By Gord Cooke
Last summer I spent an inordinate amount of time in Florida and South Carolina helping builders and mechanical contractors with high indoor humidity problems.
Of course, winter, not summer, is the right time of year for a middle-aged Canadian to be in Florida, and it is usually in winter that I am diagnosing indoor humidity problems in Canada. However, there have been important changes in houses in Florida that are impacting indoor moisture control the summer months, and we should take note to avoid similar problems here in Canada.
As you make your way into this year’s air conditioning season, it is best to be mindful that true comfort comes from addressing both temperature and humidity.
Speaking with homeowners about the two separate functions of their AC systems – lowering temperatures and lower humidity – may help you lead into other opportunities, such as switching to an energy recovery ventilation system, and ultimately to a whole-house dehumidification system.
To put a fine point on those last two items. If a household is currently ventilating with an HRV or a simple exhaust-only ventilation system, installing an ERV would reduce moisture loads by 10 to 15 litres a day on the highest humidity days at very low power consumption.
A whole house dehumidifier can remove in the order of 40 litres of moisture from the air each day, at a very efficient three litres per kWh of operation.
The difference between a nice comfortable 23°C, 50% RH indoor condition and a muggy 65% RH is just an extra three to four litres of water dispersed into the air of a typical 2,300 sq.-ft., two-storey home.
And a properly sized AC system with correctly adjusted airflows and refrigerant flows needs to operate for at least 15 minutes, and as long as 30 minutes, before any moisture will drip off the coil and down the drain line. Of course, if the fan is left on during the off-cycle, the moisture left on the coil will be re-evaporated into the air in about 30 minutes.
Feeling the challenge?
Newer homes typically have more and bigger windows than older homes, so now 40 to 60 per cent of the AC loads experienced are associated with the heat gain through windows.
This results in a high peak, but intermittent sensible load, which is what we use to size the capacity of the system. However, the base latent or humidity load is fairly uniform throughout the day and night (unless the occupancy loads vary greatly throughout the day).
Let me reiterate that; we size for a peak sensible load, but a significant element of comfort success and risk management is based on a fairly uniform base latent load.
We want to meet the expectations of our clients for a reasonable response time during hot, sunny periods, but there are pitfalls with respect to proper moisture control when oversizing AC systems. Fortunately, there are measures and opportunities for success in achieving a comfort balance.
Low E coatings on windows became a code requirement for new homes in Ontario in 2009, and energy efficiency programs across the country have encouraged Energy Star windows with Low E coatings since the mid-2000s.
Low E coatings vary significantly by manufacturer, however, so it is possible to meet Energy Star requirements with coatings that result in a solar heat gain of anywhere between 0.2 and 0.6. That is a threefold variation in solar gain through the window.
It is possible to purchase a simple detector that when placed on a piece of glass will indicate the solar gain characteristics of the window.
An optimized system should be able to remove about 1.5 litres of water per hour, per ton of AC capacity. You could, quite literally, turn on the system on a hot humid day, wait a half-hour for the condensate to start flowing, and measure the amount produced over the next hour. A three-ton system should nearly fill a five litre pail.
If you aren’t getting proper moisture removal it would be an opportunity to adjust the airflow over the indoor coil. This typically means lowering the air handler fan speed to lower the supply plenum temperature, without compromising the overall operation of the system.
It’s best to refer to the AC manufacturer’s pressure/temperature charts and measure the suction line pressure, but a quick check can be done by measuring the dewpoint temperature of the air in the supply plenum or, easier yet, at the first supply register.
If you are trying to achieve an indoor condition of 23°C and 50% RH, the dewpoint temperature of that air is 12°C, so your target for the supply air should be two or three degrees below this.
Turn the system on, let it run for around 20 minutes, and measure the dewpoint temperature of the air at the supply register nearest the air handler. If it is not approximately 10°C, then turn down the fan speed until it gets there. After adjusting the flow, be sure to check the manufacturer’s specifications to be sure the system is in an approved operating range.
Lowering the supply airflow may not be desirable during all operating periods. Fortunately, many new thermostat controls andAC systems allow for a ”humidity” cycle.
For peak cooling times, the system runs at full airflow, but when the thermostat is satisfied for temperature and the RH is higher than desired, the system would come on at that optimized (lower) airflow rate to more effectively dehumidify the space.
Alternatively, at very low cost you could adjust the thermostat setpoint dead ban zones so you get longer run times. By switching to Fan Auto mode, or setting the fan operation to turn off for 15 minutes after an AC cycle, the evaporator coil can drain and not re-entrain the moisture back into the air stream.