When specifying HRV systems in homes its important to look at the efficiency of the unit. However without looking at the overall system at the design stage, the efficiency & air quality can be dramatically affected . Here are 6 of the most overlooked elements in a HRV system design

1. No Designated Location for the HRV unit

The HRV unit needs to be accessible for regular filter cleaning & changing. If the HRV unit location is not marked on the drawings then it is often pushed into the attic. HRV units in cold attics are potentially less efficient & if they are not easily accessible then regular maintenance is usually the first casualty. As filters block up the HRV unit must work harder to force the air through the system. Out of sight, out of mind

2. Incorrect Sizing of HRV Unit

Larger homes will require larger HRV units. Its not uncommon for 2 HRV units to required. If the unit is undersized then the extract air is pushed through the heat exchanger quicker meaning it recovers less heat & is less efficient. An under sized unit will also be noisier & will have more mechanical failures.

3. Over reliance of Boost Buttons

People are very poor judges of indoor air quality. That is why people are quite happy to sleep in a bedroom with a wall vent blocked & then wonder why there is condensation on a window in the morning. The HRV system should be designed so that the correct volume of air is supplied at all times without need for homeowner interaction. If extra ventilation is required then this should be done with automatic CO2, presence or humidity sensors. There is a trend of under sizing the HRV unit for cost reasons & relying on homeowners to push boost buttons when showering. This will clear the steam but the ventilation rate for the rest of the day is often well below minimum requirements

4. No Specification on the Ducting System

We will often see a HRV unit being specified with no mention of the ducting system to be used. HRV units will need to be replaced overtime but the ducting system will remain in place for the lifetime of the house. The efficiency & noise of the unit is largely determined by the resistance in the ductwork. A good ducting system will cost more than the HRV unit itself & can potentially add an extra 500euro-1000euro compared to a ‘bad’ duct system. Branch & Tee ducting or a manifold system? Is the ductwork certified as airtight? Is the ductwork a certified system or a from a patchwork of manufacturers? Is sound attenuation required? What diameter is the ductwork?

5. Incorrect Supply & Exhaust Terminals

This is generally a form versus function issue. Undersized or poorly designed terminals are the main cause of HRV units under-performing & even failing. When exhausting through a roof, ideally we would use a stack type terminal. These give very little resistance & avoid sucking in dirt that accumulates on the roof. However they are not always used as aesthetically they lose out to low profile vents. With wall terminals, flush terminals are the neatest to use but without a rain hood on the supply side they can draw in rain through the ducting & into the unit. Fly mesh on these terminals also gets blocked up very quickly & chokes the unit.

6. Compromising duct routes

HRV ducting is bulky & can be difficult to hide within the structure of the building. If duct routes have not been planned then they often need to be made longer in order to get around joists, pipes, cables etc.This is generally at the expense of the system efficency

When locating a HRV unit within the home, thought must be given to where the units will terminate externally. The unit needs to be located as close as possible to these termination points, if not then the large primary ducts (150mm-200mm diameter) may need to be boxed in.

Also flat roof areas & vaulted ceilings should be designed to allow ducts to pass through where required.