HIGH-PERFORMANCE HVAC
IT’S THE SUMMERTIME AND THE
HUMIDITY I IT T Y ISN’T FINE Last edition, I challenged you to focus on summer humidity control as part of a commitment to ensuring healthy and comfortable indoor environments. For context, I noted that the difference between a healthy, comfortable relative humidity of 50% and a risky 65% is just an extra five to seven litres of water dispersed throughout the air in a 2,400 sq. ft. building. While we have little control over some potential sources of water vapour in the air, builders and HVAC contractors are able to influence two sources of water vapour: moisture introduced by natural air leakage and mechanical ventilation, and occupant activity load. The moisture introduced by mechanical ventilation is very controllable. We should all be ready to measure flows and adjust them to match occupant needs, and then choose ventilation strategies such as Energy Recovery Ventilators (ERVs) to reject the summer humidity load. Of course, when it comes to the highly variable occupant activity load, in my experience the only control strategy at your disposal is education; creating awareness with your clients as to the impact their activities can have on humidity levels.
DEALING WITH LATENT LOADS
Ventilation will always add to the latent load of buildings when there is more humidity in the outside air than the inside air. The dewpoint temperature of comfortable 23°C, 50% RH air is 12°C. To be most helpful to our clients, we should be conscious throughout the spring summer and fall of the dewpoint temperature of outdoor air. Whenever it is above 12°C, we should be thinking about dehumidification strategies. For example, commonly available ERVs reject 50% to 60% of the moisture difference between the incoming fresh air and the exhaust air. Thus, ERVs minimize the added moisture load due to ventilation, and they do it in a very efficient way. Rather than bringing in moisture laden fresh air and then having to run a compressor-based dehumidifier or air conditioner to remove the moisture, it can be up to four times more efficient to use the ERV core to reduce that latent load due to ventilation before it enters the building.
CREATING A STRATEGY Knowing moisture sources and their relative impact, it is possible to target strategies to achieve a healthy moisture balance. Table 1 shows potential moisture removal strategies and the impact they might have on our theoretical 2,400 sq. ft. building. The first one, making tighter buildings should be everyone’s goal and, moreover, mechanical contractors who encourage or
Gord Cooke
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ask for air tightness testing can do a better job of matching equipment to specific buildings. The table also shows the value of employing energy recovery ventilation technology for the fresh air requirements of all buildings. It should be noted that in the coming months and years there will be a focus on increasing fresh air rates in all buildings to ensure the healthiest possible air for occupants, so we need to apply the right technologies.
Gord is a professional engineer who has spent 20 years helping builders and HVAC contractors implement innovative technologies into high-performance homes. He has particular expertise in IAQ and airflow management in houses, and can be contacted at gcooke@airsolutions.ca.
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