Modeling hydronic heated and/or cooled slabs using radiant panels (method 1)
This method approximates the performance of a hydronic radiant slab using radiant heating and cooling panels within the conditioned space. There are no slab zones and nothing inside of any floor or ceiling construction. Panel performance parameters are treated much as they would with actual heating and cooling panels, with two very important exceptions: the panels representing the slab will be massive and, if the slab is a floor, the surface of the panels must be treated as a floor with people walking on it and sitting immediately above it.
Radiator and Chilled ceiling Types dialogs:
- Orientation should be horizontal if this is to mimic a ceiling or floor; vertical if an active wall element.
- Radiant fraction should be set according to the split of convective vs. radiant effect. The faction will tend to be higher for a cooled floor or heated ceiling than for a cooled ceiling or heated floor, given the convective heat transfer characteristics of floor and ceiling surfaces.
- When using a model of radiant panels to mimic a heated and/or cooled slab the reference surface-to-air delta-T and associated heating or cooling capacity must be constrained by the need to maintain slab surface temperatures within the range desired for human thermal comfort—typically 64°F (18°C) minimum in cooling mode and 75°F (24°C) in heating mode. Given room air temperature of around 75°F (24°C) when in cooling mode and 68°F (20°C) when in heating mode, the reference delta-T values would be just 11°F (6 K) and 7°F (4 K) for cooling and heating modes, respectively.
- The heat or cooling output at the reference temperature should then be set at a reasonable value for this modest delta-T. In the case of a heated floor, the convective heat transfer coefficient will be higher, and thus capacity will be better than a heated ceiling slab. In the case of a cooled floor, unless there is direct-beam sun striking the floor to present the load very directly, the cooled floor will have less cooling effect than a cooled ceiling, given convective heat transfer will be very limited in the downward direction. Sensible cooling capacity for typical slab-to-space temperature differentials is on the order of 24 Btu/hr-ft2 (~7 W/ft2 or 77 W/m2) of active surface, not including associated ventilation systems or strategies. When even a relatively low-cooling-capacity ventilation system, such as DV, is also accounted for, cooling capacities begin to approach those of conventional all-air VAV systems.
- Water capacity should be the volume in each radiant loop/zone, within reason, as the effect of this will be dwarfed by the effect of the massive floor.
- The weight should reflect the approximate mass of the heated/cooled floor construction. Again, very rough numbers can be used at this stage just to approximate the thermal inertia of the slab.
- Additional general technical information such as that provided above, along with many references for further reading, is available in a paper authored by ApacheHVAC Product Manager, Timothy Moore, prior to his joining IES. This paper is available from the UC Berkeley Center for the Built Environment at www.cbe.berkeley.edu/research/pdf_files/IR_RadCoolScoping_2006.pdf