Firing rate versus flue temperature

Steamhead (in transit)

it's possible to set a low-fire too low, which is why I asked if the low-fire rate had been checked on the jobs you mentioned, and if so, what was it and how did it relate to the boiler and the load. A too-low low fire could certainly contribute to problems, but as with everything else, we have to check it. If we don't check/test/verify, we're guessing.

I'll be interested to see that HERS report. There may be something going on there that we haven't taken into account. For example, did anyone check the run-times on those two furnasties and the modulation characteristics (how much time it spent at high fire, low fire or in-between) on the one? Are there more air changes in the house with the higher bill? Was a blower-door test or IR survey done? The figures you cite don't make sense based on the info we currently have.......

"Steamhead"

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Jim Davis_3

Flue temperature is not just representative of heat transfer!! It also represents the firing rate and how much time the heat spends in the heat exchanger.

A gas burner has a flame that reaches say 2800 degrees @ 6% O2. An Oil flame has a flame of about 3800 degrees @ 6% O2. A good flue temperature on gas would be 330 to 430. On oil 430 to 530. Of the oil flue temperature is only 350 degrees what is wrong?? Not making enough heat to cover the whole heat exchanger. Some part of the heat exchanger is getting hot and rest is not. Besides not using the heat exchanger effectively, this can cause excees stress from uneven temperatures.

Historically for the past 30 years, on tens of thousands of natural gas, propane and oil appliances, if the flue temperature did not fall within a certain high range it was proven 100% of the time people used more energy. Obviously it is assumed the combustion setup is the same in all cases. More money was saved on equipment in the 80's by raising the firing rate when it was low, than any add-on controls saved.

I imagine the exhaust pipe on a 6 cylinder vehicle is lower when it is only running on 3 cylinders but I bet it isn't getting more miles to the gallon.

Robert O'Brien

Jim

you keep beating this to death.Provide something other than anecdotes and I'm all ears! The AFUE test says all else being equal,lower firing rates are more efficient in the same appliance.Can you provide anything other than opinion to contradict this?

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Jim Davis_3

It is unfortunate that AFUE ratings mis-represent true equipment performance. I have pretty much studied combustion efficiency calculations for 30 years and they just don't add up most of the time and that is what AFUE is based on. It is tougher to check the true performance of a boiler versus a furnace because most aren't set up to check gpm. I have check thousands of furnaces that have been reted at 80+ & 90+ AFUE and found them running less than 60% in the field at factory settings. This is checking actual air flow and Delta T to get btus and comparing it to input.

Smaller flames lose their ability to radiate to the heat exchanger surface and the less flue gas cannot effectively scrub the surface without laminar flow interferring.

6% O2 and 400 degree Flue T calculates into 81.8% efficiency. But according to most efficiency charts every 1% of O2 is 1% loss of efficiency. Every 30 degrees of Flue T over ambient is another 1% or in this case 11%. That puts us at 83%. That is sensible heat. But what about the latent heat of vaporization. Most technical manuals show that 14% of gas combustion is H2O. Unless you condense it is lost. 14% from 83% is only 69%. In the field that is all we have been able to get after a precision tune-up. These are numbers for an older furnace.
An 83% AFUE furnace has the same O2, and about 100 degree less Flue T and the same amount of H2O. That is only a 3% difference in actual performance or our good old 83% furnace can't get much higher than 72.5% efficiency. Under-fire them and the efficiencies are dropping below 50%. Furnaces are easier to measure than boilers. But combustion is what it is depending on what is is. I guess?

If someone was setting up my equipment to get the highest calculated efficiency on an analyzer I would call someone else quickly.

Until everyones starts measuring delivered performance and tuning accordingly (that is how you set up A/C) equipment will continue to operate at much lower efficiencies.

I have been teaching this information for 25 years to thousands of contractors, engineers, manufacturers etc. and to date no one has come back with anything that has been proven different or I wouldn't still be teaching it.

bill_97

I would think

the most efficient boiler would be the one that is the right size and has the widest temperature difference between the combustion flame and the vented gas ? Or maybe that's just a part of the equation ?

I downfired my boiler from 1.00 gph to .60 a few years ago . I also installed an indirect and ODR around the same time . Fuel use was cut by 1/3 easily , using the same 9 year old boiler . I think it comes down to matching the output to the heatloss , and mine is still overfired at .60 .

I'd still like to know your thoughts on modulating gas boilers . They work on the same principle - matching the output to what's needed . Do you think savings can be had using a gas modulating boiler ? Why wouldn't the same principle apply to an oil boiler ? What's your thoughts Jim ? And Brad , I'd like to hear what you think too

tk_3

I will have to try and find the article from I believe it was either Cleaver Brooks or Patterson Kelly. They also stated that modulation in itself was less efficient but with reduced standby losses due to less off times it was a wash. Modulation was not a huge money saver if the boiler was properly sized but did save fuel on an oversided boiler. When not using a chimney vented boiler more money was saved bu sizinh and piping than by modulation.
Thoughts Jim.

Glen

we think of -

those cute little boilers (regardless of make) as the only way to really save fuel costs. But - having commissioned many many CI boilers, with modulating burners, that are sized correctly, piped correctly and well managed by a modern ODR controller; I have witnessed and have been given glowing reports of substantial fuel savings in the first year. Some as high as 50%. We would consider this equipment as mid efficient and non condensing. So the real answer once again is - it depends. I must reiterate that it is the skill and knowledge of the installer that really makes technically advanced equipment sing.

Unknown

Amen Glen

you hit the nail on the head.

I have worked with a lot of older equipment and been able over the years by certain modifications and getting the equipment to its correct firing rate along with some modifications to the system itself seen overall efficiency go up. To me real efficiency is reduction in fuel usage based on good measurement without a loss of comfort. In some cases comfort was increased.

By the way I do not measure flue temperature with an analyzer, I use a very accurate probe type thermocouple sensor. It is very accurate. I also do not use the draft reading on the analyzer I use a draft gauge from Bachrach I trust it more.

Jim Davis_3

Underfiring in itself is not what is saving the money if it is being saved. For years I listened to techs that said the only way they could stop a furnace or boiler from smoking was to lower the firing rate. Any appliance that is creating excess smoke and sooting is not going to be efficient no matter what. NOt a cure, just a bandaid.

The heat exchanger in an appliance is rated for a certain btu input. This should be the input that heats the heat exchanger the most efficienct. It doesn't make sense that you can heat a 100,000 btu heat exchanger with only 70,000 btu efficiently. No different than trying to heat a house with a 100,000 btu load with only 70,000 btu. On days the load is less there is no problem but the size of a heat exchanger doesn't change.

The only true way to de-rate a boiler is water temperature or in the case of steam, duty cycle.

When you make a flame smaller there is more distance from the heat exchanger. Flames radiate heat based on distance. If you hold your hand above a candle and then move your hand farther away your hand doesn't get as warm. Read any book on combustion and it will state that you get 7 to 8 times more heat from the flame than the gasses it produces.

The temperature of the metal of the heat exchanger is proportional to the rate of transfer. The hotter it is, the more heat will get into the air or water.

I have heard some contradictary info on some of these new low mass boiler which are fantastically more efficient than the old clinkers(cast iron). Haven't had as much chance in the field to do as much comprehensive testing yet.

Modulating commercial and industrial boilers on the other hand tested totally inefficient when they modulated - 33% - 45% extra energy use. A hospital in New Philadephia, Ohio had some modulating boilers. We eliminate all but the top 3rd of the modulation. Their gas usage dropped from 33,000,000 to 22,000,000 cu.ft. This is process steam and heating so the load doesn't vary much year to year.

I want to see everyone fuel usage drop to the max. I want the wear and tear on equipment to be minimized. That is what has happened over the past 30 years when I showed people how to tune their equipment to its highest firing rate, minimize modulation and or low/high fire.

If the combustion is not set up correctly with a combustion analyzer and the equipment operating within certain O2, Flue T, CO & Smoke parameters, no firing rate is efficient.
Equipment should be fired at its ability not ours.

Jim Davis_3

Never could understand how modulation reduced off cycle losses. A modulating burner keeps a boiler hotter during the off cycle rather than letting the water or steam temperature drop. Something tells me that a boiler at 180 degrees has more jacket loss than one at 160 degrees. A steam boiler at 0# pressure has less jacket losses than one at 2#. Then we hear how much heat is lost during pre-purge and post-purge. Is the fuel free when a burner is in low fire??

I did read this in a Cleaver Brooks manual " Our boilers are rated for maximum efficiency in high fire only." This was strange because these where the ones I found to be way off most of the time in the field

Glen

In some cases comfort was increased. !

and dollars were saved to boot. Many ways to skin the cat. Which make and model of temp probe are you using Tim? I have tried several but now resort to the onboard gear of the analyser - just fewer things to pack is all. But always looking at new gear of course!

Constantin

Hmmmm..

Hi Jim,

It strikes me as odd that some boiler designs expressly eschew the radiant heat transfer you are talking about in favor of convective heat transfer.

For example, the Viessmann Vitola fires into a combustion chamber that is somewhat thermally separated from the primary HX surface (i.e. no tight fit). The flame shoots from the front into the back of the chamber, the hot gases then make a u-turn, come back towards the front of the unit before making another U-turn and running over the fins of the HX and out the back. If radiant heat transfer was the whole story, I doubt that the HX would be effective... yet it is.

I continue to suspect that your experience may be valid for some of the scenarios you have been describing, while being invalid for certain types of equipment you may not be familiar with, such as some condensing boiler designs whose thermal efficiency goes up as they modulate down for a given water temperature in the HX.

For me, the answer is in the physics. The BTUs from combustion have to go somewhere.... whether by radiant, convective, or conductive heat transfer. Some HX designs may be better at dealing with modulating burners than others, hence presumably the predominant gas condensing design out there that squezes the flue gases through tight (and cool) coils, for example.

Jim Davis_3

Constantin,

Yes it is weird. Every commercial and industrial combustion seminar I have attended stressed the importance of radiant heat and how much more heat transfer it creates.

You are correct in saying I have not had much experience on the new low-mass boilers to totally evaluate their performance, but they do seem to perform quite well. The question is, can they do better?

Years ago I did an experiment with two pipe nipples 1" & 1-1/4". I put two identical flames in them. The 1" nipple got too hot to touch while the 1'1/4 didn't seem to get warm because the distance to the flame was 1/8" farther. Simple experiment for anyone to try. Sure proved to me that it takes a certain size flame to heat a heat exchanger.

BTUs from combustion do have to go somewhere. But are they radianting in a different direction. Now if the flame is 100% surrounded by the heat exchanger that would be the only place the heat can dissipate and that might leave me without explanation at this time. Heat can be absorbed by metal that is not part of the heat exchanger or absorb by air that won't be passing through which will do little to add heat to the heat exchanger. Again, still have to do alot of field testing on this new stuff.

When you mention thermal efficiency are you talking about actual field measurements or the fictitious one that IBR gives out?

Mike T., Swampeast MO

When provided, every mod-con tech manual I've studied shows boiler efficiency increasing as modulation level decreases--this regardless of HX material or general design.

Manuals also show efficiency dropping as the average temperature in the boiler increases--such change being independent of modulation level. While I cannot say how absolutely accurate my measurements have been, I can say that I have found both of these situations to be true--higher temps or higher modulation rate will reduce efficiency.

Jim Davis_3

Most of those ratings are based are based on the flue temperature difference which affects the calculated efficiency but not necessarily performance. Some how someone took the statement that the lower the flue temperature the higher the efficiency to literally.

But this would sure make steam heating equipment the least efficient you can own.

An analyzer's calculations are as follows:

O2 - 12% Flue T - 230 degrees Eff - 81%
O2 - 6% Flue T - 410 degrees Eff - 81%

Are these both the same efficiency?? The first one is less than 60% when tested in the field.

Mike T., Swampeast MO

Having never used a combustion analyzer and only seen one used once (by a pro who had to borrow from an out-of-town friend) I must admit to some ignorance, but surely proper compensation is made for the fact that oxygen makes up about 21% of the atmosphere. While I'm not looking up the specific heat of the remaining components of air, it sure seems to me that basic compensation has been made in the numbers you give--as I would expect, when oxygen is in excess, the flue temp has to drop significantly to maintain the same level of boiler efficiency.

How are you measuring, "60% when tested in the field?" If you are in any way using degree days or other reasonable measures of load please be aware that you have introduced system efficiency into the equation. DO NOT FORGET THAT AFUE IS BASED ONLY ON APPLIANCE EFFICIENCY--IT HAS NOTHING TO DO WITH THE EFFICIENCY OF THE SYSTEM TO WHICH IT IS CONNECTED!

Glen

non condensing test

Even non condensing modulating boilers will test best at the lowest mod level. In the table attached, position 2 is lightoff/low fire, and normal mod range is 3 - 7. Boiler temp during testing was 165 - 185 F. I would characterize the stack temps as a bit low - but all other parameters were acceptable. This burner was setup for low Nox, the analyzer a Kane May Quintox. Test port immediately at breech, baro on connector. This particular install (3 Viessmann boilers with indirects replacing 4 superhots and direct fired DHW in a 450 suite apartment/highrise) reduced fuel consumption by 50% this past winter. So while the combustion analysis suggests that this install is truly mid efficient - system efficiencies gained has made the owner smile - a lot!

Jim Davis_3

In this discussion I am speaking of equipment efficiency not system efficiency. System efficiency would be lower.

An analyzer measures combustion not equipment efficiency. If it measured combustion efficiency then it would always read about 99% or higher. 100ppm of CO is .01% unburned fuel. Therefore any time CO is lower your combustion efficiency(amount of fuel burned) is over 99%.

How can an analyzer calculate thermal efficiency when it never knows the btus in the fuel, gpm, cfm etc.? When AGA was certifying equipment their engineers even told me that their testing doesn't measure read efficiency but it just compares apples to apples. Unfortunately is was more like lemons to onions. Something stinks!!

When we measure efficiency in the field we measure how many btus are transferred to the air or water. Oxygen and flue temperature give us an idea how far off the combustion mixture might be and how much we can improve it and nothing more than that.

A recent study by some HERS(Home Energy Raters)found that in Energy Star homes with 95%AFUE furnaces, system efficiency averaged less than 59%. Same numbers, just backwards.

Jim Davis_3

Love factory specs. Always wondered how you can heat water to 165 degrees with only 103 degree flue gas, or 145, or 165? This is a perfect example how firing rates can be fudged to produce bogus efficiency ratings. You can not have a non-condensing appliance operate at a true efficiency above 83% and run safe. Would love to see the temperature rise of the water at the 103 degree level. Of course GPM can also be fudged to make things appear to work.
Until I see GPM X 8.33 X Delta T X 60 I don't have any clue to what actual efficiency might be, nor does anyone else.

As I tell contractors in class, after we have screwed up existing equipment all these years, anything would save a customer money. I never saw less than a 20% saving when just replacing cast iron sectional boilers with standard copperfin boilers that were standard efficiency. Even running poorly new low mass boilers save money. Furnaces are not so forgiving.

Glen

please note -

flue gas temps are nett given the ambient of 100 deg. I don't have all the flow rates of the PS loop - but return temp to the boiler was probably 10-15 Deg less. Nothing fudged - just reporting info as set up.

Constantin

You've just illustrated my point...

The wide section may have a harder time getting warmed up. However, radiant heat transfer is just part of the story. Consider how much excess air is getting entrained in the larger pipe vs. the smaller pipe - the cooler ambient air allows the HX to be protected via the boundary layer effect.

Now take the same flame and make it go through a HX with 0.8mm passages... for one, you get good turbulent flow / a good probability that pretty much all the air gets into contact with the HX (which it won't in a large flue passage with laminar flow). This is very different from a huge tube boiler with bazillions of large passages that are not extensively baffled.

Thus, some HX designs may depend on radiant heat transfer and hence suffer as the flame size decreases. However, other designs that depend on convection do a lot better under the circumstances, particularly if you meter the air flow in line with the gas flow. Slower air flow could explain in part why modulating, condensing boilers do better at low fire than high fire, efficiency-wise, it gives the flue gases more time to linger on the HX before being expelled.

Lastly, I would not get hung up about the flue gas temperature in relation to the desired water temperature. What is more important is the incoming water temperature... flue gases should be higher than that temperature, the delta indicating just how good a HX design is (i.e. the smaller the delta, the better the HX design).

Steamhead (in transit)

I would think

a modulating or lo-hi-lo burner would reduce off-cycle loses by reducing the length of the off-cycle. As long as the burner is running, it's not in an off-cycle.

Another factor is the CSD-1 mandated pre-purge and post-purge on commercial jobs. Every time the burner stops and restarts, these purges blow a LOT of heat out of the boiler. Yes, I know there's a reason for these purges, but if we can avoid so many of them, we won't lose so much heat from the boiler. Modulating or lo-hi-lo burners are one way to do this.

The same would apply where purges (a.k.a. valve-on or motor-off delay) are used in residential work. I'm a BIG proponent of adding these to oil-fired boilers- they help keep the boiler from sooting up. But there is a slight loss due to the purge. Hey Carlin- when is the H2L coming out as a retrofit?

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Jim Davis_3

My mistake. No way can you have a gross temperature of 103 degrees and water not dumping out of the boiler vent.

Variable speed inducers do make the numbers look impressive. Controlling venting on equipment easily saves 5%-15% by itself.

By fudged I didn't mean Viesmann is doing it. That is just the way the actual calculations always turn out.

I have seen some condensing boilers where the flue temperature is lower than the water temperature.

Mark Eatherton posted some numbers last year on the actual thermal efficiency of his low-mass boiler and they appeared to be right on the money. Don't know how, but I am not going to argue with Mark because I wish I was half as smart. I will continue to investigate but other than these specific boilers you can't modulate or underfire efficiently on anything else I have ever tested.

Jim Davis_3

That certainly is the old wives tale. How could a burner running use less energy that a burner not running? How can keeping an appliance hotter under no load reduce losses?

Pre-purge and Post-purge 3 minutes total, versus burner running for 3 minutes. I am pretty sure which one use the most fuel.

Tested a steam boiler years ago that was Hi-Lo. The burner brought up the steam pressure to 7# and then cycled to lo-fire until the pressure dropped to 2#. This took approximately 5 minutes.

Next cycle the pressure was brought up to 7# and the burner was shut off. It took 10 minutes with the burner off to loose the same pressure. That burner was re-controlled immediately.

Also, flue temperature is somewhat representative of the temperature of the last heat exchanger passage. If there is minimum temperature difference there is minimum transfer which is reflective in the Delta T.

Steamhead (in transit)

That is totally counterintuitive

so some questions are in order:

First, was this a heating or process boiler? What type of boiler was it (cast-iron sectional, steel water-tube, Scotch Marine, etc.)?

What was happening out in the system during these tests? What load was on the boiler while the tests were being run, and what percentage of the boiler's maximum capacity was it?

Were there automatic dampers in the flue connector that closed it off when the burner shut down?

What type of burner was in use? If oil, was it a flame-retention burner? Did the burner use pre-purge and post-purge cycles (you say "years ago", that may have been before CSD-1) which would cool the boiler down?

What were the inputs of the high and low fire settings? How do these compare with the boiler's maximum firing rate and the range of loads the boiler was expected to carry? Was the burner properly tuned on both high and low fire (from what I know of you, it probably was)?

If you don't remember all this, that's OK. But as I've said before in many different threads on different topics, any claim of fuel savings must include proper context- what the original situation was, what we did, how much fuel we saved (preferably on a degree-day basis with heating boilers). Only then can we interpret the results properly.

"Steamhead"

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Jim Davis_3

6,000,000 btu heating boiler in a High School. 50 degree day with sunshine. Not much load, just piping losses.
Fire tube boiler(old), power burner with induced draft. Don't think I ever saw any more of these since and this had to be 20 years ago or better.

Combustion was previous set by boiler/burner rep and left as is, not that it was the best but could have been worse, but typical of how things were normally set up.

Set up is a big factor. Obviously an underfired piece of equipment that is finely tuned could actually operate more efficient than one at the correct firing rate not tuned properly. If I heard it once I heard it a thousands times "We could get the burner to stop smoking so we put in a smaller nozzle." Saved some energy but a lot less than if they had fixed the problem.

Nothing I have found in 30 years, causes fire tube boiler to need re-tubing, more than modulation! Process boilers have some purpose for minimum modulation but heating equipment make no sense.

Because furnaces are so much easier to measure and evaluate it is easier to see results.

Recent study in field: One house has modulating 95% gas furnace, second house, neighbor, has single stage 90% gas furnace. First house slightly smaller, keeps temperature lower. First house weatherized and foam insulated. Second house normal builder specs, likes house warmer.

Second house fuel bill this past winter was 38% lower than first house. First house owner not very happy!!!

Steamhead (in transit)

When you say

"Nothing I have found in 30 years, causes fire tube boiler to need re-tubing, more than modulation!"

Is this because of temperature changes? I would think shutting the burner down completely causes more extreme temperature fluctuations, especially when pre- and post-purge are used as per CSD-1....

and... "Recent study in field: One house has modulating 95% gas furnace, second house, neighbor, has single stage 90% gas furnace. First house slightly smaller, keeps temperature lower. First house weatherized and foam insulated. Second house normal builder specs, likes house warmer.... Second house fuel bill this past winter was 38% lower than first house. First house owner not very happy!!!"

Were the furnasties in question metered separately from the rest of the houses, or were there other things like stoves, water heaters etc. using gas from the same meter? Were the furnasties properly sized to the houses' heat losses? How leaky were the duct systems? Can you post a link to this study?

"Steamhead"

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Mike T., Swampeast MO

How are you determining that 12% O2 @ 230F = 81% efficient
and 6% O2 @ 410F = 81% efficient? What fuel? Also, I suspect these values come from the same two-stage appliance. Is this correct?

I've spent some time researching the internal calculations made by analyzers typically used by space heating techs and found that many use the Siegert Formula for computing dry gas flue loss. (A copy of the formula is attached)

When I plug those numbers into the Siegert formula using natural gas as the fuel and assuming 70F incoming air temp I get:

13.2% dry air loss @ 12% O2 @ 230F

18.2% dry air loss @ 6% O2 @ 410F

Changing the incoming air temp for both results in different percentages but the relative difference remains nearly identical.

The typical formula used for combustion efficiency of the analyzers is simply the ratio between flue gas loss and heating value of the fuel with flue gas loss assumed to be the sum of dry air loss + loss of the water produced by burning hydrogen + loss from water contained in fuel + loss from incomplete combustion that forms CO. (I can think of at least one more loss--that of the the water contained in the combustion air itself but I found no reasonable analyzer containing a humidity sensor.)

In non-condensing appliances, the heat loss from the water produced by burning hydrogen is the difference between the high and low heating values of the fuel--about 11% for natural gas. While this value does vary somewhat depending on the actual makeup of the fuel, it is unaffected by flue temp, O2%, etc.

When I add 11% to the dry air loss, I get:

24% flue loss @ 12% O2 @ 230F

29.2% flue loss @ 6% O2 @ 410F

Sure looks to me like the boiler at low flue temp was about 5% more efficient.

Losses due to water vapor (in fuel or combustion air) are also independent of flue temp, O2%, etc. so we are left with the loss of incomplete combustion resulting in excess CO production. You didn't supply CO percentages so I can't calculate. Was the lower flue temp case producing FAR more CO?

(I've triple-checked my math, but as always appreciate verification that I didn't make the same error each time.)

Don't forget that condensing boilers operate with a minimum of excess O2 because such increases the dewpoint of the flue gasses and increasing dewpoint means condensation which means less heat recovered (or more heat lost depending on your LHV or HHV perspective of the fuel).

The real trick of modulating a condensing boiler is maintaining a consistent and low as practical excess O2 regardless of the firing level while maintaining a stable flame and avoiding resonance-induced noises. Key to this is the ability to accurately control the amount of combustion air based on the actual amount of fuel being delivered and also ensure that the fuel and air are properly mixed at the point of combustion. A few mod-cons can do this already, and I suspect such will become standard equipment before too long.

p.s. When I use fuel oil in the Siegert formula instead of natural gas the dry air losses are:

13.2% @ 12% O2 @ 230F

17.8% @ 6% O2 @ 410F

Difference: 4.6%

Jim Davis_3

2nd question first. This was reported by a HERS rater in Iowa. He has not yet printed a report, but all gas appliances were taken into account but 38% difference isn't going to be changed much by cooking and hot water. The O2 on the modulating furnace was over 14% from low to about mid fire and not adjustable. When actually Measuring btus delivered in the plenums it has been as low as 38% of the input in low fire.

Never quite sure if it had to do with temperature or corrosion, but in many cases there were also comments of frequent chimney repairs that were also reduced. Tends to be more rust inside modulating equipment because they operate at temperatures closer to condensation. I remember after awhile when I went out on demos I would ask the boiler engineer if he had to retube his equipment fairly often and he thought I was physic. All I knew was that when we minimized lower firing rates on firetube boilers the maintenance was reduced.

Jim Davis_3

These were two identical furnaces operating at different gas pressures. The one at 12%-O2 had 3.5"W.C. and the one at 6%-O2 had 4.75"W.C.

The one at 12%-O2 was delivering 48% of its input btus and the one at 6%-O2 was delivering 71% of its input btus. The actual btus of the gas was not known which make little difference the performance difference would still be the same. Notice even the good one couldn't get near 80% nor will it ever.

Mike T., Swampeast MO

What calculation resulted in the identical combustion efficiencies?

Jim Davis_3

The house that had the higher bills was the one that was blower tested and weatherized which makes the difference even worse.

On modulating furnaces the gas pressure is factory set and not adjustable. The modulating furnace ran over 38% longer to deliver less heat.

The figures I site are that facts. They make perfect sense to me but do not make sense to most who believe in the obsolete and fictitious efficiency standards and rating the HVAC industry has been using since the beginning.

It really doesn't matter what we may read in a report. What is more important is we go out and try to validate the information.

Remember our local transit authority had some waste oil boilers installed but left their gas boilers in low fire all winter for back-up to maintain minimum temperatures. There gas usage ended up higher. Stories I can tell for a long time but again that is heresay. I want people to test reality in the field. I would think after all these years just one person would have come back by now with positive proof to the contrary if there was any.

Mike T., Swampeast MO

I must question your ability, Jim, to reasonably measure BTUs delivered by the HX of a forced air furnace anywhere except a laboratory using extremely sensitive equipment.

What are you using? A manometer and temp probes in the supply and return plenums?

One: It's an open system and you cannot assume that BTUs supplied - BTUs consumed = BTUs returned.

Two: The heat carrying capacity of air is significantly influenced by the amount of water vapor it contains. During the heating season relative humidity levels (especially indoors) are typically quite low with hygrometers capable of reasonable accuracy in this range extremely expensive.

Three: I believe that it is quite difficult to measure a accurately representative--dare I say average--temperature of high volumes of air at high velocity via a single probe inserted at an arbitrary position inside of a plenum of variable geometry.

Mike T., Swampeast MO

Mark Etherton wasn't the only one to post numbers last year regarding actual thermal efficiency of a low-mass condensing boiler.

I do not own a combustion analyzer, but with the addition reasonably accurate condensate production, an arsenel of painstakingly placed temperature sensors and mathematical equations based on the exact same principles employed by typical analyzers which must derive their CO2 content. I found actual boiler efficiency at minimum modulation exquisitely close to 99%. FAR more important is that my system, during a typical heating season, will spend about 1/3 of its time operating at this level. And guess what? At the urging--or was it "Hey I won't do it for a customer, but for an experimenter?"--my mod-con is intentionally undersized via Manual-J loss and has never proved inadequate compared to the previous boiler with over 2X the input rating under my torture tests.

Glen

I'll buy that!

"All I knew was that when we minimized lower firing rates on firetube boilers the maintenance was reduced." And its worse on LPG. The struggle is always maintaining sufficient flue gas temps. (BTW - don't you guys go to work??)

Jim Davis_3

Sharing the knowledge and experience I have collected the past 30 years and teaching is my work. Looks like I worked 2 minutes of overtime. I truly want to challenge everyone to know the truth by going out and testing as I have done.

Hopefully those reading this are learning and want to learn more.