Thread: Which mantel clearance must used with this unvented firebox and log set?

I run into this fairly often, and I'd like to see what answer the Forum comes up with.

This is the clearance diagram for a universal unvented firebox listed to ANSI Z21.91, which states that any unvented gas log listed to ANSI Z21.11.2 and meeting the size requirements of the fireplace opening may be installed.

uvfrc clearance.JPG

The unvented gas logs - made by another manufacturer, have a different clearance diagram, assuming the worst case scenario on a minimally acceptable installation:

gas log clearance.JPG

The mantel clearance for the gas logs is much more restrictive.

Which do you go by, using the minimum required clearance?

Originally Posted by Tom Parks

The mantel clearance for the gas logs is much more restrictive.

Which do you go by, using the minimum required clearance?

When two code sections are applicable, referenced standards are applicable, or manufacturer's installation instructions are applicable - the most restrictive applies.

This is stated in the codes.

When there are conflicts specific requirements take precedence over general requirements and more restrictive requirements shall apply over less restrictive requirements.

Understood, Jerry.

However, if you ask the manufacturer of the unvented firebox (approved ventless enclosure), they will tell you that their product was tested and approved using those clearances for ANY unvented gas log.

Hypothetically, the firebox could be installed and finished before the homeowner selected gas logs, meeting all requirements of the firebox installation manual, which the inspector would be able to approve.

If you ask the gas log manufacturer, the clearances in the gas log installation manual are for UL127 solid fuel fireboxes or masonry fireplaces, not ANSI Z21.91 unvented fireboxes, but the manual does not specify that.

If you ask either the firebox manufacturer or the gas log manufacturer, they will tell you that the less restrictive clearances in the manual are adequate, but I don't know how an inspector would be able to accept that.

What do you think?

And you can drive one of the super cars 200 mph on any road.

Well, except for those roads which have more restive limits on speed.

Are you permitted to drive that 200 mph supercar on a road with a 70 me limit? Sure, but the most restrictive limit takes precedence.

Just ask the man driving the car behind you ... the car with the flashing lights.

This is an interesting situation. The VF universal box was listed with a max. 40MBH burner and wall/ mantel temps. at stated clearances not exceeding 117F above ambient. That's a pretty specific number. Same for hearth protection. The mfr. of the box assumes the risk since he holds a listing which states not just for his brand of gas logs but in this case ALL Vf. It used to be early universal box mfr.s would specifically deny their listing for other brands as is their right.

As Tom pointed out, the mfr. of the logs has no idea what or where his product is going to be installed in so, based upon his testing in either a masonry firebox or factory built woodburning fireplace listed to UL 127 that is also listed for use with VF, then you have numbers. Now, a mfr. will always defer to the worse case meaning if the surface temps on one box at a given distance from the opening are higher than the other, then he'll apply that distance to both. Almost all VF log mfrs. will have two sets of facing clearances: one with a hood and one without. ALL universal box mfr.s I'm aware of require hoods (usually 4") which is a main way to keep those temps down. No woodburners I'm aware of, masonry or 127 boxes come with hoods. That's the last thing a homeowner wants to see.

So, which do you go with? Ask your AHJ. If you have a universal box, I would go with the listed clearances stated by that box mfr. If I had a masonry fireplace or UL127 box, I'd go with the log mfrs. stated clearances. That's if I was contemplating installing VF logs. I won't. Too many problems. Aside from the combustion gases and humidity/ IAQ issues, nobody has ever constructed a masonry fireplace then tested it as a ventfree with the damper closed. I know of only one instance where a masonry fireplace was even tested for clearances: 1984 in Warnock Hersey labs. They constructed a masonry fireplace per NFPA 211-1980, allowed for a 28 day cure, then encased the fireplace with 3/4" plywood painted flat black with seams taped and a type J thermocouple every 6" in grid intersections snapped with chalklines and at key points such as the edges at nailing taps, top standoffs, etc. This replicated the UL12 test all factory built fireplace have to pass. The result burning as an open hearth woodburning fireplace? The test structure ignited in the lab! So, for all you wise guys who scoff at those cheap factory built fireplaces you think are Budweiser cans with Zippo lighters in them, consider this test. The chimney must pass a 1,000F x 16hrs., which no masonry chimney would pass much less the 1,700F x 10 minutes without self destruction much less offset ignition of nearby combustibles. The brand test would surely destroy the mortar in the firebox of a masonry fireplace as it did in the 1984 test.

Back to VF: VF gas logs are not allowed in any factory built fireplace built before April 1999 as that was the date the UL127 listing first offered the optional VF test. A number of mfr.s specifically forbid the installation of VF in their products and to do so will almost certainly burn the house down. Heatilator/ Heat&Glo are the biggest.

Originally Posted by Bob Harper

This is an interesting situation. The VF universal box was listed with a max. 40MBH burner and wall/ mantel temps. at stated clearances not exceeding 117F above ambient. That's a pretty specific number. Same for hearth protection. The mfr. of the box assumes the risk since he holds a listing which states not just for his brand of gas logs but in this case ALL Vf. It used to be early universal box mfr.s would specifically deny their listing for other brands as is their right.

As Tom pointed out, the mfr. of the logs has no idea what or where his product is going to be installed in so, based upon his testing in either a masonry firebox or factory built woodburning fireplace listed to UL 127 that is also listed for use with VF, then you have numbers. Now, a mfr. will always defer to the worse case meaning if the surface temps on one box at a given distance from the opening are higher than the other, then he'll apply that distance to both. Almost all VF log mfrs. will have two sets of facing clearances: one with a hood and one without. ALL universal box mfr.s I'm aware of require hoods (usually 4") which is a main way to keep those temps down. No woodburners I'm aware of, masonry or 127 boxes come with hoods. That's the last thing a homeowner wants to see.

So, which do you go with?

From the 2012 IRC (and it has been this way for decades): (bold is mine)
- R102.1 General.
- - Where there is a conflict between a general requirement and a specific requirement, the specific requirement shall be applicable. Where, in any specific case, different sections of this code specify different materials, methods of construction or other requirements, the most restrictive shall govern.


- SECTION G2408 (305) INSTALLATION
- - G2408.1 (305.1) General.
- - - Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of listing, the manufacturer’s instructions and this code. Manufacturer’s installation instructions shall be available on the job site at the time of inspection. Where a code provision is less restrictive than the conditions of the listing of the equipment or appliance or the manufacturer’s installation instructions, the conditions of the listing and the manufacturer’s installation instructions shall apply.
- - - Unlisted appliances approved in accordance with Section G2404.3 shall be limited to uses recommended by the manufacturer and shall be installed in accordance with the manufacturer’s instructions, the provisions of this code and the requirements determined by the code official.

The most restrictive shall apply. Install any other way and the installation is non-compliant with the code (does not meet the code).

Originally Posted by Jerry Peck

From the 2012 IRC (and it has been this way for decades): (bold is mine)
- R102.1 General.
- - Where there is a conflict between a general requirement and a specific requirement, the specific requirement shall be applicable. Where, in any specific case, different sections of this code specify different materials, methods of construction or other requirements, the most restrictive shall govern.


- SECTION G2408 (305) INSTALLATION
- - G2408.1 (305.1) General.
- - - Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of listing, the manufacturer’s instructions and this code. Manufacturer’s installation instructions shall be available on the job site at the time of inspection. Where a code provision is less restrictive than the conditions of the listing of the equipment or appliance or the manufacturer’s installation instructions, the conditions of the listing and the manufacturer’s installation instructions shall apply.
- - - Unlisted appliances approved in accordance with Section G2404.3 shall be limited to uses recommended by the manufacturer and shall be installed in accordance with the manufacturer’s instructions, the provisions of this code and the requirements determined by the code official.

The most restrictive shall apply. Install any other way and the installation is non-compliant with the code (does not meet the code).

Jerry, that's nice and I'm certainly well aware of that section as I'm the one preaching it frequently. My point is, we're dealing with conflicting listings--not code vs. listing. In general, the code is there when there's an absence of engineering such as a listing. The code defers to engineering. When there is no engineering to the contrary, the listing of the appliance certainly applies. The question here is, when there is engineering to the contrary that indicates reduced clearances are a tested, known value then usually that listing trumphs a more generic listing. For instance, You may have an appliance that carries a stated clearance to combustibles off the venting of let's say 6". However, you choose a venting product suitable for that class of service with a reduced clearance to say 3". The 3" would apply because the liability is transferred to or assumed by the venting mfr. That's all.

Originally Posted by Bob Harper

Jerry, that's nice and I'm certainly well aware of that section as I'm the one preaching it frequently. My point is, we're dealing with conflicting listings--not code vs. listing. In general, the code is there when there's an absence of engineering such as a listing.

Bob,

The listings and installation instructions are code by reference, thus any listings and installation instructions are code.

Additionally, the code requires it (meaning the firebox and the gas log set in this case) to be installed in accordance with its listing and installation instructions, unless the code is more stringent - then it must meet the code. That is why I posted this:
- SECTION G2408 (305) INSTALLATION
- - G2408.1 (305.1) General.
- - - Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of listing, the manufacturer’s instructions and this code.

The key word is "and". The installation must meet all three requirements.

It all goes back to the most restrictive of the code, the listing, and the installation instructions.

The firebox says you can install ANY ... blah, blah, blah ... and meet this clearance.

The gas log set says it must be installed with this greater clearance.

If you meet the clearance of the firebox and there is a problem the firebox manufacturer has a problem - unless you install a gas log set which requires a greater clearance, then the firebox manufacturer says that you exceeded the listing and installation instructions of the gas log set so they take the liability.

It is possible that the firebox was manufactured before the gas log set, in which case the "any" in the information for the firebox does not refer to gas log sets manufactured after the firebox was manufactured. Also, the "any" refers to "any" which meets the stated ANSI standard - the ANSI standard which would have been in effect at the date of manufacture of the firebox, not an ANSI standard which came later and revised the requirements/allowances.

My questions are these: Does that gas log set meet the ANSI standard referenced by the firebox and was the gas log set manufactured at the same time of, or prior to, the firebox? If not, then the two may not be compatible.

2009 IRC R102/4 Referenced Codes and standards. The codes and standards referenced in this code shall be considered part of the requirements of this code to the prescribed extent of each such reference. Where differences occur between the provisions of this code and referenced codes and standards, the provisions of this code shall apply.
Exception: Where enforcement of a code provision would violate the conditions of the listing of the equipment or appliance, the conditions of the listing and manufacturer's instructions shall apply.

Again, we have a conundrum. The code is in conflict with itself. I stand by my position. If the listed universal box is tested and listed with a reduced clearance for that specific appliance category, then ALL appliances within that product group would apply regardless of when introduced into the market place. FYI, ALL ventfree gas logs have a ceiling of 40,000 BTU max. Therefore, ALL mfrs. of universal VF boxes have that 40mbh ceiling to test against. If they fail at lower inputs they have choices:
-they can redesign it until it passes.
-they can leave it and list if for a reduced max input rating knowing it will either cut their market or result in fire losses as that is ignored

When testing a universal box they don't line up a bunch of various VF logs to test in it: they install a burner pipe set up to deliver a specific input rating. Therefore, the temps. are rather uniform and predictable. No need to test every log set in every VF univ. box or re-test when you make a revision to the logset but keep it withing the input rating.

There are times to worry about the more stringent applying but this isn't one of them. If that wall got anywhere close to being too hot, it would have been called out in the listed instructions of the VF box's clearances. As I said, I don't trust the stated clearances (most stringent) on masonry fireplaces because they are wholly untested. Now, which would you be more worried about? I think you're barking up the wrong tree here.

Originally Posted by Bob Harper

2009 IRC R102/4 Referenced Codes and standards. The codes and standards referenced in this code shall be considered part of the requirements of this code to the prescribed extent of each such reference. Where differences occur between the provisions of this code and referenced codes and standards, the provisions of this code shall apply.
Exception: Where enforcement of a code provision would violate the conditions of the listing of the equipment or appliance, the conditions of the listing and manufacturer's instructions shall apply.

Again, we have a conundrum. The code is in conflict with itself.

There is no conundrum and no conflict:
- the listing on on the firebox is a *minimum* clearance 24" above the top of the firebox for a 10" projection of combustible material ... that is not a *maximum* clearance
- the listing on on the firebox is a *minimum* clearance 16" above the top of the firebox for a 6" projection of combustible material ... that is not a *maximum* clearance
- the listing on the gas log set is a *minimum* clearance of 28" above the top of the firebox for a 10" projection ... that is not a *maximum* clearance ... and that 10" is " 10" and less " so it covers projections down to 6"
- a key factor that some seem to be missing, or ignoring, is that the gas log set also requires a heat resistant material minimum of 12" high by 1/2" thick above the firebox opening whereas the firebox is addressing the clearances as related to a combustible material in that space.
- the listing on the gas log set completely meets (does not violate) the listing on the firebox

Comparing the firebox and the gas log set clearances is almost like comparing Red Delicious red apples Granny Smith green apples ... both are clearances / both are apples, other than that there are a lot of difference, except that the gas log set clearances do not violate the firebox clearances.

Now, there would be a conflict if the gas log set had allowed for less clearance than the firebox, but the gas log set doesn't.

No conundrum and no conflict:
- Installing that gas log set (installing in according with the listing and installation instruction of the gas log set) in that firebox meets the listing, labeling, installation instructions, engineering, of that firebox, the code, and the gas log set.

No conflict whatsoever. No conundrum either.

And, yes, the most restrictive was met.

I thought this would be an interesting (if heated) debate, given the level of expertise on this forum.

As mentioned previously,what is not immediately obvious is that the clearance diagrams are apples and oranges, and do not conflict, but simply refer to different situations, and most manuals do not clarify the difference.

This one does, a little:

heatmaster clearance.JPG

And of course, regardless of the intent of the manuals, the AHJ has the final say.