AFUE - Single Pass vs Triple Pass Boiler.... ?

Unknown

I thought what you describe in your first sentence is in part what gives mod-cons their efficiency ? I know the subject isn't about mod-cons , but couldn't the same theory be applied to downfiring a conventional boiler ? If you look at the literature for oil boilers with different firing rates , the AFUE usually increases when you lower the input in the boiler's range .

Rodney Summers

AFUE - Single Pass vs Triple Pass Boiler.... ?

question:

both the Buderus G115-xx (triple pass) & the Peerless EC/ECT-xx series (roughly a single pass JOT design) list afue's of ~ 86.x % in their respective performance charts. wouldn't you think that the triple pass afue rating would at least slightly clobber the single pass design - (at least relative to one another)?

in other words, i should just keep my old severely downfired, AFGed, OA reset-ed, standby purged, happy running JOT-4??

comments welcome.... (-:

ss

Brad White_109

One would think so

the three-pass beating a single-pass. But it all gets down to retention time in the heat exchanger, the length of travel and configuration. I agree with you, any three-pass should have more turbulence, surface area and retention time on the way to the flue.

Maybe this underscores how limited AFUE is as a barometer of real efficiency? Not to start anything you understand.

Wayco Wayne_2

I recently

took a class given by Weils Mclein about their Ultra line of boilers, including their new oil fired 3 pass boiler. According to the teacher from W/M A pin style boiler and a 3 pass have the same efficiency, however the advantage of the 3 pass is that it can be vented sidewall with no additonal mechanical means. I found that very interesting. WW

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J.C.A._3

The big question.

How long will the pin boiler MAINTAIN those high numbers VS. a 3 pass? As soon as the pin boiler gets a bit misted with the products of combustion, those numbers go south real fast. The other consideration should be the ability to scrub said products(and ..it DOES happen!) from the heat exchanger effectively, when they occur. Just my .02 Chris

Wayco Wayne_2

I agree

I love the accessability of the 3 pass for cleaning. Door swings open and it's all there and easy to get too. It's what I install when given the chance. My fav is the Buderus G115 although the new W/M Ultra oil fired looked pretty good, and comes with the equivilent of the logomatic already installed. Hmmmmmm. WW

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Jim Davis_3

Wasting fuel

The first thing is that down firing any piece of equipment radically increases fuel usage.
60% of heat transfer on most equipment is radiant energy of the flame and 40% is convective from the scrubbing of flue gases. What does it take to exchange radiant energy? Flame temperature and flame exposure(size and distance)from heat exchanger. You lose up to 50% of radiant transfer with a small flame.
A three pass boiler should always be more efficient than a single pass but anything can be misfired to create a high AFUE. AFUE does not address delivered efficiency just tampered with burner operation.

Will_5

AFUE

....sure does baffle me too. I've been using a few of the Burnham MPO's as my triple pass of choice, nice boiler. I've never understood how a std pin style boiler that has a gross stack temp of 500 degrees can have the same AFUE as a 3-pass with a stack temp of 325 degrees. I usually expalin that to a homeowner and it's a no brainer.

Brad White_9

Jim

Interesting, I had not heard the 40%/60% convective/radiant ratio before.

I would think (just think, not having thought of it before) that the radiant portion would occur in the combustion chamber immediately within sight of the flame.

Further that the remainder of heat absorbtion is more conductive (or as you say, convective which is unavoidable).

In condensing boilers such as the Vitodens with the Matrix burner, radiant definitely seems to be the initial force but the corona is limited so there is hot flue gas transfer heading out between the exchanger spirals.

Others, such as my Monitor MZ, starts as a ring of blue flame, little of which glows red (infra-red) and which is invisible to my eye heading down into the exchanger.

I guess my observation is that, once away from the immediate radiant zone, out of line of sight from the initial flame, there still has to be significant transfer taking place. To go from 2200 degrees to 150 degree flue gas and rapidly so, I think there is more than that much radiant transfer occuring. The other part is, to absorb radiant energy, should the surfaces be a darker color than stainless steel?

Not a challenge, I respect your perspective and knowlege, just curious and wanting to know more.

Best Regards,

Brad

Jim Davis_7

Ron Jr.

This is a subject of much controversy but as you lower the firing rate on most equipment the calculated efficiency goes up in most cases. Unfortunately the actual btus delivered drops substantially. In most combustion engineering classes it is taught that you transfer 7 to 8 times more heat from the radiant flame than from the gases it produces. So with smaller flames less heat exchanger is getting the 7X exchange but because the flame is actually farther from the surface this is even reduced. Have not tested a lot of the new boilers yet but hope to in the future. So far though testing furnaces, 2-stage and mod furnaces deliver about 45%-55% of their actual inputs in low fire. Meanwhile the combustion analyzer is calculating a higher efficiency? I use actual measurements of performance to evaluate real efficiency. Boiler do better than furnaces but I don't have all the facts on the new ones, just older ones. Historically, commercial and industrial modulating boilers used 25% to 30% more fuel than High fire only equipment.

Jim Davis_7

I agree boilers are accomplishing something that furnaces can't and the low mass heat exchanger design is helping and most likely creating radiant enrgy from the gases. I first heard the 60/40 rule at an industrial seminar with speakers from North American Boiler/Burner, Cleaver Brooks, Kewanee etc. Also the 7X 8X rule.

Unknown

I tend to agree

with Jim on this posting. I have been doing some testing with condensing and modulating equipment (Boilers) and feel that burner design has a lot to do with how much radiant heat transfer occurs as is seen with Viesmann products. I am on my way out for a meeting and would like to post a lot more. Time permitting I will get back to this posting.

Frenchie

WOW

This, if it is true, sort of blows out of the water many of the benefits of modulating boilers then. In other words a single fire oil boiler would compare more favorably with a gas mod-con. I made this choice last winter and went with the oil boiler for longevity reasons. This seems to further vindicate my decision.

Christian Egli_2

A smile with a blush is cuter than without T^4>>T

I too attended a CleaverBrooks talk where it was mentioned that research now was all about trying to transfer heat in the radiant form rather than the convective form. It's simply better because of the non-arguable difference in the mode of heat transfer. All modes are related to the temperature difference, meaning the formula for heat transfer is computed from the multiplication of the surface area, and a coefficient and the temperature difference. For convection, the simple temperature difference, for radiation the difference between the temperatures to the power of 4. How would you like your pay check increased by the power of 4? I would. I would. I would. I would.

So it seems to be an optional question to a boiler manufacturer to choose one transfer mode or the other. But a fire is a fire? right? wrong.

Playing with an ordinary oxygen-acetylene torch will make it obvious. Turn it on, then mess a little with the valves like you do to adjust the nice flame. One adjustment will give you a flame that gives tons of light (that's visible energy). Other settings will have you feeling lots of heat (that's veering on the infra red). Lastly the correct setting for welding will give the most dot-concentrated heat; all the heat is there, but it feels different.

I love radiant heat. I love the red hot gas burners. The Schunk and Reverberay type. The same sort of burner inside the magic Viessman and many others. All these burners are advertised as low NOx, thus low temperature... Cold invisible flames. How can something cold give off lots of radiation? It doesn't. It's magic.

It's all about the substrate behind the flame. The portable burners I like the most have a ceramic platen with about two gazillion holes. I think Viessmans' and such use a refractory stainless steel cloth substrate. The point being, all these refractories start to sweat when placed so near the flame. Because of what they are, they can't transfer the heat away and so they get red hot and start glowing. The flame doesn't glow, the substrate does.

I have tried grilling myself in front of substrate-less portable burners that look like satellite dishes with a flame in the middle. I didn't cook.

Inside the boiler, once things get up to a good concentrated glow, you can bounce heat away to a larger collection surface (you need a larger collection surface because water is not all that powerful in picking up heat, you have to compensate by giving it space. Which is fine.)

In the acetylene flame, the bright glow comes from carbon particles, pure C, that appear when you starve the torch for bottled oxygen. Incomplete initial combustion turns these naked particles loose in the middle of the flame and because they find themselves embarrassed they glow wildly. Starve the torch until the carbon particles suit themselves and you get black smoke. Lots of black smoke.

OK, now I'm picturing naked carbon atoms running after each other.

Next, here is why I think perhaps too low a modulation backfires on your efficiencies. I don't know how sophisticated burner heads are, but I doubt they are subdivided into small individual plots. Residential heating is complicated enough with one head to which the gas mixture pipe is throttled. A full dose of gas gets things to a full red. A half dose would get you half red but remember how radiation grow exponentially to the power of 4. A half red color will amount to nothing compared to a full red once factored by the power of 4.

From a radiation stand point, a half burner fully red and the other half turned off (thus 50% gas use) will give greatly more radiant heat than a whole burner at half red (still 50% gas use). Subdividing burner head into plots is better.

I've spent lots of time in front of one of those portable radiant heater while playing with the valve and its four plot zones. All the heat you don't take away in radiation you loose in convective currents above the burner. The same convective heat you get above your kitchen range. Kind of hapless.

Another problem with modulating boilers has to do with any lack in draught flexibility. True of older non force fan applications where the flue opens up when the fire is on. Big fire, small fire the flue is wide open. The draught is big and grossly overpowering compared to a turned down flame, and thus very wasteful.

Big draught, deflated radiation and poor heat transfer; carbon atoms can't be having fun. There wasn't necessarily much wrong with the old idea of matching fire to heating surface and sticking with it (within a smaller range). None of us invented it.

Which heats best? the half sized really hot steam radiator or the full sized half cold hot water one? Go for maxi-radiation, T^4>>T. Beat that cold.

Doctors will tell you when a cold is eating away at you and you are prescribed some pills and you are a wimp, instead of the three pills a day, you take only one. -If three is good, then one is plenty! I often think that way when my suspicions run wild. And when you need to go back to the doctor, you'll get the lecture about how one pill only teased the mutant cold, it even divulged to it your secret health strategy. Now you're fighting Godzilla.

AFUE doesn't tell you all this. Cough.

Jim Davis_7

WoW!!

Exactly what I was saying-I think?

Jim Davis_7

Years ago I went out on an oil furnace with a contractor that was oversized for the house. It was firing at 1.25 gph but it was rated for 2.00gph. Combustion analyzer said it was 86% efficient. Brought the firing rate back up to 2.00gph which dropped the combustion efficiency down to 79%. After winter was over the homeowner called the contractor and told him he used 600 gallons less oil this winter than any of the previous 15 years prior. It was not the mildess winter on record or versus one of the other years.

Christian Egli_2

It's more efficient with lots and lots of words.

Obviously, me, I don't know how to modulate and condense my fingers. Your numbers are very interesting. Thanks Jim.

Constantin

Hmmm...

I don't claim to be an expert on the subject, but I did co-develop a water heater in a former life.

My guess is that the temperature of the combustion gases combined with the flow would yield how many BTUs are going up the stack as opposed to staying in the home. (Mass flow and all that).

A smaller flame will yield a smaller radiant output, but I'd also wager that burner design and HX design will have a large influence on how the whole system responds to changes in input.

For instance, in a mod-con the flame will modulate down, but the narrow flue passages combined with a large HX surface probably counteract the reduction in radiant transfer very effectively, at least in the one design I have seen where net output was measured across the entire modulating range. That probably also had to do with the design of the burner.

On older boilers and many furnace units, the flue passages are so wide and open that much of the output will go up the stack as they get downfired. The stack temp and mass flow should tell that story, i.e. higher BTU losses up the stack at lower output.

Rodney Summers

downfiring mysteries.....

downfiring & afue - typical case in point - this peerless chart below. the afue increases with the downfiring on all systems. obviously not linear but always appears to be approaching some (never tested) optimum match between the nozzle size & the surface area of the chamber - in fact, right up to the point where it looks like the larger EC-4 is the more efficient choice to run w/ a 1.25 then the smaller EC-3 w/ the 1.20. don't you think it's odd that they didn't just run the same afue test for the EC-4 w/ the same 1.20 that they used for the EC-3 instead of the 1.25 that was supposedly used? maybe because it keeps leading to the same observation that a smaller flame running into a larger surface area (to a point approaching some optimum HX transfer balance) makes for the most efficient transfer? obviously there has to be some limit where the trend reverses - meaning no, you're not going to find that holding a BIC lighter in the chamber produces the best transfer. in the A/C world, they say that one of the dirty little secrets about the SEER rating of an outdoor compressor is the fact that compressor & fan components are the same, but that only the greater sized surrounding coil jacket increases the surface area for the more efficient rating. are we not basically seeing some similar trend here? there are many experimenting with downfiring on some new & older oversized boilers to better match input with the load. i can't say that i've seen a whole lot of posted results on this, but for that matter, i also haven't seen too many controlled case studies posted that quantify the reduced oil usage of a brand new system over the old system while accounting for the actual degree day loads to ensure some measure of accuracy. my own downfiring results with an ibr nozzle rating of 1.55 down to .75 (and starting to go to .65) have been very positive... the best combustion efficiencies (83-84% on the fyrite pro) that i've ever been able to attain, much less CO, cleaner light off by far / cleaner HX at the end of the season, MUCH longer cycles, lower flue temp, a true zero smoke at a decent CO2, and what certainly seems to be a reduction in fuel used (i have yet to do the degree day calc so i'll let that stand for now). but if anyone can point to or provide a link to where this 'mystery' has been quantitatively studied & documented, one way or the other, i'd love to see it. otherwise it seems the hearsay continues.

ss

Bob Forand

It's smoke and mirrors

I spoke with a boiler manufacturer and was amazed to learn that AFUE is burn efficiency in a lab. That number can be +/- 3 degrees. Meaning that 86% could be 83% and the manufacturer can claim 86%. Again being in a lab the boiler does not have any insulation on the block. How SYSTEM efficient is that pin style peerless boiler with 1/2" of insulation glued to the jacket, that by the way is just so you don't burn your hand when you touch the jacket, versus 3" wrapped around the block on a Viessmann ? The point is that AFUE is a fictitious number. It clearly comes down to the system efficiency. Now let's look at stack temperatures. What is the stack temp on a pin style boiler versus 3 pass ? You can resonably expect to see 75 - 100 degrees less on 3-pass boilers. That in itself adds to system efficiency. Just my 2 cents worth...

Constantin

Allow me to disagree...

Efficiency in AC systems is not solely driven by the HX that is being used. The reason that a lot of systems currently feature larger jackets rather than higher-EER compressors probably has to do with cost/efficiency.

That is, the manufacturer is looking for the least expensive way to create a 13 SEER system, and using a larger HX is one way to do it. Other approaches that may become interesting in the future are variable-speed compressors, different kinds of heat exchangers, etc.

Besides, there are diminishing returns with HX's as with all other design approaches. For example, the XC21 series from Lennox has the same condenser coil whether you're dealing with the 2 ton or the 5 ton model. The SEER difference, IIRC, is 1 point for a matched air handler. Perhaps Lennox makes up some of the HX surface area difference with the air handler HX, but I doubt it.

Constantin

(double post, my apologies)

John Ruhnke

Good indicators are important..............

Christian,

Its great to have all of these theories. But I like to look at indicators to judge performance. One indicator I look at is oxygen content in the flue gas and stack temps. The lower the stack temp the better the heat transfer. The lower the oxygen content the more of the combustion gas is used in the combustion process. Excess air in combustion gas as indicated by higher oxygen temps carries extra heat up the chimney. This dilutes with the combustion gas and lowers the stack temps. Any time you lower the 02 levels in the flue gas you will get a higher stack temp. The trick is to lower the stack temp (best heat transfer) and lower the 02 levels. I don't care how they make the burner, the indicators will tell you how it performs. I always look for equipment that puts out the best numbers (lowest o2 and stack temps). I don't like to get caught up in the design of the unit as much. The boiler with the best numbers saves you the most in fuel bills. Most of my experience is with conventional boilers and I find the buderus to put out good numbers. I install a lot of them. I also have tuned a lot of existing boilers. Some of the older boilers can be tuned to put out better numbers then the newer boilers with higher AFUE.

JR

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Unknown

So Bob,

Where does the heat that's lost between cycles go, on a boiler with 3" of insulation?

Oh, that's right. ALL of it goes up the chimney, rather than some going through the insulation, into the room with all of the pipes (that most contractors leave uninsulated, myself included) that you don't want to freeze.

Perhaps there might be some baseboard installed in that room? That would make up for all of that heat that you prevent from leaving the boiler and staying within the building. Then, as the boiler cools down, NONE of it will be lost through the jacket.....

(tongue in cheek)

Noel

Unknown

Noel , you wry devil

It was good talking to you today .

In 20 years , this is one thing I never checked - stack temperature after the boiler shuts down . I wonder what a side by side comparison would show us ? Heat up a pin type , a steel dry base , and a triple pass and see which one holds the heat in longer . Or even better , buy a boiler with control logic that dumps the leftover heat into a water tank or a zone .

I've installed quite a few different brand boilers . This notion that a triple pass boiler is easier to clean than pin type might not be true across the board in my opinion . Leave plenty of access around any fairly new boiler and cleaning times will not vary by much . Well , I am talking about boilers with swing doors . I'll admit I don't have much cleaning experience , but it took me almost the exact same time to brush down my Peerless WBV3 under my stairs as it did to clean out a sooted up Buderus I was sent to service . Again , access to the boiler is the key .

The debate about which style boiler is preferable will go on and on . Personally I don't have a preference to style . What is most important to me is dependability , then efficiency . Will that triple pass save our customers fuel compared to conventional boilers ? How much ? This is what our customers always ask , and I never have a good answer .

Unknown

The other arguement...

is that vertical boilers lose the heat faster in the off cycle.

If the draft regulator is set the same on both boilers in the same chimney (separately tested, of course) and the same burners are set to the same small air adjustment, I'd dare say that the more efficient the boiler heat exchanger, the MORE heat you'll lose up the chimney during the off cycle.

The vertical arguement doesn't hold water, for me. Is there less flue gas in a horizontal flue connector portion than a vertical flue connector portion? Nope, the chimney pulls right through all of it, right back to the burner and the open draft regulator.

Those that run cleanly without draft regulators are onto something, in my view.

I guess you can pick your favorite spin on boilers, and use it as you may.

Noel

Steve Ebels_3

Had this conversation

A couple years ago I was on the phone with Reza from Viessmann, who is basically the MAN on the Vitodens. We were discussing gas pressures needed for the Matrix burner to operate correctly and after mowing off that topic we got onto turn down ratios. If you study the specs on a lot of the Mod/Con boilers you'll find that the Vitodens seems to lag behind many of the others in the field. It will only do about 3.5 to 1 compared with some that are 5-6 to 1. Reza told me this was done for a reason and the reason is what Jim is alluding to. Beyond a certain point the flame characteristics just go down the crapper and you lose what you are trying to accomplish. He also told me that a very high percentage of heat transfer in the Vitodens occurs via radiant energy as opposed to "flame". This also makes sense when you consider that the Vitodens drops the combustion temp from 1,700* down to near return water temp in the space of about 1 1/2". Something different is going on there that doesn't happen in a "normal" boiler. Radiant rules everywhere when it comes to heat transfer.