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  1. #1
    I'm planning on finishing the basement myself and want to make sure it is up to code according to the 2003 Michigan Residential Code. I'm just in the budgeting stages and planning at this point, and hope to start framing in a few weeks. The area I need the most help on is the code requirements for ventilation, ductwork/return air, and combustion air. I can handle the electrical/framing/plumbing as I'm pretty familiar with those codes, but really don't know much about HVAC.

    There are a few areas I have questions and need clarification on.

    1. In section 303.1 the code states that 4% of the floor area for each room much be openable to the outdoors, via windows or provide mechanical ventilation. How do I calculate this and provide this? What exactly is "mechanical ventilation"? I'm finishing an 1100 sq ft section and only have one egress window.

    2. What exactly do I need to do to make sure my return air for the basement complies with section M1602 in the code?

    3. Combustion air must meet chapter 17 (M1702.1) codes from inside the building. Specifically I have questions about installing two openings and the requirement for each opening to have a free area equal to a minimum of 1 square inch per 1000 BTU of the input of all the appliances installed within the space. How do I install and calculate this?


  2. #2
    Join Date
    May 2004
    Which Detroit suburb are you in?
    Not enough info here to properly address all your questions.
    Example: When was the home originally built?
    If after 1978, IT is considered to be unusually tight construction, if energy upgrades such as glass block windows installed in the basement if built before 1978, that also could warrant the tight construction considerations.
    A lot depends on the layout of the finished area. Naturally the bathroom will need exhaust to the exterior, 50 cfm's per toilet, and you have to replace that air.
    The Mi. code requires .35 air changes per hour. The code gives 2 ways of figuring this and you have to take the whole home into consideration here. Generally you can accomplish this with a make up air kit installed into the return duct if one isn't already installed. If a 4" MUA is presently there you will need to increase it to at least 6", (Total cubic feet of air, lenght X width X height per floor in the home X .35 divided by 60 gives you the cfm requirement for the mechanical ventilation) average static at .1 and a 4" MUA will give you approx. 45 cfms where a 6" will get you approx. 100 cfms.
    Supply air and return air are required in all habitable spaces period, and since your finishing it, the rooms other than baths and halls become habitable.
    As far as combustion air, if your enclosing the furnace room you have 3 methods you can use. The easiest way is the 1 opening method, which is 1 sq. inch per 3000 btu's of the total of the gas appliances in the room located within 12" of the ceiling. The 2 opening method gets too complex depending on vertical or horizontal ducting. Some inspectors will allow you to use a toe kick register without a damper on the furnace supply plenum if the MUA is large enough to provide excess air over and above the ventilation air requirement.
    The best thing would be to check with the mechanical inspector of your jurisdiction or post the city/town and I can give you his/her phone number.
    Hope this helps,
    Vern, City of Walled Lake Mechanical Inspector
    Vern P: 2003 MBC,MRC,IFGC,IFC
    An HVAC-Talk Michigan Chapter Mechanical Inspector, Jurisdiction-Ann Arbor

  3. #3
    Join Date
    Oct 2001
    Waterford Michigan
    Do you really plan on pulling a permit for this job?

  4. #4
    Thanks for the reply Vern, I'll PM you separately for the personal info.

    The home is just 1 year old as we had a C of O issued last December.

    What is confusing to me is I have the requirements from the city for finishing a basement, and it says right on it that combustion air must meet the requirements of chapter 17 of the MRC. Then it says, in bold, "M1702.1: All combustion air from inside of building" and goes on to say that two openings shall be installed one at the top and one at the bottom, etc.

    I do have a make up air return installed and it's at least 6" (looks bigger than that even), but there is no combustion air return. I thought it was code to require combustion air also, but why wouldn't my 1 year old home already have it and why does their handout say from inside of building only? Makes a person confused for sure.

    I will be installing heat runs and cold air returns in all habitable rooms.

    I'm still a little confused on this other line item on the list that says according to code 4% of the floor area for each room must be openable to the outdoors via windows or mechanical ventilation (section 303.1). I have about a 1000 sq foot area I'm finishing with only 1 egress window, and 1 glass block vented window. Does my existing MUA line already meet this requirement?

  5. #5
    Join Date
    Mar 2002
    Gaylord, Michigan
    If it is that recent of a home, do you have a 90+ furnace? Is it piped to the exterior with two pvc pipes? If so, you do not need combustion air for that. Water heater will require combustion air though.

    I work in northern michigan, and regardles of if the basement is finished or not we are required to have combustion air to appliances.

    Might help if we knew what you have for equipment, to properly size the combustion air.

  6. #6
    Bryant 80% builder model furnance 107,000 or 132,000 BTUs (says both on the tag?)

    Water heater is a 50 gallon 40,000 BTU

    There is only one line going to the furnance which is for the make up air that ties into the cold air return.

    I don't see anything going to the water heater but it may go staight up through the walls to the roof where I can't see it.

    Come to think of it there is a vent coming out of my roof that is pretty thick. Could that be it? I guess I was looking for a line coming in from the side of the house (which I only have one of those for the make up air).

    [Edited by dominogold on 12-29-2005 at 09:53 PM]

  7. #7
    Join Date
    Mar 2002
    Gaylord, Michigan
    Are there supply registers in the basement, or one near the equipment? How large is the makeup air 6" or more? Inspectors here used to allow using makeup air as a source of combustion air. So that could be how it passed inspection.

  8. #8
    To johnl45: "Do you really plan on pulling a permit for this job?"

    I guess I'm curious why you are asking. Are you implying maybe it doesn't matter?

    The thought has crossed my mind just how necessary a lot of this really is. I do know dozens of people that did their own basement and just enclosed their furnance/water heater in a small room with a big hole in the wall for combustion air, and their houses didn't explode or anything.

  9. #9
    Yes, I have 4 supply registers in the floor joists in the basement. The makeup air looks like a 6"... says Hart and Cooley Flexible Air duct and has 6" on it with an R-6.

  10. #10
    Join Date
    May 2001
    Kansas City
    Your really not going to save a lot by doing the ductwork yourself. You need to do a load calculation for the basement and design the ducting accordingly. If you cut supply runs into the trunk line in the wrong places you'll screw up the airflow needed to your rooms upstairs and end up with an uncomfortable house.
    The combustion air is the easy part but the most important as far as safety is concerned.
    You can probably pay an HVA/C company to consult you on how to do it if you want to do it yourself. Try the Co. that installed the system.

  11. #11
    Join Date
    May 2004
    Dom, you could make an arguement that when the home was built the basement was unfinished or wide open. Is the basement stairway wide open to the upstairs also and going to remain that way? If so I'd say they gave you credit for the MUA excess air in lieu of the combustion air requirement, just a guess but probably how it happened and as an unfinished basement not considered a habitable space. As far as M1702.1 is concerned that is IF you are using all air from inside. The chapter goes on to give you 3 methods for using outside combustion air, and since your enclosing the furnace room your much better off using the 1 opening method of 3000 BTU's per sq. inch for that room (172M divided by 3000= 57,1/3 sq. inches or approx an 8" run. Now some are going to tell you thats wrong but its what the Code states in black and white using this method.
    You may get by with a 6" outside run and cut transfer grills high and low (14 X 6) using a combination method which is also allowed.
    This work your planning REQUIRES permitting and inspection per the MRC,2003. Don't give your insurance company any reason NOT to pay off in case of an EVENT in your home. For the people that tell you different pay no attention. Its for your own saftey and investment protection to do so.
    Your MUA should be sufficient if you put the outside combustion air in.
    You can call your mechanical inspector and verify this info for your purposes since he will have the final word at some point.

    Vern P: 2003 MBC,MRC,IFGC,IFC
    An HVAC-Talk Michigan Chapter Mechanical Inspector, Jurisdiction-Ann Arbor

  12. #12
    Thanks for the reply.

    So amoung the 3 methods, how do I determine which method I use?

    I'd like to finish it without having to bring in combustion air from outside if possible. I just had an HVAC tech on another board tell me I don't need it from outside because 174,000 BTU devided by 1,000 = 174 Free square inches. One standard residential return air grille with 1/2" spacing listed size of 20X14 gives you 180 free square inches so just installing two grills one 12" from the top and one 12" from the bottom should be sufficient.

    I just read over M1701 also, and I guess I'm wondering how do you determine if you should use M1701.1 or M1701.3, which requires to get the air from outside.

    Also how do you determine if the house is "tight construction" or not. This house is only 1 year old with R-13 walls and R-38 attic insulation.

    What about the 4% mechanical ventilation (or openable to via windows) requirement? How can I tell if I have met that requirement with a 6" make up air and egress window? The area I am finishing is about 1000 square feet.

    Basically as long as I provide ventilation am I going to have a problem with this if I don't bring in outside combustion air?

    Is there a way to post pictures on this chat board? I have a rough sketch.

  13. #13
    Join Date
    May 2004
    Dom, in Michigan each and every home built since 1978 is considered to be tight construction period, no ifs, ands or buts, yours is TIGHT being built last year.
    4% is not an issue since you don't have 4% operable opening windows in each area. Mind you this is the operable opening portion not the entire window so to even use this as an option can't be considered.
    Combustion air methods are your choice which to use. Choose the easiest/cheapest method you like.
    Its up to you or your designer to come up with code compliant methods on all aspects. I can't design it or give you the step by step methods but I can tell you what the code book states is compliant.
    Once again, call your inspector when you decide your approach and run it by him/her PRIOR to starting something you may have to change later.
    Vern P: 2003 MBC,MRC,IFGC,IFC
    An HVAC-Talk Michigan Chapter Mechanical Inspector, Jurisdiction-Ann Arbor

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