ping hearthman

[rich pickering]

Just finished a hydronics design course and we were wondering about heat loss through a direct-vent fp. Should we treat the glass area as a window? Has any body studied this?

[Carnak]

To bud in it has been a while since I have seen a fireplace, but the direct vent ones, through the wall that I remember were all standing pilot, and I do not remember the glass being cold to the touch so I would not worry about it.

Maybe if something had a taller vertical vent that was coaxial, it would work like a hoyme HRV. Cold air falling in the annulus would get heated up by air rising up the vent.

Perhaps hearthman will have the answer, otherwise measure the glass temp on one next winter and let us know.

[rich pickering]

Not worried, just wondering.
Soooooooooooo......... where do you hang your stocking at xmas?

[Carnak]

Lol, the railing on the stairs

[Collin]

Just not the same......

[Carnak]

Yep not the same, and the white sand is no substitute for snow either

[hearthman]

We've talked with the IRC and Energy Star to name a few. A std. direct vent Fp is treated the same as any other exhaust appliance such as a water heater, clothes dryer, etc. The key is in sealing the vent system at the thermal envelope (wall). Be sure the listed instructions allow caulking this joint and with the specified material. We use 300 F rated caulk or better. Also, what's allowed for horizontal vent sealing may not be allowed for vertical or vice versa--check with the mfr. If you ever have to contact tech services on a ruling, get it in writing including the full name of the person and their title. This can be in an email but you want something to hang your hat on.

Vertical DVs can thermosyphon a little on their own even with just a standing pilot and therefore may seem a little cooler whereas a horizontal DV in a well insulated chase usually feels a little warmer when you stand in front of them. The problem there is almost always cold air infiltration from a) negative pressure inside coupled with B) lack of proper air sealing. Insulation is not an effective air barrier. They make filters with it that pass 2,500 cfm don't they?

There is the unusual situation with a see-thru Fp that penetrates an exterior wall. These must be rated by the National Fenestration Council as window glazing and meet the energy code requirements of the AHJ.

FYI, more and more gas DVs are switching to an Intermittent Pilot Ignition system or IPI, which is cold when off unless you have a Cold Climate function that allows you to leave the pilot standing. This also provides about 1,200 BTUs so it isn't such a cold spot in a wall. It also primes the stack and discourages bugs.

While a gas DV may have a glass area of ~2 ft.x 3 ft, the hole in the wall is usually 8 inches(50.24 aquare inches). Whether installed in a dogshed or inside a flat wall, the exterior wall or thermal envelope should be weatherized according to local code and practice. On dogsheds esp, I recommend the framing be foamed/ caulked and an air barrier applied on the exterior in cold climates. Around Philly, we recommend Thermoply laid over the fiberglass batts. I prefer the formaldehyde-free formulations to reduce the chance of odor problems. See Joe Lstiburek's http://www.buildingscience.com for information on your climatic zone and construction science.

Rich, this is an interesting topic and one that's sure to get more attention as energy costs rise. BTW, I'm scheduled to do a Dog & Pony show for an Energy Star company in NJ next month. I impressed them with the hazards of packing insulation around vent pipes, using urethane foams around hot things, and other well meaning activities that may not be consistent with long term survival of a house or its occupants.

Good stuff. I hope I answered your question Rich. Carnak, you can tell the kids Santa gets final approval on all fireplaces after they pass UL. Have a great weekend guys!

[rich pickering]

Thanks hearthman.
While it's probably not a big deal, when we start designing radiant floor systems, it might make a difference in a small room.It's interesting doing what-ifs.

[hearthman]

I'd like to see some infrared thermographic scans from the inside and outside both at standby and burning.

Maybe one day, I'll get certified and get a camera. Anyone have about $13,K lying around they don't need? :-)

If you guys do uncover anything on this please share it here on this forum.
Thanks!

[SeattlePioneer]

<<Soooooooooooo......... where do you hang your stocking at xmas?
>>


While that seems to be a frivolous question, it's really quite serious. Around Christmas I find plenty of stocking, flocking and other crap hanging off of and immedietely above gas fireplaces.

Most gas fireplace installation manuals have charts describing required clearance to combustible "mantles." But a mantle appears to be any combustible material that extend out in front of and above the fireplace. So my supposition is that flammable Christmas decorations should meet those clearances, at a minimum.

In short, keep the Christmas crap away from the fireplace!



Seattle Pioneer

[hearthman]

NFPA 97 has these definitions but basically, if it is not inorganic or will burn, it cannot be within a stated clearance. This means substances and products that carry a flame spread rating (ASTM E-84/UL753) of less than 50 might be considered non-combustible for use in fire rated assemblies such as fire walls and fire resistive construction but are not tested or approved for long term heat exposure. A flame spread rating addresses a short term piloted flame ignition. It does NOT address pyrolyis from long term heat exposure.

The test most generally accepted for non-combustibles is ASTM E-136, which is a 750C vertical tube furnace (1382F). For instance, "fire code" sheetrock is utterly destroyed in this test. Cementitious backer board passes. Think of it this way: if you threw it into your furnace for a day then pulled it out, if it still weighs the same and is structurally intact, it's probably ok to use within a stated clearance.

Good point, S/P. Same goes for all those dried flowers, lace covers, and candles. They seem to lose the point it is a FIRE PLACE. That means there is FIRE in that PLACE! Keep away stuff that will burn.