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ArticleTechnical Article

Heat Recovery Ventilators - Getting Technical

By HHI Staff

The majority of the balanced ventilation systems on the market are heat recovery ventilators (HRVs). Most HRVs consist of an insulated cabinet, a heat-recovery core, two fans, some ductwork, and a control. But not all HRVs are created equal. They have different types of cores, they are made from different materials, they handle moisture differently, and some are more efficient than others. Units that transfer moisture between the incoming and outgoing airstreams are called energy recovery ventilators (ERVs).

Cores

There are several different types of heat-recovery cores. Flat-plate cores are the most common. They consist of several thin plates spaced about an 1/8" apart. The plates are connected at the ends and edges so the fresh airstream and stale airstream can move through the alternating spaces without contaminating each other. Crossflow flat-plate cores, which have the airstreams moving at 90° to each other, are the most widely available. Counterflow flat-plate cores have the airstreams moving in opposite directions.

 

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Rotary cores have a special wheel that rotates between the fresh and stale airstreams. Heat-pipe cores are not widely used. They consist of sealed tubes filled with a refrigerant that transfers heat from one airstream to the other.

 

Flat-plate cores can be made of plastic, aluminum, or specially treated paper. All rotary cores are plastic, and heat-pipe cores are made of metal. Some cores can be removed from the cabinet. This is a good idea, because a removable core can be taken outdoors and occasionally washed off with a garden hose. Cleaning non-removable cores can sometimes be very difficult.

Leakage

Ideally, there should be no leakage between the fresh and stale airstreams. However, in reality, there is usually some leakage. Obviously, it’s desirable to obtain a model with minimal leakage between airstreams. Cross-leakage rates have been reported as low as 0% and as high as 40%. In general, rotary-core models have higher cross-leakage rates.

Efficiency

Efficiency also varies.   Some units are advertised as being over 90% efficient. However, test results from independent laboratories are often lower than a manufacturer’s advertising claims. Efficiency depends on the particular design of the unit, indoor and outdoor temperature, humidity, and fan speed. Therefore, efficiency ratings for the same unit can vary, depending on how the testing was done.

 

If a unit with an efficiency of 80% is used when the outdoor air temperature is 10°F and the indoor temperature is 70°F, then the 10° air will be warmed up to 58° (10 + {[70 - 10] x 80%}) as it passes through the core. The 70° air releases its heat and drops in temperature to 12° (70 - 58) by the time it reaches the outdoors. Of course, the 58° air will still need to be warmed up by the heating system, but the cost will be considerably less than if a window was opened and 10° air was used for ventilation.

 

Independently tested efficiencies and leakage rates for most HRVs are available in the Certified Home Ventilating Products Directory, which is available from the Home Ventilating Institute.

Fans

All HRVs have one or two small fans to move air through them. If a fan motor is within the fresh airstream, it can be a minor problem for some hypersensitive people. This is because, as a motor warms up during use, oil, synthetic materials, or lacquers heat up and volatilize, sending minor odors into the fresh airstream. Some HRVs have the motor located in the stale airstream, where this minor outgassing will be blown toward the outdoors.

Moisture

Because the temperature and relative humidity of the airstreams change as they pass the core of an HRV, water can condense in the core. When this happens, the moisture drips into a pan inside the cabinet, then it passes down a drain which must be connected to a plumbing drain. A 3/4" drain is usually sufficient. If the drip pan is not allowed to drain freely, it can harbor mold growth.

 

Rotary cores, and flat-plate cores made of treated-paper, are designed to transfer moisture from one airstream to the other. This can be advantageous in the summer in a hot humid climate where you want to bring in fresh air, but not excessive humidity. Some pollutants (such as formaldehyde) can be transferred through these cores along with the moisture, but the amount is not significant.

Defrosting

If the outdoor winter temperature is low enough, the moisture that condenses inside an HRV can freeze, blocking the flow of air through the core. If this is a potential occurrence with a particular model, or in a particular climate, the HRV should have defrosting capability. Some manufacturers simply use a small electric heater to warm up the interior of the cabinet whenever frost forms—one company uses a light bulb. Other units have a small motor-driven damper that allows warm house air to periodically circulate through the core to defrost it.

Materials

Most HRV cabinets are insulated. This is necessary to prevent condensation either on the inside or on the exterior of the cabinet itself. At one time, some companies used raw fiberglass insulation, but most are now using a foil-faced insulation to prevent contamination of the air stream.

 

In order to prevent air from leaking out of the cabinet, or around the edges of the core, gaskets are used in various places. Occasionally a sensitive person will be bothered by the odor of a synthetic gasket but, with care, gaskets can usually be covered with aluminum-foil tape, and still flex enough to form a tight seal.

 

Combining ventilation with heating/cooling

 

Ventilation equipment is often installed in conjunction with a heating/cooling system. This is only possible with a forced-air heating/cooling system. It is not possible with heating/cooling equipment that doesn’t use ductwork, such as a forced-hot-water boiler. There are both advantages and disadvantages to combining the two systems.

 

A general ventilation system moves a fairly small volume of air, usually 50-200 cfm, through small-diameter ducts, compared to a forced-air heating/cooling system which might move over 1,000 cfm through larger-diameter ducts. When heating/cooling and ventilation are separate, independently operating systems, two sets of ductwork are needed. This increases the installation cost. Combining the two systems means you only need a single system of ductwork.

 

If you interconnect the two systems and only run the ventilation fan, it won’t be able to push the air through the entire system of larger heating/cooling ducts. So, when ventilation and heating/cooling are combined, both fans must usually operate at the same time. In a typical installation, a low-powered ventilator fan blows fresh air into the ductwork just ahead of the furnace/air conditioner, where it mixes with air coming from the living space. This mixture of fresh air and house air is then heated or cooled, and blown into the living space by the larger fan. A combined system means more air movement, and more noise, than would be experienced if only a low-powered ventilation fan were running.

 

A significant advantage to combining heating/cooling and ventilation is that you can use a single better-than-average filter to clean both airstreams. When the systems operate independently, you may need two filters: one for the incoming fresh air to remove outdoor pollutants such as pollen, and a second filter to remove pollutants, such as house dust, that are generated within the living space.

 

In a combined system, during mild weather when neither heating nor cooling are necessary, ventilation will still require both fans to be running, one of which is fairly powerful. This means a higher operating cost. A few high-efficiency furnaces are fitted with energy-efficient ECM motors which can significantly reduce the operating cost compared to standard furnace motors. They are often recommended when ventilation and heating/cooling are combined into a single system, particularly when the heating/cooling fan is to run continuously. Because they have a higher up-front cost, ECM motors aren’t common, but they can be cost-effective over the life of a motor.   In fact, if a furnace motor runs continuously, an ECM motor can pay for itself in one year. They can be retrofitted to make an existing system more energy-efficient. ECM motors are variable speed, brushless, DC, permanent-magnet motors that can change speed at any time during operation. They are currently manufactured by Emerson Motor Co. and General Electric Motors and are available through major forced-air heating/cooling equipment manufacturers.

 

(This article is from the archives of the original Healthy House Institute, and the information was believed accurate at the time of writing.)

 

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Heat Recovery Ventilators - Getting Technical:  Created on February 28th, 2009.  Last Modified on February 28th, 2011

 

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