wpc
Your air compressor does it have a flow switch or temperature cutout on the circuit, also I assume this is a two stage compressor with an intercooler. You could possibly install a check valve with a five pound pressure drop before the tee to the hot water tank, this would give the circuit to the heat exchanger circuit first priority on the flow if the pressure drop through the heat exchanger is less than five pounds. You could install a pilot operated check valve in the outlet of the heat exhanger after the tee that is returning water to the water heater. What this could do is open the water flowing through the heat exchanger to the waste. To do this install a line to the line to water heater after the check valve to the pilot connecton of check valve. When the water pressure increases in supply to water heater it will open the pilot operated check letting water flow to the sewer. THE 5# check to water heater would divert water to heat exchanger, the pilot check in outlet of heat exchanger would allow flow through the heat ex. when wash unit is not used. This is a thought on the subject. Check with qualified person on this, pressures may have to be set up differently. This can be done if set up properly
