HEAT RECOVERY VENTILATORS,
(HRV UNITS)

These units were developed as a means of providing fresh air into homes while recovering the heat or air conditioning from the equal amount of air that is being exhausted by the unit. HRV units have a heat exchanger that transfers heat from the outgoing air to the incoming air. The unit also has an air filter to clean the incoming air.

If these units are installed in new home construction it is common for the washroom exhaust vents that would normally exhaust through the roof or eave to exhaust through the HRV. The HRV will also condense moisture that could be in the air from washroom showers. Many HRVs’ are set up to operate with switches in the washrooms and are interlocked with the exhaust fan of the washroom. After market sales of HRVs’ are simply installed in the mechanical room to cycle on a time schedule or operate constantly and are near impossible to connect to the washroom exhausts.

This is why this writer has a problem with HRV units. Even in new home construction we would not connect kitchen range hoods to an HRV because the normal grease produced during cooking would foul up the heat exchanger. Clothes dryers are not connected to HRV units because lint and excessive moisture would destroy the exchanger.

Few homes have only one bathroom anymore which means they could be a fair distance away from the unit that is typically located in a mechanical room. Long runs of ductwork from the bathroom fan to the HRV reduce the amount of air that is exhausted. The amount of air exhausted will not necessarily match the amount of air being brought into the home.

This writer has a problem with any home or office that is under a negative air pressure because the replacement air will infiltrate back into the structure wherever and whenever it can to neutralize the pressure differential. The air coming in through cracks, doors and windows is unfiltered and unconditioned. I see no point in having an HRV if it is only doing part of the job it is intended to do and doing that job poorly.

The ideal home or office building is one that maintains a neutral or slightly positive pressure and controls when and where the incoming air is introduced so that it can be filtered and conditioned. In the ideal structure, if you open a door in the dead of winter when exhaust fans have been operating or the laundry is being done there would be no movement of air, in or out of the doorway. The only way to do this is to bring in outside air at the same rate of speed and volume as the air is being exhausted and condition it simultaneously.

This would be a near impossible feat because of the lag in starting or stopping either the exhaust fans or the intake fans and that each fan would have a different capacity. A sensitive pressure control could initiate a supply fan when it senses the structure going negative and a thermostat could initiate a furnace or A/C to start if the incoming air was not suitable for the occupied space.

The unit I am discussing hasn’t been invented yet, but when it is it will be suitable for all applications and not just new construction. The filtration on the new unit will have to be better than existing HRV’s to clean the air for allergy sufferers. Gas fired clothes dryers will always have to be vented to the atmosphere due to the products of combustion, but the air from electric clothes dryers could easily be recirculated in the home if the lint could be effectively filtered and the humidity controlled (used during the winter and condensed during the summer). The advent of variable speed motors for the residential market is making this project more feasible.

The new unit would work just like a make-up air unit for the high rise market. So many older homes are abandoning their chimneys’ for side wall venting on new high efficient furnaces that the old chimney could be lined and used for the fresh air intake source that is above the normal intake elevation of an HRV. May-be when the unit is invented my name will be linked to the patent.