Renovation & Design

MIKE HOLMES: Clearing the air on use of HRVs in home

Postmedia/Leaky windows are a common problem during winter. When winter sets in, you find condensation on your windows and sometimes even pooling on your sills? Condensation mostly occurs when warm air inside your house comes into contact with cold surfaces, such as the glass on your windows.
Postmedia/Your home works best when the flow of air it takes in is balanced with the flow it exhausts. Any tip of the scales one way or the other can cause havoc.

Open and close doors and windows to encourage air flow, or install good ventilation fans in the kitchen and bathrooms to exhaust warm, moist air.

But if high humidity is a chronic problem in your home, one of the best ways to fix the problem is by installing a heat-recovery ventilator (HRV).

An HRV is a mechanical ventilation unit that was first introduced into too-tight homes to supply and distribute fresh air and expel excess warm, moist air. The great thing about an HRV is it captures heat in outbound air and reuses that energy to condition (heat or cool) inbound fresh air. Today, they're used in all kinds of applications to reduce humidity, promote better air flow and help cut energy costs.

Here's how it works. The HRV attaches to your home's ducting using two channels: One distributes fresh air through the house, the other receives stale air. The unit is also tied to exterior vents at two locations. An upper vent exhausts stale air; a lower one collects fresh air. Air flows through the HRV in both directions simultaneously and is drawn into the unit's heat-exchange core, which is where the magic happens. Although the inbound and outbound air supplies are always separated, they're brought so close together that (in winter) the warmth of the outbound air preheats the cool inbound air. The result? Around 70 or 80 per cent of the heat energy in the outbound air is transferred to the inbound air. That means you use less energy to heat fresh air. Your furnace runs less. Your gas bill is lower.

(As an aside, you may have also heard about ERVs, or energy-recovery ventilators. The ERV is a more complex type of HRV that transfers moisture as well as heat energy. That's great news for Canadians, since our winters are usually cold and dry and our summers are hot and humid. In extreme climates like ours, an ERV can take some of the load off your HVAC system.)

The great thing about HRVs is they're flexible and can be installed to support any type of residential heating system. Although they require their own ducting systems, they work well with radiant-flooring, electric-baseboard and even hot-water heating systems. In these cases, the HRV distributes warm air throughout the house using the direct ductwork, and collects and expels stale air via exhaust-air ducts.

On a traditional forced-air furnace system, the HRV piggybacks on existing ductwork. The HRV's fresh-air ducts attach to the furnace's return-air duct. The HRV brings in fresh air from the outside, filters it and supplies it to the furnace for heating. The unit also collects stale air from the home, passes it through the heat exchanger to pre-warm the inbound air, and vents it directly outside.

A word of caution: An HRV is designed to be in continuous use. That means you should never turn it off (unless you're about to get it serviced or cleaned, of course). By all means, take the time to change the rates of air exchange and the humidity level in your home. Adjust these to make the house more comfortable as the seasons change, but never turn off the system because it's too noisy, or because you think it's a waste of energy to run the unit all the time. These units are designed to always operate and the conditions inside your home depend on the balance the HRV strikes. By turning off the unit, you interfere with that balance and must take steps to account for the change in ventilation flow.

A quick search online will turn up thousands of websites that show how to install an HRV. But I'd never suggest you go the DIY route. These are sophisticated machines that require a lot of skill and training to install correctly. For example, you likely don't know how to ensure the unit is properly balanced. Your home works best when the flow of air it takes in is balanced with the flow it exhausts. Any tip of the scales one way or the other can cause havoc. Exhausting too much air, for example, creates negative pressure inside the house. Negative pressure will suck the carbon monoxide emitted by combustible fuel sources (such as your fireplace or furnace) right back into the home. Bad news. Introducing too much air, on the other hand, creates positive pressure that pushes the warm, moist air in your home into your walls. This leads to condensation, mould and rot.

Get a licensed professional to handle your HRV. It's the only way to be sure the job is done right.

HRVs are one of the products at the forefront of the healthy, energy-efficient housing movement. I'd recommend anyone who's thinking about retrofitting talk to an HVAC specialist about these units.

-- Postmedia News

Catch Mike in his new series, Holmes Inspection, airing Thursdays at 8 p.m. ET/PT on HGTV. For more information, visit For more information on home renovations, visit


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