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PART 7: THERMAL EXPANSION (CLOSED SYSTEMS)
Thermal expansion occurs in any hot water system when system water is heated or “recovered” during
periods of non-use.
If the system is operated in an “open” condition, such as being connected directly to the city main, the
volume of expanded water generated during
recovery periods can be dissipated back through the “open”
connection to the city main so pressure cannot increase.
However, once a back flow preventer is installed to isolate system water from the public supply, or a
pressure reducing valve is installed to protect a water meter, or any device preventing flow back into the
cold water supply is installed
, the “open” condition becomes “closed”. During non-use periods, water
expands and pressure increases until a relief valve opens, releasing hot water.
A relief valve opening on pressure releases small amounts of water. A valve relieving on temperature
releases large amounts of water.
Since water is not compressible, some provision must be made for THERMAL EXPANSION to protect the
system from excessive pressures. HTP recommends installing a properly and adequately sized
expansion tank to meet the expanding volume of water.
Service problems or parts failure due to excessive pressure ARE NOT covered under warranty.
The temperature and pressure relief valve supplied with the water heater IS NOT considered to be
protection against thermal expansion.
Your water supplier or local plumbing inspector should be contacted on how to control thermal expansion.
PART 8: VENTING
The connection from the water heater vent to the stack must be made as direct as possible and of the
same diameter as the vent outlet. The recommended slope of any horizontal breaching is at least 1/2"
rise per linear foot. A barometrically operated draft regulator (barometric damper) shall be installed in the
vent connector at a location just above the water heater.
The stack must extend at least three feet above the highest point of the roof to insure proper venting. The
stack should be provided with a weather cap of approved design.
Provisions shall be made to prevent contact of the vent pipe with combustible materials in accordance
with all codes and regulations.
A separate vent for each appliance is strongly recommended. Separate vents are required for installation
and application of multiple power vents. If combined venting of multiple appliances is necessary, or if an
unusual situation arises, consult the National Fire Protection Standard for Oil Burning Equipment NFPA
No.31, or in Canada CSA B139.