Quick answer
Yes, run your HRV in summer, but on low or an intermittent setting such as 20 minutes on and 40 off, and turn the dehumidistat on the wall control to OFF. An HRV recovers heat, not moisture, so every hour it runs brings humid outdoor air inside. Let the air conditioner remove that moisture, and keep indoor humidity below 60%.
You should run your HRV in summer if the house is newer and airtight, because the reasons it was installed do not take the season off: cooking, showers, cleaning products and people keep loading the indoor air. What changes is the outdoor air. From June to September in southern Ontario it is often wetter than the air inside, and a heat recovery ventilator has no way to keep that moisture out. The summer rule is less ventilation, on a timer, with the air conditioner doing the drying.
Why an HRV makes a house more humid in July
An HRV core passes heat between two airstreams through thin plates, and the streams never mix, so no water vapour crosses. In winter that works in your favour: dry outdoor air comes in and damp indoor air goes out. In summer the trade reverses. Incoming air is cooled a little by the outgoing air, but it arrives carrying all of its moisture, and the air conditioner then has to condense that water on its coil.
The result of too much summer ventilation is longer cooling cycles, a sticky main floor and a damp lower level; why a basement gets humid in summer covers what that looks like downstairs.
The summer HRV settings: low speed, 20/40 and dehumidistat off
- Speed: the lowest continuous setting or an intermittent mode. High speed is for a shower or a burnt dinner, not for all day.
- 20 on / 40 off: the unit ventilates for 20 minutes of each hour and rests, or recirculates on some models, for the other 40. That cuts the outdoor moisture it brings in by two thirds compared with running continuously at the same speed.
- Dehumidistat: OFF, or the highest number on the dial if there is no OFF. That dial is a winter feature. It forces the unit to high speed whenever indoor humidity is above the setting, on the assumption that outdoor air is drier, and in summer that assumption is wrong.
- Bathroom timers: keep using the 20-minute push buttons after showers. In many newer Ontario houses the HRV is the only bathroom exhaust.
- Windows open: HRV off. The house is already ventilating, and the fans are only adding running hours.
- October: turn the dehumidistat back on when the furnace season starts, and lower the number as the weather gets colder to keep the windows dry.
What the furnace-fan interlock does in summer
Many Ontario installations connect the HRV to the furnace return duct instead of giving it separate ducts to each room. In that layout the furnace blower has to run whenever the HRV does, or the fresh air has no route to the bedrooms and can loop straight back into the exhaust. An interlock wire between the two makes that happen automatically.
In cooling season the interlock has a side effect: a blower running between cooling cycles evaporates water off the wet coil and returns it to the house. That is one more reason to choose the 20/40 mode over continuous, since it limits how long the blower runs with the compressor off. If your HRV shares the furnace ducts and the blower never starts when the HRV runs alone, the interlock is missing or disconnected and worth having checked.
When to turn the HRV off in summer, and when not to
Use a hygrometer on the main floor as the decision rule. Above 60% with the air conditioner running, cut the HRV back one step; under 50%, you have room to ventilate more.
- Turn it off on wildfire-smoke days and when there is a strong outdoor odour, and read running your AC during wildfire smoke for the filter side of that.
- Turn it down to timers only for a few days of a heat wave if indoor humidity will not come under 60%.
- Do not leave it off all summer in an airtight house built in the 2000s or later. Odours and stale air build up, and bathrooms with no separate fan lose their only exhaust.
- With no air conditioner, nothing in the house removes the moisture an HRV brings in. Run it on bathroom timers and at the cooler, drier ends of the day, and consider a dehumidifier.
- In a 1950s or 60s bungalow or a pre-war brick house with no HRV, none of this applies: those houses leak enough air to ventilate themselves unless a renovation has sealed them up.
Is an ERV better than an HRV for Ontario summers?
For summer humidity, yes. An energy recovery ventilator passes heat and some water vapour between the two airstreams, so part of the incoming moisture is handed to the outgoing air and less of it reaches the air conditioner. In winter the same core keeps some humidity in the house, which suits homes that get too dry and is less suited to homes with wet windows. Our HRV vs ERV comparison goes through that choice.
On some models the manufacturer sells an ERV core that fits the existing cabinet; on others the change means a new unit. Houses close to the lake in Burlington and Oakville, where summer nights stay muggy, are the ones that gain the most from the switch. There is no published price band for this work, so it is quoted in writing after a look at the unit and its ducting.
Wash the filters and clear the outdoor intake screen in June, whatever mode you pick. ZK Mechanical checks the wall control wiring, the furnace interlock and the airflow balance, and sets the unit up for the season while you watch. For a replacement or an ERV upgrade, see HRV and ERV installation. Call (647) 801-1252 or book a ventilation check, or keep reading in the air quality and ducts answers hub.