small air-cooled chillers run at 1.6 kW/ton to 1.1 kW/ton. Lower kW per ton is better. Even for the best water-cooled units, check part-load performance as well, because the chiller spends most of its hours below full load.

   vav vs cav which is more efficient

   Variable air volume (VAV) is more efficient than constant air volume (CAV) with reheat. A CAV system serving several zones cools all the air for the zone with the highest demand and reheats it for the others, paying twice. VAV varies airflow at a constant supply temperature, saving fan energy and reheat. VAV can struggle to hold even temperatures at low airflow, which fan-powered VAV terminal units address. VAV now works for areas as small as 6 tons of cooling load.

   Glossary

   Part-load performance

   How efficiently HVAC equipment runs below its full rated output, which is where it spends almost all of its operating hours.

   Design conditions

   The extreme hot and cold days an HVAC system is sized to handle. They occur only a small share of the year.

   Turndown ratio

   How far a boiler can throttle back from full input before it must cycle off. A higher ratio means less on-off cycling at low load.

   Condensing boiler

   A boiler that pulls enough heat from its flue gases to condense the moisture in them, reaching higher efficiency and better part-load performance than conventional boilers.

   kW per ton

   The kilowatts a chiller draws for each ton of cooling it delivers. Lower numbers mean less electricity for the same cooling.

   Coefficient of performance

   COP: the ratio of cooling delivered to energy consumed. Higher is better, and it is the inverse way of reading chiller efficiency.

   Variable air volume

   VAV: an air distribution system that varies airflow to each zone at a constant supply temperature, saving fan and reheat energy compared with constant air volume.

   Waterside economizer

   A heat exchanger and controls between the cooling tower loop and chilled water loop that cool the building without the chiller when outdoor wet-bulb temperature is low.

   Commissioning

   Testing an HVAC system under all aspects of operation, correcting the problems found, and training operations staff, before the owner accepts the system.

   8

Read the rest, download, print, and take it with you.