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.
By Peter Alspach, Arup, and Julianne Laue, Mortenson Construction March 4, 2015

Julianne Laue (left) and Peter Alspach respond to unanswered questions from the Feb. 26, 2015, webcast HVAC: Alternative heating/cooling systems. Courtesy: CFE MediaPresenters 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).

Question: I have concern about the proper insulating of the variable refrigerant flow (VRF) system.Is the insulating of these systems considered when the design work is done? Installing contractors are not using the appropriate thicknesses as we have seen problems with lack of proper thickness to control condensation. Also do you recommend vapor dams on these systems?

Julianne Laue: It is important to design and install any HVAC system per the manufacturer’s recommendations. Installers typically install what is specified. I recommend working with your local manufacturer’s representative for the VRF system you are specifying to ensure all design and installation conditions are met.

Question: With regard to low delta T hydronics, dewpoint sensitivity: How good are current relative humidity (RH) sensors? Accuracy? Time between recalibration? Sensitivity to contaminants?

Peter Alspach: Dewpoint sensors/RH sensors do require calibration for best accuracy. For that reason,on-pipe condensation switches are often used. The switches can be used either as the sole means of control for supply water temperature, or as a back-up system in case the dewpoint sensor control has error.

Question: How often do failures occur?

Peter Alspach: I believe this question is in relation to failures in in-slab piping systems used for thermally active slabs. The main cause of failure is typically post installation-when anchors are driven into the piping, however even this is rare. Education is a good solution for this, but some feel that redundant piping loops are worthwhile. Obviously there needs to be an evaluation of risk vs cost. If the radiant system is a base load system, then the trimming system may be able to be modified in the event of a major failure at lower cost than installation of redundant piping.

Question: Why would one choose an active chilled beam solution over a variable refrigerant flow(VRF) solution? Use a hospital room as an example.

Peter Alspach: This answer could be quite an essay. However, a couple key reasons are:

  • An active chilled beam system uses chilled water. Central chiller plants are typically more efficient cooling sources than VRF systems, which are generally air-cooled scroll compressor technology.So for a hospital that typically has a large chilled water plant, it would make sense to leverage that system and its efficiencies.
  • VRF systems are very sensitive to room volume in order to comply with ASHRAE Standard 15:Safety Standard for Refrigeration Systems limits on refrigerant quantities. They are often a challenge when serving lots of small spaces, such as patient rooms, since the refrigerant limits of ASHRAE 15 will often mean that the VRF system has to be broken up into smaller systems (i.e.fewer indoor units for each outdoor unit). This break-up drives up costs as well as limits the ability to recover heat from zone to zone.

Question: In a variable refrigerant flow (VRF) system, how do you deal with limitations of refrigerant volume in a space, if system circulates in so much of the building?

Julianne Laue: ASHRAE Standard 15: Safety Standard for Refrigeration Systems and Standard 34 provide safeguards for refrigerant systems. It is best to use these design standards as well as consult with your local authority having jurisdiction (AHJ) as to their interpretation of the standards.

Question: Is activated slab different from radiant heating and cooling that is installed in the floor?You mentioned that they must be concrete and no other floor type.

Peter Alspach: Activated radiant slabs are different from the traditional floor systems in a couple ways.The most significant is that traditional floor (or ceiling) systems usually have the piping in a topping slab or other surface layer, with a layer of insulation to thermally separate the topping slab and the structural slab. Activated radiant slabs have the tubing embedded directly in the structural slab and are desired to provide heat transfer at the ceiling below and the floor above. By their nature of being embedded in the structural slab, activated radiant slab systems are always installed in a concrete structure. One has to be careful in activated slab systems that surface treatments (carpet, wood, ceiling finishes) do not limit the heat transfer as many act as insulators and decouple the activated slab from the space.

Question: Regarding variable refrigerant flow (VRF) systems that have the best energy (can do heating/cooling simultaneously), are there difficulties finding enough vendors to comply with government project requirements for "or-equal"?

Julianne Laue: Specifying any HVAC system should be done with great care. Specifications that are copied from an individual manufacturer create increased difficulty in meeting an "or-equal" condition.Energy performance should be looked at as a minimum performance allowing for products with performance above the minimum to be allowed.