2021 IMC Chapter 12
Hydronic piping carries hot water, chilled water, steam and condensate between the plant and the coils and terminal units of an HVAC system. Chapter 12 of the 2021 IMC covers the pipe, its joints, its valves and its tests. Two parts the loop cannot live without, the expansion tank and the relief valve, sit in Chapter 10, so this lesson pulls them in. Houston enforces the Uniform Mechanical Code rather than the IMC (Lesson 1), so check the adopted code before you write a correction.
The chapter applies to hydronic piping that is part of heating, ventilation and air-conditioning systems: steam, hot water, chilled water, condensate and ground-source heat pump loops (1201.1). Potable hot and cold water belongs to the International Plumbing Code. Components are sized for the demand of the system (1201.2). As an alternative to Sections 1202 and 1203, piping can be designed, installed, inspected and tested to ASME B31.9 (1201.3).
Materials must be rated for the operating temperature and pressure and suit the fluid (1202.3), and the outside of the pipe must be protected from corrosion. Table 1202.4 lists accepted pipe: copper tube (Type K, L or M), copper pipe, steel pipe (ASTM A53 or A106), PEX, PE-RT, CPVC, PVC, polypropylene and ductile iron, among others. Fittings must be approved for the pipe, and reused pipe and valves must be clean and approved by the code official (1202.2). Brazing uses filler to AWS A5.8 (1203.3.1). Soldered copper follows ASTM B828 with ASTM B813 flux and ASTM B32 solder, after tube ends are reamed to full inside diameter (1203.3.3). CPVC and PVC take primer and solvent cement, with a narrow primerless exception for certified yellow cement on 1/2 to 2 inch CPVC (1203.3.4). Threads follow ASME B1.20.1, with sealant on the male threads only (1203.3.5).
Shutoff valves are required on the supply and return of a heat exchanger, on the building supply and return of a central system, on the connection to any pressure vessel, on both sides of a pressure-reducing valve, on connections to mechanical equipment, and at connections to nondiaphragm expansion tanks (1205.1). Pumps, air separators and metering devices are exempt from the equipment rule. The low-pressure side of a pressure-reduced system gets a relief valve set at the system's maximum design pressure (1205.2).
The system must be drainable (1206.2), and the potable supply is protected from backflow under the IPC (1206.3). Penetration openings are larger than the pipe, concrete and masonry openings are sleeved, and the annular space is protected per the IBC (1206.4). Pipe whose outside surface exceeds 250°F needs 1 inch of clearance to combustibles (1206.5). Velocity is controlled to limit water hammer, with an arrestor near a quick-closing valve (1206.7). Support follows Section 305, and condensation on pipe exteriors must be prevented (1206.10, 1206.11). Pipe insulation is tested to a flame spread index of 25 or less and a smoke-developed index of 450 or less, except in one- and two-family dwellings, and its thickness comes from the IECC (1204). Transfer fluid must have a flash point at least 50°F above the highest operating temperature (1207.1).
Water expands when heated, so every hot water system needs an expansion tank, closed or open, rated for the system pressure (1009.1). A closed tank on a system designed to operate above 30 psi must be constructed and certified to the ASME Boiler and Pressure Vessel Code (1009.2). Where the design data are not all known, Table 1009.2 sizes the tank by system volume.
| System volume (gal) | Diaphragm tank (gal) | Nonpressurized tank (gal) |
|---|---|---|
| 100 | 9 | 15 |
| 200 | 17 | 30 |
| 300 | 25 | 45 |
| 400 | 33 | 60 |
| 500 | 42 | 75 |
| 1,000 | 83 | 150 |
An open tank sits at least 4 feet above the highest heating element and has an overflow of at least 1 inch diameter draining to the plumbing drainage system (1009.3). The boiler itself carries a listed safety relief valve set at or below its nameplate pressure (1006.2, 1006.4), mounted directly in the boiler's relief opening with no valves on either side (1006.5). Its discharge pipe is rigid and full size, without traps, valves, tees or threaded ends, and it ends at an air break in the same room within 6 inches of the floor or a waste receptor, where occupants can see it (1006.6).
Worked example. A church hall loop holds 400 gallons. Table 1009.2 calls for a 33 gallon diaphragm tank or a 60 gallon nonpressurized tank, so a 20 gallon diaphragm tank fails on volume. If the loop is designed above 30 psi, the closed tank must also carry ASME certification.
Hydronic piping is tested hydrostatically at one and one-half times the maximum system design pressure, but not less than 100 psi, for at least 15 minutes (1208.1). The 100 psi floor governs any system designed below about 67 psi, because 1.5 x 67 is 100. Worked example. A loop designed for 60 psi calculates to 1.5 x 60 = 90 psi, which is under the floor, so the test is 100 psi. A loop designed for 80 psi calculates to 1.5 x 80 = 120 psi, so 120 psi governs. PEX may be tested with compressed gas instead only where all the manufacturers' instructions for the installed pipe and fittings authorize it and no other rule prohibits it.
In the field
A parish hall gets a new boiler on an old loop. Ask three questions. Is there an expansion tank, and is it sized to the loop volume rather than to the small tank that came with the old boiler? Where does the relief pipe end (a hose into a bucket, or a tee into a sink drain, fails 1006.6)? What pressure will the test gauge show, and for how long? Reused pipe and valves must be clean and approved by the code official (1202.2).
Heating panel pipe is steel pipe, Type L copper, polybutylene or other approved plastic rated 100 psi at 180°F (1209.1). Pipe to be embedded in concrete is pressure tested before the pour and held at operating pressure during it (1209.2). A radiant slab needs a thermal break of asphalt expansion joint or similar material where it meets a foundation wall or another conductive slab, and the insulation's R-value mark must stay visible (1209.5). Ground-source loops may not use reused materials (1210.2). The assembled loop is tested with water at 100 psi for 15 minutes before header trenches are backfilled, and flow and pressure drop are then compared with design; a difference over 10 percent means finding and fixing the cause (1210.10).
Chapter 13 covers fuel oil piping and storage, with tank storage tied to the International Fire Code (1301.1, 1301.2). Chapter 14 covers solar thermal systems for space heating or cooling, domestic hot water, pool heating and process heating (1401.1). Reach for them when a rehab includes an oil-fired boiler or rooftop solar collectors.
1. A hot water heating loop in a Group B office has a maximum system design pressure of 80 psi. Under 1208.1, the minimum hydrostatic test pressure is:
2. A chilled water loop in a Group E school has a maximum design pressure of 50 psi. What test pressure does 1208.1 require?
3. A crew hydrostatically tests a new heating water main in a Group R-2 apartment building. What is the minimum time the test pressure must be held under 1208.1?
4. A 300 gallon hot water system in a Group A-3 community hall is sized from Table 1009.2 using a pressurized diaphragm expansion tank. What is the minimum tank capacity?
5. An open-type expansion tank is proposed for the hot water heating system of a small clinic. Under 1009.3, it must be located at least:
6. A closed-type expansion tank serves a hot water system in a Group R-1 hotel that is designed to operate above 30 psi. Under 1009.2, the tank must be:
7. An inspector finds the discharge pipe of a boiler relief valve in a Group B building ending 14 inches above the floor. Under 1006.6, the pipe may end not more than:
8. A boiler relief valve discharge pipe in a small church is inspected. Which feature violates Section 1006.6?
9. A chilled water system serves a Group B office. Which item is NOT covered by the requirement for shutoff valves at connections to mechanical equipment in 1205.1.5?
10. A steam line in a Group F workshop passes near wood framing, and its outside surface temperature exceeds 250°F. What clearance to combustible materials does 1206.5 require?
11. A glycol transfer fluid in a Group B building has a maximum operating temperature of 200°F. Under 1207.1, its flash point must be at least:
12. A contractor in a Group R-2 building wants to test a PEX hydronic system with compressed air instead of water. When does 1208.1 allow this?
13. Hydronic tubing will be embedded in the concrete slab of a community center for radiant heat. When does 1209.2 require the piping to be pressure tested?
14. The assembled ground-source heat pump loop for a church is ready for testing. Before the header trenches are backfilled, 1210.10 requires a water test at: