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Storage Water Heaters

Print Date : 1/15/2009 2:07:20 PM

1. Description and Overview A storage water heater is one of the most commonly founditems in a boiler inspector's working environment. However, not all water heaters will fallwithin the jurisdictional mandate requiring a periodic inspection.

Most jurisdictions use the exemption criteria outlined in ASME Section IV, paragraph

HLW-101.2. In summary, most water heaters are exempted when none of the followinglimitations is exceeded:

n a heat input of 200,000 Btu/hr (58.6kW), or n a water temperature of 210 F, or n a nominal water-containing capacity of 120 gallons.

Some jurisdictions use different limiting criteria and the inspector must be familiar withthose differences, if any, in each jurisdiction in which they are working.

Most storage water heater manufacturers offer water heaters which have a heat input justbelow 200,000 Btu/hr, such as 199, 900 Btu/hr, or a capacity just below 120 gallons, suchas 119.9 gallons. These water heaters will typically not exhibit the ASME "HLW" symbolnor a National Board number. In addition, these water heaters will be exempt fromregulation (inspection) in most jurisdictions.

Storage water heaters will be installed in almost every type of facility which needs hotwater. Facilities such as self service laundries or car washes will need a high recoveryrate so the heat input rating may exceed 500,000 Btu/hr while the capacity may only be80 or 85 gallons. Other types of facilities may not require such a high recovery rate andinstead opt for a larger water capacity. Facilities needing even more capacity mayconnect a hot water storage tank (which can appear almost identical to a storage water heater) to the water heater.

Storage water heaters may be constructed using unlined corrosion resistant material or they may be lined with acceptable materials which are listed in HLW-200.

2. Appurtenances, Settings, and Piping Storage water heaters must have at least onetemperature and pressure safety relief valve or at least one safety relief valve. No safetyrelief valve can be smaller than NPS 3/4. The set pressure of the safety relief valve mustnot exceed the maximum allowable working pressure (MAWP) marked on the water heater.

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The minimum relieving capacity of the safety relief valve must not be less than themaximum input. If the water heater is clearly marked with the rated burner input capacity,that value may be used as the basis for safety relief valve sizing. Temperature andpressure safety relief valves typically show two different capacity values; a CSA* ratingand an ASME rating. The CSA rating is always less than the ASME rating. Inspectorshave asked the same question for years "which capacity rating do I use, CSA or ASME,when determining acceptable relieving capacity?" While this guide should not be

considered the definitive answer to the question, the most widely accepted approach isthis:

n if the water heater is stamped with the ASME "HLW" symbol, use the ASMEcapacity rating shown on the safety relief valve, or

n if the water heater does not exhibit the "HLW" symbol, use the CSA capacity ratingshown on the safety relief valve.

The inspector is encouraged to ascertain and follow the instructions of the applicable jurisdiction.

* The CSA rating replaces the AGA rating used up until approximately the year 2000.From an inspector's point of view, nothing has changed except the name of the ratingorganization.

Safety relief valves on storage water heaters may be installed in a vertical or horizontalposition.

More information on safety relief valve requirements can be found in ASME Section IV,HLW-800 and HLW-801.

Each storage water heater must have:

n

an operating control (HLW-701.1);n a separate high temperature limit control with a maximum possible setting of 210 F(HLW-701.1);

n suitable primary (flame safeguard) safety controls, safety limit switches, and burnersor electric elements as required by a nationally recognized standard (HLW-703);

n a bottom drain valve NPS 3/4 or greater (HLW-810); andn a thermometer (HLW-820)

Storage water heaters should be installed on a solid floor or on a structure designed tosafely support the weight of a water heater completely filled with water. Clearancemeasurements must comply with all jurisdictional and manufacturer's requirements.

Fire codes typically require the water heater to be elevated at least 18" above the floor if installed in a garage or other location where gasoline fumes could collect.

Local building codes may require seismic bracing to prevent the water heater from fallingover during an earthquake.

Although most jurisdictions do not require inspection of the piping associated with astorage water heater, there are some water supply installation requirements in Section IVthe inspector should review. Please see HLW-805.

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n look for evidence of overheating (the cosmetic jacket may be warped or exhibitscorched paint);

n inspect the fuel burning apparatus as required by the jurisdiction;ask the owner or owner's representative to drain a few gallons of water from the bottom drain valve inorder to observe the amount of scale, sludge, or mud, if it can be accomplishedwithout damage to the water heater. A slight rusty tint to the water, which quicklydisappears is usually not a cause for concern. A large quantity of rusty water could

mean the water heater is close to the end of its useful life.

An internal inspection of a typical storage water heater is impractical during routineinspections. If the owner or owner's representative wishes to determine the extent of interior corrosion, the only openings available for inspection are those for the:

n safety relief valven water inletn water outletn bottom drainn sacrificial anoden heating elements (on electric models only)

5. Miscellaneous Information Additional information to aid inspectors of storage water heaters can be found in the following publications and sources:

n National Board Inspection Code n ASME Section IVn Manufacturer's Installation, Operation, and Maintenance Documentationn Jurisdictional Laws, Rules, and Directives

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