Very High Efficiency Commercial HVAC System Design Specification and Guidelines

January 31, 2024

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Heat and Energy Recovery Ventilation | Heat/Cooling Systems | Critical System Design Guidelines | Background 

 

 

Heat and Energy Recovery Ventilation – Minimum Performance & Features / Capabilities

  1. At the midpoint of nominal full air flow under balanced supply/exhaust flow conditions, Minimum Sensible Recovery Efficiency for HRVs shall be 85% and for ERVs shall be 75%; Total Recovery Efficiency for ERVs shall be at least 80%.

  2. Minimum fan efficacy: 2.0 cfm/Watt at 0.5” w.g. at midpoint of nominal full air flow

  3. Control capabilities: DCV, by zone; control based on time, occupancy, CO2, pressure

  4. Economizing (heat recovery bypass)

  5. Adaptive defrost, no recirculation allowed

  6. Cross-flow leakage: less than 3% (3rd party verified)

  7. BACNet, Modbus interface capability

  8. Outdoor-rated; outdoor / roof mounting capability (all climates)

  9. Dedicated ducting / zoning (separate from heating / cooling air flows, separately controlled)

  10. Filters: Minimum MERV 13 supply air filters and MERV 8 extract filters

Heating/Cooling Systems – Minimum Performance & Features / Documentation

  1. Heating (Ducted/Unducted): HSPF 9.5 (≤ 65 kBtu/hr); COP @ 47 °F 3.6, COP @ 17 °F 2.4 (> 65 kBtu/hr). Cooling (Ducted/Unducted): SEER 16.0, EER 13.0 (≤ 65 kBtu/hr); IEER 18.0 (> 65 kBtu/hr). Note: Hydronic heating and cooling systems must be heat pump-based. For systems that do not have a specified system EER or COP see annotation #1 under Heating/Cooling System below.

  2. No simultaneous heating/cooling (“heat recovery”) w/o an analysis showing cost-effective incremental savings and a zoning plan that will effectively utilize this feature (e.g., core/perimeter)

  3. System proposals must include, at a minimum: outdoor unit spec, indoor unit(s) spec, controls spec & sequence of operations, dedicated ducting, zoning; proposed system drawings

  4. In locations where the 99 percent design heating condition is 10°F or below, the VRF system heating capacity must be equal to or greater than the nominal rated heating capacity (rated at 47°F) of the outdoor unit while operating at 70°F DB indoor temperature, and 5°F DB outdoor temperature. This capacity minimum is before reductions are taken for indoor unit/outdoor unit combination ratios other than 100 percent, or for defrost operations.

Critical System Design Guidelines 

  1. All calculations to be based on a space-by-space heat loss analysis
  2. Complete separation of ventilation air from heating/cooling air, with each controlled separately (but coordinated) and zoned independently
  3. HRV/ERV sizing: individual units specified to run at a maximum of 75% of nominal rated full flow when meeting ASHRAE 62.1 ventilation rates (fully occupied, non-boosted); ducting sized for max flow.
  4. Heating/cooling sizing: no less than 600 sq ft/ton of system cooling capacity in Climate Zones 5 and 6, at least 750 sq ft/ton in Zone 4.
  5. For ducted systems: supply & return for each space served by 25 cfm or more of supply air (spaces without doors exempt)
  6. Ventilation supply air delivered to one side of space, exhaust air extracted from opposite side
  7. System commissioning, including ventilation system air flow verification3
  8. Duct-sealing and leakage testing as part of commissioning scope; ventilation duct insulation where required (in unconditioned spaces, to/from outdoors to HRV/ERV)
  9. Extract air for H/ERVs shall be taken from directly conditioned indoor areas of the zones. Extract air shall not be taken from unconditioned spaces such as plenum volumes above a suspended ceiling. Supply and extract air ducting that passes through an unconditioned plenum volume or other unconditioned space, whether or not separated from the occupied spaces by a ceiling or wall structure, shall be insulated to a level of at least R-8.

 

Background 

Heat Recovery Ventilation

1. Minimum sensible recovery efficiency. Over the last few decades, the vast majority of HRV and ERV products used for recovering ventilation air flow energy losses have had sensible recovery efficiency ratings between 50 and 70 percent. This is still true today. Most of these products are either wheel- type ERVs, or ERVs/HRVs that have crossflow heat exchangers, both of which limit sensible heat exchange efficiency to about 80 percent or less. In addition, these recovery efficiency ratings derive from a seriously flawed test procedure that simply measures the temperature difference between the outgoing exhaust and delivered fresh air to compute the fraction of energy in the outgoing air returned to the incoming air stream, without accounting for many of the other energy flows going on and their origins (like case or housing energy losses, fan/motor losses, cross-flow air leakage, etc.). When the building heating and cooling system efficiencies generating the energy being recovered in the ventilation air flows are low (COP 0.6 -0.8 for heating, COP 1.5-2.5 in cooling), recovering some of the energy this way makes at least a little sense, though cost-effectiveness would vary based on the cost of the energy being recovered. So we haven’t seen widespread use of heat or energy recovery in the market so far. In some climates, energy recovery efficiency is focused only on sensible energy and it makes sense to recommend HRVs in these moderately dry climates. In humid climates, both sensible and latent energy transfer efficiency are important to keep unwanted outdoor humidity out of the indoor environment by transferring it to the outgoing exhaust air stream, thus ERVs are more applicable. Another application for ERVs is in very dry climates, where they can help retain desired humidity indoors. In these applications, a minimum latent transfer efficiency of 50 percent is desirable, again at 50 percent of nominal full air flow rate.

In order to begin the pilot projects that generated this set of specifications and guidelines, it was necessary to acquire a much more advanced set of heat recovery capabilities. Such products are widely available in Europe, but have not been available at all in North America with capacities suitable for commercial buildings. The Ventacity Systems line of HRVs used in the pilot project derives from these European technologies, has been adapted to North American market requirements, and has been enhanced with a highly advanced suite of control and monitoring capabilities. Recently, Swegon’s GOLD RX line became Passive Haus rated and meets these specifications. We are hopeful that more products with this level of heat recovery will soon be available.

The key reason for the recovery efficiency specification set at this level is to require systems that will eliminate the need for conditioning the ventilation air stream before delivery to the building spaces for the vast majority of operating hours in Climate Zones 4-6. This not only reduces the heating/cooling load significantly, but allows a significant reduction the heating/cooling capacity required for the building, and eliminates the cost and control complexity imposed by the need to manage system components for conditioning the ventilation air that is delivered in typical HVAC systems. This level of efficiency requires the use of a counter-flow heat exchanger, which typically delivers minimum efficiencies of 85 percent and near-zero cross-flow air leakage (critical in some applications).

2. Minimum Fan Efficacy. As in the case of recovery efficiency, the vast majority of HRV and ERV products used for recovering ventilation air flow losses have had dismal fan efficiencies. Typical fan efficacies range between 0.5 and 1 cfm/Watt (1 to 2 Watts/cfm). This level of efficiency is so low that if the heating/cooling system is very efficient (as in the case of a VRF/VRV-type heat pump system), using such fans to recover ventilation energy will often consume more energy in fan power than is recovered from the ventilation air exhaust stream, rendering the expenditure on HRV or ERV systems for ventilation air inadvisable.

European HRV and ERV equipment uses some of the world’s most efficient fans, with whole- system efficacies ranging from 2-6 cfm/Watt (0.5 to .15 Watts/cfm). At this level of efficiency, it always makes sense to recover energy from the outgoing ventilation air stream, nearly eliminating the heating/cooling load associated with ventilation air provision.

The specification set at this level (2 cfm/Watt, or 0.5 Watts/cfm) ensures that the energy balance between energy recovered and energy expended is always positive. This results in a Coefficient of Performance (COP; for HRVs/ERVs the ratio of energy recovered to energy used to recover it) of about 20 in the cooling mode and 30 in the heating mode, or about 10 times the efficiency of a typical heat pump system. As in the case of heat exchange efficiency, using the most efficient fans tends to deliver a step-function improvement in system efficiency.

3. Control capabilities. When ventilation air is completely separated from heating/cooling air (the core requirement for this conversion concept), the ventilation air should then be managed for occupancy and level of occupancy (primarily, for most commercial building occupancy types). This requires a suite of control capabilities and sensors that have to do with occupancy and level of occupancy.

This element of the specification lists all of the necessary control modes required for appropriately controlling ventilation air in the building and space types that are encountered in commercial building occupancies.

4. Economizing. In current systems that combine ventilation air with heating/cooling air, provision must be made for utilizing “free cooling” during hours when outside air dry-bulb temperature is sufficiently low to provide the cooling function for the building without the added energy use for the cooling system compressor. The building energy code typically requires this function for any system larger than 4 tons of cooling capacity.

When separating the ventilation function from the heating/cooling function, the outside air connection is not associated with the heating/cooling system, so this function must be provided through the HRV/ERV in order to maintain the ability to increase the number of hours annually during which mechanical cooling (and heating, as well, in some cases) is not required.

5. Adaptive Defrost. In most climates, heat recovery systems will have a need to defrost the heat exchanger core when outside ambient temperatures approach freezing. In the most efficient systems, freezing occurs on the exhaust side of the core when warm, moist indoor air cools to near-freezing temperatures as heat exchange with the incoming air stream progresses through the length of the core. This freezing condition rarely occurs in less efficient HRVs or ERVs because these units recover so little energy that the exhaust air stream rarely reaches freezing temperatures. Defrost via inefficiency.

This part of the specification ensures that the system has the ability to apply a variable amount of energy to the incoming air stream to keep the exhaust side of the core frost-free when outside ambient temperatures are at or below freezing, while providing very high levels of heat or energy recovery. The use of a variable capacity defrost system maximizes system efficiency by applying only as much defrost energy as is required by outside ambient conditions. Partial or full recirculation of exhaust air to the incoming fresh air stream is prohibited in the specification to preclude the recirculation of exhaust air contaminants back into the building, even if during a limited number of hours per year. The recirculation of ventilation exhaust air is a very poor practice, inherently.

6. Cross-flow Leakage. In many occupancies (such as healthcare), the inadvertent recirculation of contaminated exhaust air back across the heat exchanger to the fresh supply air is a non-starter for a ventilation system. For that reason, and in view of the fact that such cross-flow leakage compromises the efficiency of the heat exchange process, sometimes significantly, this part of the specification is designed to eliminate as much as possible this kind of bad practice.

This part of the specification ensures that the HRV or ERV technology promoted by this conversion process will perform adequately in all building occupancy types, will not reintroduce exhaust air contaminants back into the fresh supply air, and will optimize the efficiency of the system.

7. BACNet/Modbus Connectivity. Most building automation control systems use the BACNet protocols to ensure that the various components of an HVAC system can “speak” to one another. In addition, the Modbus protocols are often used to allow one system to manage another, and to collect data and deliver instructions. A packaged HRV/ERV product must have this kind of connectivity in order to be part of an integrated building HVAC system.

This part of the specification ensures that the HRV or ERV will be able to manage and be managed by existing building control systems, and can be used to integrate the functions of the ventilation system and the heating/cooling system. This is critically important in operational modes such as economizing where the HRV should be able to turn off or modify the setpoints of the heating/cooling system during economizing hours.

8. Outdoor Installation. Many HRV/ERV systems or components are not designed to be installed outdoors or on the roof of a building. However, in the kinds of system conversions envisioned by these specifications, the HRV or ERV will often need to be located on a building rooftop, or in an outdoor location. In order to preserve the installation flexibility to be installed in a wide variety of commercial building applications, and to preserve the efficiency of the system in operation, the HRV/ERV must be designed to operate well in extreme weather conditions (temperature, humidity, rain, or snow), and be well enough insulated and air-sealed to prevent significant heat loss or gain to the ventilation air streams.

This part of the specification requires a system manufacturer to certify that the equipment is rated to be in an outdoor environment, even if at times extreme, and that it is configured for roof mounting and ducting.

9. Dedicated Ventilation Ducting. Many system designers, not being familiar with the type of equipment specified here or its capabilities, will attempt to direct the ventilation air stream to the intake side of the heating/cooling air stream at some point in the system, either in the mistaken belief that the ventilation air must be tempered by the heating/cooling system (something that other parts of the specification are designed to eliminate) or to save Such practices violate the intention of the specification, and will compromise the performance of the overall system. This part of the specification is simply designed to prohibit the substandard design practices that the overall system conversion concept is designed to eliminate.

10. Filters. Higher filtration levels are critical as public awareness of the health benefits of better indoor air quality increases.

Heating / Cooling System

1. Heating Efficiency, Cooling Efficiency. The test procedures for the current rating metrics are deeply flawed but have not yet been replaced by better For smaller systems (under 65 kBtu/hr), new testing and metrics became available at the beginning of 2023 as a voluntary Canadian Standards Association standard (CSA SPE07:23). No systems have been rated using this new standard as of mid-2023. Testing and rating standards for larger VRF/VRV-type systems are still under development.

The current specifications in this document use existing metrics and are set at levels that encourage downsizing of system  capacity – the larger systems in each category will most often not meet these efficiency levels.

Hydronic System Heating and Cooling Efficiency. Most hydronic systems in existing buildings are based on boilers for heating and chillers or cooling towers for cooling. A VHE HVAC system must be based on a heat pump technology, such as air- or water-source reverse-cycle chillers or a ground source heat pump system. The whole-system efficiency of such systems depends not only on the rated efficiency of the heat pump(s), but also depends heavily on how the balance of system is designed and specified (pumps, valves, piping and fittings, etc.). There are no metrics, or models that we’re aware of, that can reliably predict the operating efficiency of such systems, which is always lower than the rated efficiency of the heat pump equipment. It is extremely important to minimize pumping head loss in these systems, and to use the most efficient pumps available, with variable capacity to optimize part-load efficiency. If hydronic fan/coils are used, they should have very efficient fans. Note also that some well-based ground source heat pump systems may require more capacity oversizing to prevent the system from running continuously at design heating conditions. System off time allows well energy to recharge. Systems that have wells with good ground water flow-through characteristics can be sized with less extra capacity.

2. No simultaneous heating/cooling. This specification is in place to eliminate the significant cost of this feature of VRF/VRV-type systems if such investment will not produce measurable additional cost-effective energy savings and that any increases in system complexity and cost are justified by the additional benefits promised. The requirement for a matching zoning plan should ensure that if the investment is made, the zoning will be designed to actually deliver the savings.4

This part of the specification is present to ensure that projects deliver cost-effective energy savings, and that any increases in system complexity and cost are justified by the additional benefits promised. Early pilot project lessons learned made it clear that distributors and contractors will almost always attempt to sell the “heat recovery” feature, without any documentation of estimated energy savings, but there is no guarantee that the engineer or contractor will zone the building to properly take advantage of it.

3. System proposals. Detailed plans and specifications are required. In the world of small commercial building HVAC work, contractors (especially) typically provide very little detail about the systems they propose to building owners. While this may work well enough in the case of a simple RTU replacement, when the complete conversion of the existing system is proposed, detailed plans and specifications are needed to ensure that the system is designed and specified properly, that the ducting is properly sized and configured, that the appropriate control sequences and sensors are being used, and that project costs are reasonable. In a worst case (seen more than once in the pilot projects), the contractor will simply provide a single proposed cost and one-line description that basically says, “Provide building HVAC system according to plans.” In such cases it is impossible to know exactly what is being proposed, and if it has even a slight resemblance to the kinds of systems envisioned here.

This specification is designed to allow both the building owner and the program manager some assurance that the project proposed is likely to meet the performance required for the program, at a reasonable cost, and to provide information that might raise a red flag in critical system design and specification areas (system capacity, heat recovery, duct system design, control strategies, etc.).

4. Locations with low heating design temperatures. Balanced outdoor and indoor system capacities are key to utilizing full system capacity, especially in cold climates with significant summer humidity. If you significantly oversize the outdoor unit relative to the sum of the indoor unit capacities in order to maintain sufficient heating capacity at very cold temperatures, you will end up with a significantly oversized cooling system.  This will result in a system that won’t perform very well in dehumidifying the building during low sensible cooling load conditions.

In other words, it will be difficult to turn the system down under low-load conditions and still have good dehumidification performance (good indoor comfort). This is an important reason for using a cold-climate VRF system type, with an indoor/outdoor combination ratio near 1:1, in climates where design heating temperatures are 10 degrees F or lower and summer humidity is a challenge.

Design Guidelines

1. All calculations to be based on a space-by-space heat loss and heat gain analysis. Design load calculations are used to size the heating/cooling system. The system should be sized based on the higher of the design heating or design cooling load. Building UAs for existing buildings should be estimated conservatively, particularly for glazing which has an outsized impact on building performance. If in doubt, assume no low-e coating. Internal gains from lighting, equipment, and people should not be used to offset the envelope and ventilation heating loads because design heating conditions typically occur in the very early hours of the morning when most buildings are unoccupied or lightly occupied. The zone-level impacts of outdoor air introduced into buildings with (1) heavy occupant traffic between indoors and outdoors (whether continuous, as with certain types of office or retail occupancies, or intermittent, as in many public assembly occupancies), and/or (2) make-up air for high-volume exhaust systems must be carefully considered as VHE HVAC systems are more sensitive to untempered outdoor air than traditional systems. These conservative methods will typically result in a heating design load calculation that is 7-10 percent overestimated.

2. Complete separation of ventilation air from heating/cooling air. Mechanical engineers and HVAC contractors have rarely designed and specified systems that provide ventilation and heating/cooling separately, especially for smaller buildings. They automatically assume that the ventilation air will have to be tempered before delivering it to the space, and so tend to want to reintroduce the ventilation air into the heating/cooling air at some point downstream of the HRV/ERV. This completely negates the significant benefits and energy savings that come from controlling these two air streams independently.

This part of the guidelines simply emphasizes that at no time should the ventilation air be combined with the heating/cooling air. In the most extreme climates where a slight tempering of the ventilation air is advised, this function should be provided separately from the parts of the heating/cooling air that provide comfort conditioning directly to the building zones.

3. HRV/ERV sizing. Most HVAC designers will look at the maximum air flow capacity of a system and choose the smallest (i.e. cheapest) equipment model that can meet the design condition. Whether this is to save project cost or because the equipment they’re used to sizing does not have variable capacity capability, this is a really bad idea. Heat recovery ventilation system efficiency varies inversely and non-linearly with flow rate, both in recovery efficiency and fan efficacy. The “sweet spot” for design efficiency is in the middle of the flow range of the HRV/ERV. This results in very good efficiency (much better than at full flow) while resulting in a reasonable number of individual units for meeting the full ventilation requirements of the building. It also allows for higher-flow economizing.5 In many commercial buildings, occupancy and occupant density changes regularly, so designing for a ventilation system flow rate in the middle of the range also ensures that if occupant density increases in the future, the ventilation system will be capable of meeting the new requirements.

This guideline attempts to ensure that the HRV/ERV(s) are properly sized for ventilation efficiency (to maximize savings), and minimize noise and drafts. It also attempts to ensure that the ventilation ducting is sized for full-flow economizing.

4. Heating/cooling sizing. It has been demonstrated conclusively that the VHE HVAC conversion allows the capacity of the heating/cooling system components to be significantly downsized. This is a significant source of both energy and demand savings for the conversion. Even the most efficient systems do not cycle well (efficiently) under low-load conditions. Significant oversizing seriously compromises energy savings and increases project costs.

For the most common building occupancies (such as office and retail), in Climate Zone 4, delivered capacity sizing should most often be no less than 750 sq ft per ton of system cooling capacity, and in Climate Zone 5, no less than 600 sq ft per ton, but always based on design load calculations, as in number 1 above.

All capacities referred to here are delivered capacities, not the nominal rated capacities of the VRF outdoor unit(s). Some VRF systems will deliver substantially less capacity to the individual zones than one might assume. This is because there are several capacity adjustments (reductions) that must be considered in calculating how much capacity is delivered in each zone. These deductions include those for refrigerant piping losses, reductions due to an indoor/outdoor capacity combination ratio that is more or less than 100 percent (equal indoor and outdoor capacities), and those for capacity loss at low ambient temperatures and defrost intervals in the heating mode. Some systems will deliver only 60 percent of nominal rated sensible capacity in the zones.

5. Return ducting. Pilot project commissioning work has verified that in smaller spaces with doors that are often closed, a lack of return air inlets for heating/cooling at flow rates above 25 cfm will

pressurize the space by 10 Pa or more, which results in unwanted increases in fan power for both heating/cooling and ventilation. Because ventilation flow rates for smaller spaces are almost always under 25 cfm, exhaust inlets for ventilation air can be placed in common area spaces. Air transfer under doors generally works without excessive room pressurization for total flow rates under 25 cfm.

This guideline attempts to ensure that adequate return air paths are provided for both heating / cooling and ventilation air, even when doors are closed, by requiring a dedicated return for heating/cooling air (generally the higher air flow rates, compared to ventilation flow rates).

6. Ventilation duct configuration in a building space. While it may seem simple and obvious to some, most mechanical engineers and contractors have not designed a ventilation-only system using equipment with this level of sophistication, and so they don’t know to introduce fresh supply air to a space on one side and exhaust air from the other side, so that the people in the room (for whose benefit the ventilation air is provided) receive the benefit of the fresh In the pilot projects, it was also observed that more than one contractor had a tendency to limit ventilation duct run length to save money, to the fresh air detriment of the future occupants of the building. Monitoring of CO2 levels in the pilot project validated the inadequacy of designs that did not meet this guideline.

This guideline is there to remind designers of this simple, but often unknown or neglected, ventilation system design principle. It matters, a lot.

7. System commissioning. While system commissioning is a well-known and often-used practice now in the HVAC industry, most commissioning technicians have never commissioned a ventilation-only system, especially one as sophisticated as those provided by the HRV/ERV technology specified Special flow hoods are required to measure very low flow rates (under 100 cfm) accurately, and the controls used in these systems are unfamiliar to designers and commission technicians. But as for any HVAC system, competent commissioning is required to ensure that the system is performing in accordance with design intent and specification.

This part of the guidelines reminds project stakeholders that these unfamiliar but sophisticated systems also require good commissioning to perform optimally.

8. Ventilation duct sealing and air leakage testing, and insulation. There is a tendency of designers and contractors to dismiss these critical system performance issues because the air being moved around is at or close to room temperature. Pilot project work uncovered serious system performance compromises due to the intake of outside air (or air at outside ambient temperatures in unconditioned spaces) into ventilation exhaust air streams. Both efficiency and comfort were compromised. Especially in older buildings where ducting is being re-used or re- purposed, it’s critical to verify the air leakage rates to minimize adverse system performance impacts. Also, where ventilation ducting is in unconditioned space, insulation is required to maintain comfortable fresh air delivery temperatures. Any fresh air intake or exhaust air ducts between the HRV/ERV and the outdoors that are inside the building also need to be insulated. When outdoor ambient conditions are very cold, both the fresh air intake duct and exhaust duct will be at (supply air) or very close to (exhaust air) the outdoor ambient condition. Frost and condensation (and subsequent moisture damage) are a near-certainty unless the ducting is adequately insulated.

This part of the guideline attempts to maintain system performance (efficiency), comfortable supply air, and to prevent moisture damage to the building.

9. Extract air location. This addresses heat exchange performance degradation that occurs when H/ERVs draw extract air from unconditioned volumes of indoor spaces where the temperature is notably warmer in the cooling season and/or notably colder in the heating season than the indoor temperature setpoint. The closer the temperature of the extract air to the temperature of the incoming outdoor air, the less heat exchange takes place in the H/ERV. This can result in substantial increases in the heating and cooling loads for the zones served by the H/ERV.

1 Based on Passive House Institute certification or testing under balanced supply/exhaust flow, 50 percent of nominal full flow, and AHRI 1060 ambient test conditions.

2 When a new test and rating procedure is available, the heating and cooling specs will be translated into SCOP values.

3 Commissioning tech must use a flow hood that can measure accurately to ± 1 cfm, for both supply and return/exhaust.

4 Ventilation air flows are typically much lower than heating/cooling air flows. The heating/cooling fans typically provide the economizing function. If the HRV/ERV is to provide this function effectively, it will need to be able to run at a notably higher flow rate than typically used for the ventilation function.

Need more info? Get in touch!

David Cohan
Senior Advisor
david.cohan@imt.org