ApacheHVAC Hydronic Loops and Controls for Radiant Slabs

Radiator Types and Chilled Ceiling Types are, for the time being, used to model hydronic loops that will be placed within a slab zone. This is accomplished by effectively eliminating a number of irrelevant inputs normally used for modeling radiant panels. Guidelines for settings in the Radiator and Chilled Ceiling Types dialogs for hydronic heating and cooling loops placed within a slab zone are as follows:
- Orientation (horizontal or vertical) is relatively unimportant, but should be set to match the orientation of the slab in which the zone loop will be placed.
- Radiant fraction should be set to zero, as the loop should have only a convective (really conductive) coupling with the core of the slab.
- The reference temperature should be set to 1 K (1.8 ° F), the smallest value permitted. The reason for this is that this delta-T will, in this type of application, be referencing the difference between the controlled average water temperature in the zone hydronic loop and the negligible volume that must remain within the 3D slab zone in order for it to be included in the model. Setting a larger value as would be done for a panel in the occupied space will unnecessarily constrain the heating and/or cooling capacity of the zone loop.
- The rate at which the room loads add or remove heat to/from the slab will determine how close the core temperature of the slab is to the water temperature. In some cases, the delta-T may be a fraction of the minimum 1 K input value.
- The influence of the heating and cooling Output at Reference Temperature Difference should be effectively eliminated as a limit on heating/cooling capacity. It should therefore be set to a greater value than the maximum from boiler/chiller or the peak zone load from the loads analysis. However, because extreme values may introduce instability in the model, an appropriate value would be two to four times the anticipated peak zone load, thus allowing capacity equal to the peak zone load to be available from the loop when the water-to-slab core delta-T is ½ to ¼ of the reference temperature (above). The idea here is to let the modeling of the slab and controls on water temperature and flow rate in the loop determine the actual capacity at any given time step.
- Maximum Cooling from Chiller and Maximum Heating from Boiler values can be used as limiting factors for zone loops, but this is in addition to setting the water temperature and flow rate at the unit controller. As the latter are typically a more appropriate means of limiting the zone loop heating or cooling capacity, the value for Maximum Cooling from Chiller and Heating from Boiler can be set to a value that will be notably higher than any zone for which the particular Type is to be used.
- The weight should be that of just the water, and the water capacity should be consistent with the loop or loops for which the type will be used. As the loop mass and water volume are likely to be very small relative to the concrete slab, exact values are not important here.
To facilitate multiplexing and copying of room components representing slab zones and containing zone loop controllers (Room Unit Controllers for “Radiators” and “Chilled Ceilings”), “Room with Air Supply” components should be placed on a side branch of the airside HVAC network. From a setup and editing perspective, this is preferable to using the “Room without air supply” component (this component is, for this reason, to be removed until a multiplex compatible version is made available).
The airside network branch that couples the slab zones must have a time switch controller with Flow Rate set to zero.
Temperature sensor locations for hydronic slab heating and cooling loop controllers (“Radiators” and “Chilled Ceilings”) should be determined by design. These locations can include any combination of slab core (local or within the slab zone), slab surface, the adjacent occupied space conditioned by the slab, any other space in the building where the thermostat would be located, and/or external to the building (for modeling water temperature reset based upon outdoor temperature).
Water temperatures for heating and cooling should be set appropriate to radiant slab design parameters—i.e., to avoid thermal discomfort or condensation issues—and in keeping with any limitations imposed by low-energy heating and cooling sources used in the project.
When using room temperature sensors rather than a surface temperature sensor assigned to the surface of the slab itself, the radiant fraction in the controllers should normally be set to a small fraction of 1, or about 0.1–0.2 for a typical wall-mounted thermostat. However, if the project will use a specialized thermostat that approximates operative temperature, a more appropriate value for the sensor radiant fraction may be in the range of 0.4–0.5.
Flow rates for hydronic loop controls are, as of VE 6.4, not yet autosized; 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).