The Plate Heat Exchanger
There are a few brands of boiler that cannot be fitted with a hot-water heating coil (see Tankless Coil blog). Thus, if you have such a boiler, your options for hot water are to install an Indirect Water Heater (see blog), a Tankless or On-Demand Water Heater (see blog), or a separately fired hot-water heater.
If you select the Indirect Water Heater, you can improve its efficiency with a Plate Heat Exchanger.
A plate heat exchanger exchanges heat from one liquid to another without mixing the two. Within the exchanger there are a series of thin metal plates (a 10-plate exchanger will be sufficient for a household of up to three people; for more than three people a 20-plate exchanger should be used). The exchanger is installed above the water heater. Cold domestic water (typically about 55°F) enters from above, passes through and exits into the heater below. Hot water from the boiler (typically about 180°F) enters the exchanger at the opposite end to the cold water, passes through in the opposite direction, and returns to the boiler. As the hot water flows through the exchanger it heats up one side of each of the plates. The cold water flows by the other side of the plates and draws the heat out of the metal.
The good news is that the plate heat exchanger is simple to install, inexpensive, and requires no other expensive modifications to get it up and running.
In rare circumstances a plate heat exchanger could be fitted to a hot-water heater that has its own burner. In that instance, the heater would operate as a storage tank in winter—when the heat for the hot water is provided by the boiler and plate heat exchanger. But in summer, when the boiler is off, the heat will come from the hot-water heater’s own burner—a considerably more efficient solution than running the boiler just for hot water.
Plate Heat Exchanger Pros
Easy and inexpensive to install.
Gets its heat from water supplied by the boiler which, in the cold season, is already working.
Can be used with a boiler that cannot be fitted with a tankless coil.
Can be fitted to a hot-water heater with burner*—if there is no hot water coming into the exchanger the water heater will fire up its own burner.
Plate Heat Exchanger Cons
In most situations (see above for variation)* the boiler must be fired to provide hot water, even in summer.
Cannot be used with a furnace.