Preheating hot water

We are successfully preheating our hot water with our current Sedore (the new one is to be shipped this week :) ) There is a coil in the burn chamber that is hooked up to a water tank behind the stove a few feet. That then feeds into our electric hot water heater. For a few weeks we just preheated our hot water. Now we are leaving our elecric heater off and using the water as it is coming off the stove. It is working fine for during the day. Sometimes if we have a few showers or baths to go through in an evening for our family we will turn on the hot water heater for an hour or so to heat the water up a bit more. Not sure what we will save out of this but we seem to have been spending about $40/month on the hot water heater. This should go down substantially with this new system.