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Understanding Code Requirements for HRVs

jason_v | Posted in Building Code Questions on

I am having trouble sorting through code requirements for HRV’s.

This is regarding Part 9 – Housing and Small Buildings

Section 9.32. Ventilation

We are planning radiant heated floors, ductless AC and a dedicated HRV system.

Am I to understand that if i have air conditioning that i will require “one-half air change per hour if the room or space is mechanically cooled during the non-heating season”?

If  this is the case then that means that a 125 cfm HRV would suffice for my home during heating season but i will have to upsize to 250 cfm HRV to meet the code requirements for cooling season.

This seems contradictory, any clarification would be greatly appreciated.

Thank you for your time and consideration.

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  1. Trevor_Lambert | | #1

    Not sure exactly where in that section you got that, but no. You have a dedicated HRV system, see table for the required rate. Whether you have A/C or not is not relevant. Having said that, it is never a bad idea to have excess capacity in your ventilation system for boosting during cooking, showering or parties.

    1. jason_v | | #2

      thank you. its directly from

      9.32.2. NON-HEATING-SEASON VENTILATION Required Ventilation
      1) Rooms or spaces in dwelling units shall be ventilated during the non-heating season by
      a) natural ventilation in accordance with Article, or
      b) a mechanical ventilation system conforming to Subsection 9.32.3.
      2) Where a habitable room or space is not provided with natural ventilation as described
      in Clause (1)(a), mechanical ventilation shall be provided to exhaust inside air from, or to
      introduce outside air to, that room or space at the rate of

      a) one-half air change per hour if the room or space is mechanically cooled during the non-
      heating season, or

      b) one air change per hour if it is not mechanically cooled during the non-heating season.

      1. Trevor_Lambert | | #4

        That looks different from what I remember. Note however that the requirement only applies if you don't have natural ventilation (aka a window) in the area, or adjacent area connected by a minimum unobstructed opening per the table below the text. There's no way anyone is installing mechanical ventilation of 1ACH for the whole house. That would be upwards of 400cfm for a modest sized house, up to more than 1000cfm.

        1. jason_v | | #5


  2. Jon_R | | #3

    Note that heating system L/s is a minimum, so I don't see any contradiction. If I did the math right, it's more like a 4:1 ratio between seasons.

    1. jason_v | | #6

      for our house, we come up with 60 L/s for continuous operation (via room by room addition of rates), if we go with 1/2 air exchange per hour the math works out to 250 ish cfm, which is 117 L/s

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