Quad Mod: Improve heat output by limiting airwash

[Dwig]

I've been brainstorming ways to get more heat out of some seriously terrible pellets that I'm burning this year. Before I started burning these pellets, I thought that "low heat" claims that others have complained about were overblown, and that pellet quality had mostly to do with ash buildup. Well, I was wrong, so I had to figure out how I was going to get enough heat to stay toasty this winter.

The first thing that peaked my interest was a website that tested and compared different pellet stoves on different characteristics: heat output, cleanliness of glass, efficiency, etc. ((broken link removed to http://www.forgreenheat.org/decathlon/sefficiency.html)). The website made the following observation:

"The very high levels of oxygen in the stack of the these six stoves may indicate that pellet stove manufacturers are sacrificing efficiency in order to provide enough air to keep the glass clean. Stoves that have an air-wash for the glass are actually leaking air in, usually beneath the glass surface. This friction across the glass from the air allows the glass to stay clearer. Our most efficient stove, the Piazzetta, also had the dirtiest glass, which may be a sign that they prioritize higher efficiency combustion over seeing the flame."

The idea to mess with the airwash to increase heat output was then further solidified by a prior thread, in which some quad owners reported serious heat gains by blocking the airwash: https://www.hearth.com/talk/threads/airwash-experiment.82190/

Theoretically, by blocking the airwash (either partially or fully), four things will improve efficiency:

1. Because less room air is getting sucked into the firebox, the air headed to the heat exchangers will be hotter.
2. Because less total air is moving through the combustion system, the airflow over the heat exchangers will slow and have more time to transfer heat.
3. More air (although not as much as was pulled through the airwash) is pulled through the burnpot, improving the burn.
4. The air sucked through the airwash is no longer causing drafts in your house.

So I decided to try it. I blocked the airwash completely with a rope gasket, and turned the stove on high. I noticed three things. First, most obviously, the glass got dirty FAST (likely made worse by the dried dung pellets I'm working with). Second, the heat output was outrageous. I didn't have a thermometer on hand to take a reading, but there was a massive heat improvement. I was actually concerned I was going to damage the stove somehow. And fourth, as expected, the burnpot was ejecting a lot of large embers due to the increased airflow through the burnpot holes. In fact, zero ash accumulated in the burnpot during the test.

The large expelled embers were definitely an inefficiency in the new setup. If I wanted to pursue this experiment further, I would have partially obstructed my OAK so as to reduce airflow. This would have improved heat output even more as the burn would be more efficient and the hot air would move even slower over the heat exchangers. I did not do this, however, due to concerns that I would overheat something. And in any case, it wasn't acceptable to me to have glass that dirty all the time.

Next, I cut two 2-3 inch pieces of rope gasket and repeated the experiment with only the ends of the airwash blocked. I noticed that the heat was still significantly better than it was with an unblocked airwash, although not quite as good as when the airwash was fully blocked. The air coming through the burnpot was also not so strong that it blew large embers out of the pot. You can see in the photo below that the glass got dirty on the sides of the glass, but stayed clean in the center. In effect, I made my garbage pellets into high heat pellets by sacrificing glass clarity. This is more than acceptable to me.

Takeaways:
1) Castiles and Santa Fes can be easily tuned to trade glass clarity for efficiency.
2) Much (I think most) of the gains can be achieved by sacrificing about half of the glass area.
3) If you want to get maximum efficiency and don't care about what the glass looks like, and you block the entire airwash, you should also consider partially blocking your OAK to reduce air flow through the burnpot.



Thanks for reading!

 

[NYBurner]

Dwig - you my want to read up the ongoing 'stoich' thread. Altering the burn will have a negligeable effect on heatput.

This was my exact thought with what was posted via the for greenheat team....the information is misleading/unclear.

You would be better off facing a room fan at your stove or using turbulators in your exchange tubes than blocking your air wash to slow down air movement in the fire box. I say this because, the air in and out of the combustion chamber is controlled via a damper or whatever exhaust/intake air control method your stove runs. If anything, you diverted a LOT more air directly into you burn pot resulting in a LEANER burn. Concentrating significant heat beyond your stoves capability can deteriorate the burn platform or even damage the firebox (provided LEAN burn = HOTTER burn). Creating a larger temp delta from firebox -->heat exchanger is the biggest hurdle any of these stoves have currently.....and that is about the only data that is usable from the testing I have seen imo.

 

[Dwig]

I think this has less to do with stoichiometric ratio and more to do with the fact that the airwash does not send air through the burnpot, but rather sends cold room air directly into the firebox. This influx of cold air comingles with hot air from the fire and is then sent to the heat exchangers. By blocking the airwash, you are preventing that dilution.

I do agree that any change in efficiency from increased air through the burnpot holes is minimal. In fact, it could be negative due to larger embers being spit out from the vortex. But my experiment confirms a significant heat output increase.

 

[NYBurner]

the air wash system does not use room air(provided you have an OAK)....it diverts a portion from intake for combustion.

Your stove temps may have risen, but points in complete opposite to data posted/discussed advising choking the intake down. The method you described feeds more air into the fire, and heat exchange time relative to volume of air in and out of the stove is unchanged (perhaps a small change with higher heats/draft changes).

Realistically it sounds more that you pushed your convection blower heat exchange into compression so it could no longer hold temps steady and the entire stove temp rose......which is also discussed in the Greenteam article but not explained.

 

[Dwig]

Ah I think I see the disconnect between us. My stove draws air from the room for the airwash. The combustion blower thus draws air both from the OAK (which is pulled through the burnpot holes), and through the airwash (which is not pulled through the burnpot holes). When I block the airwash, I am essentially eliminating a leak in the firebox.

Imagine someone drilled a hole in the side of your firebox, so room air was getting sucked in. That would not be such a good thing for efficiency. I basically plugged the hole.

 

[NYBurner]

Oh OK, that makes sense but either way you are still feeding the fire with more air directly.

 

[Dwig]

True. That's why I was considering partially blocking the OAK to limit that effect. I could reduce the airflow through the burnpot to it's original level which would compensate for my blocking the airwash.

But although more air does come through the burnpot when you block the airwash, the increase in air through the holes in the burnpot is a lot less than the decrease in air from the airwash. The airwash is a pretty big gap under the window that allows for a lot of air movement; the combustion blower cannot suck the same amount of air through the burnpot holes.

 

[NYBurner]

sounds like a good experiment to try since you've already begun. It would be great if you could get some data along the way in various stages too.

 

[Dwig]

Yeah, I'll have to pick up a meat thermometer and measure the temperature difference. I have an infrared thermometer, which I can use to roughly measure the temperature of the tubes themselves, but my understanding is that measuring the air is the preferred method on this forum. I'll post some data soon.

The only problem is my experiments all have to take place in the middle of the living room and my wife thinks I'm totally crazy!

 

[DMKNLD]

Hey to a fellow Quad tinkerer !

I've partially blocked the air wash on my 1st generation Castile for the past several burn seasons and it has worked great, as many other Quad folks have found as well. Here's a past forum link where FFRKing did some IR temp testing on his Quad before and after obstructing his air wash, and he averaged about an 80 degree temp increase out of each of his 10 exchanger tubes, which cumulatively adds allot more ambient heat to the room. https://www.hearth.com/talk/threads/airwash-experiment.82190/#post-1057432

I've found I rarely need to run the stove on high, even in our drafty 1870's non-insulated balloon-walled New England farmhouse, and have been able to get heat output on the medium stove setting with the air wash obstructed that is roughly equivalent to the high setting when the air wash is unobstructed, saving significantly on pellet use.

Like you, I found that if I obstruct the air wash completely I get too much air pulling thru the fire pot and it blows partially burnt pellets (incomplete combustion) out of the pot into the firebox, depending on the type and quality of pellets I'm burning. So I use an old tadpole door gasket I cut up into about 6 - 8" long sections to partially obstruct the outside edges of the air wash, while leaving the middle section of the air wash open, which seems to be the 'sweet spot' for getting complete pellet combustion and max heat output without blowing the partially burned pellets out of the fire pot. Your pellet quality, as well as your EVL / vent configuration, would likely require more or less of the air wash obstruction to get the optimum burn efficiency for your stove.

It does definitely dirty the glass, which is largely aesthetic. I burn 100% softwood pellets that leave such a light fluffy ash that it brushes off easily with a paintbrush and the ash vac. Maybe every 4 or 5 days I wipe the glass clean with a damp paper towel. I only scrape the burn pot once a week or so, as any fly ash is completely blown out of the pot, which is when I also clean out the fire pot air holes with an appropriate sized allen wrench, which are rarely if ever ash obstructed, and I spray some non-stick cooking spray into the fire pot to lubricate the 'trap door' and reduce the carbon buildup leftover from the pellet impurities.

If you're interested in some other 'Quad mods" to increase your stoves heat output, here's a link to an excellent discussion from some fellow consummate Quad tinkerers who came up with them, and they have worked great with my Castile. The discussion wanders a bit over it's 9 page length, but it is a good read if you're up for squeezing every possible BTU out of your stove. As a standard disclaimer, any mods will void your Quad warranty, and are done "at your own risk / peril". You can Private Messaging me if you have any questions on these mods. https://www.hearth.com/talk/threads/quadrafire-castile-experiment.58216/

Regards, and happy burning !
DK

 

[Golfeur]

hi all
What would be interesting to know also how long you do with a 40lbs bag with airwash clogged or normal in the same temperature condition

 

[DMKNLD]

hi all
What would be interesting to know also how long you do with a 40lbs bag with airwash clogged or normal in the same temperature condition

The pellet feed rate will be the same regardless of whether you are running it with the air wash blocked or not - you just get a lower but hotter and more intense flame when the combustion blower is pulling more air thru the burn pot with the air wash obstructed. I actually run mine with the feed gate a bit more open than the standard setting, which seems to better balance the air to fuel ratio on my stove when the air wash is obstructed.

So I feed pellets a little faster that way, but with the air wash obstructed I get a good bit more heat output. So I can maintain the same room temperatures relative to the outside temps by running the stove on low instead of medium, or medium instead of high, which is where the net savings in pellets comes in, if that makes sense.

I only need to run my Quad on high setting when it is below 0 F temps and the wind is blowing hard, otherwise the medium setting works fine most all winter for me, and low setting for the shoulder seasons like now. I've also done a couple other Quad modifications that have helped increase my BTU output, as noted in the forum link in my above post. So as always, "your mileage may vary".

 

[Wilbur Feral]

Very informative posts, and greatly appreciated. I have an MVAE and find that very few mods are possible. But I just installed a Santa Fe insert to replace an old Whitfield for our second stove, and have been surprised at the low heat output for a stove I know to be clean and functioning properly. An examination of the stove revealed the open airwash area at the bottom of the glass, which surprised me even before I found this thread.

I am also surprised at the amount of old smoke smell I get when the stove is off and air pressure outside is changing. I have an OAK, but believe the "leaky" firebox design of these stoves is a big culprit here. Air that should be sent back out the OAK when the stove is off (reverse draft, if you will, which can happen in a cold stove with no fan running) is allowed to exit into the room. I suspect much of my next month will be spent fiddling around blocking these air leaks. Thank you for excellent information and discussion.

 

[Dwig]

What would be interesting to know also how long you do with a 40lbs bag with airwash clogged or normal in the same temperature condition

I did not change the feed rate at all before or after I did the blocking experiment. Despite not burning pellets at a faster rate, I noticed a significant increase in heat output. This means that the mod does not merely allow you to burn pellets faster to get more heat - it actually allows your stove to extract more heat out of the pellets. My understanding is that this is because the airwash mixes cold room air with your hot combustion air, and when you block the airwash, you are limiting that dilution of hot air. You do also get more air coming in through the holes in the burnpot, but there is definitely a net decrease in air moving through the system.

Measuring how long a bag lasts is hard because it is dependent on the weather. But I'll tell you, we had a relatively cold night last night and when I woke up, I noticed that the hopper was more full than I expected it to be. I expect this mod to save me a lot of pellets, and I actually don't mind the dirtiness on the sides of the glass because it's still crystal clear in front of the fire. I'll post a photo tonight of how the glass looks after three days with 2 inches blocked on each end.

Another way that I can use the mod to save pellets is by increasing the temperature swing on my programmable thermostat. Because the stove can crank out heat faster, I can set it to 63 degrees at night and then have it turn on an hour or two before I get up and it's 70. Before the mod, the stove had a harder time increasing the temp in the house from a cold state.

I am also surprised at the amount of old smoke smell I get when the stove is off and air pressure outside is changing. I have an OAK, but believe the "leaky" firebox design of these stoves is a big culprit here.

This is interesting. I haven't experienced that, but then again my exhaust goes vertically out of my chimney (insert). Perhaps if your exhaust exits horizontally out of the wall then it would allow the wind to blow in. I think partially blocking the airwash would help this. Also, and I discussed in a previous post, you can seal off the ash pan, which makes your OAK more airtight.

So I use an old tadpole door gasket I cut up into about 6 - 8" long sections to partially obstruct the outside edges of the air wash, while leaving the middle section of the air wash open, which seems to be the 'sweet spot' for getting complete pellet combustion and max heat output without blowing the partially burned pellets out of the fire pot.

DMKNLD - What have you found to be the sweet spot in terms of how much of the airwash to block? I've only tried blocking about 2 inches off each side. I'd be interested if you found that, say, 3 inches worked better. I'm also always glad to encounter other tinkerers!

 

[moey]

Ive put aluminum foil in the air wash on the outside. The increase in temperature of the glass always worried me.

 

[NYBurner]

Hopefully you can take measurements in controlled intervals. Making changes to fuel feed, airflow, burn time, etc will all affect the readings.

 

[DMKNLD]

My understanding is that this is because the airwash mixes cold room air with your hot combustion air, and when you block the airwash, you are limiting that dilution of hot air. You do also get more air coming in through the holes in the burnpot, but there is definitely a net decrease in air moving through the system.

Besides less cold air being pulled in with the air wash obstructed, you get a 'blow torch' effect of increased forced draft through the fire pot holes, instead of the exhaust blower drafting air through both the air wash and the fire pot. This essentially gives an increased "bellows effect", where you inject more oxygenated air directly into the hottest portion of the fire to mix it with the partially combusting gases, which increases the combustion and gives a hotter fire.

 

[DMKNLD]

Ive put aluminum foil in the air wash on the outside. The increase in temperature of the glass always worried me.

Hey moey, *theoretically* the #3 snap disc should trip before the stove over-fired enough to reach the 1300 degree F temp rating of the ceramic glass used in pellet stoves. Certainly wiping the hot glass with a wet rag or paper towel when cleaning is a more likely expen$ive way to crack the glass.

Knock on pellets here, but I've never had my stove over-fire enough yet to trip the #3 disc, and have no signs of stove metal warping, running mine all winter with the air wash partially obstructed. But I imagine you could if you were burning high end 100% softie pellets with the stove setting on high, with the feed gate WAO (wide azz open), and the air wash completely blocked, which I never have done. I haven't been willing to do that 'grand experiment', ie "hey Bubba, watch this....." on my 14 y/o vintage Quad stove !

 

[Dwig]

Hey moey, *theoretically* the #3 snap disc should trip before the stove over-fired enough to reach the 1300 degree F temp rating of the ceramic glass used in pellet stoves. Certainly wiping the hot glass with a wet rag or paper towel when cleaning is a more likely expen$ive way to crack the glass.

Knock on pellets here, but I've never had my stove over-fire enough yet to trip the #3 disc, and have no signs of stove metal warping, running mine all winter with the air wash partially obstructed. But I imagine you could if you were burning high end 100% softie pellets with the stove setting on high, with the feed gate WAO (wide azz open), and the air wash completely blocked, which I never have done. I haven't been willing to do that 'grand experiment', ie "hey Bubba, watch this....." on my 14 y/o vintage Quad stove !

Yeah, I did this (High setting, feed gate removed, air wash fully blocked) with North American Pellets, arguably the worst, lowest heat pellets known to man, and I was concerned the stove was going to melt. I definitely wouldn't do it with high-octane super premiums!

 

[Dwig]

So I took some measurements over the week on different settings, confirming my observations (and the observations of others) of having more heat output with partial airwash obstruction. Using my infrared thermometer, I measured the temperature of each of the tubes after the stove had running for a while ~ about a half hour. I did this both with the 2-inch airwash blocks in place and with them removed. I found that the 2-inch airwash obstructed setup had tubes that were on average 20 degrees hotter. And for the tubes that have the most airflow, the temperature increase was generally 30-40 degrees hotter.

Unobstructed airwash - average tube temperature: 365
Tube-by-tube data: (1) 380, (2) 381, (3) 358, (4) 368, (5) 391, (6) 376, (7) 351, (8) 334, (9) 361, (10) 358

2-inch (at each end) airwash obstruction - average tube temperature: 386
Tube-by-tube data: (1) 414, (2) 420, (3) 401, (4) 392, (5) 409, (6) 385, (7) 349, (8) 339, (9) 373, (10) 379

As a caveat to this data, I have to add that after running the stove for a while with an unobstructed airwash, I noticed that I had a layer of black soot/creosote on the heat exchangers. This is a common problem with my very dirty North American Pellets. It seems that by obstructing the airwash and thereby increasing air flow through the burnpot, I have been able to improve the burn quality of the North American Pellets and eliminate the creosote problem. When I removed the airwash obstruction for this test, the creosote came right back.

This creosote problem means that the outcome of my experiment might have been different with normal pellets, because most pellets burn cleanly even without the airwash obstruction.

And as a further lesson, if your are burning very dirty pellets (i.e., creosote forming), airwash obstruction might be your way to a cleaner burn.

Here is what my stove looks like after a couple of days with 2-inch blocks at the ends of the airwash:

 

[DMKNLD]

Excellent 'evidence-based research' Dwig! I'll concur on all your salient points.

One added caveat / disclaimer is that Quads are built with high quality cast and metal components, which likely can withstand higher heat temps than stove brands that have cheaper component part compositions that could make them more prone to an over-fire and / or heat warping.

Although theoretically the #2 snap disc should trip if firebox temps exceed the snap disc parameters, increased heat on the door frame in particular when the air wash is obstructed could be an issue on non-Quad stove brands. As always, any stove modifications will void the dealer warranty, and are done "at your own risk", as after-market DIY stove mods are neither homeowner insurance coverage approved nor Quadrafire sanctioned !

That said, given that I'm heating a square footage area that is at the upper end of my Castile's BTU output, I'll take the increased room temps and improved burn efficiency to the bank all-day, any-day, given the reduced pellet use and more complete combustion that the Quad mods have provided. And as always, "YMMV".

 

[NYBurner]

Congrats on the success! and thank you for sharing the data!
You've maintained the airflow in and out of the stove, yet redirected the mass directly into the fire netting you substantially higher exchange temps. This points to running a leaner fire is hotter, correct? I was not quite sure if that transitioned over to pellet stoves burning wood under vacuum.

 

[twill]

Excellent 'evidence-based research' Dwig! I'll concur on all your salient points.

One added caveat / disclaimer is that Quads are built with high quality cast and metal components, which likely can withstand higher heat temps than stove brands that have cheaper component part compositions that could make them more prone to an over-fire and / or heat warping.

Although theoretically the #2 snap disc should trip if firebox temps exceed the snap disc parameters, increased heat on the door frame in particular when the air wash is obstructed could be an issue on non-Quad stove brands. As always, any stove modifications will void the dealer warranty, and are done "at your own risk", as after-market DIY stove mods are neither homeowner insurance coverage approved nor Quadrafire sanctioned !

That said, given that I'm heating a square footage area that is at the upper end of my Castile's BTU output, I'll take the increased room temps and improved burn efficiency to the bank all-day, any-day, given the reduced pellet use and more complete combustion that the Quad mods have provided. And as always, "YMMV".

I hate to sound stupid, I keep reading about blocking the airwash but no one says where to find it! My door has a gasket on all sides so I am a bit perlplexed...

 

[Ssyko]

What stove do you have?

 

[moey]

If your not sure take a piece of aluminum foil and run it along the edges of the glass it will slip in at some point to the metal. You should be able to see it though where the glass meets metal.