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JRMichler 03-26-17 02:02 PM

Inadvertent HRV bypass
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I built a new house five years ago, and had a single speed Fantech SH704 HRV installed. The HRV moved more air than needed, and made a lot of noise, so it was mostly left off. I sort of noticed that the air in the house was cleaner than what would be expected for a tight house without mechanical ventilation. The house had been air door tested to be 0.85 ACH50.

The HRV had been installed to pull air from both bathrooms, and supply air to the cold air return. The HVAC system has a pleated air filter, which has higher pressure drop than the cheapo fiberglass filters.

The negative pressure in the cold air return was pulling air through the HRV when the HRV was off, and causing unbalanced flow when it was running. I had paid for an HRV, and was getting simple mechanical ventilation. The HRV installation was in accordance with the manufacturer's recommendation.

The remedy was simple. I disconnected and taped off the supply air duct to the cold air return.

The handy box with the knob on the upper right corner of the HRV is a speed controller I added. A ceiling fan three speed controller is wired into the common wire to the two HRV fan motors. It works very well. On low speed it moves enough air for good air quality and can barely be heard when the house is completely quiet.

The supply air is dumped into the utility room, from where it flows to the entry area, after which it flows past the cold air return.

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