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Renovation & Design

Understanding heating and ventilation systems

Question: I am a homeowner of a relatively new house here in Winnipeg and I googled about the HRV system and I checked on an article you wrote in the Winnipeg Free Press in 2017. With this, I would like to ask for an advice with regards to HRV settings during summer and winter.

Please let me know how to set it during the summer and also how to set it correctly in the winter. For your information, the builder checked our house just after possession and he set the thermostat to 19 degrees, the system to cool, and the fan setting to auto. The dehumidistat, which the installer set to 80% from the date installed, was changed by the builder yesterday to 50 per cent. The builder also told us to keep the HRV turned on for a year after we originally took possession. After the builder’s settings, the system has been operating continuously and cool air is coming in from the outside. Should I leave it that way or can I somehow program the system to pull air inside intermittently?

Thank you and will wait for your expert advice,

F. Salon.

Answer: Setting your heat recovery ventilator (HRV) main control should be a simple procedure, as long as the dehumidistat control is engaged and the unit is not set to run in a continuous manner when the power is turned on. Turning the main control off during the non-heating seasons should be done and an average setting of 30 per cent RH on the dehumidistat should be employed when the furnace is in use.

This may seem like an unusual time of year to be talking about HRV settings, with air conditioners still in use due to high outdoor temperatures and humidity. Unfortunately, I still find that several clients of mine who move into newly built homes have issues with their HRVs. Some of this stems from a simple lack of understanding for this system, like in your question, and others are from units that are not properly set up for use in our climate. Even more concerning are some newer controls that seem overly complicated, or appear to have timed cycles which override the dehumidistat’s function. In many cases, the units have been initially set, at the request of the builder, to run continuously to help dry out the building materials near the end of construction, normally during the heating season. In that scenario, if the controls are not reconfigured to allow the dehumidistat to function properly, by the HVAC technician, they will not be able to be correctly set for use by the homeowner.

If the controls are fully functional, then my recommendations for setting of the HRV unit are very straightforward and easy to follow. For the non-heating season, meaning any time the thermostat is not set to heat mode, shut the HRV main control off. This may be done by switching the main control power button off, on some units, or turning the dehumidistat dial or electronic control to the highest setting. At that time of year, you may have some or all of your windows open, which will be natural ventilation, negating the need for the mechanized system to operate. While there may be some debate over this advice, following this will simplify your HRV use and uncomplicate the entire discussion. If you wish to turn the unit on during these seasons you may quickly and easily do so by engaging the timed controls in one of the bathrooms. That will do the same thing as manually switching on the unit from the main control, but will automatically turn it of when the time setting is exceeded.

The rationale for non-use of the HRV in the summer, and portions of the shoulder seasons, can be explained by understanding how the system is intended to work. When running, the HRV brings in fresh outside air into the building and exhausts stale indoor air, at the same time. We need this air exchange to remove indoor air pollutants, produced from our normal living habits, such as CO2, products of cooking, dust, dander, and other undesirable components dissolved in our air. Because our new homes are very airtight, for maximum energy efficiency, we need this mechanical system to accomplish this critical task, but only when natural ventilation is not available. So, when the windows are open, the only areas that need additional ventilation are the bathrooms and kitchen. You should have a range hood in the kitchen for this purpose, to remove unwanted odours and other biproducts of cooking. In the bathrooms, the manual HRV control should be used when taking a shower or bath, or to remove unwanted odours.

The other associated function of the HRV is to remove excessive moisture from our homes, which is critical when we shut the windows in colder weather. This is a necessary requirement to prevent high relative humidity (RH) in our homes, which can lead to mould growth and moisture damage to components. For this to work properly, the amount of moisture in the outside air must be lower than the indoor air, when the unit is operating. Since cold air can hold only a small percentage the water vapour that warm air can, bringing in warm, humid summer air to replace the cooler indoor air is senseless. With the exception of the moisture from a hot shower, which is normally at or above 100 percent RH, this should not be done. In the heating season, the outside air is much colder than the indoor air, so it’s RH will drop significantly when drawn inside and warmed up by the heating system, lowering the overall RH of the indoor air.

Setting your dehumidistat at 30 per cent, plus or minus five percentage points dependent on outside temperature, should maintain a healthy indoor air environment during the heating seasons. When the ambient temperature is a little warmer, increasing the setting above 30 should be warranted and below that mark in the very coldest winter weather. Turning the power off at the same time you start to open your windows in the spring, and back on with the heat setting, is also the right move.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

trainedeye@iname.com

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