Radiant Heating and Cooling with Dedicated Outside Air System (DOAS)
HVAC Systems: Radiant Heating and Cooling with Dedicated Outside Air System vs. Conventional Forced Air HVAC
Provides space conditioning by circulating heated or cooled water through radiant ceiling panels, floor, or walls while required ventilation air is provided by a dedicated air handling unit.
Item ID: 74
HVAC--Other HVAC Systems
Technical Advisory Group: 2010 HVAC TAG (#3)
Average TAG Rating: 2.7 out of 5
TAG Ranking Date: 06/29/2010
TAG Rating Commentary: FROM 2010 HVAC TAG RANKING SURVEY RESPONSES:
1. Expensive to retrofit. This technology is mostly for new construction buildings.
2. Need much more experimentation/case study on the control side. DOAC needs intelligent reset to be cost effective. Also issues of morning heat/afternoon cooling to be resolved. Demonstration would be good.
3. I am concerned when heating coils pass through surfaces with high thermal mass, because the ability to set back and warm up the building is seriously compromised. I realize this proposal covers other options (such as wall radiators) as well. I also want to see a few installations of radiant cooling to see whether condensate is handled adequately. It seems these are often considered for applications where the minimum air flow rates are high for other reasons than heat transfer... A confusing situation out there. Has strong potential, but needs some careful consideration.
4. I love this technology from a comfort perspective, but what is the source of the savings? Perhaps a lower temperature setpoint because of comfort. Am I missing something?
5. New buildings only, application specific. Ventilation air needs to be conditioned. Some cooling systems may require a cooling coil for dehumidification. Slab systems provide good opportunity for off-peak cooling, and cooling tower can be used to provide water-side economizer.
6. Too hard to turn into an incentivized technology.
Technical Advisory Group: 2009 HVAC TAG (#2)
Technical Advisory Group: 2015-1 Commercial HVAC TAG (#11)
Average TAG Rating: 3.1 out of 5
TAG Ranking Date: 03/10/2015
TAG Rating Commentary:
- High opportunity for energy savings. We used this approach in a local fire station and it uses about 1/3 of the energy of a typical station and half the energy of the next most efficient station in the region. However, radiant can be more expensive then other systems and it is not universally applicable to all occupancy types. Other ductless distribution systems such as VRF paired with DOAS can offer similar savings with more flexibility and at less cost.
- While this technology is more suitable for new construction and most of the heating savings would be non-electric, it could be helpful to identify good retrofit applications and guidelines for estimating and verifying energy savings.
- This is really just a generalization from the chilled beam case. More likely to be useful for new construction than retrofits.
- I am not sure if this technology is suitable for retrofits.
- Radiant systems can be much more efficient and there are big savings by not moving so much air - particularly good with zoned DOAS
- May not be applicable to retrofit projects.
Radiant systems circulateheated or chilled water through radiant floors, walls, or ceiling panels, whichcannot be covered in insulating carpets, cabinets, or tiles without sacrificingsystem performance and efficiency. These systems have been around for decades,especially in Europe, but have not caught on in the Pacific Northwest. Thesesystems may provide energy savings and improved comfort with less noise than forcedair HVAC, but are generally less nimble in varying space temperatures amongsmall zones over time. This description focuses on commercial applications, butthis technology can also be used in residential applications.
Fresh air issupplied to the spaces based on their maximum occupancy, and typically amountsto about 20% of the overall air delivered to a space by conventional forced-airHVAC systems. Reducing the size of the fans to supply only the ventilation airand meeting the heating and cooling needs with radiant systems will save energybecause pumping energy use is less than blower energy use. Duct losses arealso eliminated, resulting in improved efficiency of delivered conditioned air.
The first cost ishigher by about 25%, ranging from about $16/sf to about $20/sf. For the climatezone that includes Seattle, energy savings from this technology are estimated tobe approximately 20% better than for a system with high-performance variableair volume (VAV) air handling units. However, success of this technology dependson the specific application (building type and climate). This technology issuitable for new construction. A building retrofit would be quite expensive. Itcan be used in both commercial and residential buildings.
The radiant elementsof the system are passive devices and should have an extended life similar tothat of a piping system – perhaps 30 to 40 years. Active components will lastabout as long as conventional equipment – about 25 years.
Baseline Description: Forced air heating and cooling
Baseline Energy Use: 10.5 kWh per year per square foot
The 2009 CommercialBuilding Stock Assessment (CBSA) gives the actual electrical building Energy UseIntensity (EUI) values for various types of heating and cooling systems (CBSA TableD-EA5). Office buildings with electric heating and cooling have an EUI of 20.1kWh/sf/year. Office buildings with no electric heating or cooling use only 8.2kWh/sf/year, indicating that the combined HVAC heating and cooling energy useis 11.9 kWh/sf/year. (For all commercial buildings, the corresponding valuesare 19.9 and 9.4 kWh/sf/year, respectively.) Because this technology can beapplied to many types of non-office commercial buildings, a baselineenergy use of 10.5 kWh/sf/year is assumed (NEEA, 2009).
Manufacturer's Energy Savings Claims:
"Typical" Savings: 45%
Savings Range: From 45% to 45%
(ThermoSoft, 2013) states 45%
Best Estimate of Energy Savings:
"Typical" Savings: 20%
Low and High Energy Savings: 18% to 73%
Energy Savings Reliability: 4 - Extensive Assessment
Energy modeling of a medium-sized office building in Seattle indicates energy savings of approximately 20% for a package of efficiency measures with radiant heating and cooling compared to the same package with a VAV system (Thorton 2009). References show higher savings for other locations. Assume savings will be at the lower end of the savings range for the Northwest. More experience with radiant heating and cooling is needed to confirm the savings potential for the Northwest.
Based on a literature review conducted by the U.S. DOE Building Technologies Office, the estimated heating/cooling system energy savings are 20%, based on a comparison of chilled beams (another form of radiant cooling covered in ET #316 Active Chilled Beam Heating and Cooling) with a standard VAV system (Navigant Consulting, 2012).
Research by Green Building (Green Building, 2009) indicates that energy savings can be 10% to 15% of the total energy bill for residential applications.
In general, the savings are due to fan energy savings and comfort at a larger deadband. Standing on a warm floor that radiates warmth upward makes occupants feel as warm as if they are in a space with forced-air HVAC and a warmer air temperature. Specific energy savings, however, depend on the following factors: climate, operating times and temperature setpoints, system components, and equipment efficiency, such as use of variable speed drives (VSDs) and VAV systems.
The system is not recommended for climates with high humidity during the cooling season, either from internal sources (such as cooking, showering, or Jacuzzis) or external sources, due to the potential for condensation caused by infiltration of outdoor air. This technology will achieve greater savings in climates with higher heating degree days and/or cooling degree days, where more energy is expended to heat and cool non-ventilation air. In addition, this technology is more appropriate in climates with smaller diurnal temperature swings so occupants do not require a big early morning warm-up and then afternoon cooling. Buildings located in Western Washington and Oregon, where the temperature can vary little throughout the day for much of the year, are good applications for this technology.
The size of the cooling tower will determine the extent to which a waterside economizer cycle can be utilized. Depending on the climate and economics, this may drive the sizing of the cooling tower beyond the size required by the chiller.
Application of demand-controlled ventilation (DCV) is particularly effective with this system. Resetting the supply air temperature of the dedicated outside air (OSA) ventilation system is necessary and can be complex. Supplying the ventilation air to all spaces at a neutral temperature can result in unneeded heating of the air, while supplying the air at a cooler temperature can result in sub-cooling of interior spaces. Optimizing controls with feedback from each zone should be used to reset the supply air temperature.
Energy Use of Emerging Technology:
8.4 kWh per square foot per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
This measure is a design strategy that can best be applied to new construction or major renovation. It is not a likely retrofit to existing buildings. CBSA projections of commercial building square footage indicate an annual growth rate of approximately 1%. If we assume existing buildings have a 50-year life, then 2% of buildings will be replaced or have major renovations in a given year. Thus, new construction/renovations reflect about 3% of the existing building stock in a given year. Assuming the next 10 years of construction represent the new construction/major renovation potential, this totals 34% of the existing commercial square footage (3,118,000,000) or 1,060,120,000 sf. Applying market shares from existing buildings, consider only conditioned square feet (85%) and electrically heated space (27%) = 1,060,120,000 x 0.85 x 0.27 = 243,297,540 sf (NEEA, 2009).
This technology can also be applied to residential buildings – single family and multifamily, although the energy savings would be different. Accounting for this would increase the technical potential.
Regional Technical Potential:
0.51 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: square foot
Emerging Technology Unit Cost (Equipment Only): $19.00
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $1.00
Baseline Technology Unit Cost (Equipment Only): $16.00
For the climate zone that includes Seattle, the proposed technology had a cost premium of approximately $4/sf over the system of VAV packaged rooftop HVAC units. The numbers above use $16/sf for a 50,000 sf medium-sized office building in the baseline. The ET uses $4/sf more, or $20/sf for the same 50,000 sf medium-sized office building. (Thornton, 2009)
Simple payback, new construction (years): 15.9
Simple payback, retrofit (years): 105.8
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Payback will depend on use of building, weather conditions, etc.
This technology uses radiant panels for heating and cooling. Space heating and cooling is accomplished by circulating heated or cooled water through radiant ceiling panels or through tubing embedded in the floor or walls of each space. The ceiling panels could be chilled beams (detailed in ET #316 Active Chilled Beam Heating and Cooling). Water can be heated by a water heater or boiler, and cooled with a chiller and cooling tower. During appropriate weather, water from the cooling tower may be used for radiant cooling directly (known as a water-side economizer).
Ventilation is accomplished by supplying the required quantity of outside air (OSA) to each occupied space via a small air handling unit that is used solely for ventilation, supplying 100% OSA. If radiant panels are used, they need to be strategically located to condition spaces that are most often occupied and used in spaces where occupants are relatively stationary. These systems are most applicable in climates that do not have large summertime diurnal temperature swings, which require morning heating and afternoon cooling.
In general, the savings are due to fan energy savings and comfort at a larger deadband. Standing on a warm floor that radiates warmth upward makes occupants feel warm as warm as if they are in a space with forced-air HVAC and a warmer air temperature. Specific energy savings, however, depend on the following factors: climate, operating times and temperature setpoints, system components, and equipment efficiency such as use of VSDs and VAV systems.
The system is not recommended for climates with high humidity during the cooling season, either from internal sources (such as cooking, showering, or Jacuzzis) or external sources, due to the potential for condensation caused by infiltration of OSA. This technology will achieve greater savings in climates with higher heating degree days and/or cooling degree days, where more energy is expended to heat and cool non-ventilation air. This technology is more appropriate in climates with smaller diurnal temperature swings where occupants do not need a big early morning warm-up and then afternoon cooling. Western Washington and Oregon, where the temperature can vary little throughout the day for much of the year, are good locations to apply this technology.
The size of the cooling tower will determine the extent to which a waterside economizer cycle can be utilized. Depending on the climate and economics, this may drive the size of the cooling tower beyond the size required by the chiller.
Application of DCV is particularly effective with this system. Resetting the supply air temperature of the dedicated OSA ventilation system is necessary and can be complex. Supplying the ventilation air to all spaces at a neutral temperature can result in unneeded heating of the air, while supplying the air at a cooler temperature can result in sub-cooling of interior spaces. Optimizing controls with feedback from each zone should be used to reset the supply air temperature.
The standard practice for providing HVAC in medium to large buildings is to use central, variable volume air handling units to deliver mixed air to terminal units with hydronic or electric reheat. OSA is mixed with return air at the air handling unit to deliver the required volume of ventilation air to each space. Most codes require the capability to supply 100% OSA when ambient conditions are appropriate for cooling (economizer cycle). Also, most codes limit the amount of primary air that can be reheated at the terminal unit, which can be a difficult design challenge and can add to the installed first cost.
The proposed technology is quite common in Europe, and has been for decades. It has been tried in North America in warmer climates, but has not reached full market acceptance in the Pacific Northwest, primarily because of lack of available suppliers and concerns about condensation of the radiant panels in cooling mode. Controls are available to minimize condensation but this technology has still been slow to move in this market.
Hydronic radiant heat is “softer” than other HVAC systems; that is, it is slower to cool and heat, so it provides a more uniform temperature over time. Occupants also enjoy reduced fan noise and warm feet in the winter if radiant floors are used. In addition, this technology does not have ductwork, which requires maintenance.
End User Drawbacks:
Drawbacks include higher initial construction cost, issues with indoor or outdoor humidity (lack of filters, air cleaners, and dehumidification), lack of expertise in the design and construction industry, lack of familiarity among building owners and occupants, and a lack of actual energy use data. If radiant panels are used and not strategically located, they will not provide adequate comfort to occupants, particularly if occupants are not stationary. Due to the use of thermal mass, these systems are not nimble enough to provide effective morning warm-up and afternoon cooling; in the Pacific Northwest, diurnal temperature swings can require this, particularly on the east side of the Cascade Mountains.
Operations and Maintenance Costs:
Baseline Cost: $1.00
per: square foot per year
Emerging Technology Cost: $1.00
per: square foot per year
The O&M costs are about the same for this ET and VAV, just different equipment to be maintained.
Anticipated Lifespan of Emerging Technology: 25 years
The radiant elements of the system are passive devices and should have an extended life similar to that of a piping system – 40 years – as listed in ASHRAE. A potential concern is the long-term integrity of the flexible hoses that are used to connect each panel. Associated boilers and chillers should have about the same life as conventional systems – about 15 years. The dedicated OSA ventilation system is a typical air handling unit, which should have a useful life of about 25 years.
High performance VAV system with DCV; energy recovery ventilator; indirect evaporative cooling; supply air temperature reset; static pressure reset; high-efficiency fans and low static pressure drop casing, coils and filters. Expected energy savings from the proposed technology are approximately 20% better than the system of high performance VAV air handling units.
Another technology that will be a competitor in many applications is variable refrigerant flow systems, which can heat using compression technology and offers internal/external heat recovery between zones.
Reference and Citations:
Thornton, et. al.,
Technical Support Document: 50% Energy Savings Design Technology Packages for Medium Office Buildings
Pacific Northwest National Laboratory
This document shows a 50% energy savings when dozens of technologies are implemented. It does not show the savings attributed to radiant heating and cooling over a baseline, so it is inconclusive.
Summary Report: Simulation of Radiant Cooling Performance with Evaporative Cooling Sources
UC Berkeley, Center for the Built Environment
Energy Saving Electric Floor Heating Systems
Green Building Energy Savings,
Underfloor Radiant Heat - A Historic and Modern Overview
Green Building Energy Savings
Northwest Commercial Building Stock Assessment (CBSA): Final Report
Prepared by the CADMUS Group for the Northwest Energy Efficiency Alliance
Modeling a Radiant Slab Coupled to a Cooling Tower using the New National Energy Analysis Program
Cooling Frontiers Symposium, Arizona
Raftery, et. al.,
Analysis of a hybrid UFAD and radiant hydronic slab HVAC System
Proceedings of the International Conference on Applied Energy (ICAE 2010)
Potential and Limitations for Hydronic Radiant Slabs using Waterside Free Cooling and Dedicated Outside Air Systems
Third National Conference of the International Building Performance Simulation Association (IBPSA)
Bourne & Hoeschele,
Applying Natural Cooling to Slab Floors
ACEEE Summer Study on Energy Efficiency in Buildings
Energy Savings and RD&D Opportunities for Residential Building HVAC Systems
U.S. DOE Building Technologies Office
ASHRAE, REHVA Jointly Publish Guide to Chilled Beam Systems
Air Conditioning, Heating, Refrigeration News