FM Global DS 3-2: Fire Water Tank Heating and Freeze Protection

In cold climates a frozen fire water tank or tank piping takes the water supply out of service; FM DS 3-2 Section 3.8 notes that ice in or on tank structures has directly caused several collapses. FM Global DS 3-2 (Water Tanks for Fire Protection, October 2015, Interim Revision April 2026) tied freeze assessment in its July 2022 revision to the 100-year return period daily minimum temperature (100-year DMT) zones of the FM Worldwide Freeze Map (Appendix B), with further guidance in DS 9-18, Prevention of Freeze-Ups.

When to Insulate and When to Heat (2.2.6, pp. 13–14)

• The need for protection follows the site’s 100-year DMT zone on the FM Worldwide Freeze Map; use local data where conditions are known to be more severe, for example in mountainous areas (2.2.6.1).
• Heating and insulation, where required, must keep the water in the tank or piping at a minimum of 42°F (5.6°C) on the coldest design day; insulation is protected from weather (2.2.6.2).
• Zones of 20°F (−6.7°C) or colder: frost-proof casing or insulation for exposed risers under 3 ft (0.91 m) diameter or those within pedestal tanks, and for exposed suction tank piping, with minimum R-values of 3.5 hr-ft²-°F/Btu (0.62 m²-°C/W) for 20°F to −5°F zones, 5.5 (0.97) for −10°F to −20°F and 7 (1.23) for −25°F and colder (2.2.6.3).
• Zones of 5°F (−15°C) or colder: heat those risers and the exposed suction piping (2.2.6.4).
• Zones of −5°F (−20.6°C) or colder: heat pump suction tanks and steel gravity tanks and their risers; insulation may be added to conserve heat (2.2.6.5).
• Piping in unheated buildings: frost-proof casings, insulation of R 3.5 (0.62) or more and/or heating (2.2.6.6).
• A low-water-temperature alarm set at 40°F (4.4°C), connected to a central station or a continuously manned control room (2.2.6.7).
• Break tanks, fill valves, the fire pump and piping in a heated area; float valve pipe wells and fill valve pilot lines protected from freezing (2.2.2.12).

DS 3-2 notes that 100-year DMT values are usually 10–15°F (5.6–8.3°C) colder than the lowest one-day mean temperature (LODMT) used previously (Section 3.8). A site’s zone is read from the FM Worldwide Freeze Map at www.fmglobal.com, which is not among the sources for this article.

Heating Systems (3.8)

• Heat demand: the heating system must replace the heat lost from the tank and piping when the coldest water is just above freezing and the outside temperature is at its lowest for the locality; heat losses are taken from Tables 6–9 by 100-year DMT zone.
• Circulating systems (3.8.2): cold water from near the tank bottom or the discharge pipe passes through a heater and returns through a separate hot water pipe. Manually controlled gravity systems need close supervision; automatic control costs more initially but needs less supervision, with the sensor placed to read the true tank water temperature. Forced systems use an electric motor-driven pump, are always automatically controlled and need a reliable electric supply; they are necessary when the tank is far from a heated building and the pipes are below grade. The pump should have a rated head at least 5 psi (0.34 bar) above the pipe friction loss at rated capacity and a rated capacity of at least 10 gpm (38 L/min).
• Steam heaters: FM Approved, needing a constantly available steam pressure of 10–50 psi (0.69–3.45 bar); do not use hot water in a heater designed for steam, and connect multiple heaters in parallel at the same level with symmetrical piping (3.8.2).
• Vertical radiator heaters (3.8.3) and steam coils inside tanks (3.8.4) are further options.
• Insulation (3.8.1): exterior insulation is preferred and protected from weather; interior insulation is allowed only behind a flexible liner, with the net tank capacity reduced accordingly. R-values depend on the material chosen (Table 10).
• Air-bubbler systems (3.8.5): they keep marinas and reservoirs ice-free because those bodies of water are very large and in contact with ground below the frost line. DS 3-2 stresses that fire protection tanks do not have that much available heat, and the method does not appear among the data sheet’s recommendations (Section 2).

Operation and Maintenance (2.3.10–2.3.13, pp. 14–15)

• In 100-year DMT zones of 20°F (−6.7°C) or colder, follow DS 9-18 and 2.3.11–2.3.13 (2.3.10).
• During freezing weather, check daily (or more often) that the coldest water is at least 42°F (5.6°C) (2.3.13.1), that pressure tank and other enclosures are at least 40°F (4.4°C), and that gravity tanks, their structures and roofs below are free of ice (2.3.11).
• Flush the circulating pipe and heater in the fall and about monthly during the heating season, extending to no more than two months depending on sedimentation (2.3.13.2).
• In the fall, test the heating system and check thermometers, gauges and low temperature alarms, relief valves, steam regulators, pressure-reducing valves, thermostats and safety pilots (2.3.13.3).
• At the end of the season, clean and overhaul heaters, traps and strainers and renew gaskets; have gas- or oil-fired heaters serviced during the summer (2.3.13.4).
• Every five years, or at the manufacturer’s interval, carry out major maintenance (2.3.13.5), cleaning radiator pipes and replacing badly corroded pipe with copper tubing, brass (85% copper) or cast iron (3.8.6).
• If the tank water is partly or fully frozen, provide an adequate emergency water supply and follow Section 3.8.8 (2.3.12).

Quick Checklist

• 100-year DMT zone established from the FM Worldwide Freeze Map
• Casing or insulation (≤ −6.7°C), pipe heating (≤ −15°C) and tank heating (≤ −20.6°C) decided by zone
• Heating sized from Tables 6–9 to hold at least 5.6°C on the coldest day
• 40°F (4.4°C) low-water-temperature alarm connected to a supervised location
• Reliable power and adequate pump capacity for forced circulation
• Exposed suction piping and break tank connections protected
• Fall test, monthly flushing, end-of-season and five-yearly maintenance recorded

Frequently Asked Questions

What temperature must the tank water be kept at?

FM DS 3-2 2.2.6.2 calls for heating and insulation that keep the water in the tank or piping at a minimum of 42°F (5.6°C) on the coldest design day. Section 2.2.6.7 adds a low-water-temperature alarm set at 40°F (4.4°C), connected to a central station or a continuously manned control room, and during freezing weather the coldest water temperature is checked daily (2.3.13.1).

What is the difference between forced and gravity circulation?

Per DS 3-2 3.8.2, forced circulation uses an electric motor-driven pump, is always automatically controlled and needs a reliable electric supply; it is necessary when the tank is far from a heated building with below-grade piping, and the pump needs a head at least 5 psi above pipe friction and at least 10 gpm (38 L/min). Manually controlled gravity systems need close supervision, while automatically controlled ones need less.

Does the suction line need freeze protection too?

Yes. Under DS 3-2 2.2.6.3 and 2.2.6.4, exposed suction tank piping needs a frost-proof casing or insulation in 100-year DMT zones of 20°F (−6.7°C) or colder, and heating in zones of 5°F (−15°C) or colder. Break tanks, fill valves, pump and piping belong in a heated area (2.2.2.12).

How often should the heating system be serviced?

Per DS 3-2 2.3.13: test the heating system and check its instruments and alarms in the fall, flush the circulating pipe and heater in the fall and about monthly during the heating season, overhaul at the end of the season, and carry out major maintenance every five years or at the manufacturer's interval.

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