HIGH-PERFORMANCE HVAC It’s the summertime and the humidity isn’t fine

IT’S THE SUMMERTIME AND THE

HUMIDITYYIT

ISN’T FINE Last edition, I challenged you to focus on summer humidity DEALING WITH LATENT LOADS control as part of a commitment to ensuring healthy and comfortable Ventilation will always add indoor environments. For context, I noted that the difference between a to the latent load of buildhealthy, comfortable relative humidity of 50% and a risky 65% is just an ings when there is more extra five to seven litres of water dispersed throughout the air in a 2,400 sq. ft. building. humidity in the outside air than the inside air. The dewpoint temperature of comfortable 23°C, 50% RH While we have little control over some potential air is 12°C. To be most helpful to sources of w ater vapour in the air, builders and our clients, we should be conscious HVAC contractors are able to influence two throughout the spring summer and fall sources of water vapour: moisture introduced of the dewpoint temperature of outdoor by natural air leakage and mechanical air. Whenever it is above 12°C, we should ventilation, and occupant activity load. be thinking about dehumidification strategies. The moisture introduced by For example, commonly available ERVs reject 50% to 60% of the moisture differmechanical ventilation is very ence between the incoming fresh air and controllable. We should all be the exhaust air. Thus, ERVs minimize the ready to measure flows and adjust added moisture load due to ventilation, them to match occupant needs, and and they do it in a very efficient way. then choose ventilation strategies such Rather than bringing in moisture laden as Energy Recovery Ventilators (ERVs) to fresh air and then having to run a comreject the summer humidity load. pressor-based dehumidifier or air conditioner to remove the moisture, it can be Of course, when it comes to the highly variable occupant activity load, in my up to four times more efficient to use the ERV core to reduce that latent load due to experience the only control strategy at your disposal is education; creating awareness ventilation before it enters the building. with your clients as to the impact their activities can have on humidity levels.

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 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 Cooke 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.

Potential Removal Strategies

1. Create tighter buildings

Potential Reduction per Day

5 - 10 litres per day from 3 to 1 ACH@50Pa.

2. Use an ERV with proper ventilation rate Power use: 125 watts per litre moisture removed.

3. Use a properly sized AC unit Power use: 660 watts per litre moisture removed.

4. Use a whole building dehumidifier. Power use: 330 watts per litre of moisture removed. 15 - 25 litres per day at 50 L/s (100 CFM) ventilation rate. ERVs remove approx. 50% of the moisture difference between outside and inside air. They can only impact dehumidification of the ventilation air.

25 - 35 litres per day for a 2.5-ton AC unit running for at least 12 hours a day, with proper set up of airflows and refrigerant.

35 - 45 litres per day for commonly available whole house dehumidifiers. Dehumidifiers can work independently of AC or ventilation systems such that they can deal with all types of moisture sources.

DEHUMIDIFICATION IN NON-DUCTED BUILDINGS

Water vapour in air disperses relatively quickly throughout a space. More technically water vapour moves from areas of high vapour pressure to low vapour pressure until the pressure is equalized throughout a conditioned space. These means that even in buildings without central forced air heating or cooling distribution, ducted or non-ducted dehumidifiers can be very helpful.

As you remove water vapour from one part of the space, water vapour from areas of higher moisture levels will be drawn to the area where a lower vapour pressure has just been created. I often hear that in climates with hot, humid summers, radiant floor heating doesn’t make sense because immediately a central forced air-cooling system is needed to manage summer humidity and air conditioning needs. Of course, an option with in-floor heating would be a standalone, ducted or non-ducted dehumidifier and mini-split AC units targeting specific cooling needs throughout the building.

USING AC FOR DEHUMIDIFICATION

The most common approach to dehumidification in homes and workplaces is to rely on the central air conditioning system. This requires the system to be set up such that the discharge air off the cooling coil is cold enough for long enough to condense sufficient moisture out of the air to maintain a balance in the building. Knowing the target dewpoint of the conditioned space is 12°C, verifying a discharge air temperature off the AC coil of approximately 10°C is a first simple check to assess dehumidification capacity. Beyond knowing whether the air is cold enough, we also need to ensure that the system runs long enough – long enough for moisture to condense on the coil, accumulate into beads of water heavy enough to drain off the face and collect into drops big enough to run out of drain pan and into the drain. In a typical residential or small commercial application, the system has to run for at least six minutes before any water drains out of the pan. Moreover, when the cooling call ends, any moisture still on the coil face or in the drain pan will get re-evaporated into the air stream. The example in the table shows that a 2.5-ton system can remove approximately 25 to 35 litres per day, but only if it is running for at least 12 hours. In the U.S. southeast, they employ two-stage air conditioners, with stage one triggered by a humidity sensor as a way to increase run times. They also program fan operation to shut off for a few minutes at the end of a cooling cycle to encourage better drainage of water off the coil and down the drain. In a Canadian context, however, relying on large central air conditioners to provide ever more important humidity control is not particularly effective or efficient. The real opportunity for HVAC contractors is to apply whole-home or building dehumidification equipment. There are great new pieces of equipment available, with capacities starting at 15 litres per day and as high as 100 litres per day.