Boiler and Steam Fitting (C-4) · Lesson 1 of 7

Sizing a Closed Expansion Tank by CMC Section 1004.4

"Sizing a Closed Expansion Tank by CMC Section 1004.4" is lesson 1 of 7 in the Boiler and Steam Fitting (C-4) Learning Hub. It opens with "A Closed Hot-Water System Must Have an Expansion Tank" and works through 5 short steps, then a worked example you can check yourself against.

Open the free trialSee pricing

One-time purchase, no subscription. Law & Business included.

Steps in this lesson

  1. A Closed Hot-Water System Must Have an Expansion Tank
  2. Open Versus Closed Tanks
  3. Pick the Sizing Method the Code Lets You Use
  4. Table 1004.4(2) Is a Shortcut With Fixed Assumptions
  5. The Diaphragm-Tank Equation

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Expansion volume" through "Compare with the table shortcut".

What you'll be able to do

  • Explain the main idea of "Sizing a Closed Expansion Tank by CMC Section 1004.4" in plain language you can use on the exam.
  • Apply this step with confidence: "Pick the Sizing Method the Code Lets You Use".
  • Apply this step with confidence: "Table 1004.4(2) Is a Shortcut With Fixed Assumptions".
  • Apply this step with confidence: "The Diaphragm-Tank Equation".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Sizing a Closed Expansion Tank (CMC 1004.4)DIAPHRAGM EXPANSION TANKAIRWATERto systemAir cushionPf = fill pressure12 psigDiaphragmWater sideaccepts the 8.16 galof expansionPRESSURES (absolute = gauge + 14.7)Pf (fill) = 12 + 14.7 = 26.7 psiaPo (max operating) = 30 + 14.7 = 44.7 psia1 - Pf / Po = 1 - 26.7 / 44.7 = 0.403Vs = 300 gal system, t = 180 F averageTANK SIZEExpansion = (0.00041 x 180 - 0.0466) x 300= 0.0272 x 300 = 8.16 galVt = 8.16 / 0.403 = 20.3 galPlain closed tank (Eq. 1004.4(1)): 36.8 galVt = (0.00041 t - 0.0466) Vs / (1 - Pf / Po) CMC Equation 1004.4(2)Vs = system gallons without the tank, t = average operating temperature in F, Pf = fill, Po = maximum operating pressureTable 1004.4(2) basis: 195 F, 12 psig fill, 30 psig max. A 300 gal system lists 25 gal (diaphragm) or 45 gal (non-diaphragm)Gravity systems: Table 1004.4(1) by EDR (650 sq ft = 24 gal). Open tank: 3 ft above the highest point (CMC 1004.2)Choose a diaphragm tank of at least 20.3 gal (about 25 gal from the 195 F table row)Source: 2025 California Mechanical Code 1004.4, Equation 1004.4(2), Tables 1004.4(1) and 1004.4(2), 1209.4CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: A Closed Hot-Water System Must Have an Expansion Tank

Heated water takes up more room, and in a closed loop the extra volume has nowhere to go. 2025 CMC §1004.1 requires an expansion tank in a hot-water-heating system to control the pressure increase from thermal expansion, and §1209.1 requires one in each closed hydronic system, of the closed type and rated for the pressure of the system. The tank must be supported to carry twice the weight of the tank filled with water without straining the connecting piping (§1004.1, §1209.2), and it must be piped so it can be drained without emptying the system.

Step 2: Open Versus Closed Tanks

An open-type tank must sit at least 3 feet above the highest point of the system, with an overflow at the top of at least half the size of the supply pipe or at least 1 inch, discharging through an air gap into the drainage system (§1004.2). A closed-type system needs an airtight tank or other approved air cushion, designed for a hydrostatic test pressure of 2-1/2 times the allowable working pressure; a tank for a system operating above 30 psi must comply with ASME BPVC Section VIII.1 (§1004.3, §1209.3). The shutoff to the tank must stay open in operation, with the valve handle locked open or removed (§1209.2), and non-diaphragm tanks get isolation valves at their connections (§1212.7).

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

More lessons for this trade

Browse all lesson previews

Ready to practice for real?

Try the real classroom free, no sign-up, or get the full course for your trade.

Open the free trialSee pricing

Independent exam preparation. Contractors Classroom is not affiliated with the Contractors State License Board, and no course can guarantee a passing result.