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How Figure The Best GPM For Oil Burner?
Tom_22
Member Posts: 108
Since most (all?) oil fired boilers have a range of about three recommended firing rates, how do you figure the best one to use?
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Comments
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The only efficient firing rate is the highest the equipment can properly fire based on its installation. Anything less will use more fuel.0 -
Not really Tom
There are maybe a few makers that offer two firing ratings in one specific size boiler and one is always at an efficency cost. That's why every firing size usually has an increase of sections on a CI boiler. Probably some boilers woldn't make the 80% AFUE min at higher firings. I think I know of only one residential steel boiler that spans 3 fire rating sizes: Laars Max0 -
How does it change the amount of fuel? If you fire it lower it runs longer at less gph. Upfire and it runs shorter at more gph. You need the same btu's to heat your space with both firing rates. I don't believe there is a noticeable difference in oil consumption.
BTW there are many boilers which have multiple firing rates. Peerless, Thermodynamics, etc...
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When considering just the length of the burner cycle.....
the longer burn cycle is more efficient. The most inefficient part of the burner cycle is when it first fires up, so the fewer the stop/starts, the higher the efficiency.
Steve0 -
Longer burner cycles
One of the biggest fallacies ever conceived. If this is true why not fire 20000 BTU into a 200000 BTU boiler or furnace. When first learning combustion on industrial boilers I always heard that pre-purge and post purge caused major losses. But when a hospital started to cycle their 600 HP boiler on and off instead of idling in low fire, they cut their gas usage $100,000. Another plant that had 200 HP boilers eliminated modulation and saved $35,000. What was amazing is that these boilers had to be re-tubed every year prior to doing this and have extensive chimney repair. Firing in high fire eliminated most of this maintenace entirely. A home owner with a oil furnace rated at 2.00 GPH was told to fire at 1.25 GPH which he did for 10 years. When he went back to 2.00 GPH his oil usage dropped 600 gallons. Burners run longer when you reduce input because they can't tranfer the proper amount of heat. A contractor in Sacramento who exclusively sold 2-stage equipment eliminated the first stage of his furnace after my seminar. He told me it cut his bill 40%-50%. For 25 years no one has been able to prove any different in the field. Facts are the true measure of success.0 -
Sat what?
I've done enough large commercial boilers/burners to know the exact opposite is true. The lower the burner input, the higher the eficiency.
Taking care of over 20 schools and 500+ HP boilers in 10+ apartments has shown the exact opposite of your suggestion. The larger the burner - the more important full-mod becomes.
Do you also suggest Outdor reset is also a waste?
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Resetting temperature is the correct way to save fuel if all other conditions are correct.
If what I am saying is wrong then all laws of physics and thermal dynamics need to be rewritten. Reducing input only creates false combustion efficiency calculations.
What you are saying is that if you have 500,000 BTU of radiation a 250,000 BTU boiler would be more efficient to use. A boiler is a radiator. If you don't fill it with heat it can't transfer. Does undercharging air conditioners work when they are too big?
How long have you been working on these units?
Where is the logic that the smaller the flame and the less flue gases you fill an appliance with, the more efficient?0 -
Firing Rate
> Since most (all?) oil fired boilers have a range
> of about three recommended firing rates, how do
> you figure the best one to use?
Choose your nozzle based on the heat loss of the structure. Generally a lower firing rate will increase steady state efficiency. Don't fire lower than the manufacturer's recommendations as you may get condensation.0 -
Firing rate
Jim, how do you account for two stage gas furnaces? Are you saying the operating cost is going to be HIGHER when they're in low fire?0 -
Firing Rate
Select your nozzle based on the heat loss. I'm assuming the boiler was sized properly here. Generally, a lower firing rate will increase the steady state efficiency. Don't fire lower than the manufacturer's recommendations as you might get excessive condensation.0 -
Our method in the U.S....
of calculating the efficiency of a boiler is rather lame - basically just CO2 and flue temperature are analyzed. But in Europe they look at the true thermal efficiency - the percent of the energy that gets transferred to the water. As far as I know - a lower fuel input, put into the same heat exchanger area, at the same air/fuel ratio, should almost always give a higher boiler efficiency (no matter how you calculate it) than the higher fuel input (unless somehow the heat transfer is relying greatly on some turbulence effect that only the higher input flow creates). I've read that you have alot to offer, Mr. Davis, and we all want to learn more - but the a/c charge comparison is certainly not of value to your point. And general fuel cost savings can be affected by so many variables, as has been discussed at length here recently. Can you please provide more relative information?0 -
Jim, what is it to \"downfire\"
Jim, I appreciate reading your comments about lower total consumption with higher firing rates. A bigger hotter flame for more radiant transfer, more Delta T, and more conductive transfer with the greater volume scrubbing against the exchanger.
A boiler (System 2000) ships from the mfg with a .85 GPH rate. The manual specifies a range of of firing rates. The others are .68, .74, and 1.0 GPH rates.
So to operate most eff at it's highest rated capacity - should I automatically use 1.0 GPH?
Is there any place for trying to lower stack temps (min 300F)?
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Firing rate and total boiler combustion efficiency
Humble woolgathering:
As one might see by comparing an oxy/acetylene flame with that of an oil burner, combustion with ordinary air is imperfect. Also, in addition to good burner design, boiler combustion chamber design is fundamental to good combustion. Should downfiring compromise the chamber's assistance to combustion, losses occur.
With oil, or example, excessive downfiring may fail to heat the refactory to sufficient temperature to help vaporize the spray. If so, excess air will be needed to do the job, with losses.
Modest downfiring offers the tradeoff of fewer "cold chamber" starts, which may help compensate for slightly reduced refactory temperature. The boiler manufacturer knows all the combustion factors that make up a boiler's ratings across various firing rates and fuels. Deviating from the recommendations would seem to compromise the many tested design factors built into the boiler to assist imperfect combustion.
Boiler reps and others here on The Wall can bring much more information to this discussion thread.0 -
Firing Rates
I kind of got to go with Jim on this. I have found that downfiring too much causes more fuel to be used. You don't want to be going off on high limit, but you don't want to run forever, either. A tough thing- most houses have too much rads or too little. Bad ductwork. You have to work around all factors.0 -
Firing rate
We have been putting in boilers for years based upon the actual heat loss of the structure-calculated by the buildings actual consumption during a cold period of time-if oil we use the K factor- if gas use the meter readings during a cold spell. The number will vary with the area that you are in. In Boston we use 2.65 divided by K to get minimum nozzle size- then we adjust as this often gives a smaller size than is commonly available- If there is domestic water on the boiler we use an indirect heater or a storage tank as this usage is a very heavy short term demand-If there is steam, then we fire the EDR of the installed radiation including any uninsulated pipe. This approach gives us the long runs necessary for better efficiency and fewer starts and stops- this is similar to city versus highway driving. Of course you do not want to get your flue gas temperature too low as then the gases will condense and cause more problems. In Massachusetts, the energy code ststaes that you are not allowed to oversize a heating appliance by more than 25% over the heat loss of the structure.In steam systems firing the load is vital so that you get good distribution and max efficiency.
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A little clarity on this...
Since the original question was for oil-fired equipment - aren't many residential oil-fired boilers way oversized for the space heating load, due to the minimum practical size of nozzles and most installers' rule of thumb? If the under-firing does not create a damage problem with the chimney or boiler, and the domestic hot water demand can still be met - are there still those who claim it is less efficient to do so? This is an interesting subject that could use more discussion here, I think!0 -
My boiler's experience...
I have found in my own home that my boiler was initially oversized by about 20% or so. I was firing at 1.00 gph and with the blessing of Dunkirk, my Ultimate 4-section is now at 0.75 gph.
That was last year, and Glenn's readings on AFUE have went from 85.5% to 86.7% and the boiler is cycling longer...especially with the Ergomax feeding my radiant and the additional 15,000 BTUH in the 650 sq. ft. addition done this year.
It's not a fair comparison, but I personally am using nearly exactly the same fuel as last year (with piping improvements and the Ergomax with additional 15k heatload at design), and less than the year before (when I had 1.00 gph feed).
Just one example, and I'm not a contractor.
Hope this helps. Take Care, PJO0 -
firing rate
Firing rate is boiler input in btu's/hr. divided by 140,000
The heat content of #2 fuel oil in btu's per gallon. this will give you nozzle gph.Nozzles are rated at 100 psi so check your pump pressure. You can make a more efficient fire by downsizing a nozzle and increasing pump pressure. Go to Delavan nozzle sight for a great lesson on oil combustion.
John0 -
Jim Davis, I don't doubt you are right.............
........but my point was that when the burner first fires up its the most in-efficient part of the burner cycle. Is this right?
I was not talking about vastly under-firing the unit. I'm sure that you will agree with me that short cycling, and very short burner cycles are not very efficient either, and have to be harder on the equipment too. Or am I wrong?
Steve0 -
Proper firing
If firing at a lower rate was efficient the industry would teach us to set all gas furnace pressures at 2.5" instead of 3.5". Ever ask your car mechanic to reduce the amount of fuel going into your carborater because you don't drive fast? AFUE is a calculated value not a measures value. It can be attained by falsely lowering flue gas temperature to get a high calculation. If 14% of combustion is water, the highest AFUE attainable without condensing is 86% at 0% Oxygen in the flue gas and a net stack temperature of 0 degrees. Based on efficiency formulas every 1/2%-1% of Oxygen in the flue gas drops efficiency 1%. Every 30-40 degrees of net stack temperature lowers efficiency 1%. In my seminar I explain that based on all rules of combustion that are taught by the industry, and with the lowest acceptable readings in the flue gas, the highest efficiency attainable in the real world is about 73% without condensing. Every appliance in the field that I have studied with my customers(50,000+), has shown a significant reduction in fuel usage when it was fired at its mechanical ability, with the best combustion readings attainable. In most cases when making adjustments in the field, flue temperatures were raised, not lowered. Comfort issues that needed to be addressed were solved using others means. After my seminar in Long Island next week, Mark, Maddog and a few other students may have a few comments to add. When I am in Boston a few weeks later there will be some more good people from here to challenge this. I often tell my students I would feel guilty teaching half truths and hearsay versus facts and facts are weirder than fiction.0 -
I thought the original question was for "oil Burners"?
If the "MAXIMUM" is the ideal, then why do so many boilers come with multiple firing rates? If I only need 75K and my boiler is rated for 75, 85, and 95K, how is it more efficient to fire at 95? Seems to me I would get short cycling and the associated issues with that.
Please convince me that overfiring an appliance for its application is sound practice.
Thanks,
Ken0 -
Not overfiring-Proper Firing
In one of my original posts I stated that an oil furnace firing at 1.25 gph was raised to 2.00 gph like it was rated and the home owner saved over $600 versus any previous year. I have sold oil boilers and furnaces for over 25 years to contractors. Based on oil usage, the equipment that fired at its highest mechanical ability, used the least amount of fuel, regardless of the BTU loss of the building. The most common reason I hear people say they want to keep their commercial equipment running is because they are afraid it may not light the next cycle. Rule of "heat transfer"-The greater the temperature difference the greater the rate of transfer. The hotter you make the whole mass of the heat exchanger, the more heat you will tranfer. Firing less than maximum causes the surfaces of the heat exchanger to be cooler. One assumption that I am making is that the burner is set correctly in the first place-Lowest Oxygen, best flue temperature(not lowest)and proper amount of CO and/or smoke. Raising flue temperatures may go against the grain, but they must meet certain minimum parameters. Very few people, including manufacturers have ever argued this point in my class and if they have they got very frustrated in the field trying to prove it wrong. I didn't invent this information, I have only tried to prove and verify it and explain it.0 -
Oil heat firing rate
In my experience of oil heat firing rates, you would
go by the size of the boiler, the size of the house, and the size of the burner.0 -
So, if I understand you correctly. You don't consider heat loss, actual need, manufacturers, etc. You simply fire the boiler to the max?
Why bother to rate boilers for multi firing rates at all? I am fairly new to this field and am trying to learn. Nearly every post on this site speaks of properly calculating heat loss and matching equipment to that. I still don't grasp the concept of "damn the torpedos" fire at max.0 -
Ken
Proper heat loss calcs must be done, Jim is not discounting them.
What he is saying is that youm should fire the equipment to it's full potential. Manufacturers aren't going to make seperate castings for every nozzle size, so they allow you to fire one at two or three different rates. Well which is the optimal? They can't all be the same. Same applies to furnaces.
Heat goes to cold and it goes faster when the difference between the two is greater.
If you ever get a chance, take one of Jim's classes.
Mark H
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That was my point exactly. Fire to what you need not to the boiler max!!!!!!0 -
I understand
what you're saying, but what you need may not be the optimal firing rate for the boiler or furnace.
AFUE numbers are not the same a thermal efficiency or combustion efficiency.
So you will spend more money in fuel when you underfire the equipment than if you fired it to it's capacity. Note I said fired to capacity, not overfired. Boilers and furnaces are designed around a MAX capacity and then they are allowed to be underfired from there.
Mark H
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If firing maximum
input 100% of the time is most efficient; why are Viessmann, Munchkin, Peerless, Weil, & many other modulating condensing gas boilers selling like hotcakes?
Are their fuel saving numbers false? Are the folks who install, and those who monitor & report those savings, spinning yarns? And. Why are the Euros now making a fully modulating oil burner for the residential market?
As for the hospital gross boilers; there is probably more there than meets the eye. Were the electromechanical draft dampers operating in conjunction w/ the burner modulation? Are the boilers sized properly and staged to meet load requirements? Or are they so oversized that the burners, operating in low fire, had such low stack temperatures that the boilers were condensing, particularly in the 3rd or 4th passes? That is a guaranteed tube eater. Sometimes the solution is to reduce the turndown ratio from the much ballyhooed 3:1 to 2.5:1 or lower.
Most hospitals, manufacturing operations, shopping malls, etc. operate full mudulation burners. They can't wait for the burners to pre-purge 90 seconds & then take another 5+ minutes to respond to a significant load fluctuation. And. It does lower wear & tear on motors, transformers, electrodes, etc. Boiler tube sheets like it slow & easy too.
I don't disagree that downfiring is inefficient, when the load is static. But, in the commercial world, as well as in the home, loads change. Until recently residential equipment could not respond to that change. Now the gas folks can. Soon the oil folks, over here, will be able to do the same. That's why I believe a blanket "fire the maximum all the time" is an oversimplification of a complex question.0 -
Air / fuel ratio clarity...
Jim Davis - for the issues in this discussion, we are agreeing that the air / fuel ratio (CO2 for simplicity) is being maintained (manually adjusted by whatever means allowable) for the various firing rates, aren't we? If not, then I could certainly see your point, in many instances. I'm sure you are aware of all of the negative effects that simply lowering the gas manifold would have, never mind the efficiency!
Obviously you are not a proponent of the AFUE test, in terms of it's accuracy and accounting for all factors, but who is? It's got a lot of "holes", but we are stuck with it here in the U.S.. But don't you think that if a certain piece of equipment is tested for several input rates, and the AFUE or combustion efficiency tests show a trend toward a better result as the input is raised or lowered, that there is some substance to that? Do you believe that thermal efficiency tests are also totally invalid? Good discussion here, though.0 -
I wish...
I could figure out why some of my replies wind up in the wrong place on these threads, and why I can't break up my posts into shorter paragraphs. Please bear with me.0 -
Hey Jim
To put your posts higher up or in a different place, simply open the post you want to reply to and click on reply and it will put it there. As far as breaking up your post into different paragraphs, just hit enter twice after each paragraph and it will leave it spaced. Hope this helps.
Glenn0 -
Hit \"Enter\" 2x...
Thank you Glenn - let's try that!
So, what's your take on this discussion?
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Jim,
I totally believe thermal efficiency tests are the only valid way to even come close to BTU's as long as all values are measured. Whether its pump gpm or blower cfm X Delta T etc(not the whole formula). There is one number we can never measure and that is BTU created(input). In the real world 1 cu.ft. of gas cannot produce 1000 btus. 1 gal of oil cannot produce 140,000 btus. We can only truly measure what we deliver. If a boiler is rated for 100,000 input and 80,000 output, 100 cu.ft. of gas cannot do the job. By the way I recommend modular boiler installations over 2-stage 100%0 -
Are you referring to...
the European method of calculating efficiency, where the input side of the equation uses a lower number for the calorific value of the gas (which leads to exagerrated efficiencies over 100 %)?
Or are you referring to the burner's inability to convert all of gas to heat energy for the heat exchanger to absorb(either way, the burner and heat exchanger are considered all part of one system that we are analyzing anyway)?
So what does it matter how falsely we calculate the input, anyhow - doesn't it all "wash out" when we compare tests at various inputs on a piece of equipment, to see what is more efficient?
If 100 cubic feet of gas cannot "do the job" on a 100,000 BTUH input rated boiler (at sea level, sized correctly for the heat load), does it have to be over-fired to do the job?0 -
What I am eluding to is that natural gas has a ultimate btu or caloric rating at perfect combustion-no excess air-flame temperature at 3600 degrees. We operate at 2800 degree flame temperatures which is something less than perfect at 6% oxygen and not full caloric value. I do not recommend overfiring but firing correctly. When I talk about tuning equipment I refer to natural gas, propane, oil, wood, coal etc. One standard method needs to be applied to all fuels for proper firing adjustment. Clocking the meter only works on one, checking gas pressure works on two, but what about the rest? The ability of the fuel and air to mix and burn completely and the vent to carry the products of combustion up the chimney. A combustion analyzer measures the actual ability of the appliance in its installed environment. I believe that if an appliance says the output to the air, water or steam is 80,000btu, then that is what we should try to attain within good combustion guidelines.0 -
It was my experience with tests
run by the American Gas Association back a few years was that underfiring or as it was called "derating" actually caused the cubic foot usage based on degree day analysis to increase. These test had a two year running time which was considered adequate time to determine what types of adjustments to equipment saved the most energy. in some cases it covered three heating seasons.
Other low end energy savers:
Vent Dampers 2 to 3% savings
Two Stage heating on boilers and furnaces only one system actually saved at all. The rest either did not change or actually showed increased consumption. It is my theory that folks did not get adequate comfort with two stage and kept turning the thermostat up.
Programmable thermostats showed 4 to 5% savings after customers stopped overriding the thermostat.
Insulation increase and new windows showed the highest savings windows 7 to 8% and insulation as high as 12% in a couple of cases.
We also went in and fine tuned several boilers and furnaces along with some steam systems.
Boilers after tuning and testing 4 to 6% savings
Furnaces only 3%
Steam boilers (several were underfired) went to 7% savings.
Conversion from standing pilot to spark ignition 3%
Conversion from standing pilot to spark ignition with adding a vent damper 3 to 5%
These tests had a accuracy rating of + or - 2% based on heat value, measuring methods etc. Each heating system was seperately metered with special flow type test meters which we used in our meter shop to measure meter accuracy.
We also did 4 different design gas systems with barometrics and blocked or removed draft hoods. We combustion tested and set draft to a -.02 on all four. The overall results on the four jobs was 7 to 8% savings. One boiler actually had a 10% savings.
Here is the interesting thing. Several homes we did nothing but determine that equipment was operating safely. We did not change anything. We did not tell the customers that we had done nothing. All of those houses showed a savings of 3 to 4%. It shows that when you make people aware of the need to conserve they start watching their usage.0 -
Just sittin' back
and enjoying it for now. I have some thoughts much along the lines as yours, but since I'll be attending Jim's class in Boston next month, I'll keep my thoughts to myself for now and remain open minded to all of the discussions.
Glenn0 -
Timmie, some of these points are well taken...
1- Your theory on people turning up their thermostats, because of not feeling satisfied while their 2 stage equipment runs at minimum - I had the same thought, but was reluctant to express it (without any proof).
2- The replacement of the fixed draft hood with a barometric would obviously decrease stack losses, if the installer adjusts it correctly. Whether it is appropriate to do this was discussed here a few months ago.
3- Your reference to the AGA program where the gas boilers were derated again leads me to believe that the air /fuel ratio was altered - as most times this is done, the gas input is reduced, but the airflow through the unit is left basically the same. An oil burner, however, has an air intake adjustment means - when you drop the oil input, you can also decrease the air intake opening, to (in most cases)give the same CO2 as the original firing rate. In each of my replies to this thread, I have tried to include the importance of maintaining the AIR / FUEL RATIO. Jim Davis' replies to this snake their way around this issue, therefore all bets are off, I feel.0 -
When working on oil equipment what creates the .02" pressure drop? The volume of gases produced in combustion. Without this drop there is no scrubbing of the surfaces and because the volume of gases is reduced, laminar flow is enhanced. Does the gpm of the water or the cfm of the blower have anything to do with how much heat is transferred? If not all the hydronics schools need to change their teaching. I have never seen the pump gpm changed as a oil boiler in de-rated. What about radiant heat transfer? If the flame is smaller it heats less surface and is farther from it. If you hold your hand a foot above a candle, odds are it won't get hot no matter how long you hold it there. But make the flame bigger or bring your hand closer and heat transfer will occur. When it comes to controlling air, the flue is in charge. Two stage gas boilers have been made for years so because the CO2 cannot be controlled, they have been misrepresented.
The key to this whole discussion is not believing theory or I heard someone say or that's the way its always been done, but instead, to actually go into a real field situation and prove it-Yay or Ney! Still taking all bets!0 -
I don't think that...
The minor drop in oil input rate we are considering is going to greatly affect the heat exchanger pressure drop (how accurately can you measure it?) or the turbulence effect within the heat exchanger (especially "tight" or baffled design). The major fuel input drop that occurs in modulating gas boilers doesn't seem to hurt the efficiency of those units (but I was trying to avoid going there).
Can you repeat your candle test, using an insulated tube between the flame and your hand (to simulate the combustion chamber)? I don't think you will have the time to say "heat transfer", before you will yell another word that is not printable here!
Is that a "real field" situation?
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