Energy & Heat Recovery Ventilation

With the high insulation values and effective air sealing techniques that are a large part of building your low-impact home, comes the responsibility of installing and maintaining an adequate fresh air ventilation system. This system must be sized and installed properly to remove pollutants (chemicals, spores, allergens) and excess moisture from your home. The way to accomplish this is with Heat or Energy Recovery Ventilators.

Heat Recovery Ventilators (HRV's) or Energy Recovery Ventilators (ERV's) operate by venting stale air and replacing it with fresh air from outside your home. These two streams of air are tempered by each other, in that the fresh cold air (in Winter) is pre-heated by the exhaust air by passing through a heat exchanger core. Both HRV's & ERV's operate the same way, except the ERV is used in hot & humid climates. The core on ERV's is vapor permeable so as to allow the transfer of moisture across the exchanger core, as well as heat. Both types of exchanger cores achieve about 65% heat transfer rates.

HRV & ERV System Design

So, which type do you need? At our location in northeastern Pennsylvania we use HRV's featuring a defrost cycle due to the Winter temperatures. If you need help deciding which to use, as well as general help on system layout and design, just give Timberworks a call. Which system to use really is dependent on your local climate and home configuration, and Timberworks can help with these decisions.

If you want to try and size your HRV/ERV yourself, there are some rule-of-thumb calculations you can make. To start, most Indoor Air Quality (IAQ) references suggest minimum air changes per hour of between .35 & .50. At Timberworks Housewrights we tend to reference closer to the .50 number, unless we have been integral in the design and material choices that will go into the home. If you have selected natural materials with low emmisivity of chemicals, properly sealed the home against soil gases and taken active measures to limit moisture sources in the design, then you may be able to use lower numbers.

The next step is to calculate the volume of your home in cubic feet. Let's use a simple one story home with flat ceilings at 8' and an area of about 2,000 square feet. 2,000 square feet x 8' = 16,000 cubic feet in volume. Since we are dealing with coming up with a Cubic-Foot-per-Minute (CFM) figure, take your volume and divide it by 60 (60 minutes per hour) to arrive at about 267. Now that you have that, multiply that by your chosen air change rate...for .35 air changes per hour that would be 267 x .35 = about 94 CFM required. An exchange rate of .5 would yield about 135 CFM. Got it?

You can then take your decision of whether to use an HRV or ERV, and this number and look at the specifications on the systems to select which to purchase. Most homes can be serviced by one unit quite well. Very large, or spread-out designs may sometimes be better served by using two independent systems. And any structure housing a spa or pool, forget the rule-of-thumb approach. These areas always require help from a professional Mechanical Engineer! We can design your system if you like, as well as consulting with our engineers if it is required.

To see our selection of HRV's & ERV's please Click Here. You will also find accessories needed for the installation such as controls and vent hoods. Ductwork, fittings and registers are widely available from HVAC supply sources so we do not cover them here. If all of this leaves your head spinning, just contact Timberworks, toll free, at (866) 203-3136.