Now you have all your design spaces created and respective parameters, conditioning systems, and space constructions defined. The next step is to pull all this data together into a heating and cooling load analysis report to tell you how this particular building will perform throughout the year. You will then use that data to refine your conditioning systems further as well as size the
equipment you will assign to your HVAC zones.
You have already seen how the building and space construction can be modified to suit your building. But how do the different construction options affect the heat transfer into or out of the space? The engine that performs the heating and cooling load analysis in Revit MEP 2012 uses a Radiant Time Series (RTS) method to determine the building and space peak heating or cooling loads. This method takes into account the time delay effect of heat transfer through building envelopes, from the outside, and into spaces. A brief explanation of this method follows, but the RTS method of calculation is defined in detail in Chapter 30 of the 2005 ASHRAE Handbook:
Fundamentals, as well as in the Load Calculation Applications Manual, also published by ASHRAE (visit www.ashrae.orgfor details).
The RTS calculation method determines cooling loads based on an assumption of steady periodic conditions, such as occupancy, design-day weather, and cyclical 24-hour heat gain conditions. Two time-delay effects are addressed during cooling load calculations:
Delay of conductive heat gain through opaque massive exterior surfaces, such as exterior •u walls, the building roof, and floor slab on or below grade
Delay of radiative heat gain conversion to cooling loads •u
Figure 8.11 shows a flowchart summarizing the RTS calculation method.
Exterior building elements conduct heat because of a temperature differential between indoor and outdoor air; solar energy is absorbed by exterior surfaces as well. Because each surface has a mass and an associated thermal capacity of the materials that make up its construction, a time delay occurs from when the heat input of the outdoor and solar loads becomes heat
gain to the interior space. The majority of energy that is transferred to a space as heat occurs through a combination of convection and radiation. The cooling load immediately picks up the convective part of the energy transfer, and any radiant heat is absorbed into the surrounding space constructions and interior room finishes.
Series (RTS) calculation flow chart
The radiant heat is then transferred via convection from those surfaces to the space at a delayed time. Interior loads contribute to both the sensible heat gain of the space as well as a latent heat gain that is given off by people′s activity within the space. The latent heat gain contributes to the instantaneous cooling load of the space, while the sensible heat gain from internals is absorbed and retransmitted by radiation to the space.
The engine sums up the calculated cooling loads to determine a total cooling load per each design hour, and it selects the highest load, or peak, for the design of the air-conditioning system.
Note that Revit MEP 2012 uses, for the standard calculation, the hours of 6 a.m. to 6 p.m. for the design day, notthe full 24 hours, and only the months of April through November (October through May for southern hemisphere locations), notthe full calendar year. The design day is derived from weather data for the location that you set during project establishment. You will
revisit weather data later in this chapter.
Heating loads are calculated much the same way. The major differences are the obvious lower outdoor air temperatures in the heating design day, solar heat gains and internal heat gains are ignored, and the thermal storage effect of the building construction is not included.
Negative heat gains, or heat losses, are considered to be instantaneous; therefore, heat transfer is
dealt with as conductive. Latent heat gains are treated as replacing any space humidity that has been lost to the outdoor environment.
The worst-case load, as determined by the Revit MEP engine, is based on the design interior and exterior conditions and loads due to infiltration or ventilation. Although solar effects are ignored, assuming night or cloudy winter day operation, Revit does recognize internal heat
gain from people, lights, or miscellaneous equipment to offset the heating load needed. These additional factors can be edited, as shown in Figure 8.12. Occupancy/Heat Gain and Electrical Loads for Lighting and Power can either be specified as <Default> Values or edited to suit that particular space