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Bosch On Demand Water Heaters Information
(Bosch Tankless Water Heaters or Instant Hot Water Heater)
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Water heating accounts for 20% or more of an average household’s annual energy
expenditures. The yearly operating costs for conventional gas or electric storage
tank water heaters average $200 or $450, respectively. Storage tank-type water heaters
raise and maintain the water temperature to the temperature setting on the tank (usually
between 120° -140° F (49° -60° C). Even if no hot water is drawn from the tank (and cold
water enters the tank), the heater will operate periodically to maintain the water
temperature. This is due to "standby losses": the heat conducted and radiated from
the walls of the tank—and in gas-fired water heaters—through the flue pipe. These
standby losses represent 10% to 20% of a household’s annual water heating costs.
One way to reduce this expenditure is to use a demand water heater (also known as
"tankless water heaters" or "instant water heater").
Bosch On Demand water heaters are common in Japan and Europe. They began appearing in
the United States about 25 years ago. Unlike "conventional" tank water heaters,
tankless water heaters heat water only as it is used, or on demand. A Bosch tankless
water heater has a heating device that is activated by the flow of water when a hot
water valve is opened. Once activated, the heater delivers a constant supply
of hot water. The output of the heater, however, limits the rate of the heated
water flow.
Gas and Electric On Demand Water Heaters
Bosch On Demand water heaters are available in propane (LP), natural gas, or electric
models. They come in a variety of sizes for different applications, such as a
whole-house water heater, a hot water source for a remote bathroom or hot tub,
or as a boiler to provide hot water for a home heating system. They can also be
used as a booster for dishwashers, washing machines, and a solar or wood-fired
domestic hot water system.
You may install a on demand water heater centrally or at the point of use,
depending on the amount of hot water required. For example, you can use a
small electric unit as a booster for a remote bathroom or laundry. These
are usually installed in a closet or underneath a sink. The largest gas units,
which may provide all the hot water needs of a household, are installed
centrally. Gas-fired models have a higher hot water output than electric
models. As with many tank water heaters, even the largest whole house
tankless gas models cannot supply enough hot water for simultaneous,
multiple uses of hot water (i.e., showers and laundry). Large users of
hot water, such as the clothes washer and dishwasher, need to be operated
separately. Alternatively, separate demand water heaters can be installed
to meet individual hot water loads, or two or more water heaters can be
connected in parallel for simultaneous demands for hot water. Some
manufacturers of tankless heaters claim that their product can match
the performance of any 40 gallon (151 liter) tank heater.
Selecting a Bosch Demand Water Heater
Select a Bosch On demand water heater based on the maximum amount of hot water
to meet your peak demand. Use the following assumptions on water flow
for various appliances to find the size instant hot water heater that
is right for your purposes:
Faucets: 0.75 gallons (2.84 liters) to 2.5 gallons (9.46 liters) per minute.
Low-flow showerheads: 1.2 gallons (4.54 liters) to 2 gallons (7.57 liters) per minute.
Older standard shower heads: 2.5 gallons (9.46 liters) to 3.5 gallons (13.25 liters) per minute.
Clothes washers and dishwashers: 1 gallon (3.79 liters) to 2 gallons (7.57 liters) per minute.
Unless you know otherwise, assume that the incoming potable water temperature is
50° F (10° C). You will want your water heated to 120° F (49° C) for most
uses, or 140° F (60° C) for dishwashers without internal heaters. To determine how
much of a temperature rise you need, subtract the incoming water temperature from the
desired output temperature. In this example, the needed rise is 70° F (39° C).
List the number of hot water devices you expect to have open at any one time,
and add up their flow rates. This is the desired flow rate for the demand water heater.
Select a manufacturer that makes such a unit. Most demand water heaters are rated for a
variety of inlet water temperatures. Choose the model of water heater that is closest to
your needs.
As an example, assume the following conditions: One hot water faucet open with a flow rate of
0.75 gallons (2.84 liters) per minute. One person bathing using a shower head with a flow rate
of 2.5 gallons (9.46 liters) per minute. Add the two flow rates together. If the inlet water
temperature is 50° F (10° C), the needed flow rate through the demand water heater
would need to be no greater than 3.25 gallons (12.3 liters) per minute. Faster flow rates or
cooler inlet temperatures will reduce the water temperature at the most distant faucet. Using
low-flow showerheads and water-conserving faucets are a good idea with demand water heaters.
Some types of tankless water heaters are thermostatically controlled. They can vary their output
temperature according to the water flow rate and the inlet water temperature. This is useful when
using a solar water heater for preheating the inlet water. If, using the above example, you connect
this same unit to the outlet of a solar system, it only has to raise the water temperature a few
degrees more, if at all, depending on the amount of solar gain that day.
Cost
Bosch Demand water heaters cost more than conventional storage tank-type units. Small point-of-use heaters
that deliver 1 gallon (3.8 liters) to 2 gallons (7.6 liters) per minute sell for about $200. Larger
gas-fired tankless units that deliver 3 gallons (11.4 liters) to 5 gallons (19 liters) per minute
cost $550-$1,000.
The appeal of Bosch demand water heaters is not only the elimination of the tank standby losses and the
resulting lower operating costs, but also the fact that the heater delivers hot water continuously.
Gas models with a standing (constantly burning) pilot light, however, offset some of the savings
achieved by the elimination of tank standby losses with the energy consumed by the pilot light.
Moreover, much of the heat produced by the pilot light of a tank-type water heater heats the water
in the tank; most of this heat is not used productively in a demand water heater. The exact cost of
operating the pilot light will depend on the design of the heater and price of gas, but could range
from $12 to $20 per year. Ask the manufacturer of the unit how much gas the pilot light uses for the
models you consider. It is a common practice in Europe to turn off the pilot light when the unit is
not in use.
An alternative to the standing pilot light is an intermittent ignition device (IID). This resembles
the spark ignition device on some gas kitchen ranges and ovens. Not all demand water heaters have
this electrical device. You should check with the manufacturer for models that have this feature.
Life Expectancy
Most Bosch tankless water heaters have a life expectancy of more than 20 years. In contrast, storage tank
water heaters last 10 to 15 years. Most tankless water heaters have easily replaceable parts that can
extend their life by many years more. Thus, for many applications - on demand water heaters will save
money, space and energy and with an endless amount of hot water.
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