2021 IMC Chapters 9 and 10
A boiler is a pressure vessel with a fire under it, so the code treats it differently from a furnace. Three things keep it from becoming a hazard: a relief valve that can always open, a low-water cutoff that stops the fire when the water is gone, and controls that were set and tested by the installer. This lesson follows 2021 IMC Chapter 10 for boilers, water heaters and pressure vessels, then samples the appliance rules of IMC Chapter 9. Gas-fired boilers also answer to IFGC Section 631, which sends installation back to the IMC. Lesson 1 explains why to confirm the boiler and mechanical code your city has adopted, and note that Section 1001.1 leaves out any boiler or pressure vessel already subject to federal or state inspection, so check who else has jurisdiction.
Chapter 10 governs the installation, alteration and repair of boilers, water heaters and pressure vessels (1001.1). Its exceptions include small unfired vessels of 5 cubic feet or less at 250 psi or less in Groups B, F, H, M, R, S and U, refrigeration vessels covered by Chapter 11, and any boiler or vessel subject to federal or state inspection. Boilers are built and certified to Section I or IV of the ASME Boiler and Pressure Vessel Code. Their controls and safety devices meet ASME CSD-1 when input is under 12,500,000 Btu/h, and NFPA 85 at 12,500,000 Btu/h or more (1004.1). The installer follows the manufacturer's instructions, leaves permanent operating instructions on the boiler, and sets, adjusts and tests all controls. The nameplate stays attached, and the installer furnishes a complete control diagram (1004.2, 1004.7).
Leave room to inspect, service, repair and replace the boiler, and to see every gauge. Passageways around all sides are at least 18 inches wide and unobstructed unless otherwise approved (1004.3). The top clearance to the ceiling or an overhead obstruction comes from Table 1004.3.1: 3 feet for boilers with manholes on top, and 2 feet for boilers without them. The table raises that to 3 feet for steam-heating and hot water boilers with input above 5,000,000 Btu/h or heating surface above 1,000 square feet, and for high-pressure steam boilers it asks for 3 feet or as much as 7 feet depending on capacity, heating surface or input. Boilers sit on a level base and are secured per the manufacturer (1004.4), on noncombustible floors unless listed for combustible flooring (1004.5). A boiler room complies with the IBC and Chapter 3 of the IMC and needs a floor drain or other approved means of disposing of liquid waste (1004.6).
Every boiler has a shutoff valve in its supply and return piping, and each boiler in a multiple installation gets its own; a single low-pressure steam boiler is the exception (1005.1). A steam boiler is protected with a safety valve, and a hot water boiler with a safety relief valve (1006.1, 1006.2). These valves are listed and labeled, have at least the rated capacity of the equipment, and are set no higher than the nameplate pressure rating of the boiler (1006.4). The valve goes directly into the boiler's own valve opening, nothing is placed on either side of it, and it discharges by gravity (1006.5). The discharge pipe is rigid, approved for the system temperature, and high-pressure steam valves discharge outdoors. Discharge piping may not connect directly to the drainage system, must end at an air break in the same room as the boiler, and is at least the size of the valve outlet, full size to the air break. It serves one relief device only and ends at the floor, the boiler pan, a waste receptor or outdoors where occupants can see it. It has no traps, valves, tees or threaded end, flows by gravity, and ends not more than 6 inches above the floor or receptor (1006.6).
Boilers carry the controls and limit devices the manufacturer and the listing require (1006.7). Control power comes from a two-wire grounded branch circuit or an isolation transformer, and control voltage is 150 volts nominal at most. Controls interrupt the ungrounded side, and there is a readily accessible disconnect that can be locked off (1006.8). Steam and hot water boilers need a low-water cutoff, except coil-type and water-tube boilers that force water through and are protected by a flow-sensing control (1007.1). It automatically stops combustion when water falls below the lowest safe level, or when circulation stops (1007.2). Steam boilers have a bottom blowoff valve that discharges to a safe place, and two valves in series where the working pressure exceeds 100 psig (1008). A hot water boiler has a pressure gauge and a temperature gauge, or a combination gauge, and a steam boiler has a pressure gauge and a water-gauge glass whose midpoint sits at the normal water level (1010).
Every hot water system has at least one expansion tank, closed or open, rated for the system pressure (1009.1). Closed tanks for operating pressure above 30 psi are ASME constructed. Where the design data are not known, size a closed tank from Table 1009.2: 100 gallons of system volume needs a 9 gallon diaphragm tank or a 15 gallon nonpressurized tank, 200 needs 17 or 30, 300 needs 25 or 45, 400 needs 33 or 60, 500 needs 42 or 75, 1,000 needs 83 or 150, and 2,000 needs 165 or 300. An open tank sits at least 4 feet above the highest heating element and has an overflow of at least 1 inch, piped to the drainage system as the IPC directs (1009.3).
A potable water heater or storage tank is listed and labeled, installed per the manufacturer, the IPC and the IMC, and must be removable without taking out a permanent part of the building. Relief valves and water connections follow the IPC (IMC 1002.1), and for gas heaters sizing, relief valves, drain pans and scald protection also follow the IPC (IFGC 624.1.1). A combination unit for drinking water and space heating must be listed for both and sized so heating does not eat into the potable capacity. Where space heating water runs above 140 degrees F, an ASSE 1017 mixing valve holds the potable side to 140 degrees F or less (1002.2). Chapter 9 adds rules by appliance:
| Appliance | Key rules (IMC Chapter 9 unless noted) |
|---|---|
| Vented wall furnace (909) | No ducts; door cannot swing within 12 in of an air inlet or outlet, and no door stop to fake it; manual shutoff ahead of controls; no bathroom air moved to other rooms |
| Floor furnace (910) | Not in an aisle or egress path of a public hall or assembly; 6 in from walls; drapery 12 in from the register; not in a slab on grade; 6 in above grade (2 in if the bottom is sealed); 12 in clear on all sides, 18 in on the control side |
| Unit heater (IFGC 620.4) | Suspended: 18 in sides, 12 in bottom, 6 in above the top with an internal draft hood; floor-mounted: 6 in at back and one side; noncombustible hangers |
| Forced-air furnace (918) | No volume damper that cuts the air the furnace needs; circulating air in continuous airtight ducts |
| Cooling tower (908) | Ready access; plume at least 5 ft above or 20 ft from any ventilation inlet; drift eliminators at 0.005 percent of flow at most; drains indirect |
| Infrared heater (912) | Fixed independent of its fuel or power lines, noncombustible hangers, maker's clearances |
When assembly and installation are complete, acceptance tests follow the ASME code or the manufacturer's requirements and must be approved. The code official receives copies of the test documents and the manufacturer's data reports (1011.1). The test gauge connects directly to the boiler where the operator can see it, is calibrated and certified by the test operator, and has a scale from at least 1.5 to at most 4 times the maximum test pressure (1011.2).
Worked example. A community center boiler is rated 2,000,000 Btu/h, under 12,500,000, so its controls and safety devices follow ASME CSD-1. The hydronic system holds 300 gallons, so Table 1009.2 asks for at least a 25 gallon diaphragm tank, or 45 gallons if the tank is nonpressurized. Between listed volumes the text gives no interpolation, so do not round down and ask the designer for the calculation. The acceptance test runs at 60 psi, so the test gauge scale must run from at least 1.5 x 60 = 90 psi to no more than 4 x 60 = 240 psi. A 0-60 psi gauge is too small and a 0-300 psi gauge is too large, but a 0-100 or 0-150 psi gauge fits.
In the field
A rehab crew replaces the relief valve on an old hot water boiler in a fellowship hall and, to keep the pipe away from the wall, caps the discharge with a threaded fitting and tees it into a floor sink line. Every one of those choices breaks Section 1006.6: threaded end, tee, and a direct drain connection. If the valve ever lifts, hot water has nowhere safe to go, and the person standing nearby cannot see it. Trace the discharge from the valve to its end: rigid, full size, one device, air break in the room, 6 inches or less above the floor.
1. A hot water heating boiler in a Group A-3 community hall is being inspected for pressure protection. Under IMC 1006.2, hot water boilers must be protected with:
2. The nameplate of a hot water boiler in a Group B clinic shows a maximum pressure of 30 psi. Under IMC 1006.4, the safety relief valve must be set at:
3. A relief valve discharge pipe on a boiler in a Group R-2 mechanical room ends 14 inches above the floor. Under IMC 1006.6, this discharge:
4. An inspector finds a shutoff valve and a tee on the relief valve discharge pipe of a boiler in a Group E school. Under IMC 1006.6, the installation is:
5. A boiler relief discharge in a Group B office is piped straight into the building drain with a solid connection. Under IMC 1006.6, this connection:
6. A steam boiler in a Group F-1 plant is inspected for controls. Under IMC 1007.1 and 1007.2, the low-water cutoff must:
7. A hydronic heating system in a Group I-1 facility holds 200 gallons and uses a closed pressurized diaphragm expansion tank. Under IMC Table 1009.2, the minimum tank capacity is:
8. An acceptance test of a boiler in a Group B building will run at 80 psi. Under IMC 1011.2, which test gauge scale is acceptable?
9. A boiler is being installed in a Group A-3 community center mechanical room. Under IMC 1004.3, the unobstructed passageway around all sides of the boiler must be at least:
10. A hot water heating boiler with no manhole on top, an input of 1,000,000 Btu/h and less than 1,000 square feet of heating surface, is being set in a Group B mechanical room. Under IMC Table 1004.3.1, the minimum clearance from the top of the boiler to the ceiling is:
11. A package boiler in a Group F-1 plant has a fuel input of 15,000,000 Btu/h. Under IMC 1004.1, its controls and safety devices must meet:
12. A boiler control circuit in a Group B clinic is fed from a 277 volt source without an isolation transformer. Under IMC 1006.8, what is the maximum control voltage?
13. A gas boiler in a Group R-2 building will sit on a wood-framed floor. Under IMC 1004.5, this is acceptable only if:
14. A combination water heater serves both a Group R-2 building's domestic hot water and a space-heating loop at 170 degrees F. Under IMC 1002.2.2, the domestic supply must: