Storage Water Heaters

Description and Overview

A storage water heater is one of the most commonly found items in a boiler inspector's working environment. However, not all water heaters will fall within the jurisdictional mandate requiring a periodic inspection.

Most jurisdictions use the exemption criteria outlined in ASME BPV Code Section IV, paragraph HLW-101.2. In summary, most water heaters are exempted when none of the following limitations is exceeded:

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

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

Most storage water heater manufacturers offer water heaters which have a heat input just below 200,000 Btu/hr, such as 199, 900 Btu/hr, or a capacity just below 120 gallons, such as 119.9 gallons. These water heaters will typically not exhibit the ASME "HLW" designator nor a National Board number. In addition, these water heaters will be exempt from regulation (inspection) in most jurisdictions.

Storage water heaters will be installed in almost every type of facility which needs hot water. Facilities such as self service laundries or car washes will need a high recovery rate so the heat input rating may exceed 500,000 Btu/hr while the capacity may only be 80 or 85 gallons. Other types of facilities may not require such a high recovery rate and instead opt for a larger water capacity. Facilities needing even more capacity may connect 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 ASME BPV Code Section IV, Paragraph HLW-200.

Storage water heaters must have at least one temperature and pressure safety relief valve or at least one safety relief valve. No safety relief valve can be smaller than NPS 3/4. The set pressure of the safety relief valve must not exceed the maximum allowable working pressure (MAWP) marked on the water heater.

The minimum relieving capacity of the safety relief valve must not be less than the maximum 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 and pressure safety relief valves typically show two different capacity values; a Canadian Standards Association (CSA)* rating and an ASME rating. The CSA rating is always less than the ASME rating. Inspectors have 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 is this:

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

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

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

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

Each storage water heater must have:

Storage water heaters should be installed on a solid floor or on a structure designed to safely support the weight of a water heater completely filled with water. Clearance measurements 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 falling over during an earthquake.

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

Part 1, Section 3 in the National Board Inspection Code addresses installation requirements and must be followed when mandated by the applicable jurisdiction.

  • if the water heater is stamped with the ASME "HLW" designator, use the ASME capacity rating shown on the safety relief valve, or
  • if the water heater does not exhibit the "HLW" designator, use the CSA capacity rating shown on the safety relief valve.
  • an operating control (HLW-701.1);
  • a separate high temperature limit control with a maximum possible setting of 210° F (HLW-701.1);
  • suitable primary (flame safeguard) safety controls, safety limit switches, and burners or electric elements as required by a nationally recognized standard (HLW-703);
  • a bottom drain valve NPS 3/4 or greater (HLW-810); and
  • a thermometer (HLW-820)

Storage water heaters usually have an effective life of approximately 7-8 years before they start to leak. Depending upon the water conditions, this life span could be less or more. Most lined water heaters have a sacrificial anode which will help protect the water heater shell should any part of the lining fail. Although the sacrificial anode is replaceable, very few, if any, water heater owners have ever done so.

The most common problem associated with storage water heaters is a build-up of sludge or mud in the bottom of the water heater shell. Since the water heater is used for potable water service with 100% makeup, there is no permitted form of water treatment other than filtering, softening or other non-toxic processes. The most effective way of preventing sludge or mud accumulation is for the water heater owner to periodically drain a few gallons of water out of the bottom drain valve. This simple procedure performed every 6-12 months will help the efficiency of a water heater fired with oil or gas since the sludge or mud acts as an insulator and inhibits heat transfer. The sludge or mud can also cause premature failure of the lining due to overheating because of a lack of heat transfer.

If the bottom drain valve is used infrequently or never, it may be seized in the closed position. Attempts to force it open could cause it to break or seize in the open position. The water heater owner should be aware of this possibility and arrange to have qualified personnel perform the periodic flushing procedure if the owner does not have this capability.

Upon entering the area where the storage water heater is operating, the inspector should perform a general assessment of the water heater, piping, controls, fuel system or electrical supply, and combustion air supply for water heaters fired with gas or oil. The inspector should then:

An internal inspection of a typical storage water heater is impractical during routine inspections. 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:

  • review the current operating certificate (if one was issued in the past) and compare the information to the associated water heater and its nameplate;
  • compare the safety relief valve nameplate data (set pressure and relieving capacity) with the water heater nameplate to ensure the safety relief valve is adequate for this installation;
  • inspect the safety relief valve operation as described in the National Board Inspector Guide for Pressure Relief Devices;
  • check for evidence of "weeping" from the safety relief valve outlet. If present, it could be the result of thermal expansion of the water combined with a high water supply pressure or a check valve on the cold water supply pipe. In these cases, an expansion tank is strongly recommended;
  • inspect the operating control and the high temperature limit control;
  • check the thermometer reading (if there is a reason to question the accuracy of the thermometer, it should be replaced or recalibrated);
  • look closely for leaks at all pipe connections associated with the water heater;
  • look closely for evidence of leaks from the water heater. If leaks are observed from the water heater shell, welded repairs are not economically practical if the water heater shell is of the lined variety;
  • look closely for evidence of corrosion between the cosmetic jacket and the water heater shell (the insulation in this area can sometimes trap moisture or condensation);
  • look for evidence of overheating (the cosmetic jacket may be warped or exhibit scorched paint);
  • 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 in order to observe the amount of scale, sludge, or mud, if it can be accomplished without damage to the water heater. A slight rusty tint to the water, which quickly disappears 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.
  • safety relief valve
  • water inlet
  • water outlet
  • bottom drain
  • sacrificial anode
  • heating elements (on electric models only)  

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

  • National Board Inspection Code
  • ASME BPV Code Section IV
  • Manufacturer's Installation, Operation, and Maintenance Documentation
  • Jurisdictional Laws, Rules, and Directives