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What is that metal box hanging from my basement ceiling?


In some homes, particularly newer homes, there may be an HRV or ERV installed. HRV’s, otherwise known as Heat Recovery Ventilators or Energy Recovery Ventilators, become a vital appliance to any home, especially in tighter, well insulated and sealed homes.




Homes need fresh air. We need fresh air. It has been found that people spend more time indoors now than ever before. We are also building and renovating to make our homes better insulated and sealed against air leakage. Building products and practices have changed over the years. More attention is given to air/vapour barrier continuity and sealing, and products such as spray foam are used more often and performing extremely well when installed properly.


What is the end result? Many homes now do not have enough fresh air entering the building naturally. In older homes, this is easily attained due to the fact that homes are leakier and fresh air easily enters through older windows, house framing, poorly insulated areas and areas without any moisture/air barrier.


Ideally, the volume of air inside your home should be completely exchanged with fresh air once every 3 hours. In other numbers, 1/3 ACH or 0.3 Air Changes per Hour. This is to promote healthy indoor air for its occupants and to prevent mold growth or other moisture issues from excess stale/humid air lingering inside. Newer or tighter homes that cannot achieve this air change rate naturally need help. That is why we must provide mechanical ventilation to force outside fresh air into our house. In many regions, it is now code to have these in new homes.




How the HRV Works:

The air exchange is actually a simple procedure. A central fan extracts stale\humid air from the house either from each bathroom and sometimes the kitchen and laundry area (newer installations) or from the return air duct of the furnace system (usually retrofit installation). At the same time, another fan working at the same rate, brings in air from the exterior usually through a screened opening through the exterior wall. Of course this air will be cold or even well below freezing in most parts of our country. The HRV is designed to have the indoor warm air and the cold fresh outdoor air criss-cross a heat exchanger so that the heat energy from the indoor air that is about to go outside gets transferred (to a certain point) to the cold air. Thus the term heat recovery. This slightly warmed fresh air is then introduced to the ducting of the furnace to blend in for distribution throughout the house. Of course different models have different efficiencies. Some models have two cores, or heat exchangers, for higher degree of heat recovery.




Controls for the HRV vary depending on installation. Each bathroom may have a switch to activate it just like a typical bath fan. Same in the kitchen and or laundry area. In addition to these independent controls, a central control is also present to set the desired humidity level, fan speed, intermittent or continuous operation. The central control will operate the HRV automatically when it senses that the humidity in the home is above the level set on the panel.


So what do I do with this thing?


-Ensure that the settings are set properly for your house. Humidity levels in a home typically should not linger beyond 45%. Get familiar with the system and the controls. Your home Inspector should take time to explain the system and its maintenance. Monitor the Relative Humidity level in your house.

-The biggest issue we see with HRV is lack of maintenance and understanding of how they function. These systems have filters that need cleaning as often as the furnace filter. Blocked or dirty filters may restrict air flow and making the air exchange unsuccessful. High humidity and mold growth are now possible results. There are typically two filters one for each side of the HRV. Clean filters are crucial. They are usually washable/reusable filters that slide out easily beside the heat exchanger from inside the cabinet.

-Just as crucial is the screened opening at the exterior of the home. Many times, the inlet screen gets completely clogged by dust and debris making it impossible for fresh air to enter the system. A quick cleaning and regular monitoring is recommended.


-The HRV is a balanced system, in other words the “in” and “out” fans work at the same rate. Maintenance is sometimes needed to ensure the system stays balanced. The core may also need cleaning depending how well the filters are performing.

-Make sure the tube is draining. There is a defrost feature in HRVs to ensure the system does not freeze up to much or get blocked when the cold air meets the warm humid air in the heat exchanger. This condensation is led from the bottom of the unit to a drain. A kinked line will cause pooling water at the unit and leaks on the floor.

-Make sure your furnace fan is communication with the HRV. Whenever the HRV is called upon to operate, the furnace fan should also kick in (usually at low speed for muti-speed furnace fans). This is to ensure that the air brought in by the HRV is in fact distributed throughout the house. In many cases, it is not wired this way and a heating contractor can usually improve the wiring connections depending on make and models.


HRV or ERV what is the difference?


HRV’s are generally used in colder climates as they are primarily used to recover heat from the air this about to be exhausted to the exterior and keep that recovered energy indoors where it belongs.

ERV’s also recover heat from the exhaust air but also helps exchange and control the amount of moisture in that air. In regions of higher humidity and air conditioned homes, where the humidity outside is typically higher than the interior of the home, the ERV limits the humidity levels entering the home. In dryer regions, the ERV help to limit the amount of humidity that exits the home.


HRV’s are a great feature in any home. A few simple regular maintenance steps can go a long way to ensure proper fresh air in your home.



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