Continuing our series on water heating, this week we’ll look at two options for heating water with the home’s central boiler. First some terminology: boilers heat water or produce steam for distribution in baseboard units or steam radiators, while furnaces heat air for distribution through ducts and registers. Integrating water heating with a standard hot-air furnace is not possible; if you have a furnace, you have to stick with a stand-alone water heater.
There’s a lot of appeal to the idea of using your boiler to also heat water. You can get by with just one burner for both heat and hot water, so there’s less to maintain or to go wrong, and when you’re heating the house anyway, it doesn’t take all that much extra energy to heat water also.
There are two common options for using a home’s boiler for water heating. The most common is referred to as a “tankless coil.” This is a relatively small heat exchanger that fits into a gas- or oil-fired boiler. These are popular for people with hot-water baseboard heat because they’re inexpensive options or retrofits for standard boilers. A copper-coil heat exchanger extends into the boiler, and water is heated as it flows through this coil. It is very much like the “tankless water heater” described last week, except that the heat source is your boiler.
The problem with tankless-coil water heaters is that the boiler has to be hot to produce hot water. In the winter in a cold climate, the boiler is hot much of the time and may not even need to fire up to supply hot water (due to residual heat in the boiler). But during the summer the boiler is less likely to be hot, and the on-off cycling to heat water wastes a lot of energy. It is not unusual for water heating using an oil-fired boiler and tankless coil to use 200 to 300 gallons of oil during the summer months, and the average efficiency during that period may be as low as 25%. Thus, tankless-coil water heaters can make sense in the winter months, but they aren’t a good idea in summer.
The other primary option for using a heating system to produce hot water is to install an “indirect” or “indirect-fired” water heater. This is a separate insulated water tank that is heated using hot water from the gas- or oil-fired boiler. The indirect tank is usually plumbed to be a separate “zone” on the heating system. One advantage of this option is that during the warmer months the boiler only has to fire up occasionally to heat water in the indirect tank. Because water in the tank remains “thermally stratified” (with the hot water at the top, where it is drawn off to supply your shower or dishwasher), you can draw hot water from the tank multiple times before the boiler turns on to heat water in the tank.
Another plus is that because there is no burner on the storage tank, you can wrap it with extra insulation without having to worry about an air supply or venting (as you do with a gas-fired, storage-type water heater).
The downside to indirect water heaters is the cost. A good one will likely cost over $1,000, including installation of the additional zone on the heating system. This is much more expensive than a tankless coil or a conventional gas or electric storage-type water heater.
Interestingly, both kinds of integrated water heaters can help to keep your boiler in good working order. If your boiler is in a basement that stays fairly damp during the summer months, having it fire periodically—as will occur with either a tankless-coil or indirect water heater—is recommended by many heating contractors as a way to minimize corrosion. That was one of the reasons we put in an indirect water heater when we installed a Buderus boiler at our home ten or twelve years ago. Otherwise, I would likely have stuck with the stand-alone electric water heater that we had operated using off-peak electricity prior to installing a central heating system (more on off-peak electric water heating in a future column).
Some people choose to use a tankless-coil water heater during the winter months and then switch over to a stand-alone electric or gas-fired water heater during the summer. Except for possible boiler corrosion problems in damp basements, this can be a good compromise. Ask your heating contractor or plumber for advice on these different options for your particular situation.
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