Chapter 5
Domestic Hot Water
In this section, we’ll examine various ways
in which Oilheat systems generate domestic
hot water; sizing water heaters; piping and
maintenance recommendations; safety concerns;
and energy conservation strategies.
There are two basic types of water heaters:
direct-fired and indirect.
Direct-fired water heaters are basically dry
base vertical tube boilers; they utilize an oil
burner that creates hot combustion gases that
heat the water inside the tank. The gases and
the water being heated are separated by the
wall of the tank.
Indirect water heating uses boiler water to
heat domestic water. This type features a heat
exchanger that draws heat from the heating
system water into the domestic water. For
many years, the only indirect water heater
available was the tankless coil, today there are
additional options, including a separate “Indirect
Water Heater” consisting of a storage tank,
circulator, control and related accessories
Direct-fired water heaters
Oil-fired water heaters are typically available
for residential use in sizes that range from 20
gallons to over 100 gallons. They are available
in two configurations, (Figure 1), based on the
path of the flue gases:
• Center flue heaters feature a path for the
flue gases to travel through the inside of
the tank.
• Rear flue heaters have a path for the flue
gases to travel around the outside of the
tank.
Direct-fired heaters are operated by an aquastat
which turns the oil burner on and off to
maintain water temperature.
36 National Oilheat Research Alliance
Figure 1
Most direct-fired water heaters feature an insulated
glass-lined tank. The inside of the steel
tank is coated with ceramic lining material that
protects the tank from rusting and corrosion.
The material is not totally impervious to water,
and thus the tank can rust.
Manufacturers provide protection for the
steel casing by installing replaceable sacrificial
anode rods in the tank. These rods should be
checked during tune ups and replaced when
necessary.
Indirect Water Heating
Indirect water heating involves the use of a
heat exchanger to transfer heat from the boiler
water to the domestic water.
Tankless coils—there are two types of tankless
coils: internal and external. The biggest
disadvantage of tankless coils is that a household
can easily outrun their ability to generate
domestic hot water. Depending on the temperature
in the boiler, and the size of the coils in the
unit, a busy household may not have sufficient
hot water for peak usage.