Renovation & Design

Heat-recovery ventilator should handle steam-shower moisture

QUESTION: I just purchased a 10-year-old house that has a Lennox HRV unit. I want to convert a bathroom tub and add a steam shower. Is the HRV sufficient for the steam or do I require an exhaust fan? If so, the problem is that it is in the fully finished basement.

— Donna Bradley Katz

Answer: The heat-recovery ventilator (HRV) installed in your home should be sufficient to remove the steam from the new shower, as long as the system is designed and installed correctly. This should be easily evaluated by a licensed HVAC technician, who may be able to make modifications, if required, to ensure it works effectively.

HRVs are designed to provide proper ventilation for new homes, as well as reducing relative humidity (RH), by exhausting damp air from several areas in the home and replacing it with dryer outside air. The main areas of moisture generation are the bathrooms, due to water vapour created primarily from bathing and showering. If the system is designed and set up properly, a timed switch in the bathroom should be manually engaged that will run the unit on high speed, when required. By activating the switch before you use the retro-fitted steam shower, it should draw the saturated air in the bathroom into the register mounted on the wall or ceiling in the bathroom. This will move the moist air through the ducting to the HRV unit, where it will be passed through the heat-recovery core, before venting to the exterior.

The HRV should already be adequately sized for your home, and the only concern with additional moisture generation from the steam shower would be that the unit runs for a longer period of time to accommodate it. It may only take a few minutes to clear the damp air from your current bathroom after a shower.

The damp air will likely increase significantly with the new steam-generating unit. Since most bathroom HRV controls are timed to run for a minimum duration before automatically shutting off, you may only have to check to ensure your control time is adequate. In fact, many of these controls have multiple choices for run times, so setting it at a longer run time may be all that is required to properly dissipate the additional water vapour.

One thing to ensure is that the HRV unit is properly balanced, no matter what your decision is with the steam shower. This technical adjustment needs to be done by a trained professional, with specialized equipment.

Balancing is primarily ensuring that when the unit is running, the same amount of air enters the home as is being exhausted. This can be adjusted by use of a balancing control, which will ensure your home is not placed under negative or positive air pressure due to the operation of the HRV. An HVAC technician who installs HRVs should have the testing equipment available, but requesting this specialized service should be done when booking an appointment.

The other item to check is the dehumidistat function on the main HRV control, normally located near the thermostat on the main floor. This control will have a dial or other method of setting the desired RH in the home, before the HRV unit engages. This should only be used in the heating season, but should also help prevent moisture-related issues with the new steam unit.

If you set this control to a normal winter setting, typically around 30 per cent RH at room temperature, it will engage the HRV if the indoor humidity gets too high. This may help if the steam shower is raising the RH in the home, even temporarily, when in use. Also, using this in the winter may help prevent condensation on windows and other basement cold areas, preventing mould growth.

The last thing to be aware of is that the HRV will not only help control air moisture levels in the bathrooms, it will work for the entire home, if the dehumidistat is functioning and used. Since there are additional registers near the kitchen, possibly the laundry room, and all the bathrooms, damp air will be removed from all these areas whenever the unit turns on.

So, if you have a small amount of residual moisture or condensation in the basement bathroom well after showering is complete, this may be removed when a control is activated in another area. This may be done with a manual control in another bathroom, or the main control through the dehumidistat function. This is why there is a register near, but not normally inside, the kitchen.

If the range hood over the stove is not successful in removing all the moisture from cooking or sink use, the HRV should help when the RH on the main floor exceeds the setting.

Additional moisture control in your basement bathroom should be no problem for your existing HRV unit, as long as all the controls are functioning properly and used. Calling a licensed HVAC contractor to check it all out should ensure it prevents excessive condensation and mould growth in the home due to the renovated bathroom.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at



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