Sounds like a wiring issue. Look inside the furnace, there should be 2 wires from the hrv to furnace, 2 from a/c to furnace and 3,4or5 from T stat to furnace. From the hrv to furnace one wire should go from the r terminal and one wire should go to the g terminal. If this wire went to the y terminal by accidient, this is the problem you would have. The wire to the Ac should go from the c terminal and one from the y terminal. If this wire was accidiently ran to the g terminal, same problem would happen. As well if this was the case, Ac would run if you switched the furnace to fan on position.
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