Randy,would pressure spikes be a concern even though we have never had leaks around the fittings or releases from the T&P valve (which I just checked and it is working)?
Larry, it’s not a gravity flow system – the household water is under pressure as is typical and the water flow from the wood burner is constantly circulated by a pump, although it is not a pressurized system. The two water supplies never mix. The hot water from the wood burner flows through a water-to-water heat exchanger (which I’ve been calling a sidearm) and the household water flows from the bottom of the water heater through a tee on the drain valve, up through the heat exchanger and dumps back into the tank at the top. The “passive” part of the system I mentioned was the closed loop where water is drawn off the bottom of the tank, rises by convection in the heat exchanger then flows back into the tank without the aid of a pump or thermostat or anything. Air in the system would not stop flow from the wood burner or to the house, but it could stop the circulation through the heat exchanger. To prevent that there is an air release valve at the high point of the circulation loop that I purge after draining and refilling the water heater. I’ve never known air to accumulate there any other time. The T&P valve is on the side of the heater about 7 inches down from the top. I would be afraid that running the circulation loop back into the heater there would affect the flow since the only thing driving it is the heating that occurs in the sidearm.