THVAC Optimization: Bigger Isn't Always Better
By Andrew Oding
Over the last 15 years, energy use in new homes has been reduced by nearly 40 per cent.. Voluntary, market-driven programs like EnergyStar, Built Green and R-2000 have provided builders with the technology and construction practices needed to build more comfortable, healthy and efficient homes.
Today, we have building codes that require increased insulation values, mechanical efficiencies and air tightness. In many provinces, a new home built today delivers the energy performance of R-2000 homes built in the early 2000s.
One very direct consequence of these changes is that heating and cooling loads have dropped substantially in new homes across Canada. However, the national standard for appropriately evaluating the heat loss/heat gain of a Part 9 building (CSA F280-M90) has remained largely unchanged for nearly 20 years!
This can, and does, cause problems for builders. An unintended consequence of using the old CSAF280-M90 standard when designing HVAC systems for today's new homes is chronic over-sizing of heating and cooling equipment, as well as over-sizing of air duct delivery systems.
Within the industry it's well known that oversized systems in new energy efficient homes often result in comfort issues for occupants and can lead to homebuyer complaints and warranty claims. Somewhat like putting shoes on a child that are too big, oversized HVAC systems result in homes that run "sloppily" and inefficiently.
Depending on a home's layout, short cycling of HVAC equipment can lead to cold rooms on the second floor, and in rooms over garages with exposed floors. During the cooling season, the problems change, but include poor thermal circulation and inadequate dehumidification.
Short cycling also results in decreased mechanical efficiency, and compromises the performance capacity of today's more efficient heating and cooling equipment.
These problems can be avoided by the use of the new CSA F280-2012 standard. Some designers/contractors will also recognize potential cost savings while optimizing system designs under the new standard.
The new CAN/CSA F280-2012 Standard Determining the Required Capacity of Residential Space Heating and Cooling Appliances is a freshly updated standard. It provides a tremendous opportunity for homebuilders and the HVAC industry to rationalize new mechanical system design.
Here is a brief synopsis of the more critical changes.
The calculation method can now accept objective airtightness indicators such as blower door air tightness tests. This will be important in both new and existing homes where energy audits or specific air tightness targets have been verified by site testing.
The interaction between different types of ventilation systems and air leakage is accounted for. For example, a home with an exhaust-only ventilation system creates a slight negative pressure that changes the leakage patterns in a home and the new standard makes allowance for this.
In the old Standard the total heat loss for the building was assigned to individual rooms as a function of the heat loss of that room. In the new Standard, recognition of the stack effect (warm air rising) in a home will mean that the assignment of air leakage heat loss will be a function of the floor level of specific rooms. In other words, rooms on the first floor of a home will be assigned a greater portion of the air leakage component.
The U factors and solar heat gain coefficients reported by window manufacturers in their CSA A440 compliant labeling can be directly put into the calculations now. This is important given the increase in window/wall ratios and the resulting heat gain load on air conditioners.
Finally, the new Standard will allow designers to take credit for the impact of heat recovery ventilation devices employed in a home.
The new Standard will result in more accurate and potentially lower load calculations given the efficiency changes in new homes.
In applying the new Standard, designers and mechanical contractors will need to recalibrate old "rules of thumb" for sizing equipment in today's new homes:
Equipment with optimized controls could be more compact, programmed to operate at peak efficiency over longer cycle times and make use of newer and extremely efficient fan motors and pump controls.
Smaller loads with traditional forced air systems require reduced fan capacity. The delivery systems (i.e. air duct design) smaller sized ducts can deliver optimal comfort to every room.
The new Standard is formally recognized by the Ontario Building Code as of January 1, 2015, and is expected to be referenced in the National Building Code of Canada in the near future.
In the last two years, the new Standard has been applied on multiple Net Zero projects across Canada with great success. Occupants of these Net Zero houses are raving about the "comfort" of their homes. Besides being quieter, and often smaller, "right sized" equipment delivers ambient temperatures which are nearly identical on every floor and in every room. The mechanical systems also operate at peak efficiency further reducing the cost of operation.
The new CSA F280 Standard results in more accurate and potentially lower load calculations that reflect the efficiency improvements in today's new homes.
The table illustrates the results of applying both the old and new standard to a reference home assumed to have an HRV air tightness of approximately 2.5ACH50, January design temperature of -20°C, and summer design temperature of 30°C.
Andrew Oding is the Senior Building Science Associate at Building Knowledge Canada. He is Chair of the Ontario Homebuilders OHBA /EnerQuality Technical Committee, 2013 Chair of Natural Resources Canada’s ENERGY STAR for New Homes Technical Committee, and Chair of the new CHBA 2014 Net Zero Energy Housing Council. He may be reached at firstname.lastname@example.org.