Installation & Service Manual
7Domestic water heaters
Domestic Water Heaters 90,000 - 500,000 Btu/hr Models
This section applies only to those units used to supply direct fired domestic hot water and installed with a storage tank(s). The use of a properly sized pump and the control of water velocity, as explained in the Water Velocity Control section, are important for correct operation of your hot water heater.
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. Water heaters are designed for installation with a storage tank. The use of a properly sized pump and the control of water velocity, as explained below, is important for correct operation of your water heater.
Water Velocity Control
IMPORTANT - To ensure proper velocity through the heat exchanger, it is necessary to regulate the temperature rise across the heat exchanger from inlet to outlet. This must be done on initial installation and periodically rechecked. With the correct temperature rise across the heat exchanger, you may be assured of the proper velocity in the tubes. This will yield long life and economical operation from your water heater. Excessive lime
1.The pump must run continuously when the burners are firing.
2.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.
3.Turn the water heater 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.”
4.Compare the temperature rise on the heater with the required temperature rise in Table 7A. 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.
4.Be sure the circulation pipes between the water heater and storage tank are not less than 2" diameter for a single unit installation.
5.Common manifold piping for multiple unit installations will require larger minimum pipe sizes to tank tappings to ensure proper flow. See Table 7B on page 47.
6.Multiple unit installations may also require a larger circulating pump to achieve the specified flow against the increased head loss of multiple unit common manifold piping.
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 7A.
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
Required Temperature Rise
Temperature rise is based on the hardness of the potable water to be heated. The temperature rise specified is for water with a hardness of 5 to 25 grains per gallon. The total dissolved solids shall not exceed 350 ppm.
TABLE - 7A
REQUIRED TEMPERATURE RISE
| Btu/hr | Flow Rate | Temperature Rise °F |
|
| INPUT | GPM |
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| 90,000 | 30 | 5 |
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|
|
|
|
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| 135,000 | 30 | 7 |
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|
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| 180,000 | 30 | 10 |
|
|
|
|
|
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| 199,999 | 30 | 11 |
|
|
|
|
|
|
| 225,000 | 30 | 12 |
|
|
|
|
|
|
| 270,000 | 30 | 15 |
|
|
|
|
|
|
| 315,000 | 30 | 17 |
|
|
|
|
|
|
| 360,000 | 30 | 20 |
|
|
|
|
|
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| 399,999 | 30 | 22 |
|
|
|
|
|
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| 500,000 | 30 | 28 |
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|
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|
|
46 
