Residential HVAC System Selection

Discussion in 'HVAC' started by aardvark, Jul 5, 2010.

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  1. Jul 5, 2010 #1





    Jul 4, 2010
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    I have recently joined the forum and noticed that there has been some discussion about residential HVAC system selections. Alternatives are being discussed (such as rad to forced air conversions) spanning decades of technologies and topics that have challenged home owners since the beginning of habitation.

    Below are some general thoughts regarding the selection of HVAC systems for the home. Also below is the system selection thought process that I’m currently using for my home… all recorded for your scrutiny and general entertainment.

    Residential System Selection

    HVAC systems are intended to provide comfortable and healthy indoor environments. Many options are available to achieve a diverse array of goals and design intents. In the analysis and selection of a system, criteria and project specific parameters need to be identified and prioritized. However, a fundamental requirement for residences is to ensure health and safety. This means ventilation must be provided when the indoor spaces are occupied and to remove building/environmental-generated pollutants at all times as needed. The required rate of ventilation varies widely depending on codes and politics. In practice, the larger challenge seems to be ensuring that it is provided and not so much on the rate of delivery. Consideration should also be given to sustainability such as responsible energy and environmental design, as well as constructability.

    In simple terms, all indoor environmental control systems include the following common functionality, regardless of building construction and virtually across all climates:
    1. make or remove ‘heat’
    2. transport and deliver ‘heat’ or ‘cold’ to the space
    3. add or remove moisture from the space
    4. move ventilation air around the building
    5. filter air to remove particulates and exhaust pollutants directly

    The best system would be one that has discrete components that fulfills each of these functions individually. This allows each single function-dedicated component to be optimized and easily controlled without the compromises inherent in multi-role components. Be cautious of complex systems that combine multiple functions into one device, they are invariably difficult to control and often end up using more energy than their simpler counterparts.

    Occupant comfort and ventilation criteria considerations include:

    □ Temperature: 20 to 24C (68 to 75F) is generally comfortable
    □ Humidity: 20 to 60% rh is generally comfortable; 35 to 50% rh is a good target.
    □ Air Movement and Space Pressure Requirements
    □ Air Purity or Quality
    □ Mould and Mildew Prevention
    □ Air and/or Water Velocity Requirements
    □ Local Climate
    □ Sound and Vibration

    System criteria considerations include:

    □ Capacity Requirements
    □ Redundancy
    □ Maintenance Management
    □ Spatial Requirements
    □ First Cost
    □ Operating Cost
    □ Maintenance Cost
    □ Reliability
    □ Flexibility
    □ Level of Control
    □ Constructability
    □ Commissioning
    □ Measurement & Verification

    Consideration must also be given to how each of the above criteria affects each other. The relative importance of these factors varies by project and owner.

    Look into HVAC system issues that may affect selection such as:
    □ Existing conditions
    □ Maintaining occupancy during the upgrade or retrofit
    □ Budget
    □ Schedule
    □ Ability to phase system installation
    □ Equipment availability

    Detailed quantitative evaluation of all alternatives is not possible. Common sense, historical information, and subjective experience are used to narrow down to a few options. For example, no matter how efficient and economical a system is to install and operate, it can only be considered if it maintains the desired building space environment within acceptable tolerances under all conditions and activities and physically fits into the building without being objectionable.

    Example - Selecting an HVAC system for my home:

    My family just moved into our dream home (based on location and layout) in May of this year. It is a 30 year old, 2000 sf bungalow built above a seasonally vented crawlspace (with no vapor barrier between the home and crawlspace). The house is heated with electric baseboards (1.5 kw units). There is one wall mounted air conditioner (1-Ton). There is no ventilation system and no humidifier or de-humidifier. Actually there was a heat recovery ventilator (HRV) surreptitiously hiding in the crawlspace, ducted in such a way that it pumped crawlspace air into the home (shortly after moving in I shut down the HRV and installed temporary fans to exhaust the crawlspace)! I suspect the HRV was an attempt at mitigating the musty smell emanating from the ground… the crawlspace may become a separate post later.

    My wife and I decided that we would invest in a new HVAC system that supported our family’s health and safety. For us, temperature control, adequate ventilation, moisture control, and reliability are the prime design goals, therefore choosing between energy-efficient-source technologies such as ground-source heat pumps, passive solar systems, etc are secondary to the choice of how the HVAC functions will be fulfilled.

    We also decided to abandon the aging electric baseboard system. Visibly the baseboards are not attractive and make furniture placement a challenge. They are also noisy as the metal snaps due to expansion. The bi-metal strip line voltage thermostats are also dated and allow large space temperature swings.

    We are considering the typical single-zone natural gas furnace coupled with a split air-conditioning unit. This system separates the heating and cooling production, and heat rejection functions with individual pieces of equipment. The heated and cooled air that is generated is distributed via shared ductwork through the home. We would also fit the unit with a filter that removes airborne particles. A separate humidifier and de-humidifier can be included as separate function-specific devices. From my experience, the lack of humidity control is a major source of concern in climates with humid summer weather. A conventional air-conditioner by itself only provides a limited and random amount of de-humidification if and when the cooling operates long enough to allow water to collect, build up on coils and drain down and away. A separate central de-humidifier may be the best choice to ensure adequate moisture removal. To provide ventilation, a heat recovery ventilator can be added to the furnace ductwork, again as a function specific device.

    It is likely true that heating and cooling functions would ideally be delivered to the space via radiant floors and ceilings, as these can provide stable temperature control with no moving parts in the space, with the lowest energy penalty and sound level. However, in a reasonably insulated and air-tight home, the benefit of radiant conditioning is significantly reduced as the loads are relatively small and air-based delivery becomes a practical means of approaching optimal performance at a fraction of the cost of a radiant system.

    We are confident that a natural gas furnace, split AC, filter, humidifier, de-humidifier and HRV combination will address our goals and will be energy efficient, easy to control and trouble shoot and ultimately increase the value of our home.

    Your thoughts and ideas are welcome. Do you agree this is the best selection? What else should I consider? Be brutal in your evaluation.

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