How to plumb in a storage tank to an existing boiler for domestic hot water?

[mlrautiola]

Hello
I have a Benjamin CC500 combination wood/oil boiler with tankless coils for domestic hot water. As soon as we get warmer weather I will be installing an electric water heater for the summer. When I switch back to the boiler in the fall how could I plumb it into the water heater tank so I could use that as a hot water storage all winter? I would at that point shut off electricity to unit.

Thanks

 

[Eric Johnson]

Welcome to the Boiler Room, mirautiola.

The easiest way to accomplish what you're trying to do would be to connect the hot water heater tank to the coil with 1/2- or 3/4-inch copper pipe, depending on the size of the coil (probably 1/2-inch). Then put a circulator in the line controlled by an aquastat in the boiler. When the boiler temp hits, say, 140 degrees, the pump kicks on and circulates the water. This will heat your hot water and provide a small buffer for your boiler, which should make a noticeable difference in how it operates. If you want to get fancy, you can put another temp probe of some sort on the water tank that shuts the circulator off when the temp hits a setpoint. If you don't, your hot water will eventually become about the same temp as your boiler, which is OK if you have a tempering valve on the hot water heater's hot water outlet.

You hook the lines up to tees screwed into the water heater's pressure relief valve opening on the top, and the drain on the bottom. The flow could be in either direction, but I'd run it clockwise, so that hot water from the coil enters the upper connection. You put your pressure/temp relief valve and drain back into the tees.

One consideration is that you should have a bronze circulator to avoid corrosion from the fresh water constantly flowing through it. That's the right way to do it. I've used regular cast iron circulators in that application (in my own house), and the circulators lasted a good long time, with no rust stain problems.

Hope that's a reasonably clear description. If not, I'll try again. And, I'm sure others will share their thoughts, and probably come up with an even better idea. Another approach would be to use a heat exchanger, like a sidearm, as another zone on your system. Then you really don't need the coil or the extra circulator.