Replacing the Water Heaters in a 1,120-Cubic-Foot Mechanical Room
A 1970s school is replacing its domestic hot water plant. The mechanical room is 14 feet by 10 feet with an 8-foot ceiling (1,120 cubic feet). Two existing natural-draft water heaters, each 100,000 Btu/h, are being removed. The new design installs one 399,000 Btu/h Category IV condensing boiler with a two-pipe PVC sidewall vent and retains one 100,000 Btu/h natural-draft water heater, for a total input of 499,000 Btu/h. The room has a louvered door to the corridor with wood louvers, 20 inches by 80 inches, rated at 25 percent free area, plus a 6-inch combustion air duct from the roof that has never been cleaned and contains a bird screen and a closed damper. The water heater continues to vent into a 12-inch by 12-inch masonry chimney that previously served both appliances; the chimney top is 8 feet from a rooftop air intake for the gym HVAC unit. A carbon monoxide alarm in the corridor has activated twice this winter.
1. Determine whether the mechanical room is a confined space and calculate the combustion air required.
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Confined space check: the standard threshold is 50 cubic feet per 1,000 Btu/h of total input. 499,000 / 1,000 x 50 = 24,950 cubic feet required. The room provides 1,120 cubic feet, so it is a confined space and must draw outdoor air.
Opening sizing: with two permanent openings, each opening needs 1 square inch of net free area per 4,000 Btu/h: 499,000 / 4,000 = 125 square inches net per opening, one within 12 inches of the ceiling and one within 12 inches of the floor. If horizontal ducts are used, the requirement doubles to 1 square inch per 2,000 Btu/h, or 250 square inches per opening.
Existing conditions fail on every count:
- The 6-inch duct has only about 28 square inches of area, far short of 125, before deducting for the bird screen and the closed damper.
- The wood louvers at 25 percent free area provide about 400 square inches net (20 x 80 = 1,600 gross x 0.25), but they are a single opening and they connect to a corridor, not outdoors. Corridor air is only usable under the code's specific conditions, and a corridor is not an outdoor air source.
- The closed damper is an obstruction, not a control.
The practical fix is a listed engineered combustion air system, or converting the water heater to a direct-vent unit so the room's air requirement drops. A plumber should not simply open the door and call it combustion air.
2. The carbon monoxide alarm has activated twice. What is causing it, and how should the water heater venting be corrected?
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The chimney is now orphaned and oversized. It was sized for 200,000 Btu/h of natural-draft appliances; it now carries 100,000 Btu/h. An oversized flue cools the products of combustion before they reach the top, so the gases condense on the masonry, the chimney never develops proper draft, and flue gases spill at the draft hood into the room. The CO alarm activations are consistent with spillage, not with a nuisance alarm.
The Category IV boiler is not sharing that chimney - it is sidewall-vented - so the boiler is not the direct cause, but its operation can depressurize the room and make spillage worse if combustion air is inadequate.
Corrections:
- Reline the chimney with a listed liner sized to the water heater's input, or replace it with a listed Category I vent sized for the appliance. Do not leave a 100,000 Btu/h appliance in a 12 x 12 flue.
- Verify draft with a combustion analyzer and a draft gauge after relining, and confirm the vent terminates at the required height above the roof and distance from openings.
- Check the 8-foot separation from the gym air intake against the manufacturer's clearance and the code's minimum distance from forced-air inlets. Eight feet may be acceptable for some appliances but is a design concern that should be documented.
- Install and maintain CO alarms per the code and investigate each activation as a real event.
A spill switch or draft proving device is a reasonable additional safeguard, but it does not replace proper vent sizing.
3. What gas piping and appliance start-up items must be verified before the new plant is placed in service?
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The gas load changed from 200,000 to 499,000 Btu/h, so nothing about the existing piping can be assumed. The plumber should:
- Recalculate the piping by the longest-run method at the available pressure and the code's allowable drop, and verify meter and regulator capacity. A pipe that served two 100,000 Btu/h heaters is almost certainly undersized for 499,000 Btu/h.
- Provide the appliance connections correctly: a shutoff valve in the same room and within the code's distance limit of each appliance, a sediment trap at each connection, a listed connector, and proper support and clearances.
- Verify manifold pressure at each appliance under flow, not static, and confirm the boiler is set for the correct gas and altitude.
- Condensing boiler details: condensate trap primed, neutralizer installed if required, vent slope back to the boiler, and PVC terminations at the manufacturer's clearances from windows, doors, and air intakes.
- Combustion verification: a combustion analysis on both appliances after the combustion air corrections, with the room door closed and the exhaust fans running - the worst-case condition.
Start-up is not complete until the CO alarm stays silent under worst-case depressurization and the combustion readings are within the manufacturer's range.