Heat recovery ventilation

Posted Wednesday, November 23, 2011 in Sustainable Maine

Heat recovery ventilation

by Paul Kando

´╗┐Energy-efficient buildings are more airtight, consequently they are less well-ventilated. Opening windows would provide ventilation; however, the building's heat and humidity will then be also affected: lost in the winter and gained in the summer. This is undesirable both for indoor comfort and for energy efficiency, since the building's heating and cooling systems must compensate for such losses or gains. Heat Recovery Ventilation (HRV) offers the optimal solution: fresh air, better comfort, moisture control, and energy efficiency.

The heart of any HRV system is an air-to-air heat exchanger, which transfers heat from warm exhaust air to the incoming stream of fresh air. So called energy-recovery ventilators (ERVs), on the other hand, also transfer the humidity level of the exhaust air to the incoming air. There are several common types of heat exchangers used in HRV, as shown in Figure 1. Cross-flow heat exchangers are 60-70 percent efficient. Manufacturers using cross-plate or countercurrent heat exchangers claim efficiencies in excess of 80 percent.

The temperature of the incoming air must be above freezing, to prevent the humidity in the outgoing air from condensing, freezing and blocking the heat exchanger. This can be achieved by recirculating some of the exhaust air (causing loss of air quality) when conditions require it, by using a very small heat pump to warm the inlet air above freezing, or by heat from a heat source such as a hot-water circuit from a wood-fired boiler.

Ground-to-air heat exchange is the preferred option in highly efficient “passive houses,” especially in Germany and Austria. An earth tube, 100 to 120 feet long and 8 inches in diameter, is typically buried about 5 feet deep and connected to the intake of the HRV’s heat exchanger (Figure 2). Since the ground temperature 5 feet down is a steady 45-50ºF, the intake air traveling through the earth tube is pre-warmed. In high humidity areas where internal condensation could lead to fungal mold growth in the tube, there are several ways to prevent contamination of the air. These include making sure that water drains out of the tube, regular cleaning, a bactericide coating such as silver ions (non-toxic for humans), fine air filters that trap mold, and air purification by ultraviolet light. The earth tube must be airtight from the surrounding soils to prevent radon contamination. Research indicates that ground-to-air heat exchangers reduce indoor air pollution and do not support the growth of bacteria and fungi.

At certain times of the year it is more thermally efficient to bypass the HRV system’s heat exchanger. During the winter at the depth of the earth tube the ground is much warmer than the air, so the air is pre-warmed before reaching the HRV’s air-heat exchanger. However, in summer, the opposite is true and the air is cooled in the earth tube. Passing it through the heat exchanger would only re-warm the air by the warmth of the outgoing air. In autumn and spring, there may be no thermal benefit from the heat exchanger, as it may heat or cool the air too much, so it is better to use external air directly. A differential temperature sensor with a motorized valve is used in these cases to have the air bypass the heat exchanger.

Heat-recovery ventilation may be accomplished using stand-alone devices that operate independently, or it can be built in, or added to existing heating and cooling systems. For small buildings in which nearly every room has an exterior wall, the HRV/ERV device may be sized to provide ventilation for a single room. Larger buildings require either many small units, or a large central unit, which may prove more economical.

blog comments powered by Disqus