Innovative Energy Engineering



HVAC

  1. Heating: A "Heating Load" exists when all heat transfer to the outside and for ventilation exceeds all internally generated and solar-gained heat. Typically, this load is sensible only.
  2. Ventilation: control air exchange in order to exhaust CO2, odors and other gases generated in the building to the outside and to replace exhausted air with fresh outside air. That exchange of air can generate a heating and cooling load
  3. Air Conditioning: A "Cooling Load" exists when all heat transfer to the outside and for ventilation is less than all internally generated and solar-gained heat. Most of the time this load is sensible and latent.

Basics

Steps to design a HVAC System

Pre-Selection

Deciding on System

For the remaining choices perform an energy simulation, estimate up-front cost, maintenance cost, and life expectancy. Keep in mind that projections over the estimated life span of over 20 years may be vague. System types and components have different life expectancies with varying replacement costs. Future replacements not only cost money, it also inconveniences occupants and may have unintended cost if access is not good or if downtime is expensive.

The best system won't operate effectively and efficiently if it is not maintained properly. Therefore the personnel maintaining the system need to "buy in". Most mechanics like what they already know, but also should be open to newer technologies. In retrofits the equipment they "know" may be 30 years old and current equipment will be "different" and require training. Differences in maintenance cost can cancel out any energy savings. Maintenace also should be possible with little impact on occupants. All equipment should be easily accessible including easy access for wear and spare parts. Roof top equipment should be avoided. It usually never fails when the weather is nice. Design the system as if you woudl maintain it and responsible for repairs. Typical mistakes include but are not limited to:

Central Heating and Cooling Plant or Zone Units?

The heating or cooling device can be located in the zone or in a central plant outside the zone. In central heating or cooling systems, air and/or water is heated or cooled and moved to the zone in question. An example for a zone unit is a water-source heatpump unit that is located in the space it serves. One problem with that is the noise and maintenace requirements in occupied areas. An example for a central system is a central boiler that provides heat to the zones by running water pipes and dissipating heat via zone radiators. One problem with that coudl be the space requirement for a central boiler or the long piping distance.

There are multiple reasons to use the in-zone device, or central plant approach. Which approach to choose depends on how many zones are served, what comfort levels are required, if noise is acceptable, and if maintenance can happen in the space. In many cases it is clear which approach is better. For example an office would not accept a gas-fired unit heater for aesthetic, noise and comfort reasons.

Moving Air or Water?

With any central plant air or water needs to heated or cooled and transported to the zone. In case of cooling we need to move air at some point to dehumidify. If latent load is non-existent, water alone could be used or a combination of air and water can be implemented. Power required to move a fluid depends on the volumetric flow and the pressure differential. The more volume and more pressure is required, the more energy is used. Water has a higher density and heatcapacity than air. In fact, it requires 3,268 times the volumetric airflow to move the same energy as with water at given dT. Even when accounting for about up to 100 times the pressuredrop (compared to air systems)in water systems, moving air still requires 32 times the energy at given dT. This also explains why piping systems typically are significantly smaller than duct systems. In addition, air systems have a certain leakage, requiring even more air to be moved, and the larger surface of ducts allows for more heat losses and gains. Whenever possible we want to avoid air movement, but it can't be avoided all the time.

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