Alternative HVAC systems Q&A
Unaddressed questions from the Feb. 26, 2015, webcast on alternative heating/cooling systems include topics like variable refrigerant flow, energy modeling, and natural ventilation.
Presenters Peter Alspach, PE, LEED AP BD+C, Arup, and Julianne Laue, PE, LEED AP BD+C, BEMP, Mortenson Construction, respond to unaddressed questions from the webcast on alternative heating/cooling systems.
Question: What energy modeling software is used?
Peter Alspach: There are a number of energy modeling tools available for use in evaluating nontraditional systems. However, care should be taken when selecting a tool to ensure it can accurately or reasonably simulate the alternative systems-not all tools can simulate all systems.
Question: What is the minimum well depth for ground source heat pump (GSHP) systems for refrigerant and for water?
Julianne Laue: Minimum well depths are dependent on each site as well as the type of installation (horizontal, vertical, pond, etc.). The International Ground Source Heat Pump Association has manuals,design, and installation standards and much more information available on their website.
Question: How do you filter the air in natural ventilation?
Peter Alspach: Outside air is not filtered in traditional natural ventilation approaches due to the pressure drop of typical media filters (the pressure drop is much greater than the natural driving forces). Despite this, most studies indicate that indoor air quality is higher in naturally ventilated buildings than mechanically ventilated/air conditioned buildings. Of course, a site with significant outdoor pollutants should be carefully examined to determine if it is suitable for natural ventilation.
Question: Have you seen many variable refrigerant flow (VRF) systems used in cold climates providing heating and cooling? Have they provided a back-up heat source for below zero operation?
Julianne Laue: Check the heating capacity ratings of the VRF manufacturer you are designing around. As outdoor air temperature decreases, the heating capacity will decrease. You have options for heating including a non-simultaneous heating and cooling system; you can oversize the system by designing the system for the heating load at the heating design day. Another option is to provide an auxiliary heat source.
Question: ASHRAE Standard 62-2010 requires naturally ventilated spaces to have a way to ensure the operable openings to remain open during occupied hours. This excludes the possibility of just relying on operable windows and using natural ventilation as the sole means of ventilation for a space. The openings have to be motorized windows or dampers in order to have natural ventilation as the sole source of ventilation. How does this impact your practice of choosing natural ventilation?
Peter Alspach: For a bit of background, I believe this language was inserted into Standard 62.1 due to studies that have found that air quality and effective ventilation rates in some naturally ventilated buildings has been inadequate, mostly due to occupants not opening the windows, particularly in cold weather. There are a few approaches I have seen used in this scenario:
- Base ventilation (i.e. the ventilation needed to meet ASHRAE 62.1) is provided via motorized/automated openings. These openings are often controlled off CO2 sensors or occupancy sensors. The additional openings generally required for cooling/temperature control are then manually operable.
- Base ventilation is provided through a mechanical ventilation system. The additional openings generally required for cooling/temperature control are then manually operable. I see this approach used quite often in colder climates as the mechanical ventilation system can use heat recovery (as well as preheat the air for better comfort).