I have a water storage tank attached to my oil fired furnace. I also have an electric water heater that I use at the same time. Is it ok to turn off the water heater to save money in the winter and only use the furnace and storage tank?
If the tank and water heater are hooked up in parallel with your main inlet and hot water line and you can isolate the water heater, you can turn it off. If they are hooked up in series and water flows from the water heater through the tank, you can turn off the heater and you'll have hot water until the tank runs out of it. If the water flows from the tank through the water heater you can't turn the heater off because all you'll get is the cold water until the heater water is replaced with the tank water. If you determine that you can turn off the water heater, I would advise against draining it only because if you turn it back on before filling it, you'll burn up a heating element in it. I have seen this done a lot.
I'm assuming the water heater is indoors and kept above freezing.