[TIGHTER MITER]

We are finishing a basement that contains 1 furnace/AC and 1 Water Heater. We are enclosing the two units in a closet and are required to provide "make up" air for them. Rather than louvered doors for the closet we plan to install 2 transfer grills (return grills) in the wall of the closet that adjoins a large room, one high and one low- see drawing below.

My question is---What size grills will be sufficient to provide required make up air?

Furnace is 60,000BTU
W H is 40,000 BTU

Room that transfer grills will be in is 24' x 14' (total of 2700 Cu Ft)

 

[flashheatingand]

Should be about 100 square inches. Is it feasible to bring in combustion air from the outdoors?

 

[TIGHTER MITER]

Should be about 100 square inches. Is it feasible to bring in combustion air from the outdoors?

So really 1 - 14" x 14" grill would do it? The furnace is a direct vent - so it is possible to get combustion air from outside with pvc but it is a really difficult area to drill through to the outside. On that, the other thing I was told was that the combustion air intake pipe needs to be within 18" of the vent pipe because of equalized pressure or something..is this true?
We are unable to get the W H fresh air from outside anyway so I figure wall grill is the way to go at this point-

 

[flashheatingand]

I don't know about the 18" from exhaust pipe, l don't think that's a fact, but, maybe someone will call me out. The rule is 1 square inch per 1,000 btu.

We recently had a death here, due to carbon monoxide poisoning, and they said the reason was vent pressure negative pressure. So, it would behoove you to find a way to bring in outdoor air. It was an apartment complex, and, odds are, you would be ok with a transfer grill, but....

I would go with something like a 4x14 grill up high, and leave a 2.5-3 inch gap with the bottom of the door as a plan B

 

[Inner10]

So really 1 - 14" x 14" grill would do it? The furnace is a direct vent - so it is possible to get combustion air from outside with pvc but it is a really difficult area to drill through to the outside. On that, the other thing I was told was that the combustion air intake pipe needs to be within 18" of the vent pipe because of equalized pressure or something..is this true?
We are unable to get the W H fresh air from outside anyway so I figure wall grill is the way to go at this point-

I believe the exhaust and intake has to be separated by a minimum of 18" so it doesn't suck in exhaust. Check the manual for the furnace it's in there.

To have a high efficiency furnace and not bring in outside air is completely counter productive. Every inch of air that gets pumped out is being replaced with outside air...very inefficient.

 

[beenthere]

2700 cubic foot does not meet code for combustion air for 100,000 BTU total input. Code requires 50 cubic foot for every 1,000 BTUs of input.

 

[BCConstruction]

We are finishing a basement that contains 1 furnace/AC and 1 Water Heater. We are enclosing the two units in a closet and are required to provide "make up" air for them. Rather than louvered doors for the closet we plan to install 2 transfer grills (return grills) in the wall of the closet that adjoins a large room, one high and one low- see drawing below.

My question is---What size grills will be sufficient to provide required make up air?

Furnace is 60,000BTU
W H is 40,000 BTU

Room that transfer grills will be in is 24' x 14' (total of 2700 Cu Ft)

You have to bring air in from outside the building envelope it can't be from another room. That's what the make up air is for. If your house is sealed tight like most are now you need the same amount of air coming in as the furnace and water heater are burning. If you don't bring in make up air then you create a negative pressure and the fumes won't leave the room and you die.

This is the same reason they require make up air for range hoods. As they can pull the exact gases back down the vents.

 

[beenthere]

You have to bring air in from outside the building envelope it can't be from another room. That's what the make up air is for. If your house is sealed tight like most are now you need the same amount of air coming in as the furnace and water heater are burning. If you don't bring in make up air then you create a negative pressure and the fumes won't leave the room and you die.

I think he is calling combustion air, make up air. They are not the same things.

 

[BCConstruction]

I think he is calling combustion air, make up air. They are not the same things.

Make up air is any air that being used from the sealed area of a home. So water heaters, furnaces, bathroom fans, dryers, range hoods etc etc

They all have to have the same amount of air coming in as they use. The more you add the more negative pressure you create and the air is pulled through the furnace vent and water heater vent. Very bad idea. It don't take a lot of negative pressure to pull fumes down a flue pipe.

 

[flashheatingand]

Or, air is pulled in through various gaps within the building envelope... Bottom line, he should figure out how to bring in outdoor air. But...

I was in a house that was sealed to the 9's, and it had a bad smell, to it. Something like stale air. Don't get me wrong, insulating is imortant and all, but making the house too tight causes other issues.

 

[Inner10]

Or, air is pulled in through various gaps within the building envelope... Bottom line, he should figure out how to bring in outdoor air. But...

I was in a house that was sealed to the 9's, and it had a bad smell, to it. Something like stale air. Don't get me wrong, insulating is imortant and all, but making the house too tight causes other issues.

I'd bet the best house it tighter than a steel drum, outdoor air supply for all combustion and an HRV for fresh air.

 

[BCConstruction]

Your right about them being air tight. These new windows and doors are sealed up pretty well. Add in the caulking of all joints and construction adhesive to hold plates down you end up with a very air tight in closure. If I turn on my TD I can't watch my carbon monoxide meter rise if I don't open a window. It don't rise to dangerous levels because my furnace don't fire up that often but if you had a faulty water heater or furnace that didn't shut of in combination to a appliance pulling air out the house you could fill that house fast with carbon monoxide.

 

[beenthere]

Make up air is any air that being used from the sealed area of a home. So water heaters, furnaces, bathroom fans, dryers, range hoods etc etc

They all have to have the same amount of air coming in as they use. The more you add the more negative pressure you create and the air is pulled through the furnace vent and water heater vent. Very bad idea. It don't take a lot of negative pressure to pull fumes down a flue pipe.

No. Make up air is for powered exhaust systems. And can be brought into the comfort duct system if you want. Combustion air can not be brought in through the comfort duct system. It may be brought into the room to the appliances by a duct system that is independent of the structure's comfort duct system. Combustion air has to meet the requirements of the fuel burning appliances it serves.

 

[BCConstruction]

No. Make up air is for powered exhaust systems. And can be brought into the comfort duct system if you want. Combustion air can not be brought in through the comfort duct system. It may be brought into the room to the appliances by a duct system that is independent of the structure's comfort duct system. Combustion air has to meet the requirements of the fuel burning appliances it serves.

What on earth are you talking about! It makes no difference if its combustion air or not. Any system that pulls air from the home as an exhaust creates a negative pressure. It makes no difference if its a 30cfm bath fan, 2000cfm range hood, direct, power or power direct venting they all have to have make up air. Some will have there own dedicated intake like a concentric intake but they all as I say need make up air to replace the air they are exhausting. It's one of the first things I was taught as a plumber and the requirements in the UK are vastly more strict than here as the homes are sealed up much better in the UK.

 

[beenthere]

Show me where in the IFGC it says make up air.

Thee are many homes that have a make up air system, and a combustion air system.

They are not the same.

 

[BCConstruction]

Show me where in the IFGC it says make up air.

Thee are many homes that have a make up air system, and a combustion air system.

They are not the same.

I'm not saying they are the same. It's a simple concept of how it works that even a HVAC guy can understand it. What ever you pull from the house you need to replace. The way this guy is wanting to design the system it's not replacing the air. It's pulling a vacuum which is negative pressure. You need to have "make up air" bought into the house so carbon monoxide can leave. Which is what concentric vents were designed for.

A general rule of thumb is using more than 200CFM requires "make up air" I have even seen this written on instructions when installing furnaces, dryers, hoods etc etc so there must be a reason they mention it.

 

[beenthere]

If his basement had a volume of 5,000 cubic foot. He would not be required to have outside combustion air brought in. Nor from another room.

 

[BCConstruction]

If his basement had a volume of 5,000 cubic foot. He would not be required to have outside combustion air brought in. Nor from another room.

I have never once seen that as a spec. All I have ever seen is anything over 200CFM requires make up air. Even if it was a rule that 5,000CF does not require make up air how do they know how many CFM are being used up by other appliances. For all they know you have a 2000CFM hood running and a tumble dryer whilst your using your vacuum cleaner. That would create negative pressure in seconds in a house. My 1200cfm extractor can put out pilot lights it pulls so much air down the flu when it's running that's in an area vastly more than 5000CF.

 

[beenthere]

I have never once seen that as a spec. All I have ever seen is anything over 200CFM requires make up air. Even if it was a rule that 5,000CF does not require make up air how do they know how many CFM are being used up by other appliances. For all they know you have a 2000CFM hood running and a tumble dryer whilst your using your vacuum cleaner. That would create negative pressure in seconds in a house. My 1200cfm extractor can put out pilot lights it pulls so much air down the flu when it's running that's in an area vastly more than 5000CF.

Your still confusing combustion air with make up air. The OP is asking about combustion air. Not exhaust hood make up air.

IFGC(International Fuel Gas Code) states 50 cu ft per 1,000 BTUs of input. 100,000 BTUs of input needs a volume of 5,000 cu ft to not require a combustion air source from another room, or from the outside. Its code across the USA. Weather or not its enforced.

If his home needs "make up" air for an exhaust hood. It would only be sized for the exhaust hood. And would have absolutely nothing to do with his gas fired furnace or water heater's requirement for combustion air.

 

[BCConstruction]

Your still confusing combustion air with make up air. The OP is asking about combustion air. Not exhaust hood make up air.

IFGC(International Fuel Gas Code) states 50 cu ft per 1,000 BTUs of input. 100,000 BTUs of input needs a volume of 5,000 cu ft to not require a combustion air source from another room, or from the outside. Its code across the USA. Weather or not its enforced.

If his home needs "make up" air for an exhaust hood. It would only be sized for the exhaust hood. And would have absolutely nothing to do with his gas fired furnace or water heater's requirement for combustion air.

I understand what your saying. I'm not saying they are the same technical name. What I'm trying to say is if your using a furnace or a water heater and pulling "combustion air" from the building envelope then you are creating a negative pressure. It's not a lack of oxygen to use to burn that's the problem its the lack of new air coming into the building to allow the exhaust to leave the building envelope. This is why I always used concentric vents as they pull in as much intake as they exhaust. As you prob know no one has "make up air" units installed. I have never seen one installed over here not once. The issue comes when you turn on anything that's pulling CFM then those fumes in the exhaust are pulled right back down the vent and into the home. It's not just furnace not having "make up air" that's the problem is the combination of all of them not having it.

Now if your house was done right and "make up air" units were installed on every appliance so it meets the same CFM as exhausting then it's prob not a problem. I think even the IRC has a section for "make up air" but I'm not sure what CFM they state before you need "make up air".

The same example goes for fireplaces. We used to install air bricks into the walls that were sized to the size of the fireplace BTU. We would then do a smoke test where you heat up the chimney and use a smoke match to make sure it was getting enough air into the building to create an updraft. When we tested boilers and fireplaces where there was no air bricks installed the smoke didn't pull into the chimney at all. With the correct size air brick to get you the right amount of "make up air" it would pull the smoke from the smoke match from about 10" away from the top lip of the fireplace. When doing this test you also have to shut all doors and windows to make sure the "make up air" is just coming from the air brick and no where else. Boilers we used digital meters to measure the exhaust gases and flow rates to figure out if there was enough "make up air" but like I have said the UK is vastly stricter with the requirements for these kinds of things.