National Oilheat Research Alliance 37
Internal tankless coils
Internal tankless coils (Figure 2) are heat
exchangers that are installed inside a boiler to
generate domestic hot water. These devices can
operate rather efficiently during cold weather
when boilers are running to produce heat, but
are much less efficient during warm weather.
The coil is surrounded by the boiler water,
which heats the outside of the coil; domestic
water is then heated as it flows through the
inside of the the coil. Tankless coils generally
requre that boilers be operated at 180–200°F.
Figure 3 shows a properly piped tankless coil
and includes the following:
1. Shutoff valve—typically a globe or ball
valve
2. Domestic pressure relief valve—Relieves
excessive pressure caused by thermal expansion
3. Flow regulator or restrictor—This device
automatically limits the flow of water so it
doesn’t exceed the capacity of the coil.
4. Drain valves—These are installed to simplify
periodic flushing of the coil.
5. Mixing valve—Thermostatic mixing
valves blend hot water from the coil with cold
water to deliver a desirable and selectable
temperature and help prevent scalding injuries.
The recommended maximum hot water
temperature setting for normal
residential use is 120°F. NORA
recommends that thermostatic
mixing valves (also known as
tempering valves or anti-scald
valves) be installed and used
according to the manufacturer’s
directions to prevent scalding.
Tankless coils supply hot water
on demand, BUT only up to a
limit that is determined by the
Btuh output of the appliance in
which the coil is installed. For
example, 50,000 Btuh is required
to raise the temperature 1 gpm
(one gallon per minute) of water
from 40 degrees to 140 degrees.
If you do the math, you’ll see
that 150,000 Btuh is required to
Figure 2
Figure 3