HVAC Questions? Ask an HVAC Expert for Answers ASAP
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Thanks Phil, The exposed metal flue has a 10" diameter (metalbestos) and the boiler is rated at 130,000 BTU input. Greg
Hello again.The code requirement is 1 square inch for every 2,000 btu of heating capacity input rating.So that for the the wood burner alone you would need 65 square inches of combustion air intake grill, then you need an additional inch of open grill area for the unvented space heater per every 1,000 btu's of input rating. These heaters all need an ODS pilot light to prevent ignition when the oxygen content of the room drops below 18% (ODS= oxygen detection safety, or oxygen depletion safety). I would not bet my life on any of them however. The manufacturers are vague on the issue, in many cases they just say 'be sure to leave a window in the house open 2 inches' counting on their ODS and cracks in the houses envelope to admit sufficient fresh air... many people consider the use of these unvented heaters unsafe in any circumstance.CLICK HERE FOR A REFERENCE ON UNVENTED HEATER OUTSIDE AIR REQUIREMENT ISSUES. ...and the maximum size unvented heater for a room. 3850 square feet per 3850 btu/hr input rating. The article is by a registered professional engineer. It is however not well written or accurate, and vague about outside air requirements and refers you to the heaters owners manual for fresh air requirements. I posted it for the broader content related to unvented heaters and that range of issues. Each jurisdiction inspector will have a slightly different interpretation of what he or she sees as allowable.In summary: read that article for its notes and warnings and discussion of the broader issues. In answer to your question, personally I would install 150 square inch open area fresh air/ combustion air intake to the room assuming the BTU rating on the unvented heater is less than 50,000 btu's input capacity.. such a vent needs to be unobstructable and placed low on a wall or in the floor..... and I would make sure the heater was built after 1982, and has and ODS pilot light.I would store no flammable liquids in the room and insure that the propane fuel tank if any is stored outside the room.We need more discussion on this before I feel you will have a valid final answer, read the link I sent, and let me know more about the BTU rating on the unvented heater.We can go from there.Phil
Hello, I am a Home Inspector. The garage is a residential garage and the ignition source is 18" above the floor. Typically NFPA is acknowledged in VT but other than a couple of towns in VT there is no code enforcement. Greg
Hello again, there are a wide range of safety issues here. I thank 'Electrofier' for mentioning the 18" rule for garages. I was focusing on your question regarding adequate combustion air however.. Beyond that there are the issues of storing flammables in the room now.... or later, and clearance of the wood burner from adjacent walls. The existing clearance is most likely safe enough but needs a discussion...fire rated wall, ceiling and door construction is another issue we have not addressed. If possible it will not be a bad idea to put two half sized combustion air grills near the floor, to afford some circulation in the event that some time later, someone stores flammables in the room. Regarding the 18" clearance, the fire box, particularly the hot ash pan at the bottom can easily be closer to the floor than 18"... that needs to be addressed, and measures taken in that case to isolate the bay with the boiler and wood burner in it from the garage area. I had assumed there would be a local inspector to look at the job, since there is none, perhaps we should discuss the full range of variables. Let me know if you require that or not.( My background is in mechanical engineering nationally, and in California as a mechanical contractor, with an unlimited boiler license... 52 years, recently retired.)If I read you correctly the gas fired boiler is rated at 130,000 BTU's and all we have on the wood burner is its 10" diameter stack. In that case code for the gas fired unit alone is 70 square inches of *open area combustion air intake grill. Then we have to estimate the peak btu rating on the wood burner, judging by its stack diameter... and I will double that to account for over stoking etc.
The wood burner fresh air intake alone should exceed the diameter of the vent stack by 100% ..area of the vent stack is 5 x 5 x 3.14 or a bit under 80" of open grill space x 2, or 170 square inches to be safe for the wood burner alone... then we add 80 or 100 square inches of open air intake for the gas fired water heater...( if it is that you are referring to as 130,000 btu/hr).
I would split that up into two 150 square inch combustion air intake grills... with 1/4" galvanized wire mesh over them and rain hoods over the top.
We need to consider a fire rated door on the bay as well, if the fire box or ash tray on the wood burner is anywhere near the floor.
Let me know what you think, we can go from there. Perhaps the other expert would like to comment on the fire rated walls, doors and ceiling construction as they might apply in this case.
Thank you for your time and effort. I called it out at the inspection and recommend a qualified wood-burning heat technician perform a safety check and maintenance to the boiler to ensure it is properly installed and there is safe combustible makeup air. I also recommended that the technician determine if there should be a fire rated wall, door and determine if the enclosed garage space be sealed from the adjacent garage.
Thank you again, Greg