An Introduction to Accurate HVAC System Modeling

HVAC (Heat, Ventilation and Air Conditioning) is all about comfort, and comfort is a subjective feeling associated with many parameters like air quality, air temperature, surrounding surface temperature, air flow and relative humidity. For example, while it is easy to understand how the temperature of the air in your living impacts how good you feel, the surfaces with which you are in contact also strongly affect your comfort. For example, last night I got out of bed to clean up after my dog who thought it would be a good idea to swallow (and give back) her chew toy. If I was wearing my slippers, it would have been much easier to go back to sleep between the warm bed sheets without the discomfort of waiting my cold feet warm up to normal temperature.

Speaking of sleep discomfort, many stem from HVAC imbalances.  If you wake up in the middle of the night quite thirsty, then you should probably check how dry your bedroom is. The recommended range is 40-60% relative humidity. A higher humidity puts you at risk for mold while lower humidity can lead to respiratory infections, asthma, etc.

Now that we know how HVAC contributes to our comfort, let’s look at the HVAC unit as a system and see its role, functioning and simulation at a high level. The following examples provided are for a house, but similar concepts apply to residential buildings, offices, and so on.

Controlling Temperature

The easiest parameter to control is the air temperature. It can be set by a thermostat and regulated according to a heating or cooling flow distributed from the HVAC unit to the different rooms through ducting. Without the introduction of thermally-different-than-ambient air, the house will heat or cool itself based on a combination of outside conditions and how well the building is insulated. Therefore, to keep a constant temperature a certain amount of energy must be used to provide heating (or cooling) at the same rate the house is losing (or gaining) heat.  This is a match of the house load and heating/cooling capacity. Figure 1 provides a graph of the energy needed.

Illustration of dependency of house load and heating capacity on outside temperature
Figure 1 Illustration of dependency of house load and heating capacity on outside temperature

The ACCA (Air Conditioning Contractors of America) Manual J (Residential Load Calculation) computes the heating and cooling load—the BTU’s and cubic-feet-per-minute volume of hot and cold air, respectively—which must be delivered to each room to maintain a comfortable temperature. Calculations are based on location; orientation, especially of windows; envelope; duct and envelope tightness; and internal gains like occupancy. Once the loads have been determined for both latent and sensible heat, the entire heating and cooling (refrigeration) cycle can be designed and optimized, both at the component level and at the system level to ensure the best performances possible. For refrigeration cycles the COP (Coefficient of Performance) is typically used for this as it relates to the ratio between the compressor energy usage and the amount of useful cooling at the evaporator. Figure 2 provides a schematic of a simple refrigeration cycle.

Example of refrigeration cycle in AxCYCLE to determine component boundary conditions required to do their design
Figure 2 Example of refrigeration cycle in AxCYCLE™ to determine component boundary conditions required to do their design, calculate COP, etc.

Zoning for Temperature Control

However, using a single thermostat to control the air temperature effectively in multiple rooms can be challenging, due to different rooms having different loads based on occupancy, number of windows, furniture, carpet vs. hardwood floor, etc. That’s where “zoning” comes in. Zoning represents the idea of grouping multiple rooms which have similar demands. Therefore, if your basement is to be kept cooler than the living room and kitchen with their big windows while at the same time having some intermediate temperatures in the bedrooms, zoning allows designers to group these rooms and set different thermostats for each zone. A basic schematic of this example is presented in Figure 3.  The air coming from the furnace is at the basement temperature and split with a portion feeding into the basement. The rest goes to the living area and bedrooms after passing an in-duct heater (heating coils) to locally control the temperature of the air provided. Note – typically the furnace is in the basement and takes up the majority of the space in an HVAC system, being used to move air from the heat exchanger into the air ducts.

Example of simplified duct zoning for HVAC system in AxSTREAM NET
Figure 3 Example of simplified duct zoning for HVAC system in AxSTREAM NET™ used to determine load requirements for the fan(s) and heating coil(s) based on the ducting and GRDs geometry, location, etc.

Other Considerations for Temperature Control

The different branches in the model above are of varying physical lengths, cross-section shapes, and dimensions. They are sized to provide the desired amount of air to each outlet. Additionally, the grilles, registers and diffusers (GRDs), which are not presented in the schematics above, can be modeled to determine more accurately how much flow will be delivered at each location. Such level of detail also allows designers to determine the GRDs and ducting pressure losses (flow resistance) which are essential data points needed to properly size the fan and motor. Different types of GRDs are shown in Figure 4. Oversizing the system would result in increased purchase costs, reduced operating efficiency, excessive wear/maintenance, noise, etc. Undersizing the system would lead to improper control of the comfort conditions such as not being able to provide the desired amount of cold air on a very warm day.

Figure 4 Examples of GRD geometries

Also, although air recirculation (vs. typically 10-20% of fresh air on average) was not presented here it is a very essential function of HVAC units. Indeed, fresh outside air renew and maintain the indoor air quality for the occupants. Yet, it is important for the system not to be wasteful both in terms of energy usage and financial cost. By allowing the majority of the indoor air to be recirculated after going through filters, the system ensures that the conditioned air (hot or cold) does not get rejected to the outside after a single use.

In this post, we have taken a brief look at what comfort corresponds to for us and how to look at a high level overview of the process.  We traveled from “I have a house” to “I require this level of heating and cooling loads” . This blog also showed what a typical refrigeration cycle looks like and how it is used. Finally, we showed how to accurately model the complete system, including ducting, to ensure the various components are all properly sized for maximum efficiency at a minimum purchasing and operating cost.

One thought on “An Introduction to Accurate HVAC System Modeling

  1. Every wall in the house can be modelled by a transfer function of heat flow.

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