Water Heaters (DHW) Just about every occupied building in the Western world has an appliance for heating what the plumber calls DHW, or "Domestic Hot Water." That DWH is usually a steel tank with an attached or built-in heat source, and it is an important part of the energy equation. (If you wonder why a cast-iron boiler lasts for 30 or more years, while a DWH fails in 8-12 years, just ask franergy@verizon.net. When Oil is the source of heat: As pointed out in Space Heaters, if the space heater is an oil-fired boiler, it is practicable to heat the DHW with a tankless heat exchanger in that boiler. That avoids the first cost, and the replacement expense, of the separate rust-prone steel tank. In the summer, however, it can be very wasteful to have that boiler turning on automatically, day and night, just to be ready for the occasional hot-water demand. So, when space heat is not needed, the homeowner can leave the (red?) safety switch in the Off position, turning it on only five minutes before hot water is needed. This can save a lot of money! When Gas is the heat-source: When the space heater is a gas-fired boiler, an internal tankless exchanger for the DHW may not be satisfactory, because the gas flame is not always hot enough to heat the water as it rushes through the heat exchanger. Here, a separate water heater is almost essential, but if that heater is the traditional glass-lined steel tank, rust will ruin it all too soon. In this situation, the indirect water heater, described below, is a good investment. When Electricity is the heat source: An electric DWH works slowly. That calls for a tank of 60-80 gallons, because its recovery rate is so slow. The tank's immersion heaters may work more than two hours in order to recover, after someone takes a shower. Electricity is an expensive heat source, particularly in the Northeast. Even Bottled gas will be cheaper, saving the added installation cost in a few years. The Indirect Water Heater: To go with a space-heating boiler, several companies offer an indirect DWH, guaranteed for as long as the original buyer owns it. The heart of the indirect system is a heavily insulated stainless steel tank that holds 40 or more gallons of water preheated by the boiler. There is no flame in that tank. Instead, there is a heat exchanger inside the tank, piped to the space-heating boiler. The tank’s aquastat turns on a circulating pump, and the boiler, only when repeated demands have dropped the tank temperature significantly. This system costs a little more than the conventional glass-lined tank, but it is guaranteed for as long as you occupy that home, saving fuel cost and reducing the risk of a flooded basement. Domestic Water Heater as Space Heater: A full-sized house, with conventional insulation and windows, needs a space heater that can provide 75,000 or more BTUs per hour during a cold spell. That requires a regular boiler or furnace. A smaller house, or one in southerly climes, or a medium-sized house that meets the specifications in Setting, Design, and Shell might need only a heater that can meet a peak demand of 30,000 BTUs or less. An apartment, with walls heated by others, might need less than 25,000 BTUs per hour. In these low-load situations, the output of a conventional gas-fired domestic water heater can be used to heat the whole space! There is no need to buy a boiler or furnace. The flue can be smaller, and side-vented thru' the basement wall. The building codes won't require a costly brick chimney or double-walled stainless-steel vent up through the building. Using the DWH as space heater works particularly well with a
type of factory-built air-handler that came on the market around
1985. These uniquely useful air-handlers come ready to
be fitted into the building's ductwork. Each comes from
the factory containing: Try it! You'll like it! |