Putting a shroud over gas termination cap - bad idea?

[Oliver Simpson]

So in the interest of aesthetics my remodeling contractor put a shroud over collinear termination cap. This was my suggestion, though as client feasibility isn't really my department. The shroud has open sides and closed roof kind of like embedded photo (representative, not the real one). Now the fireplace company says (and they make sense) this is a terrible idea because it will impede good supply air flow because exhaust gases are no more likely to return via supply. Second consideration is whether heat dissipation is impeded in a dangerous way with this in place. Third could be warranty issues.

The proposed remedy: cut a rectangular opening (6"x8") right across the top ridge of the shroud to allow better venting. This will not be visible from the ground, so while I'm kind of up in arms about damaging something I paid hundreds of dollars for, I can see that it will work. My thought is OK, if you're going to cut holes in it, at least leave the ridge intact and cut two smaller rectangles (3"x8") to either side of the ridge and still very near the top. I don't know if that ridge has any structural importance and even if it doesn't it just seems better to leave intact.

Any input as to whether it's truly necessary or other solves? Thx

 

[coaly]

The intake is for OXYGEN to the burner. Any displacement of oxygen molecules from mixing exhaust gasses and water vapor in the incoming air creates a rich air / fuel mixture. You are likely to get soot accumulation on logs or glass.

Was the termination cap UL listed? Any modification of any heating system component looses the UL rating when it isn't used and installed as tested. Does the appliance installation instructions call for a specific part?

 

[Oliver Simpson]

Thx. Just to be clear, you say soot accumulation that would build up over time, even from a GAS (not a gas-assist wood) fireplace right?

No parts are replaced or removed. As I tried to convey above, this is an ADDITIONAL item - there is an architectural shroud over the existing (ICC) termination cap.

 

[coaly]

Yes, when you burn propane or natural gas it must be mixed with the proper proportion of air, about 70 buckets of air to a bucket of fuel or 70 to 1. When richer than that it creates black soot just like diesel or burning any fossil fuel too rich. It is actually worse since the soot is very light and can look like a black wasp nest. Touch it and it becomes airborne like the black floaties you get from a acetylene torch without oxygen. Huge mess. It is unburned fuel / carbon with higher than normal carbon monoxide emissions.

All burners have an air mixing tube with an air adjustment. The mixing tube is the portion before the holes where the gas / air mix comes out to be ignited. Gas under pressure PUSHES atmosphere (air) out of the tube and atmospheric air pressure PUSHES into the tube to mix with the fuel. In a direct vent appliance, this is accomplished by hot and lighter than air gasses rising up the exhaust. As it rises this creates a low pressure (vacuum) area in the fireplace (combustion area or chamber) and atmospheric air pressure PUSHES into the intake supplying oxygen to the burner tube. Any restriction (smaller intake than square inch area of inlet) or longer length than the manufacturer recommends reduces oxygen and atmospheric air pressure at the burner and changes the air / fuel mixture. Anywhere cooler than above the flame will condense the water vapor from combustion and allow unburned fuel particles to stick accumulating soot rapidly. (cooler glass condenses moisture on it allowing particles to stick) I've had many blacken the glass in an hour and make a big mess. Do you have the installation instructions? You can't add components that it wasn't tested with. That is what a UL listing means - used and installed as tested.

 

[coaly]

I should add, gas burns blue when mixed properly with oxygen to burn clean like the burners on a stove top or oven / BBQ grill. When used as a fireplace, the burner is detuned with richer than normal areas to create yellow flames. These flames would be soot producers if not for the hotter areas of the burner tube that is blue. It should always have a bright blue flame close to the burner which is the hotter flame to burn off the carbon created by the yellow flames. A good example is how you can make a ceiling black with the yellow flame from a candle. This is a yellow carbonizing flame similar to what you would have without the hotter blue flame to clean it up. The pressures I'm referring to with atmospheric pressure pushing in compared to rising out, are measured like blood pressure with a U-tube gauge with water instead of mercury. (W.C. or inches of Water Column) They are very minute pressures that you need the proper equipment to verify you have the correct pressures. As an example 10" W.C. is about 1/2 psi (normal propane gas pressure - Natural Gas is half that pressure) which is less than your breath. Pressures required to get oxygen to your burner can be in the range of .02 or .03" W.C. This differential pressure (from atmospheric) is draft. The appliance requires a certain amount which you will change by adding components which is why they told you it is a bad idea. It is impossible to know if your draft is within limits without the proper test equipment, and it's against code anyway to modify the appliance or vent since ALL appliances must be UL Listed. (installed AS TESTED) That is why termination caps and vent parts will have a UL listing label on each part and be made for exact applications for each appliances needs.

 

[Oliver Simpson]

Thx. I guess, your discussion of internals (mixing valve, inlet surface area) means I'm not fully connecting the dots since we're talking about a shroud at chimney's top in proximity to the term cap. What you say all makes sense and yes I've RTFM'd a few times but doesn't directly get to the shroud's effects (or non effects) on termination cap performance and by association overall system. The install manual says very little; makes no mention or prohibition of a shroud per se, though does allude a (permitted) integrated cap / shroud from RLH Industries, apparently a private label of that from the Homestyle Collection. Such as this product (or one similar).

 

[Oliver Simpson]

Wrote above reply before you added your second reply which is also very useful. As you say, impossible to know. Obviously, a shroud on stilts say 10' above the termination cap (to contrive a silly example) would have zero influence, but where does influence begin and who gets to say.

 

[coaly]

No mixing valve - mixing tube on burner where air and fuel are mixed within the burner tube.
Inlet area is the square inch opening that atmospheric air pressure pushes into. What ever the square inch inlet area on appliance is, the inlet pipe will not reduce or restrict the size of the opening. So the termination cap can't restrict the square inch opening as well.
The codes are taken from "standards" written by NFPA (54 and 58 in relation to gas systems and appliances) and the phrase used is "air intake must be in direct connection with atmosphere". In your case the separation between exhaust and intake must remain constant to the point of termination.