COMBUSTION/MAKE UP AIR

FeelingColdColdCold

Have fan in can for boiler. Inspector requires louvre door (not wanted) or a 12x12 grate in the wall to the adjoining open roo,. But then asked for a wall grate up high and one down low. What is the logic of needing both for a boiler?

Larry Weingarten

Hi, I can only answer for fuel fired water heaters in closets. For these, you want grates or louvers high and low for a few reasons. First, it gives the heater combustion air without any depressurization. Second, it allows for convective currents, cool air coming in the lower grate and hotter air leaving through the upper grate. This prevents overheating in the closet. Those are the reasons I've been told.

Yours, Larry

RayWohlfarth

@FeelingColdColdCold Perhaps you can show the inspector the International Mechanical code which allows mechanical makeup air. I have enclosed a copy Good luck

MikeL_2

Feeling cold,

If the fan in the can is providing the required amount of combustion air for the boiler and is wired correctly in series with the primary control, ask the inspector to provide the written code for his additional requirements.

Bob Harper

There are few topics in HVAC more shrouded in lore, stupid codes that defy physics, and ASSumption than MUA.

First of all, if the MUA is provided 100% by an interlocked mechanical system, no addition provision is required. If you want to discuss 'ventilating' a space, then call it that. However, 'ventilation' requires stuff in and stuff out, namely room air. So, how is the air from the adjoining space going to enter the Combustion Appliance Zone or CAZ if the Fan in the Can is pressurizing the CAZ? You need a pressure gradient to move air from one space to another. It's stupid. It needs arrows to tell it where to go. If your CAZ has a FiaC, the adjoing room air will not migrate through louvered openings into the CAZ. Rather, CAZ air fill exfiltrate through said openings into the adjoining space, both top and bottom. Now, since the CAZ is full of warm ambient air, where is it going at that top louver? Out into the adjoining space. This fights draft rather than improve it. The theory is to provide a relief so that any fugitive CO or combustible vapors can ventilate into the home where lungs will filter them.

If there is negative pressure in a CAZ, how are you going to generate draft pressure against it? You want slightly POSITIVE CAZ pressure to create that differential in air densities, flue vs. CAZ to create a 'draft' or, more appropriately, 'draft pressure'. Mass flow up the chimney is entirely a separate matter.

Ask Hector the Inspector what would happen if you instead installed two 6" air ducts from above, one off the floor and one near the ceiling? Measure the air temps in each then measure the pressure inside with a manometer. You will see a negative pressure meaning 'draft'. They will both depressurize that CAZ. Codes can get you killed.

Ask Scooter if the boiler was a mod-con direct vented, would the room still need wall louvers. Functionally, what's the difference btw a two pipe CAT IV furnace, a concentric vented boiler or a CAT I boiler with an interlocked MUA system that is tested and proven to provide 100% MUA with a pressure switch and a spill switch on the boiler? Remove the draft hood, install a x2 barometric damper w/ spill switch and tell me which is safer- a CAT I boiler installed per mfr. with a dopey draft hood and dumb holes in walls or a controlled CAZ with safety switches and the chimney actually connected to the combustion appliance? Think about it.