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Duct heater required with an HRV in a cold climate??

user-1021589 | Posted in Mechanicals on

I’m in the beginning stages of building a super-insulated house near Ottawa, Canada. The foundation is completed and we’re about to start framing. One of the last details / sub-contractsI need to tie up is the HRV system. One of the firms providing a quote insists a 1500-2000w duct heater is required.

I recognize that the fresh air is likely to be somewhat cooler than the temperature of the house. However, this suggestions a little over-the-top given that the house will have R50 walls, R80 attic, tripled-glazed fibreglass windows and will be tight at less than 1.0 ACH50.

I’d appreciate suggestions.


John Scime
Lanark County, Ontario

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  1. user-869687 | | #1

    I suppose the question is how cold the incoming air could be. Take an anticipated delta-T, multiply by the heat recovery efficiency, and estimate the incoming air temperature. For example if the indoor temp is 68° and the outdoor is 18° then the delta-T is 50°. If the HRV is 76% efficient, you keep 38° of that delta-T, so incoming air would be around 56° (that is, assuming my math is not way off here, and it wouldn't be the first time.) Is that too chilly?

  2. GBA Editor
    Martin Holladay | | #2

    In your climate, an HRV or an ERV needs controls that handle low outdoor temperatures to prevent the heat-exchanger core from icing up and to maintain comfort. Every HRV or ERV that I know of includes such controls. Usually, manufacturers include a temperature sensor that will shut down the HRV periodically in cold weather to allow the core to warm up; some manufacturers circulate interior air through the core periodically to keep the core warm. These controls work, and no additional electric resistance heater is needed.

    In general, the need for mechanical ventilation decreases slightly in very cold weather, because the stack effect increases. Homes generally leak more air in very cold weather than in mild weather.

    So, don't install the heater -- unless there is an unusual technical reason for continuous mechanical ventilation in your home.

  3. user-1021589 | | #3

    Thank you very much for the information. I will not install the duct heater. The unit is a Venmar EKO 1.5 ECM HRV, so it has all the controls I will need, as well as good efficiency.



  4. gingerenergy111 | | #4

    Just happen to stumble upon this post as i was searching for an electric heater model for a VanEE Vigor Plus for a customer of mine in Ottawa.
    To be clear, the defrost control mentioned above is in most Canadian distributed units, so its not a worry. BUT the point of a post-HRV heater is to temper what may be very cold air coming into the home. It doesn't matter what your house insulation is, you are bringing fresh air directly from outside, via the HRV. If you look at the Apparent Sensible Effectiveness in the HRV specs, you will find the number you need to determine what the incoming air temp will be at your outdoor design temperature. This is not to be confused with the Sensible Recovery Effectiveness. It should also be at your outdoor design temp and at least 60 cfm.
    For some HRV's such as the Vigor Plus, in Ottawa, the cold air temp is about 6C after the HRV. Anything under 12C is a risk in terms of condensation inside the home, which is why location of the HRV is best on an outside wall, so that you have a short fresh air run to insulate and less risk of damage to the vapour barrier outside the insulation for that short distance.
    For a short run to the furnace, on a simplified HRV connection, and assuming you deliver the air back at least 6 feet from the furnace, you will get a mixed air temp (you have to calculate this) that may be ok for the furnace (ie 15C or better) and then the furnace becomes your "post heater". BUT in a Fully-Ducted HRV - with no furnace, or in an EnergyStar home, for example, you MUST consider and calculate for a post heater. For both the purpose of tempering the air to reduce condensation potential as the air is distributed to bedrooms and living spaces, and for occupant comfort.
    There are a few HRV models with really high ASE - above 89% that would negate the need for a post heater. These are my "go-to" units, however, sometimes it is more affordable to use a smaller HRV and a heater.
    I hope this clears up some of the misconceptions around HRV heaters.

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