Other than the restriction of the water heater, I have found that using the furnace at the same time as the fireplace will cause it to shut down. This is because the thermostat for the fireplace is in the back of the unit, in the cabinet with the furnace. The furnace heats up the cabinet and shuts the thermostat shuts down the fireplace. The fireplace works great in conjunction with the heat pumps.
Vicki and Mike, Aria 3901- The Wined Up Bus