Modulating burners??

captainco

Just trying to create a conversation on modulating burners and why people think they are more efficient. Is it because that is what someone told you? Are you basing off of a misleading efficiency calculation? Are you basing it on actual field performance comparing results from single stage versus modulating with the same equipment with the same burner and same settings?
Heat transfer is affected by 5 mechanical factors:
1. Temperature
2. Mass
3. Residence time
4. Line of sight
5. Specific heat capacity

Which of these is reduced with a modulating burner?

Certain Mod-Con boilers might be a slight exception to the rule because of low mass heat exchangers.

Do not want to argue, just discuss.

ChrisJ

captainco said:

Just trying to create a conversation on modulating burners and why people think they are more efficient. Is it because that is what someone told you? Are you basing off of a misleading efficiency calculation? Are you basing it on actual field performance comparing results from single stage versus modulating with the same equipment with the same burner and same settings?
Heat transfer is affected by 5 mechanical factors:
1. Temperature
2. Mass
3. Residence time
4. Line of sight
5. Specific heat capacity

Which of these is reduced with a modulating burner?

Certain Mod-Con boilers might be a slight exception to the rule because of low mass heat exchangers.

Do not want to argue, just discuss.

I know close to nothing, but wanted to ask.

Would decreasing the burner output but keeping the heat exchanger the same size increase the amount of transfer into the exchanger?

Does decreasing the burner output reduce cycling and if so, does that increase overall efficiency of the system?

GGross

I have never had the opportunity to test a modulating burner against a single stage burner on the same appliance before, my guess would be they would have the same efficiency, because why would they not? I think most people, such as myself, only really deal with modulating burners where they are installed on condensing boilers. So the test is almost always "mod-con" VS "single stage, non-condensing" When I hear someone say a modulating burner is more efficient I assume they mean because it is installed on a condensing appliance. Burner modulation is primarily there as far as I know to limit short cycling.

I would be far more interested in hearing your information though @captainco This is after all your area of expertise and I am but a lowly salesman, eager to learn something new today

captainco

I have spent thousands of hours in manufacturing plants, hospitals, high-rises etc. Also read a lot, Most burner manuals state that their burners are most efficient at the high fire ranges. Most burners when setting up combustion require higher amount of O2 or excess air to burn clean. Keeping equipment operating when it isn't needed can't be efficient. Maintaining higher temperatures in equipment also increases jacket losses. This would also include two stage equipment.

captainco

Chrisj - One of the biggest causes of efficiency loss is the higher O2 reading needed at lower firing rates. In high fire the O2 can be 3% and the flame temperature is 3100 degrees. In low fire the O2 needs to be 6% and the flame temperature is 2800 degrees. Before 1 degree of heat is transferred, 300 degrees is already lost. Subtract the flue temperature from the flame temperature and now you have an idea of how many degrees of heat were used.

ChrisJ

captainco said:

Chrisj - One of the biggest causes of efficiency loss is the higher O2 reading needed at lower firing rates. In high fire the O2 can be 3% and the flame temperature is 3100 degrees. In low fire the O2 needs to be 6% and the flame temperature is 2800 degrees. Before 1 degree of heat is transferred, 300 degrees is already lost. Subtract the flue temperature from the flame temperature and now you have an idea of how many degrees of heat were used.

I'm guessing these modulating burners do not vary the air vs the burner output?

captainco

There are fuel linkages and air linkages. However, based on the mechanics of burners, they all need more air in the lower firing rates to stay clean than in the higher firing rate. What I have encountered almost 50% of the time in the field was the flue temperature in low fire was cooler than the water or steam in the boiler which meant they were using fuel to cool themselves down. That doesn't include the ones in high fire with lower temperatures, especially on steam! But wow does that make combustion analyzers calculate high bogus efficiecny numbers,

STEVEusaPA

@captainco What do you think about the 2 stage oil burners that adjust pump pressure and air for each stage? I think I know the answer considering you have to set them up on high fire, and the measured 'efficiency' is worse on low fire.

captainco

Steve you answered your own question. Two stage or modulating burners are needed so we can light off smoothly on larger burners but beyond that they waste a lot of energy in the lower firing rates.
Years ago I worked with an energy auditor and they calculated two stage burners used almost 20% more energy per degree day than single stage.

captainco

When I got started in testing and learning how equipment operated, I was totally ignorant. I read everything I could get my hands on. Fortunately, the wholesaler received just about every publication for HVAC, residential, commercial and industrial. I searched everyone for pertinent or useful information and then tried to apply it in the field. When that didn't work then a safe trial and error method was used (whatever that means).
Just like the 1986 ASHRAE study that stated equipment can be oversized up to 200% with little or no increase in fuel consumption, It is not size of the equipment that is the problem, it is the way they get set up.

Again, just trying to create some thinking and input before the fall season.

Hot_water_fan

Is the claim that only the burner is more efficient or that the system is? I think the system is more important, and it makes sense that modulation can reduce boiler temp and that'd increase total efficiency.

captainco

How does modulation reduce boiler temperature? If I need 140 degree water or 180 degree water the boiler is the same temperature. A modulating burner only takes it longer to get there, plus it can keep the boiler hotter when it is not needed. The system is the system and the firing rate of a burner will not change that.

GGross

This is my understanding as well, the efficiency we see in a mod-con is generally achieved through the "con" not the "mod" the reality is the modulation allows for a little more slop in sizing the equipment. Now I have done low fire tests on many of my Viessmann boilers and on the models in the last 5 years or so I generally see efficiency within a point or 2 of high fire, but reading many of the captain's posts always has me questioning whether I understand fully what I am doing or not (I probably don't)

Hot_water_fan

How does modulation reduce boiler temperature? If I need 140 degree water or 180 degree water the boiler is the same temperature. A modulating burner only takes it longer to get there, plus it can keep the boiler hotter when it is not needed. The system is the system and the firing rate of a burner will not change that.

I would posit that a modulating boiler keeping the water at 120 will have lower heat loss than one that is larger and exceeds 120 before shutting off, since there's going to be some degree of over/under shoot. I agree that if systems were exactly identical, modulation probably wouldn't matter but usually they're not identical. A good example of modulation not mattering that much would be the high mass boilers like the Vitocrossal. These larger mass boilers also seem to have lower turn down ratios too.

captainco

You cannot use the calculated efficiency on a combustion analyzer for any meaningful reason. I did say some mod-cons are exceptions. I look at the O2 reading and the flue temperature and determine a efficiency and efficiency difference. I did measure a mod-con at a museum in Akron, Ohio a few years ago. It was horrible base on the O2 readings (18% in low fire) but the calculated efficiency was still in the 9o's - BS!!

SlamDunk

We use modulating burners to maintain steady pressure (industrial). Anything that would cause the burner to turn off (Pressuretrol) would give us setpoint pressure +/- 20 psi. And if the burner flames out on re-start, we would lose even more pressure. You can see from attached photo from December, we are pretty tight to 80 psi. The bumps you see is lead boiler becoming lag. I think @RayWohlfarth wrote a piece for a trade magazine explaining why firing at a higher rate makes boilers more efficient after you account for heat losses that are a constant.

captainco

Modulating boilers were created for multiple reasons but not efficiency. Many early boilers had to be manual lit. Some had very few safety controls. Then there are those that are afraid if the boiler shuts down it won't relight.

I have been in many industrial plants and hospitals that absolutely need a certain amount of steam. But what percent of the day don't they need that much, and their equipment is sitting in low fire or somewhere in between.
I ask what is the minimum amount of steam pressure do you need? Then we start to figure out what type of differential we can use and what the recovery time of the boiler is. Of course, the burner has to be set up correctly in the first place. Hospitals do energy audits annually, so I have gotten a lot of feedback from them on fuel savings. Anywhere from $30,000 to over $100,000 a year.
One additional note. Rear tube sheets on firetube boilers warp and leak because of underfiring and modulation. Of course, this might cut into some peoples annual service income.

ChrisJ

Hot_water_fan said:

How does modulation reduce boiler temperature? If I need 140 degree water or 180 degree water the boiler is the same temperature. A modulating burner only takes it longer to get there, plus it can keep the boiler hotter when it is not needed. The system is the system and the firing rate of a burner will not change that.

I would posit that a modulating boiler keeping the water at 120 will have lower heat loss than one that is larger and exceeds 120 before shutting off, since there's going to be some degree of over/under shoot. I agree that if systems were exactly identical, modulation probably wouldn't matter but usually they're not identical. A good example of modulation not mattering that much would be the high mass boilers like the Vitocrossal. These larger mass boilers also seem to have lower turn down ratios too.

So use the burner at it's full output and turn it off at 120F.

The burners output isn't what controls the water temperature an aquastat is. No?

captainco

Yes, the aquastat determines the water temperature and turns off the burner when reached. The firing rate of the burner and its set-up determine how efficient this is done.

Someone may ask what about short cycling. That is determined by how you set your on/off differential. If you are cycling on and off for less than 5 minutes then the control settings need to be adjusted.

Hot_water_fan

The burners output isn't what controls the water temperature an aquastat is. No?

Of course the aquastat controls the temperature, but there's always going to be a differential. The size of the differential means either short cycling or higher temperatures than required and a higher heat loss. A modulating burner can help with that balance, to a point. So could multiple fixed output boilers of varying sizes.

Dave Carpentier

Isnt the ideal situation a steady state burn into a steady load ? Any cycling pushes latent and building heat outdoors during pre and post venting and heat transfer surfaces heat up and cool down, possibly unevenly, affecting heat transfer rate ?
Would it be good to have a modulator controlled by some type of ODR/IDR instead of by setpoint/current water temperature ? Unless I misunderstood the manuals Ive read, it seems like most just go to full (or programmed capped max) modulation and then ramp down as the current water temp approaches the setpoint. Whats the rush ?

captainco

The point here is still that modulating burners (exception -some modcons) are less efficient than a single stage burner in high fire. On conventional boilers modulating burners create more boiler maintenance and repair.
Short cycling is caused by improper setup which may be caused by not addressing the actual needs of the equipemt.

captainco

I have sat in seminars with industrial boiler engineers, industrial boiler techs, watched videos, read manuals and they pretty much are repeating theories and opinions. Last spring, I listened to an industrial boiler manufacturer for two hours on the internet. 40 years of industry participation but still repeating theories and opinions from before his time. He had no idea how to determine the actual efficiency of his own equipment. When he closed with the best way to determine the efficiency of their equipment was whatever the combustion analyzer said it was, I about crapped in my pants. (Sorry - Depends)

SlamDunk

Sorry, I thought you were talking steam, not heating hot water. But you are correct, not very efficient, especially during the summer. I dropped the pressure to 40psi for a year and there was enough savings that HQ sent an e-mail asking what we were doing. It was enough to ask for and receive a 250hp boiler for summer use. It carries the load from April to November and runs around 60-70% firing rate. Higher in spring and fall.

captainco

There is a law of thermal dynamics that states the greater the temperature difference the greater the transfer of heat. In other words the hotter the heat exchanger gets the less heat is transferred from burner to the heat exchanger. Obviously the hotter heat exchanger transfers more heat to the water or steam, but then again the flue temperature keeps rising because the longer it runs the less is being transferred. A burner flame reaches its peak temperature within the first few minutes of operation. Why does the flue temperature keep rising if we are transferring more heat. Wait, did we just determine that is it better to let things cool down so when they come back on they are more efficient?

captainco

SlamDunk said:

Sorry, I thought you were talking steam, not heating hot water. But you are correct, not very efficient, especially during the summer.

Water and steam.
I love the commercial building that use a large firetube steam boiler and a shell in tube heat exchanger to make domestic hot water in the summer. Use a modulating burner here and the gas company will love you!

SlamDunk

captainco said:

SlamDunk said:

I love the commercial building that use a large firetube steam boiler and a shell in tube heat exchanger to make domestic hot water in the summer. Use a modulating burner here and the gas company will love you!

Then you would love my building! Four 600 & one 250 hp boiler, steam to hot water HEx for heating hot water and shell in tube Hex for domestic HW plus another for process hot water. I have to believe the gas company loves us.

I know the next is question is why? The answer is because we need it to manufacture drugs and drug manufacturers are anal about control. The safety and efficacy of drugs hinge on the room environmentals they are made in and process demands. But you are probably asking why five boilers? The answer is Redundancy. Five boilers are cheaper than losing one batch of drug. We usually run one boiler at a time. If there was a way to run more efficiently without compromising our manufacturing, i'll be reading with great interest!

I havent mastered the quoting others feature. Sorry!

EBEBRATT-Ed

If you don't modulate a large burner, it will be constantly short cycle on light load. Causes a lot of wear and tear on the burner components. And stand by loss

The combustion efficiency can suffer on the low end of the burner range but this is partially offset by the lower stack temperature

HVACNUT

captainco said:

How does modulation reduce boiler temperature? If I need 140 degree water or 180 degree water the boiler is the same temperature. A modulating burner only takes it longer to get there, plus it can keep the boiler hotter when it is not needed. The system is the system and the firing rate of a burner will not change that.

I'm totally lost, which isn't all that difficult for me. When you say whether the water temperature is 140° or 180°, the boiler is the same temperature. Do you mean the fire side? Can you explain? I thought if you have a 100K BTU boiler with a 10:1 turn down, and utilizing ODR, the modulation would automatically be more efficient. To be able to accurately match the BTU's to the heat loss on a specific day instead of max input and short cycling has to have some benefit, no? Otherwise why are we doing it? And who do we blame, Germany?
I prefer oil anyway. Anything I need is an arm's length away. 

JakeCK

This sounds like a good argument for simple boilers heating big buffer tanks...?

And dare I say even oversized boilers(compared to building heat loss) heating really big buffer tanks.

captainco

The hospital boiler I mentioned earlier were way oversized for the hospital and were underfired and modulated. I helped the hospital engineer raise the input as high as possible, eliminated modulating. It was cycling about 5 minutes on under minimum load. They saved over $100,000. I agree that cycling a large motor on that big of a burner could cause premature failure. But if it failed every year, would they have any extra money to replace it? II only kept track of them for ten years and they never had a parts failure. Starting and stopping our vehicles causes excess wear also but I see very few sitting outside idling when not in use.

Jamie Hall

Seems to me that it's very important to differentiate the efficiency gain from allowing the boiler flue gas to condense the water in it from any possible gains from modulating.

The efficiency gain with condensing can be over 10%. It is, theoretically, the difference between the higher heating value of the fuel and the lower heating value. But... to get the full higher heating value, you have to drop the flue gas temperature to be the same as the intake air temperature while at the same time getting the heat released to go somewhere useful. Right. Good luck with that. But so long as you manage to keep the flue gas temperature below its dewpoint -- often around 140 to 150 -- you will get some gain from condensing, and the cooler you can get it the better.

At any rate, that's where mod/cons get their high quoted efficiency.

Modulation may or may not get you an increase in efficiency -- and most likely not. There will be one firing rate for a given burner, firebox, and overall boiler design which will give you the best efficiency. In principle, with very close control of fuel firing rate and draught you can maintain decent results over some range on either side of that optimum firing rate -- but that requires very close control. At either lower or higher rates, the efficiency will drop off (the absurd point comes, of course, when the firing rate drops so low that it equals the boiler's inherent heat loss and the useful output becomes zero!).

Now -- and this is more a question for the guys who work with them. I know how to do that with aircraft engines and automotive engines (and nowadays,, railroad engines!): very tight feedback loops between mass intake air flow, intake air temperature, oxygen levels in the exhaust stream, fuel injector duration and timing (recips) and pressure (turbines), and spark timing (spark ignition engines only), all under computer control. How is this done with residential modulating boilers? (by the way, on engines it isn't cheap -- the complete sensor, computer, and driver package can easily run several grand, even in a low end automobile).

captainco

When I sold shell in tube heat exchangers their specs showed that you recovered 50% of the btus flowing through it. Now take that efficiency and the supposably 80% efficiency of the boiler, you have a 40% efficient water heater.

Do you think the actual performance of that equipment or system was checked out thoroughly by the architect or engineer or was it selected from a salesman that had football tickets.

I have been involved in enough job specification to realize the designer is just looking through a book someone gave them along with some additional perks to use their equipment. Oversizing is definitely the rule of thumb in most commercial applications, or at least for heating, cooling and hot water.

Hot_water_fan

Seems to me that it's very important to differentiate the efficiency gain from allowing the boiler flue gas to condense the water in it from any possible gains from modulating.

https://www.bnl.gov/isd/documents/41399.pdf

@Jamie Hall I read this differently - as run times increase, efficiency improves. What’s a good way to increase run times? Modulation. Seems to give benefits above and beyond condensing alone. 

JakeCK

Hot_water_fan said:

Seems to me that it's very important to differentiate the efficiency gain from allowing the boiler flue gas to condense the water in it from any possible gains from modulating.

https://www.bnl.gov/isd/documents/41399.pdf

@Jamie Hall I read this differently - as run times increase, efficiency improves. What’s a good way to increase run times? Modulation. Seems to give benefits above and beyond condensing alone. 

I'm sorry but I must disagree. Sure the efficiency increases those first few minutes as everything gets up to temp but once there that's it. The laws of thermal dynamics dictate that as return water temperature increases the rate of heat exchange between the fire and the water must go down. Obviously the longer it runs the hotter the return temps unless it's cold enough in the house to obsorb it all.

This of course only really applies to a residential house. I can't speak to a massive building like a hospital. But with a conventional ci boiler I would think the most efficient system that doesn't condense would be one where the boiler is sized to heat in a reasonable amount of time a very large buffer tank that is set between 130-150f, and has a thermostatic valve for boiler protection set to 130. The heating zones would be ran on outdoor reset with a mixing valve. This of course requires enough emitter to maintain temp at design with such a low supply water temp. We see variations of this all the time. Of course you always have to work with what ya got so if you have a bunch of bb that needs 180f supply water to heat at design, thats what you have to do.

captainco

Some mod cons are efficient but not all. The ones that operate at 3%-4% O2 from low to high fire are amazing. However, I have never seen this on power burners.
The longer you run equipment the efficiency begins to lower. That is the point where it there is equilibrium, maybe after 3-5 minutes.
Understand that everything related to efficiency on heating equipment is based on bogus combustion efficiency calculations and then some other points added or subtracted, but none are based on actual equipment performance.

What do you think the actual maximum efficiency of standard steam boiler, with a flue damper and spark ignition? Non-condensing equipment loses 14% latent heat right off the bat. Every % of O2 is another 1% loss, That is 20% off so far if the O2 is 6%. Every 30 degrees the flue temperature is above the combustion air temperature is another 1% loss. A boiler with 180 degree water temperature out should have a flue temperature of 450 degrees minimum. 450 minus 70 = 380 divided by 30 = 12.6% 20 + 12.6 = 32.6%
67.4% is the maximum possible efficiency. Does that agree with the DOE or IBR rating? Now they add additional points for two-stage or modulating with no actual testing verification.

When people ask what the efficiency of their equipment is when we are done we say maximum, greatest possible, the most etc.

It is the 21st century and we should have an updated knowledge of what going on versus that's the way its always been or I said so. Okay there is also they said, he said or she said.

Hot_water_fan

Sure the efficiency increases those first few minutes as everything gets up to temp but once there that's it. The laws of thermal dynamics dictate that as return water temperature increases the rate of heat exchange between the fire and the water must go down. Obviously the longer it runs the hotter the return temps unless it's cold enough in the house to obsorb it all.

Aren’t we both right here? Efficiency levels off but initially is worse. Since boilers are so oversized, gains are available here with modulation. Second, return temps can remain low with modulation. 

JakeCK

Hot_water_fan said:

Sure the efficiency increases those first few minutes as everything gets up to temp but once there that's it. The laws of thermal dynamics dictate that as return water temperature increases the rate of heat exchange between the fire and the water must go down. Obviously the longer it runs the hotter the return temps unless it's cold enough in the house to obsorb it all.

Aren’t we both right here? Efficiency levels off but initially is worse. Since boilers are so oversized, gains are available here with modulation. Second, return temps can remain low with modulation. 

I have to agree with captainco. Modulation does not increase efficiency any more then letting off the gas increases the efficiency of the engine in your car. The gains from driving slower are in spite of the loss of engine efficiency. You would be better off running the engine at full output for a length of time and then coasting but that is not practical. The other option is to right size the engine to the job at hand. But we run into the same kind of problems on the road as we do when trying to heat our houses with a fuel. The load varies, by a lot. This is why hybrids are so efficient, the small electric motor and battery is able to power the car at low load, the engine stays off until it's needed.

And temps can remain low with out modulation too with the correct design. 

@captainco what's the flue temp supposed to be at with a water temperature of 130, and what's the maximum efficiency there?

Dave Carpentier

captainco said:

However, I have never seen this on power burners.
The longer you run equipment the efficiency begins to lower. That is the point where it there is equilibrium, maybe after 3-5 minutes.

Earlier, I poo-poo'd the way modulating boilers light off at high rate and then step down as the output temperature gets closer to setpoint. But in a low mass boiler, does this help it stay in a heat gain situation as long as it can ? The combustion air fan slows down, also increasing the dwell time of that heat package.

retiredguy

My line of work was mostly in steam systems with inputs up to 30,000,000BTU's and steam pressures up to 300PSIG, so in these installations, a modulating burner is a necessity. In a processing business a steady steam pressure and flow is a necessity especially if the process is using steam as the heat source to manufacture the product. A steam temperature and flow below a certain level will render the finished product unusable and becomes a waste of material and man hours that effect the bottom line. As we all know or should know, a burner's most vulnerable time for failure is on a start up or relight, so most industries will gladly give up a few % of boiler efficiency to have a constant steam flow. A burner utilizing fuel oil would fail more often on start up than a burner utilizing nat gas and a burner using #6 fuel oil for example failed more often than a burner using #2 fuel oil.

In most industries or even very large buildings with multiple boilers the best operating conditions were when the lead boiler ran at full input, 24/7, with a 2nd unit modulating the input to keep a stead steam pressure. Of course, with a multiple boiler installation utilizing more than 2 boilers, any number of boilers could be running at full input, 24/7, with the remaining boilers firing as necessary.

This may not address the question posed as to what is the most efficient way to fire a boiler and the need to have a modulating burner but it was what I saw in my line of work. For example a hospital with most or all operating rooms being used for surgeries must have a constant supply of steam in order to sterilize their instruments. A hospital without a steady supply of steam can not perform surgeries and will have to transport those surgeries to a hospital that can do the work. It does not happen often but has happened.

I hope this fits in with your discussion.

ratio

It would seem that the efficiency of process boilers needs to be evaluated on something other than combustion efficiency. Perhaps fuel dollars to net sales dollars makes more sense.