Any reason not to use 4" vent piping (other than cost)?

[acammer]

Good morning all. I am looking at options for upgrading the venting on my pellet stove. The stove is a Timber Ridge 55-TRPAH (Englander 25-PAH) installed in a corner operating in the finished 600sqft section of my basement. Open stairway, open floorplan upstairs with another 1,500sqft. I also have the HVAC system setup with a cold air return in the finished basement, pulling the hot air off the ceiling and pushing it around the house. It heats that whole 2,100sqft space to 70*F down to 20*F outside before it needs any help from the propane appliances. Venting is left over from my old stove. It's 3" pipe, 45* right off the stove, then a 90* elbow up, 36" of vertical rise to another 90* elbow out horizontally, with a run of 24" and a downturn termination. I run a separate OAK as well, 2" flex pipe included with the stove that runs a similar path but with gentler bends. I calculate the EVL of my current venting setup to be 16.5', slightly over the maximum recommended EVL of 15' for 3". The stove performs pretty good, but I'm seeking to maximize breathing performance so I can run the feed trim as high as possible without a dirty burn. Presently with a good clean exhaust and stove I can run it about 30-40% of the way up with the air trim maxed out - I'd like to do better and get that fuel trim at the max the stove will deliver, if possible.

So, I have a few options. The cheapest option available is to simply extend my horizontal run another 8 - 10 inches inside the home and eliminate the 45* bend, going right into the 90* up bend off the back of the stove. That should be a net gain of about -2' EVL and get the total system down to a 14.5' EVL. That's still very close to the maximum recommended for 3", and I'm wondering if I should rebuild the whole system with 4" pipe to increase my breathing potential. If I did the same layout as my proposed cheapest option above all in 4" I should think that would provide a much less restrictive exhaust. I also like the option of then doing something like Selkirk Direct Temp that can pull the outside air down the vent pipe, eliminating my separate thimble for the OAK, and providing much better flow than that 2" flex hose I have now. I would think the gains of such a change would help me increase my feed trim some more and squeeze the last 1-2000BTUs out of this stove.

Is there any reason not to use 4" vent piping in this application? I know when sizing a performance exhaust on a car the whole "bigger is better" theory can be taken too far. In that application there is a sweet spot in size for velocity and scavenging. I don't understand biomass burning draft quite as well, but I understand there is a similar principal at work with hot air seeking to rise, even with an exhaust blower doing a lot of the air moving. I appreciate any thoughts you guys have as I work to squeeze every last bit of heat out of this install.

 

[funflyer]

Just brainstorming but is there any way you could use two 45s to replace the two 90s. Instead of a straight run between 90s the run would angle up on a 45. The stove may have to be repositioned to accomplish this. A picture of your setup might help to get more ideas.

 

[MadMax31]

Mine is a straight shot up and out roof from back of stove ( same one as yours ) cleanout tee, up, up and away. 18'. I ran 4". IOM states 15' max for 3". Id just run 4" and be done.

 

[Pelleting In NJ]

There is no "scavenging" effect with a pellet stove, as the airflow from the combustion fan is relatively continuous, unlike a car engine, where the exhaust flow is pulsating. There is only pellet stove performance benefit resulting from the use of the larger 4 inch pipe, rather than 3 inch.

 

[SidecarFlip]

There is no "scavenging" effect with a pellet stove, as the airflow from the combustion fan is relatively continuous, unlike a car engine, where the exhaust flow is pulsating. There is only pellet stove performance benefit resulting from the use of the larger 4 inch pipe, rather than 3 inch.

There certainly is so, I'll disagree with you on that but I'm not in an argumentative mood today.

 

[acammer]

Just brainstorming but is there any way you could use two 45s to replace the two 90s. Instead of a straight run between 90s the run would angle up on a 45. The stove may have to be repositioned to accomplish this. A picture of your setup might help to get more ideas.

I'll try to grab a picture at some point. There isn't a way to go with a pair of 45*'s like you suggest, my hearth pad is pretty well maxed out on space for the current install. I can drop the one 45* pretty easily and be down to just two 90*'s which buys me a 1-2' EVL gain, and costs me only a 1' piece of 3" to extend the horizontal run. It's not that the stove runs bad, it certainly runs quite well, burning cleanly on the highest heat setting with the combustion blower trim maxed out and the feed trim up to 4 or 5. I'm really just exploring the very top end of performance I can get out this stove, acknowledging that I've moved away from peak efficiency with the combustion blower running full tilt to handle as much fuel as possible. If I can move the air a little better, I can go up a little more on the feed trim. I'm sure that the direct temp flows the intake air better than the 2" flex tube that came with the stove, so I should be picking up flow gains there as well. It seems a little silly to put together a nice $4-500 vent on a $1,000 stove, but if it gets me a couple thousand more BTU, I might go for it.

 

[acammer]

There is no "scavenging" effect with a pellet stove, as the airflow from the combustion fan is relatively continuous, unlike a car engine, where the exhaust flow is pulsating. There is only pellet stove performance benefit resulting from the use of the larger 4 inch pipe, rather than 3 inch.

There certainly is so, I'll disagree with you on that but I'm not in an argumentative mood today.

Pelleting in NJ - are maybe thinking about there is no resonance tuning to leverage in a pellet stove exhaust, as there is a steady stream instead of a pulse? I would completely agree that is the case, so there is no "tuning" like with the primaries and collector in a set of headers for a car. I would think that you still get some scavenging - that's essentially what draft is all about in these, right?

 

[pageyjim]

I am addressing the title only and not the OP's application. I know with furnaces, water heaters and boilers you can oversize tlue. The fix there would be to make it longer or smaller. It can cause sooting and failure of heat exchamger plus various lesser problems. I do not know how this applies to burning pellets.

 

[SidecarFlip]

My 4" precludes any smoke shoud the stove go down from lack of power or my stupidity. If the inner pipe is warm (as it is when firing), I have excellent natural draft all the time. In fact, I can keep my draft shutter pretty well closed up and still pull lots of air (combustion air) in through the outside air kit, in fact, I can lower my CA fan values to almost nothing and let the natural draft keep the fire burning just fine....

Additionally, I run a pair of 45's on my outside vent to clear the eave and gutter and allow a continued vertical to the vent cap (20 feet above the cleanout Tee), which is, most ,likely much more vent than most people run but I wanted my vent termination well up in the air vertically and away from the house siding.

If I pull my cleanout cap on the cleanout Tee (with a warm vent) and shut the stove off and place a lighted match in the cleanouit opening, the draft immediately extinguishes the match (just like a normal flue would do).

I'll have to take some pictures and post them I guess.

 

[Pelleting In NJ]

Pelleting in NJ - are maybe thinking about there is no resonance tuning to leverage in a pellet stove exhaust, as there is a steady stream instead of a pulse? I would completely agree that is the case, so there is no "tuning" like with the primaries and collector in a set of headers for a car. I would think that you still get some scavenging - that's essentially what draft is all about in these, right?

Yes, an automotive exhaust system can benefit from resonant tuning, due to the variable frequencies (engine RPM) of the exhaust gas pulses. For a pellet or wood stove, the natural draft of a vertical pipe carrying hot gases in a cooler external environment does add some scavenging, but for a pellet stove, the scavenging effect is negligible compared to the powered airflow from the combustion blower. Of course, with no combustion blower, like a woodstove or fireplace, the natural draft of a vertical flue rise is critical.

The lower flow restriction of 4 inch pipe (versus 3 inch pipe) is significant, and that alone (without any scavenging effects) can be beneficial for a pellet stove.

 

[SidecarFlip]

Additionally, it wasn't a cheap date by any means. 4" Duravent isn't cheap, especially 20 vertical feet. The venting (from the stove, through the wall to the 3-4 cleanout Tee and vertically 20 feet to the cap, cost almost as much as the stove did.15 years ago but I expect the venting (and possibly the stove) to outlast me.

 

[bcarton]

No guesses as to whether you'll realize any performance increase for your $4-500, but my perspective is that it makes sense to spend as much as you need to for a quality vent system that meets manufacturer spec and makes your stove run its best. But since your stove is already running really well, maybe think about taking that money and either improving someinsulation/doors/windows or even adding a 2nd small, used stove?

 

[acammer]

No guesses as to whether you'll realize any performance increase for your $4-500, but my perspective is that it makes sense to spend as much as you need to for a quality vent system that meets manufacturer spec and makes your stove run its best. But since your stove is already running really well, maybe think about taking that money and either improving someinsulation/doors/windows or even adding a 2nd small, used stove?

It's an excellent point - there are other ways to spend that money. I'm in no way convinced I need to go 4", obviously I don't if I get good performance on 3". I have to decide if I'm satisfied with the peak output I can get now, or if I want to squeeze the last bit I can from the stove. A 2nd stove is always on my mind too, I wouldn't mind replacing the LP stove in my living room for a pellet stove some day - but that means major changes to the existing setup. And there are some LP advantages - it's fast on heat, and it is practically no maintenance. I appreciate the input guys, it's good food for thought. I'll probably get the unnecessary 45* bend out of the existing system and run with that till it presents me with a more compelling reason to replace it.

 

[SidecarFlip]

No guesses as to whether you'll realize any performance increase for your $4-500, but my perspective is that it makes sense to spend as much as you need to for a quality vent system that meets manufacturer spec and makes your stove run its best. But since your stove is already running really well, maybe think about taking that money and either improving someinsulation/doors/windows or even adding a 2nd small, used stove?

That got handled years back when we took the old pegged together farmhouse (it's put together with dowel pins and hand hewn timbers), gutted it and made it over just keeping the frame and floors.... about the same time the new stove and vent came to be.... New windows, doors, deck, siding, insulation (wet cellulose and foam), new roof (metal of course) an extra bathroom, new furnace (don't use it much), water heater, central air, you name it, we did it..... Don't take much to heat. One pellet stove keeps both stories toasty. Upstairs we had the contractors keep the laterials exposed for the rustic look. Came out real nice and it's extremely efficient too.

We lived in our motorhome for 4 months while the house was being done btw.

 

[Pelleting In NJ]

Also, 4 inch pipe can go much longer between cleanings, before the ash restriction begins to degrade stove performance.

 

[SidecarFlip]

Also, 4 inch pipe can go much longer between cleanings, before the ash restriction begins to degrade stove performance.

Thats a non issue with me. My venting stays clean. It gets sucked out via leaf blower and brushed out via pellet vent brush at least monthly. No ash inside to speak of.