Last week I reviewed some of the general strategies used for ventilating buildings -- or not. This week, I’ll zero in on the types of balanced ventilation in which heat is recovered from the outgoing airstream to preheat the incoming fresh air.
Two fans, two airstreams
As noted last week, balanced ventilation requires two fans: one bringing fresh air into the house and one exhausting indoor air. By balancing these two fans and the airflow through their respective ducts, the house is maintained at a neutral pressure -- which is important for avoiding moisture problems or pulling in radon and other soil gases.
In a heat recovery ventilator (HRV) the two fans are in the same box, and they force air through a heat-exchanger core made of a corrugated plastic or aluminum. There are several types of heat exchanger cores in HRVs, and these effect efficiency and cost.
HRVs can have cross-flow heat exchangers or counter-flow heat exchangers. With cross-flow, the incoming and outgoing air streams are typically at 90-degree angles to each other. The heat transfer efficiency is good but not great: typically 50 percent to 70 percent.
With a counter-flow heat-exchange core, there is a longer pathway in which heat exchange occurs, so the efficiency is typically higher.
Our Zehnder HRV
The HRV we installed in our new house is a Zehnder ComfoAir 350 Luxe. This is a Swiss-made, highly efficient HRV utilizing a counterflow heat exchanger.
There are various ways to measure efficiency of HRVs. Apparent sensible effectiveness (ASEF) is the most commonly reported number for reporting heat transfer from the outgoing airstream to the incoming airstream. The HVI-listed ASEF is 93 percent -- which is among the highest in the directory (though not quite the highest).
Another measure reported by HVI is the sensible recovery efficiency (SRE). This is a measure that corrects for waste heat from the fan motor that may be going into the incoming airstream, cross-flow leakage from the outgoing to the incoming airstream, and "case leakage" or heat transfer from the outside of the box to the airstream inside. These factors make it seem as if the heat transfer efficiency is higher than it really is; thus the more accurate SRE number. With our Zehnder ComfoAir 350 the SRE is 88 percent -- the highest that I found in the HVI Directory.
In reviewing the HVI list of certified products, I found some other HRVs with higher ASEF values, such as a Broan-NuTone HRV with a lists ASEF of 95 percent, but that product had a SRE value of only 58 percent. With this product and most other HVI-listed HRVs with very high ASEF values, the SRE values are considerably lower, indicating that waste heat from the fan motors using high wattages per cfm is boosting the ASEF values.
Another measure of efficiency is how much air is moved per unit of electricity consumption. Here we can look at the cubic feet per minute (cfm) of air flow per Watt of electricity consumption. Here, the Zehnder ComfoAir really shines, with a remarkable 2.58 to 3.25 cfm per Watt (depending on the fan speed). The Energy Star criteria for HRVs to be listed as EnergyStar is 1.0 cfm/W, and most good HRVs have air-delivery efficiencies only in the 1.0 to 1.5 cfm/W range. I was able to find only a few others with cfm/W values exceeding 2.0.
Nearly as exciting as the superb energy performance of Zehnder HRVs is the ducting that is provided with them. The company produces ComfoTube ducting with a three-inch outside diameter and two-and-a-half inch inside diameter. The outer surface is ribbed for strength and the inside smooth, for optimal airflow and quiet operation. The material is 100 percent high-density polyethylene, which is the most environmentally friendly plastic, in my opinion.
The ducting diameter is small enough to fit in two-by-four interior walls. Because the airflow through the ducts is relatively low and sharp bends are eliminated, the airflow is very quiet. In fact, noise control is a key feature of all Zehnder products, and this is one reason the HRV itself is so quite large.
While some ducting systems for heating and ventilation are branched -- with larger trunk ducts stepping down to smaller distribution ducts, Zehnder ComfoTube ducts are designed to be installed in a "home run" configuration -- with a single, continuous duct extending from each supply and return diffuser all the way to the HRV. This feature also helps control noise, though it can make for a complicated spaghetti-like installation.
Three operation settings
Our HRV has three speeds, plus an extra-low "away" setting. Labeled 1, 2, and 3, the primary settings can be custom-set to deliver between 29 and 218 cubic feet per minute (cfm). As configured on our system, setting 1 consumes 18-20 watts, setting 2 consumes 30-35 watts, and setting 3 consumes 80-85 watts. An Away setting uses just 7-10 watts.
There is a frost-protection cycle that goes on periodically in cold weather to prevent condensate from freezing in the heat exchanger core. This draws about 800 watts. The need for this can be greatly reduced by adding a ground-loop preheater. This circulated an antifreeze solution through a simple ground loop (tubing buried along the house foundation typically).
In my opinion, Zehnder makes the best HRVs and ERVs (energy-recovery ventilators) in the world. But you pay for that quality and performance. The system we have, a Zehnder ComfoAir 350 Luxe with ten supply ducts and ten return ducts, with their respective registers, and two remote controllers (for the upstairs and downstairs bathrooms) costs about $6,000. The geo-exchange loop, which we did not include, adds another $2,000.
While this is a lot to spend on ventilation, this integrated whole-house ventilation system obviates the need for separate bath fans, which can cost $300 to $600, installed, and savings can be realized with the Zehnder from reduced operating energy, compared to standard HRVs.
The super-quiet, highly dependable operation is a nice bonus. You can find out more at www.zehnderamerica.com.
Next week I’ll talk about commissioning our HRV system.
Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to email@example.com.