Sizing a domestic water heater system constitutes an art
form more than an exact science. For anything other than a small residential
system, you cannot simply go to a table or chart and make your selection. Codes
provide only generalizations such as “Hot and cold water shall be supplied to
all sinks, lavatories, showers, etc.” They do not indicate how much hot water
is required. Codes do, however, give information about pipe materials and
minimum and maximum flow rates to fixtures (combined cold and hot water), and
they address safety concerns such as maximum temperatures and required safety
devices.
form more than an exact science. For anything other than a small residential
system, you cannot simply go to a table or chart and make your selection. Codes
provide only generalizations such as “Hot and cold water shall be supplied to
all sinks, lavatories, showers, etc.” They do not indicate how much hot water
is required. Codes do, however, give information about pipe materials and
minimum and maximum flow rates to fixtures (combined cold and hot water), and
they address safety concerns such as maximum temperatures and required safety
devices.
A number of factors must be considered when sizing a system,
and experience plays a big part. Every system is different, so the path that
leads to final selection is also different. Two buildings can be physically the
same (i.e., have the same number of fixtures), but the quantity of water
required may be different if the buildings will be occupied by different groups
of people. For instance, an apartment building for older people would have a
different use profile than one that primarily houses families.
and experience plays a big part. Every system is different, so the path that
leads to final selection is also different. Two buildings can be physically the
same (i.e., have the same number of fixtures), but the quantity of water
required may be different if the buildings will be occupied by different groups
of people. For instance, an apartment building for older people would have a
different use profile than one that primarily houses families.
Information Gathering
So how do you size and select a domestic hot water system?
The first step is to collect the information needed to define the system parameters
and narrow the options. Some of the information will be readily available, but
some may require a little digging. The following is a fairly long list of
questions, but not all of them apply to every situation.
The first step is to collect the information needed to define the system parameters
and narrow the options. Some of the information will be readily available, but
some may require a little digging. The following is a fairly long list of
questions, but not all of them apply to every situation.
• Where is the building located?
• What code or codes should be followed?
• Do any local amendments apply?
• Does the building’s owner or operator have any unusual
requirements?
requirements?
• Are there any other hot water systems in the building?
• What is the area used for?
• How many plumbing fixtures
will there be?
will there be?
• What kind of fixtures will there be?
• Who will be using them?
• Are there any high-usage fixtures, such as hot tubs or
laundry machines?
laundry machines?
• Are there plans to expand the facility in the future?
• Will there be a laundry or health club area?
• If so, how many areas will be used simultaneously?
• How much space is available for the system?
• What energy sources are available? (Natural gas? Propane?
Steam? Hot water? Fuel oil? Heat reclamation?)
Steam? Hot water? Fuel oil? Heat reclamation?)
• Which energy source is most economical for the location?
• Where in the building will the equipment be placed?
• Will flues or combustion air be a problem, due to the
location?
location?
• What is the cold water source?
• What is the water hardness?
• Will the system be inactive for long periods of time?
Calculations
With the information you have gathered, you can calculate
the load (i.e., recovery rate and storage volume) required for the facility.
Once again, this is not a straightforward process. There are several methods
for calculating the load of a building. One method is not always better than another,
although one may be better suited for the particular application. Several methods
are available; most of them are from the American Society of Plumbing Engineers
(https://www.aspe.org/).
the load (i.e., recovery rate and storage volume) required for the facility.
Once again, this is not a straightforward process. There are several methods
for calculating the load of a building. One method is not always better than another,
although one may be better suited for the particular application. Several methods
are available; most of them are from the American Society of Plumbing Engineers
(https://www.aspe.org/).
Heater System
Selection
Selection
With the demand and storage volume established, the next
step is to select the type of equipment to be used. This is another point at
which experience plays a key part. Several factors have to be weighed—the most
practical or economical fuel source, expandability, available space, location
with respect to roof and wall locations, heater efficiency, budget constraints,
and owner preference are just a few of them. Add the various types of equipment
available, and you may find that equipment selection can be a somewhat
intimidating venture. The best approach is to address each of the parameters,
and narrow the possibilities to one or two that fit the application. Make the
final selection based on your experience.
step is to select the type of equipment to be used. This is another point at
which experience plays a key part. Several factors have to be weighed—the most
practical or economical fuel source, expandability, available space, location
with respect to roof and wall locations, heater efficiency, budget constraints,
and owner preference are just a few of them. Add the various types of equipment
available, and you may find that equipment selection can be a somewhat
intimidating venture. The best approach is to address each of the parameters,
and narrow the possibilities to one or two that fit the application. Make the
final selection based on your experience.
Safety Devices
All HVAC systems are
required to have safety devices. Water heaters and storage tanks are required
to have temperature and pressure (T&P) relief devices, which are intended
to prevent catastrophic failure of the vessels. If the system fails, T&P
devices relieve pressure due to expanding water inside the storage tank and hot
water system. When the building service has a backflow prevention device on it,
often the code requires a thermal expansion tank on the cold water inlet to the
heaters; this compensates for the expanding water within the system. It does
not affect the operation of the T&P relief valve. Many codes also require
that the maximum temperature allowed at a lavatory or shower not to exceed
115°F. To properly maintain the temperature, a thermostatic mixing valve is
required. The thermostat on the heater most often is not considered an
acceptable temperature regulatory device. The mixing valves can be located at a
central point or at the individual fixtures.
required to have safety devices. Water heaters and storage tanks are required
to have temperature and pressure (T&P) relief devices, which are intended
to prevent catastrophic failure of the vessels. If the system fails, T&P
devices relieve pressure due to expanding water inside the storage tank and hot
water system. When the building service has a backflow prevention device on it,
often the code requires a thermal expansion tank on the cold water inlet to the
heaters; this compensates for the expanding water within the system. It does
not affect the operation of the T&P relief valve. Many codes also require
that the maximum temperature allowed at a lavatory or shower not to exceed
115°F. To properly maintain the temperature, a thermostatic mixing valve is
required. The thermostat on the heater most often is not considered an
acceptable temperature regulatory device. The mixing valves can be located at a
central point or at the individual fixtures.
Experience is vital to properly sizing a domestic water
heater system. There is only one way to get that experience—by trying new
things. When you size a system in a building that is a little different from
your norm, try out a couple of different methods to calculate the size. Talk to
some of your coworkers or perhaps a sales representative. Look into the ways
different approaches to the same problem yield different results. You may be
surprised to learn that a tried-and-true method is just not the best fit for
the system.
heater system. There is only one way to get that experience—by trying new
things. When you size a system in a building that is a little different from
your norm, try out a couple of different methods to calculate the size. Talk to
some of your coworkers or perhaps a sales representative. Look into the ways
different approaches to the same problem yield different results. You may be
surprised to learn that a tried-and-true method is just not the best fit for
the system.
