High performance homes require better HVAC design strategies than code level homes. Builders frequently find that the high performance HVAC learning curve can be challenging. HVAC contractors who work with high performance builders can’t use the same strategies that work for mainstream builders. Here are a few design concepts and strategies to help builders better understand HVAC design for high performance homes.
Historically, many homes in the Pacific Northwest have only had heating – this is changing. In the Pacific Northwest we live in a heating dominant climate. We design our HVAC systems to meet the heating needs of a building first. Once we know that the system will provide enough heat, we look to meet the air conditioning needs of a building second. Other parts of the country where they are in a cooling dominant climate go at the heating and cooling load calculation from the other direction. Mini split systems provide both heating and cooling in one energy efficient system.
High performance homes have significantly lower heating and cooling requirements than code level homes. A 1,500 square foot home in Bend, OR might have a heating load of approximately 35,000 Btu if it is built to current codes. The same home built to EPS level 5 might have a heating load of approximately 26,000 Btu. If this same home were built to the Passive House level the heating load might be below 15,000 Btu.
Mini split systems are a great solution for high performance homes with low heating and cooling needs. They are energy efficient and come in a range of capacities to match any size home. Mini split systems have a variety of ductless and ducted indoor units to serve any space. They can combine multiple indoor units on one outdoor compressor system. Many builders and HVAC contractors are not up to speed with all the indoor units for mini split systems that are now available. Mitsubishi now offers a one-way cassette that fits between standard joists and is flush mounted on the ceiling when homeowners/builders want to blend the HVAC design aesthetic into a room and slightly conceal it.
The first step in designing an HVAC system is doing a room by room heating and cooling load calculation. This is referred to as a “manual J” room by room load calculation. This is generally done by the HVAC contractor after the builder has given them the envelope details and a set of plans. Load calculations may also be done by mechanical engineers. HVAC contractors and mechanical engineers use software tools such as Wright Soft, Adtek, or Spec Pro to do these HVAC calculations.
The next step in the HVAC design process is selecting the right size system (Manual S). Undersized systems will not provide enough heating to provide comfort on the cold winter days. Oversized systems increase costs needlessly and will not perform properly due to short cycling.
If you are using ducted mini split indoor units (commonly referred to as horizontal ducted systems) with compact duct systems, you need to do proper duct design. Duct design is done by doing a Manual D (duct design for static pressure and cfm flow) and Manual T (termination design for duct grills) calculation. Duct design can be challenging and is often where comfort problems arise.
In high performance homes, all ductwork should be inside of conditioned space. 2018 IECC will allow ductwork to be buried in insulation. Any time ductwork is run outside of conditioned space (including buried ducts), there is an energy penalty and the risk of condensation in/around the ductwork increases. I have seen homes where builders/HVAC contractors put a wall hung head in every bedroom without any consideration of the actual heating and cooling load in each bedroom. This is likely to increase system costs and lower system performance.
Here is an example of how to solve the low load challenge in the bedrooms of a high performance home:
A Net Zero (DOE Zero Energy Ready certified plus solar system) home built by Greenstone Homes in the Spokane, Washington area has a total heating load of 21,749 Btu (for more information on the Greenstone Net Zero Demonstration Home click here). Bedroom 2 is a corner with two exterior wall surfaces and has a heating load of 3,249 Btu. Bedroom 3 is an interior room with a short exterior wall and a heating load of 1,709 Btu. Neither of these bedrooms needs much heating and cooling. In order to right size the heating and cooling delivered to both rooms the builder and HVAC contractor decided to use a wall hung head in bedroom 2 with a transfer fan to share the heating and cooling with bedroom 3.
Load calculations are based on the coldest and hottest day of the year and done to ensure that comfort will be provided at the extremes. Most of the year we are not at the extremes, we are somewhere in the middle. This middle territory is referred to as the shoulder season for HVAC design. The ability to go from maximum capacity to a low capacity output is called the “turndown ratio”. Value priced mini split systems frequently do not ramp down to lower capacities and therefore have poor turndown ratios. If a system has a poor turndown ratio, it is more likely to short cycle. Short cycling degrades comfort, energy efficiency, and equipment life.
Oversizing systems is something we see in the field. It indicates a poor job of doing the room by room load calculation or shortcomings in the distribution design. Heating and cooling loads are getting lower and lower as building codes become more stringent. In high performance homes, we see very low heating and cooling loads that require new thinking to ensure the HVAC system is just as high performance as the building envelope.
Earlier mini split systems (including unitary air source heat pump systems) had poor cold climate performance. In the last 10-15 years Mitsubishi and other manufacturers have created air source heat pump technology that is designed to perform efficiently in cold climates. In cold climate areas (most of the Pacific NW) that have design temperatures below 30°F it is important to select a system which will perform efficiently without supplemental heat. Mitsubishi Hyper Heat systems use technology that will provide energy efficient heating in the winter down to -13°F. The Northeast Energy Efficiency Partnership (NEEP) has a cold climate air source heat pump qualified product list that is a great resource to ensure you are selecting a system which will perform well in cold climates. You can find the list here.
Some HVAC contractors focus on commercial HVAC work, some focus on residential remodel projects, some focus on residential new construction for mainstream builders. High performance builders should look for an HVAC contractor who specializes in high performance new construction. Teamwork between the builder and the HVAC contractor in the design process is essential. There are also other allies who can be very helpful in the HVAC design process. Energy raters and HVAC manufacturers want to help high performance builders find better HVAC strategies. Teamwork is important as builders move up the high performance home building curve. In the Greenstone Net Zero Demonstration project teamwork was one of the major factors contributing to the success of the project.
Mitsubishi Cooling and Heating understands that high performance builders and their HVAC contractors need help with new strategies as the heating and cooling loads for residential new construction continue to come down. Over the last few years Mitsubishi has formed the Performance Construction Team to provide this support. The Mitsubishi Performance Construction Team is partnering with groups like Earth Advantage, EEEBA, and Earthcraft to provide the high performance building community with the support they need. You can find more information about the Mitsubishi Performance Construction team here. In the Pacific Northwest you can contact Greg Davenport for assistance with high performance projects. gdavenport@hvac.mea.com or 206.719.2448