Appendix G: Hydronic Radiant Heating and Cooling Systems
There are two methods for modeling radiant cooling and heating systems in the VE. Which of the two methods is most appropriate in any given project depends on a number of factors, ranging from design phase and desired level of accuracy to system type, configuration, and parameters to be investigated.
While there are even quicker and further simplified methods provided by the ApacheSystems module, these are primarily intended for UK compliance tools and early schematic design studies. The following, on the other hand, describes methods appropriate to supporting design decisions, exploration of control strategies, modeling specific opportunities for integrated operation of building systems and passive thermal strategies, and detailed documentation of potential system energy savings.
- The first method models the radiant systems as hydronic heating and/or cooling panels or radiators in the occupied space. This amounts to the straightforward use of the ApacheHVAC Radiator and/or Chilled Ceiling components as described in the User Guide. Examples of radiant cooling panels modeling are provided in the following pre-defined ApacheHVAC Prototype Systems:
- 09e Radiant Heat-Cool panels - DCV [EWC chlr - HW blr] .asp
- 09f Rad Heat-Cool - DCV - Heat pipe [EWC chlr - HW blr] .asp
- 09g Rad Heat-Cool - DV - DCV - Ht pipe [EWC chlr - HW blr] .asp
Please note that water flow rates are not yet autosized for hydronic room units; however, they can be relatively easily calculated using the standard formulas [gpm = Btu/h / (500 x delta-T)] and results of the ASHRE Loads analysis (either from the VE directly or in the Loads Data spreadsheet generated for a custom version an otherwise pre-defined prototype ApacheHVAC system).
Note also that only the last of these three pre-defined systems includes a stratified zone for modeling thermal displacement ventilation (DV) or similar environments.
- The second method models the radiant systems as hydronic heating and/or cooling within a separate slab zone above or below the occupied space. Because ApacheHVAC does not yet, however, have dedicated zone loops for this purpose, heating and/or cooling panels or radiators are placed within the slab zone and some adjustments made to the type definitions to remove characteristics of those devices that are not relevant when modeling hydronic loops in a concrete slab.
All chilled ceiling panel systems, all four-pipe heating/cooling panel systems, all radiator/panel and fin-tube convector heating, and most heating-only slabs can use the first and simpler one of these two options. In the case of heating-only slabs, the panel-based method is often adequate given that the thermal and energy performance for most heating-only systems does not depend significantly on taking advantage of thermal mass and off-peak operation of equipment. However, there are cases, such as a space with a heated floor slab that also receives significant direct solar gain, for which is will be important to use the second method to accurately model the thermodynamics of the floor slab. In other words, if the slab is likely to become thermally saturated by direct solar gain, this will both provide buffering of peak solar gain and alter the delta-T between the hydronic loops and the slab material, thus affecting the load profile for the heating source.
Modeling radiant cooling systems requires further considerations, and thus drives the need for using the second method in all cases where there are cooled elements of the building fabric (e.g., a floor or ceiling slab) rather than a cooled metal ceiling panel. Furthermore, it is common for cooled slabs to have surface temperature sensors, and these can be modeled only with the second method. Combined heating/cooling systems are subject to the same considerations as cooling-only systems, plus additional care with respect to sensors signals and controls to avoid inefficient consecutive operation of heating and cooling modes.
There are seven essential considerations in selecting the appropriate method. The last three of these are significant mainly with respect to actively cooled concrete slabs or similar elements of the building fabric:
1. Type of hydronic radiant system: lightweight panels vs. thermally massive slabs?
2. Design phase: in conceptual or schematic design, simpler panel models may more often be sufficient
3. Location of cooled surfaces or building elements relative to the occupied space
a. Location of panels relative to the occupied space: horizontal (overhead) or vertical panels
b. Location of slabs relative to occupied space, ground, and outdoors: ceiling, floor, and/or walls
4. Operating modes: heating-only, cooling-only, or both?
5. Interaction of chilled slabs with solar loads: will active slabs frequently receive direct solar gain?
6. Level of accuracy: model the slab cooling capacity or use pre-determined value per delta-T?
7. Significance of slab material thickness: e.g., is there intent to assess potential for pre-cooling?
8. Physical coupling with other systems and building elements; for example: Is there an underfloor air distribution (UFAD) plenum on top of the radiant slab through which the supply air will exchange heat with the slab? Is there a warm return air plenum under a chilled floor slab? Is the use of thermal displacement ventilation (DV) or a UFAD system likely to increase the delta-T, and thus the convective heat transfer, at the surface of a chilled ceiling?
Modeling of radiant cooling, with and/or without heating, should facilitate or account for the following:
- Radiant and convective heat transfer to and from cooled panels and/or slabs
- Orientation of panels (horizontal or vertical) and physical location and orientation of active slabs
- Differing characteristics (capacity at delta-T, radiant/convective split) of various panel options
- Thermal mass and absorption of direct solar gain for actively cooled slabs
- Rate of heat transfer resulting from hydronic tube spacing and depth in radiant slabs
- Slab surface and occupied zone temperatures
- Control of water temperatures and flow rates in zone-level hydronic loops according to slab core or surface temperature, occupied space temperature, and/or outdoor temperature resets
- Modeling of chilled and hot-water sources, loops, and pumps, including heating and cooling sources
- Potential effectiveness of low-energy heating and cooling water sources that take advantage of the moderate water temperature usable (and often required) in radiant systems—e.g. indirect evaporative cooling, waterside economizers, condenser heat recovery, condensing boilers, and solar hot water.
- Potential for nighttime and early morning pre-cooling of chilled slabs
- Assessment of occupant thermal comfort, including dry-resultant or operative temperature both during peak cooling demand and in early morning occupied hours where pre-cooling is included
- Integrated operation and control of hydronic and airside systems, including dedicated outside air systems (DOAS) for ventilation, latent loads/condensation control, and energy recovery
- Controls for mixed-mode systems (natural ventilation plus radiant), where applicable
- Capability for modeling stratified thermal environments, with or without displacement ventilation, where appropriate to space type and or system design
- Controls for automated shading devices and/or of modeling user-operated shading, daylight-based dimming of electric lights, and other active load-control strategies
All of the above are supported within the IES <Virtual Environment>.
To the extent they are present in the system, all heated or chilled ceiling and wall panels, four-pipe heating/cooling panels, radiators, fin-tube convector heating, and most heating-only radiant slabs can be appropriately modeled using the first (panel-component-based) method. With this method, only radiant slabs require special considerations to mimic the performance of the massive slab and to appropriately constrain output when the active slab is a floor or similar surface that occupants will be in contact with. These considerations are described in the next section below. All of the other hydronic terminal unit devices mentioned above can be modeled without any special considerations.