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Boiler in Finished Basement III (*quiz)


Jeff Remas
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You are inspecting a 20 year old home that is not considered to be unusually tight construction. The family has recently finished the basment and has framed around the utility area creating a separate, nice size utility/mechanical room of 12'x12' with 8' ceilings. The room is sealed from the rest of the basement. All combustion air comes from within the room. In the room is an electric hot water heater and an oil fired hot water boiler for the hot water baseboard. The boiler has a 60,000 BTU input rating and a 52,000 BTU output rating.

We now know that there is not enough combustion air available in the room. The boiler needs 3000 cu feet of combustion air and only has 1152.

The homeowner has decided to bring in fresh air from the outside for combustion air.

What size openings to the exterior are needed to accomplish this?

Hint, this is now an oil boiler VS a gas fired boiler so the answer will be different than that of Quiz II. The point being we must pay attention to the type of fuel for several reasons.

Good luck.

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Originally posted by Jeff Remas

Wow, we just asked the exact same question on the previous thread using a gas fired furnace and there was no shortage of supplies and a correct answer. What's up with this one?

The people who know are holding back. The people who don't know don't have code books.

- Jim Katen, Oregon

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Quote: Originally posted by Jeff Remas

Wow, we just asked the exact same question on the previous thread using a gas fired furnace and there was no shortage of supplies and a correct answer. What's up with this one?


The people who know are holding back. The people who don't know don't have code books.

- Jim Katen, Oregon


Or are too lazy to look it up.........
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Unlike fuel gas in the previous quiz, an oil burning appliance will need two openings (or ducts) to the exterior. But since this is a simple question and I am only asking for the opening sizes we will not worry about ducts.

One opening to bring in outdoor air will have to be located within 12" of the top of the enclosure and the other will have to be within 12" of the bottom of the enclosure.

Since the openings will be directly communicating with the outdoors we will need 1 square inch per 4,000 BTUs of appliance input rating PER opening.

Therefore, with a 60,000 BTU input we would need 15 square inches (net free air) per opening.

So if you used metal grills with a 75% opening rating. Each grill would have to be a minimum of 20 square inches.

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Said link, I learned when trying it for the first time in a year or so, has assumed room temperature. Here's what it said, though.

NFPA 31 - Installation of Oil Burning Equipment, 1992, section 1-5 Air for Combustion and Ventilation

Covers minimum requirements for safety to life and property from fire in the installation of oil burners and the equipment used in connection with them.

Oil fuels:

Minimum No. openings required 2

Outdoor opening (sq. in./Btu h) 1/4000

Vertical ducts (sq. in./Btu h) 1/4000

Horizontal ducts (sq. in./Btu h) 1/2000

Ducts same as opening yes

Minimum duct dimension (inches) 3

Louver & grill free area opening

Free area allowance, wood* 20-25%

Free area allowance, metal* 60-75%

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