16.3 Fixture, Vent & Gas Isometric Plan Analysis

One building, two views, 20 scored pictures

Quick Answer: Lay the floor plan next to the isometric and walk fixtures → traps → waste → vents → water → gas. Fail the sheet if a vent is on one view only, if the T&P has a shutoff or dumps more than 12 inches above the floor, if a gas connector exceeds 48 inches, if CSST bonding or 18-inch burial is missing, if combustion air is missing, or if a Category IV vent is tied into a Category I Type B chimney. You have 160 minutes for 40 Part II items; spend about six minutes on each isometric.

Section 16.1 taught you to read the picture. Section 16.2 put 248 CMR 10.15 / 10.16 numbers on the DWV. This section is the coordination item: the same Massachusetts house on a plan and an isometric, with water-heater and fuel-gas mistakes sitting on the same sheet as a missing lavatory vent. The water-heater, CSST, combustion-air, and appliance-venting chapters already taught the lectures. Here you only spot those defects on a drawing.

How to overlay plan and isometric in 60 seconds

1. Count fixtures on the plan. Count fixtures on the isometric. If the plan has four trapable fixtures and the isometric shows three wastes, you already have the item.
2. For each fixture, point to its trap and vent on both views. A dashed vent on the isometric that has no corresponding roof terminal or stack-vent tie-in is unfinished. A vent through the roof on the plan with no dashed line on the isometric is a missing vent.
3. Only then run the 16.2 DFU takeoff on the isometric sizes.
4. Only then read water-heater T&P, gas connector, CSST notes, burial depth, combustion-air openings, and vent category.

PSI loves a legal 4-inch building drain sitting next to an illegal gas connector. Candidates who fall in love with Table 2 never look at the heater closet.

Missing vent — the plan/isometric mismatch

Drawing. Plan: bathroom group plus kitchen. Each fixture has a vent symbol (VTR or a 1½-inch vent leaving the lavatory wall). Isometric: 2-inch wet vent from the lavatory serving the tub and WC on that floor, kitchen 1½-inch individual vent omitted. The kitchen trap arm is drawn 4 feet of 1½-inch pipe into the 3-inch waste with no vent fitting.

248 CMR 10.16(10) requires a vent within Table 1 distance. A kitchen sink is not on the bathroom wet vent (10.16(5)(b)). Combination waste-and-vent is prohibited without Board special permission (10.16(1)(a)). Air admittance valves are not permitted without special permission (10.16(1)(e)). There is no Massachusetts “island sink exception” you can invent on Part II. The kitchen is unvented. That is the answer, even if the bath wet vent is drawn perfectly.

The reverse drawing is just as common: isometric shows the kitchen vent; the plan shows the island sink with no vent chase and no bow vent. 248 CMR 10.16(9) allows a bow vent when a sink or lavatory cannot rise 6 inches above the flood-level rim before turning horizontal — that is a plan geometry problem. If neither view shows a bow vent, loop, or individual vent, fail it.

Wrong water-heater T&P

Drawing. Vertical gas storage water heater. T&P screwed into a tee in the cold inlet 18 inches down from the top, with a ball valve on the ¾-inch copper discharge, which turns and dumps 30 inches above the floor into a service sink.

248 CMR 10.14(6)(f)2, in the form you need on a picture:

• Required on storage equipment that can heat water above 212°F.
• In a tapping in or on the tank, within 12 inches of the top of a vertical tank (6 inches on a horizontal tank), no fittings between valve and tank except a bushing, or in the hot outlet not more than 5 inches from the top.
• Discharge full size, non-ferrous or tubing rated for the relief temperature, no shutoff, to a fixture or to a point within 12 inches of the floor.

The isometric fails three ways: location (cold inlet, 18 inches down), shutoff on the discharge, and termination 30 inches above the floor. M.G.L. c. 142 still sits behind water-heater work in Massachusetts; you do not need the statute text on Part II if 10.14 is drawn wrong. Do not pick “T&P must dump to the building drain with a trap” — that is not the 10.14 termination language.

Gas connector too long, and CSST is not the connector

Drawing. Water heater shutoff on the wall. A 72-inch listed corrugated appliance connector runs from the shutoff to the heater. A second option on some sheets: CSST continues past the shutoff and screws straight to the appliance as if it were the flex connector.

248 CMR 5.00 amends NFPA 54 Chapter 9: the maximum length of a connector is 48 inches, with the stated exceptions for appliances that 9.6.1(4) / 9.6.1.1 already cover — not a storage water heater in a closet. CSST shall not be used as a flexible appliance connector downstream of the appliance shutoff device. Fixed appliances may use CSST to a termination fitting and gas cock under the fixed-appliance rules; that is not a 6-foot yellow connector. If the isometric shows 72 inches of connector, it is illegal in Massachusetts even if a national NFPA study book said 6 feet.

CSST bonding note and underground burial

Drawing. CSST from the meter along the basement ceiling, then under the slab to a garage heater. Notes on the isometric: “bond per manufacturer” blank; burial 12 inches below grade; CSST in the earth without a sleeve.

248 CMR 5.07 (NFPA 54 7.13.2 as amended): CSST systems are bonded in accordance with 7.13.1 unless that bonding would conflict with manufacturer instructions. If the manufacturer requires additional bonding that is electrical licensure work, the plumbing/gas inspector may not approve it without proof of an electrical permit. On Part II, a CSST run with no bonding note and no 7.13.1 jumper shown is the defect you can score. Do not write a 6 AWG lecture here — that is the CSST chapter. Look for the note and the jumper on the drawing.

Underground CSST (248 CMR 5.00):

• Sleeve that can take the superimposed load; free movement of the CSST in the sleeve (unless a Board-accepted pre-engineered sleeve).
• Sleeve ends sealed liquid tight.
• Warning tape not more than 6 inches from the top of grade.
• Minimum depth 18 inches from the top of grade.

12 inches of cover fails. Bare CSST in earth fails. Tape missing fails. The isometric may show only one of those — that is still the item.

Combustion air on the same sheet

Drawing. 40,000 Btu/h Category I water heater in a tight closet, louvered door not drawn, no ducts to outdoors, no openings to the adjoining basement. The DWV isometric is otherwise legal.

Combustion air is NFPA 54 Section 9.3 as amended — indoor air, outdoor air, or a combination, with opening sizes from the fuel-gas chapter you already studied. On Part II you are not recomputing 1 in² per 1,000 Btu unless the item prints the input and the opening dimensions. You are failing a sealed closet with a draft-hood appliance and zero openings. Direct-vent Category IV appliances that take combustion air from outdoors through a coaxial or two-pipe vent are a different picture: if that two-pipe vent is drawn, do not demand a louvered closet door.

Category I versus Category IV on the same drawing

Drawing. Existing Type B vent (Category I, non-positive, typically natural draft or fan-assisted that still tests non-positive) in the chase. New condensing (Category IV, positive pressure, plastic vent) water heater with a PVC vent teed into the Type B above the draft hood of the remaining atmospheric furnace.

That tee is the fail. Category IV products pressurize the vent with condensate. Category I Type B and masonry chimneys are not that system. 248 CMR 5.12 also keeps you from stuffing a fan-assisted replacement into an existing chimney under the old atmospheric rules: existing chimneys may not be used when replacing an atmospheric appliance with a fan-assisted appliance under the amended 12.6.4.2 exception. PVC used as vent must be marked as vent (12.7.6.1). If the isometric shows Schedule 40 PVC labeled only “drain” climbing from a condensing heater into a B-vent, you have both a category clash and a marking/use clash.

Do not mix this with DWV plastic. A PVC T&P drain next to a PVC Category IV vent can look identical on a sloppy isometric. Read the labels: T&P versus vent.

Time strategy: 20 isometric items inside a 40-item / 160-minute part

Part II is one sitting: 40 practical items, 160 minutes, 70%, closed book, non-programmable calculator, PSI scrap paper only. You cannot bring a printed Table 2. You cannot bring your own isometric pad. You can dump Table 1 DFUs, Table 2 building-drain numbers (180 / 216 on 4-inch, no ⅛ on 3-inch), trap-arm lengths (5 / 6 / 8 / 10), and 48-inch connectors onto PSI paper in the first minute.

70% of 40 = 28 correct. You can miss 12 items on the part. You cannot budget to miss most of the 20 isometrics and “make it up” on the other 20 unless those other 20 are perfect. If an isometric is still opaque at six minutes, pick the defect you can defend (wrong pitch, missing 4-inch drain, connector length, T&P shutoff), flag it, and move. A blank item is a zero; a reasoned T&P fail still might be right.

Order inside one isometric (use the 16.1 sequence, timed):

• 30 seconds — fixture count versus plan.
• 90 seconds — traps, wet vent, trap arms, illegal fitting.
• 90 seconds — DFU, branch, stack, 4-inch building drain / ⅛-inch-on-3-inch trap.
• 90 seconds — T&P, connector, CSST bond/bury, combustion air, Category I vs IV.
• 60 seconds — match the stem (“which is wrong,” “minimum size,” “developed length”) to one finding.

That is five minutes of work plus a minute to read options. It fits the 160-minute part if you do not redraw the house on scrap paper. Sketch only the centerline you are measuring.

Independent OpenExamPrep practice for this chapter is drawing-based reasoning, not a substitute for 248 CMR print from the State Bookstore. On exam day the book stays home. The 20 isometric items do not.