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Hydronic Heating Boilers and
Domestic Water Heaters
52
DOMESTIC WATER
HEATERS
This section applies only to those units used to supply potable
hot water for domestic use. The hot water supply boiler must
be installed with a storage tank.
This section contains specific instructions for those units used
to supply domestic hot water. All warnings, cautions, notes and
instructions in the general installation and service sections
apply to these instructions. Hot water supply heaters are
designed for installation with a properly sized storage tank.
The use of a properly sized pump and the control of water
velocity, as explained below, are important for correct
operation of your water heater.
Water Velocity Control
1. With the pump running and the water heater off, the inlet
and outlet thermometers should read the same
temperatures. If they do not, an adjustment must be made
to your final calculation.
2. Turn the water heater boiler on and allow time for the
temperature to stabilize. Record the difference between the
inlet and outlet temperatures. This difference will be the
“temperature rise.”
3. Compare the temperature rise on the heater with the
required temperature rise in TABLE–W. Should
adjustment be needed, proceed as follows:
If the temperature rise is too high, the water velocity
is too low. Check the following:
1. Check for restrictions in the outlet of the water heater.
2. Be sure all valves are open between the water heater and
the tank.
3. Check the pump to be sure it is running properly and that
the pump motor is running in the proper direction (see
arrow on volute housing).
4. Be sure the installed circulation pipes between the water
heater and storage tank are not less than 2" in diameter on
399,999 - 750,000 Btu/hr models and 2 1/2" in diameter on
990,000 - 2,070,000 Btu/hr models.
5. Common manifold piping for multiple unit installations
will require larger minimum pipe sizes and tank circulating
tappings to ensure proper flow. See TABLE–X, page 53.
If the temperature rise is too low, the water velocity
is too high. Adjust as follows:
1. Slowly throttle the valve on the outlet side of the water
heater until the temperature rise is steady at the required
temperature rise as noted in TABLE–W.
2. Sustained high water velocity and low temperature rise
may result in pitting or erosion of the copper tubes in the
heat exchanger. This is a non-warrantable failure.
Temperature rise must be properly adjusted to achieve the
specified flow rate.
Required Temperature Rise
Based on heating potable water with a hardness of 5 to 25
grains per gallon and/or total dissolved solids not exceeding
350 ppm. See Water Chemistry, page 52.
TABLE–U
Water Chemistry
The required temperature rise and the standard pump sizing are
based on the heating of potable water with a hardness of 5 to
25 grains per gallon and a total dissolved solids not exceeding
350 ppm. Consult the appliance manufacturer when heating
Btu/hr Temperature
Input Rise
o
F
399,999 12
500,000 15
650,000 20
750,000 23
990,000 19
1,260,000 24
1,440,000 27
1,800,000 34
2,070,000 39
TABLE-W
Temperature Rise Chart
IMPORTANT: To ensure proper velocity
through the heat exchanger, you must
regulate the temperature rise across the
heat exchanger from inlet to outlet. Do this
upon initial installation and periodically
recheck. The correct temperature rise
across the heat exchanger ensures proper
velocity in the tubes. This will yield long life
and economical operation from your hot
water heater. Excessive lime build-up in the
tube is caused by too little velocity through
the tubes. Excessive pitting or erosion in
the tube is caused by too much velocity
through the tubes. Take care to measure
temperature rise and maintain a velocity as
follows: