air intake on sealed combustion furnaces

Timco

I have 2 knight boilers that suck from the boiler room, but the room has shutters and a huge air intake grille. Contact mfg, and get their opinion. It is not always required, and depends on the mechanical room's needs. If you put your hand over the intake, you will see they pull a lot of air...

Tim

RL Refalo

Air intake on sealed combustion furnaces

Guys, I was at a customers house today to look at a basement finish job and noticed the two furnaces in the basement did not have their combustion air intakes piped to the outside.
I had never seen this done. How common is this practice?
Should I recommend that they have the outside air piping installed?

SpeyFitter

Not a bad idea

Does the house have a piped air intake into the room from outside that feeds the furnace air intakes that suck from the room, or is the house relatively old and leaky and the use inside air?

It could probably save them a little bit of $$$ on their gas bill to have them piped to outside, when you factor the negative pressure they create if there is no air intake piped into the room.

The other thing worth noting - if there is some kind of make up air piped into the room that the boilers draw their air intake room from, it might not be a bad idea to put some kind of cold air stop - I'm trying to remember what they are called, an arctic bucket or something like that. Basically when the air intakes aren't sucking from the room, the cold air will not be allowed to enter the room via convection from the duct as it relies on the stratification of cold & hot air to keep air from infiltrating the room when the furnaces aren't running (the just of it anyways).

EricAune

boiler manuals/local mechanical code

I just checked three different boiler manuals, Triangle Tube and Weil-Mclain both state the combustion air intake must be piped from the outside. The NTI manual does not specifically say that, but, describes and illustrates the inlet piping coming from the outdoors only.

We have Minnesota specific mechanical and building codes that address this issue. The building code refers, of course, to the mechanical code in stating that the proper venting of approved appliances must meet the code & manufacturers specifications.

If a boiler or furnace has an intake (direct vented appliance) and is not piped to the outside then there should be a properly sized cold air intake to the space. I have seen this done before too, and all I can say, it just seems to me that it was a corner the previous contractor decided to cut in what was probably an attempt to make a tiny profit on a job he was likely not qualified to do in the first place.

Sorry so long....Peace

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RL Refalo

Thanks Guys, I will recommend they have them piped.
Rich

Bob Harper

dangerous set up

I have tested installations where one furnace backdrafted the one next to it. These installations are hazardous and should not be allowed even though the listing allows some to do it.
Passive MUA has been proven unreliable and does not work. Twin pipes per the listing is the only way to go.
BTW, single pipe systems cause cold air infiltration thus lowering efficiency.

RL Refalo

Thanks Bob, and all who replied. I will advise my prospect.

tk_3

As far as the code is concerned it is up to the manufacturer. I say if all pictures and test shows and talks about 2 - pipe then it needs to be 2-pipe. If it shows or talks about both then either one is OK.
If it is a direct vent than it is 2-pipe and power vented than it is 1-pipe.

Jim Davis_3

2-pipe

For every 30 degrees the outside temperature is below indoor temperature a 2-pipe appliance is losing 1% efficiency. Also the colder the combustion air, the poorer the fuel/air mixture. Efficiency is not the reason for 2-pipe.

Here is where a problem occurs when you run one-pipe. Assuming the inducer is single speed versus variable, it is sized for two-pipe. Without the intake attached the inducer is over ramping and removing heat from the appliance too fast. When running single pipe, a restrictor needs to be installed on the intake opening.

For the record, every 100,000 btus input requires a maximum of 25 cfm of combustion air. This is less than most bathroom exhaust fans. Can lights that aren't insulated can leak this much or more. It has to be a pretty tight room for a one-pipe to depressurized it. If there is a problem with depressurization on a one-pipe it is because the room is already in a negative, not because the room is tight, but because it is loose. On furnaces it is more likely return duct leakage than a furnace venter.

If the manufacturer says 2-pipe than it probably should be, but not for the reasons most think.