Good Morning. A Home Inspector just finished an inspection on a home I just completed and told the homeowner that all 92% Eff. Furnaces need outside Combustion air. The Carrier website says that the unit is actually a “Sealed Combustion Unit”, but doesn’t say wheter or not we need to pipe air into it. The unit is located in a good sized crawlspace. My HVAC sub is very competent and I believe that they installed the unit correctly (without an outside vent for it), but just wanted to get another opinion to offset the Inspector’s.
At what point do we need an outside vent for combustion air? Does it normally connect directly to the unit or can we use the air in the crawlspace? The crawlspace is 200SF with 3-4′ headroom.
Any thoughts?
Replies
Well you migth want to look at the installation manual that came with the furnace and see exactly what it says about combustion air sources.
But a sealed combustion air system means that the burner is closed off so that combustion air can be drawn from outside via a pipe. One of the reasons for doing this is that if the unit is in conditioned spaced then it does not "burn up" conditioned air and also it will not backdraft or lead to other appliances backdrafting.
Possibly if this is in a "freely vented" unconditioned crawlspace it might not need the instake piping, but you would need to verify that wiht the installing instructions.
But the question arises, if this is an unconditioned freely vented crawlspace, WHY IS IT?
>>Good Morning. A Home Inspector just finished an inspection on a home I just completed and told the homeowner that all 92% Eff. Furnaces need outside Combustion air.
As a home inspector, I am 99.92% certain he is wrong, especially if he means "outside combustion air in a sealed pipe delivery system". (All furnaces need "sufficient combustion air" but not necessarily from outdoors.)
My 0.08% uncertainty arises because Category IV furnaces are those which are generally referred to as 90+ furnaces - I am not aware that there is a distinct sub-group called "92% efficiency" but will allow a 0.08% chance that is possible and that such a group has such a specific rule. <G>
Having read a lot of installation manuals for all types of residential furnaces, I can assure you that not all 90+ (aka Cat IV) furnaces need an outside air supply.
In fact, I don't recall seeing any that do require that in all cases, but I can't rule it out.
You have to read the installation manual carefully. Usually, they won't explicitly say if outside combustion air is required, but they provide installation variations depending on whether outside air is used.
When a furnace (or water heater) uses outside air applied directly to the combustion chamber in a sealed system, and exhausts using a sealed system, it is called a direct vent system.
But, just because a unit CAN be a direct vent system doesn't mean it HAS to be.
If you read the installation manual carefully, you're likely to find (particularly in the flue/venting section) some permitted variations of installation depending on whether it is a direct vent installation or not.
>>The Carrier website says that the unit is actually a "Sealed Combustion Unit", but doesn't say whether or not we need to pipe air into it.
Per above, if it only refers to "sealed combustion" than it has to have a sealed air intake system - but read the installation instructions carefully.
>>The unit is located in a good sized crawlspace. My HVAC sub is very competent and I believe that they installed the unit correctly (without an outside vent for it), but just wanted to get another opinion to offset the Inspector's.
Personal opinion: a "very competent" HVAC sub will fight installation in a crawl space. Crawl spaces are a crappy environment for both furnaces and the people who service them.
I want life to be as comfortable as possible for my service guy: good light, soft music etc <G>
>>At what point do we need an outside vent for combustion air? Does it normally connect directly to the unit or can we use the air in the crawlspace?
A different issue.
Assuming a 100,000 btu unit, it needs to be in a space of at least 5,000 cubic feet to not require an addition air supply. Assuming 4' high in the 200 sg ft crawl space, you aren't anywhere close.
Depending on where the additional air is coming from and how it is coming into the combustion appliance zone (vents/pipes from outside/rest of the house) you need as much as 1 sq in of free vent space per 1,000 btu (and a 10x10 with a register cover over it isn't 100 sq in of FREE vent space.)
In my opinion, if your HVAC guy put a furnace of any size (well, at least bigger than 16,000 btu, and I doubt if any furnaces are that small) in a 600-800 cubic foot area without providing a source of combustion air, he is disqualified from the category of "competent."
If the crawl space has vents through the foundation, he's probably OK, so long as they can't be closed.
Or, of course, has air supply vents into the basement.
Please keep us posted as to what develops.
Fat fingers got me again.
The crawlspace is actually 2000SF with 3-4' ceilings, or about 7000 CuFt. 15 fixed open foundation vents (16x8), so fairly good venting. (the inspectors in our area are not very up to speed on sealed crawlspaces. I like the idea, but don't want to be the pioneer with our code officials!). Judging by the foundation vents I think there is at least 4 CuFt of vent space.
I will check the manual. I'm not sure why they put the unit in the crawlspace either, it was their idea, not mine. Now I get to go under the house.
Thanks for your reply, I'll post my findings.
My MIL's relatively ancient 90+ Amana (I think) unit draws combustion air from the basement, and there is no separate combustion air fitting on it to do otherwise. The only question in her case would be whether there is sufficient makeup air (which really isn't a question in the basement of a ca 1950 home).I would think that the first question here would be: Does the unit have a fitting for combustion air. If not, then the only requirement would be for makeup air, and an openly vented crawlspace would seem to meet that requirement. If there is some question that the crawlspace might not be sufficiently vented, then about a 6" duct run from outside to the vicinity of the furnace should satisfy things. (However, there are other code requirements for venting of a combustion furnace "enclosure", and these must be satisfied regardless.)If there is a combustion air fitting on the unit, then the installation instructions should be consulted for requirements as to its use.One thing you said about "direct vent" could be confusing. Any 90+ furnace MUST have a corrosion-proof (ie, plastic) flue pipe, whether or not it is a "sealed combustion" unit which draws outside combustion air. I'm not sure about the legal definition, but I suspect a lot of people equate "direct vent" with the horizontal plastic flue, regardless of whether a corresponding combustion air duct is present.
Good points.
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Sojourners: Christians for Justice and Peace
Dan,
"I would think that the first question here would be: Does the unit have a fitting for combustion air. If not, then the only requirement would be for makeup air, and an openly vented crawlspace would seem to meet that requirement."
Yes it would seem that way, (and in reality it is vented well enough for this application), but the first question should be "What are the combustion air requirements of this appliance?". Second question would be "Are they met by my plans/intentions?". BTW, makeup air has nothing to do with combustion.
"One thing you said about "direct vent" could be confusing." True, many people may be confused about this but the legal definition is quite simple. Per the International Mechanical Code, 2003:
"DIRECT-VENT APPLIANCES. Appliances that are constructed and installed so that all air for combustion is derived from the outside atmosphere and all flue gases are discharged to the outside atmosphere."