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Discussion Forum

make up air for a mechanical room

popawheelie | Posted in General Discussion on September 19, 2007 11:25am

I’ve been reading the posts about positive VS negative air. I have a small room with a furnace ( it’s the type with two PVC pipes sticking out of the house), a gas water heater, and an electric clothes dryer in it. Most of the time the doors are closed so the cat doesn’t go in there and piss on the clothes.

Outside there is a vent that has various pipes sticking though it going to the room. Was this a make up vent at one time. Should I open it up again?


Edited 9/19/2007 9:50 pm ET by popawheelie

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  1. McPlumb | Sep 20, 2007 12:17am | #1

    Do you notice a problem with cold air coming through that exterior vent?

    If it isn't allowing much outside air in you could put a vent panel in the lower part of your mechanical room door.

    A vent in the door with the exterior vent working can become a major heat loss area.

  2. homedesign | Sep 20, 2007 02:40am | #2

    pop,

    It looks to me like that vent never had much net-free area...I can see the bricks right thru it.   You may have a safety risk. I can't tell you what to do... but I think you should have it looked at. If your water heater and or your furnace are not vented properly....There may be a chance of Carbon monoxide poisoning.

  3. roger g | Sep 20, 2007 02:40am | #3

    You don't need any make up air for the furnace because it is bringing in air through one of those pipes. The gas HW does need air but usually we don't worry about that unless it is in a really small and sealed room.

    You don't say anything about a dryer, electric or gas, and they do take out a lot of air which may of may not affect the venting of the HW.

    In summary, the furnace does not affect the pressure of the house but the HW does and so does any other exhaust fans but unless you have some huge exhaust fans I wouldn't worry about it. Most houses have enough leakage to bring air in. Also if the HW tank is on and the house goes into negative pressure(unlikely) the house will draw outside air through unused dryer vent and exaust fans etc etc. The flappers might slow it down but they will let air in

     

    roger

    1. User avater
      popawheelie | Sep 20, 2007 05:01am | #4

      I edited my original post. I meant to say dryer and said washer instead. So the clothes dryer and the water heater are on at the same time. Is that a condition I should think about fixing. There are two doors to the room with the normal 1/2" gap underneath. I really was wondering about the vent outside. Maybe they just used it to cover up the hole. I imagine behind the vent is a hole in the rim joist and a chase to the mechanical room.

      There isn't a corresponding hole/vent in the mechanical room. But there might have been one at some time.

      If you look closely there is a garden hose in that bunch of pipes. I have no idea what that is doing in there.

      1. roger g | Sep 20, 2007 05:01pm | #5

        When we talk about make up air, it means that when  combustion unit burns it needs not only a lot of air to burn but a lot more to make it go up the chimney. So as I said the furnace brings in all its own air but the HW and dryer don't. I've seen many rooms like your ( don't know the size) and don't know anyone who has died. If the dryer is sucking exhaust fumes back down the HW vent it is really being sucked into the dryer and out anyways. Not right but it works. There usually is enough air in the room and coming through the cracks to make it work. I hope the dryer isn't right beside the HW because the dryer definitely will suck in the HW gases. The only way around that is to put an air pipe and bring it right between the two at the bottom. If it was mine I don't think I would worry about it.

         

         I might cut a couple of vents into the two doors at the bottom so air can be drawn from the rest of the house.

        The vent that you show looks like an air vent into your crawl space which doesn't have anything to do with your gas appliances. Maybe the water hose is used as a drain from somewhere. Maybe airconditioning condensate or maybe the drain from the HW or HRV (heat recovery ventilation)

         

        roger

        1. User avater
          popawheelie | Sep 20, 2007 05:27pm | #6

          Thanks for the reply Roger. I'm not that concerned about it. I like to get imput for projects down the road. I will be replacing the water heater and upgrading the electric service ( 100 up to 200 ) for the house which is in the room. When I do projects I like to line up all my ducks before i start. If makeup air into that room was part of it i can start thinking about the best way to do it.

          1. roger g | Sep 20, 2007 07:29pm | #7

            I'm like that also. I build things a dozen times in my head before I actually do it. I try to go over each step and visualize the problems before I get there. Right now I'm trying to figure how I'm going to ceramic tile around a fibreglass tub. The only fibreglass tubs I've seen have upper lips that are designed for half inch board and then the tile. I have a 3/4 inch plaster wall plus tile. The tile, when it comes down onto the tub is not going to fit into the lip and be relatively flush with the side of the tub. I've got to spend some time looking at tubs to figure how I'm going to make this work.

             

            roger

          2. User avater
            popawheelie | Sep 21, 2007 02:01am | #8

            Where there is a wiil, there is a way.

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