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November 15, 2018
The more you promote the high-performance features of your homes, the higher the expectations of your customers will be. In my world, those expectations for thermal home comform are dramatically increased. So much so that in our Construction Instruction learning centre in Phoenix builders have become very interested in the instrumented total HVAC system that is able to demonstrate strategies for better home comfort control. Fortunately, in the opinion of this engineer, there are science-based resources and a variety of products, new and old, that are available to help you achieve the expectations of your home buyers. Let's explore a few of these.
The first step is to become familiar with the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) Standard 55 Thermal Environmental Conditions for Human Occupancy, or at least be aware of the existence and the value it can offer you as we implement better HVAC solutions. I found Standard 55 very valuable as it outlines the six factors that affect thermal comfort; two are characteristics of your homebuyers - clothing and metabolic rate but four are characteristics of the environments your build. These are air temperature, humidity, air speed and radiant temperatures. That last one probably needs a little explanation. Our bodies transfer approximately 50% to 60% of its sensible heat via radiation to surfaces around us. Realizing that modern home designs include dramatically more windows, you can imagine the impact this can have on the comfort of your homebuyers. Fortunately, there are some strategies available to address the radiant temperature factor.
Start with a conversation with your window supplier. They should have at least 3 or 4 glazing options available for you that can meet the code requirements for window performance while minimizing the impact of cold glass surfaces in winter and hot glass surfaces in summer. For comfort reasons alone, triple glazed windows will be on your list of technologies to include in your high-performance homes. The costs have been coming down nicely and they perform very well in energy performance modeling to offset other more expensive energy upgrades. Even with double glazed windows, investigate the impact between high solar gain and low solar gain Low E coatings. Some window manufacturers use high solar gain coatings to optimize winter energy gains at the risk of creating comfort concerns in the summer. Have your energy evaluator work with your window supplier to find a better balance between energy and comfort performance.
Recognize as well that the solar gain from windows is a highly variable or intermittent load. It's no longer good enough to measure and try to control temperatures from one thermostat located in a hallway. Great new, cost-effective technology, such as the EcoBee4 thermostat, offers specific remote room sensors for the main thermostat to allow quicker response in occupied spaces. Ultimately you will be offering zoned heated and cooling solutions, but immediately start offering these multi-sensor approaches to help offset the comfort implications of intermittent window loads.
We have covered one strategy for the radiant temperature comfort factor and one for the air temperature comfort factor, allow me to remind you of a simple change to impact the humidity comfort factor. You have undoubtedly seen the literature that says a healthy, comfortable indoor relative humidity level is 30% to 50%. We need to fine tune that a bit to read 30% to 40% in winter, recognizing that at over 40% window condensation would be an issue on cold winter days (another great reason for triple glazed windows). Then 45% to 50% in summer, recognizing that getting lower than even 50% is difficult in most parts of Ontario with our high outdoor humidity levels. In fact, through much of the spring and early summer dehumidification is at least as important as cooling. So much so that credible information sources such as the Canada Mortgage and Housing, suggest strongly that dehumidifiers or air conditioners be used to control moisture during non-heating seasons in most Canadian houses. In addition, you will find that Energy Recovery Ventilators (ERVs) are very useful in helping to manage moisture loads.
Let's start with a comfortable indoor condition of 23 0C and an RH of 50%. The Dew Point at this condition is 12 0C; the higher the Dew Point, the higher the moisture content. Compare this to the outdoor design conditions for Toronto. When considering humidity factors, we won't use the hottest day design condition, but rather the most humid design condition. In Toronto, on that day the Dew Point temperature of the air is a whopping 22 0C and yet the simple dry bulb temperature (what the thermometer would read) would be just 27 0C. These are days when an air conditioner wouldn't be running very much and yet there would a need for dehumidification as a separate function to control indoor RH levels. As you ventilate at say 60 cubic feet per minute (CFM), the typical code requirement for a 3 bedroom house, with a Principal Fan or HRV the ventilation system will introduce an additional 24 litres of water vapour into a home per day. Using an ERV rather than an HRV would cut this design day moisture load in half. This can be very important given that in a typical 2400 sq.ft. home the difference between a comfortable and healthy 50% at 23 0C and a sticky, risky 65% RH condition is just an extra 3-4 litres of water dispersed throughout the air in a home. While you should be reminding homeowners to run a dehumidifier, the standard portable dehumidifiers only have a moisture removal capacity of 15 to 30 litres per day and even the whole house dehumidifiers that professional HVAC contractors promote have capacities of about 30 to 60 litres per day. It should be obvious that a simple, cost-effective change to an ERV from an HRV is an important comfort strategy ERVs help manage dehumidification loads in non-heating seasons and avoid over-drying houses in winter.
Three simple, cost-effective strategies to get you started on the path to providing better comfort controls for your homebuyers:
For more information on the Ci Live Learning Center in Phoenix, go to www.constructioninstruction. com