small nozzel (long run time) vs. larger nozzel (shorter run)

Jim Davis_3

If anyone actually checked they would find that the flue temperature of an appliance keeps rising the longer it runs which means more heat is going up the chimney.

A condensing appliance condenses more at the beginning of a cycle before it is hot and condenses less as it gets hotterso somebody is really misinformed.

The problem with comparing firing rates as just input does not identify the actual reality of what is happening.

Without knowing O2 or CO2, Flue temperature, Air flow and Delta T, it is easy to fudge numbers. On the other hand if combustion is set the same at either firing rate, the only difference is that flue temperature will be higher with the larger nozzle, but we will also see a substantial change in the Delta T of the plenum or water which will interpret into actual btus when CFM or GPM is also known.

Longer run cycles means the equipment isn't delivering enough heat to handle the load. 80% of the winter all equipment is oversized. Except for mechanical deficiencies of certain equipment all equipment is designed to deliver maximum efficiency at maximum mechanical input. Underfiring heating equipment is no different than undercharging an air conditioner.

In the last 28 years, 100% of equipment that has been brought up to its highest firing rate has shown a decrease in fuel usage, much of this usage was audited by utilities and in house energy engineers.

The only way underfiring could save energy would be that the firing rate was set up incorrectly.

When the campfire gets smaller you don't stay warm no matter how long you sit there.

Live/Learn

run time vs gallons per hour

Hi Wallies, I have a question on the most economical way to run a hot water oil fired boier system. I'm heating a 1200 sg ft. home with a burnham v83 with a becket burner. I have been using both a .65-60% & a .85-60% with pump @ 140. I get good readings using either nozzel. My run time is quite abit longer with the .65 but heats the home well. Is there a trade off with run time and gallons per hour with the two nozzels? Thanks in advance. Live/Learn

Dave_4

You are onto something few understand...

The longer the boiler runs, the better!

Ideally, the coldest day of the year - the boiler should run for 24 hours.

This is because - every time the boiler comes on and goes off - heat goes up the chimney - wasting energy during the off cycle. If the boiler runs non-stop, there is no "wasted" energy going up the chimney - only the energy that results from combustion.

Use the smallest nozzle that continues to attain the best efficiencies, coupled with maintaining comfort.

There are some potentially complicated other "issues," like making the boiler stay below 140F water outlet temperature much of the time, but without knowing your controls setup, that is a very unlikely scenario anyhow. I bring it up to suggest there may be some problems using a nozzle so small that it might still make the house comfy on a design day, but never get into the 160F water range - which could adversely affect the boiler insides (which at under 140F becomes a rain forest inside the boiler block)

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Live/Learn

Thanks Ken!

Thanks Ken for your respounce to my question. My boiler recovers quite well after initial call for heat. I set back about 7 degrees over night and return water comes back hot (boiler temp @ 140) in about 8 minutes. It takes a total of 35 minutes to reach high limit (190)with cirulator running and the home is up to temp in about 80 minutes. Once high limit is reached the burner stays off for 5 min. and then on again for 6-7 min.until therostat is satisfied. I have a monoflo system pushing heat through slantfin type emitters. I used this set-up for awhile last year but changed over to the larger nozzel mid way through the winter as I was concerned about the run time issue. What you state does make alot of sense to me. Thanks again. Live/Learn

sootmonkey

run time

Here is my math. All numbers are hypothetical.

Lets say you have a 500,000 btu load to meet.

A 1 gal. per hour fire rate (140,000 btu per gal.) @ 82% would give a net of 114,800 btu per hour. It would take 4.36 hours of run time to meet the load, and burn 4.36 gals of fuel.

A .85 gal. per hour fire rate (140,000 btu per gal.) @ 84% would give a net 99,960 btu per hour. It would take 5.02 hours of run time to meet the load, and burn 4.27 gals of fuel.

So if the smaller fire rate would increase the eff by 2% you would save about a tenth of a gallon of fuel but power the burner motor, igniter circ, ect.. for about an extra half hour.

In order to make a true comparison of nozzle size verses eff., you would have to get the real numbers from your combustion test and make sure that the combustion tests with the different nozzles are taken with the same boiler temp. Just a monkey chewin on a bannanna

Steamhead (in transit)

Ahhh, but

there are fewer starts and stops with the smaller nozzle, and more long burns which will reduce sooting. The start-ups and shut-downs are the dirtiest operating periods on an oil burner, especially if it does not have valve-on delay.

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Jim Davis

Smaller nozzle equals higher fuel consumption

A heat exchanger has a certain mass and volume. Part of it is heat with radiant heat(flame) and the balance with convective heat(gasses). Maximum firing rate is the only way to attain maximum efficiency. De-rating 20% will use 20% more energy. If a smaller flame is better than why don't we just heat with a candle?? Two stage and modulating equipment use considerably more energy than single stage equipment. The only time people see savings when this equipment is installed is because the old equipment was set so bad anything would save energy. Even at many factory settings oil equipment runs under 70% efficiency regardless of what it may be rated, and drops lower at less firing rates. Most gas furnaces barely make 60% efficiency even though they are rated at 90%+.

Combustion analyzers almost never calculate efficiencies anywhere close to accurate!!!

Unknown

Jim

Interesting hypotheses, one that I seriously question.

Dave

sootmonkey

Jim

"Maximum firing rate is the only way to attain maximum efficiency." I belive that.

"Part of it is heat with radiant heat(flame) and the balance with convective heat(gasses)." Hmmm, I never thought of the radiant part of the equation. That will give me something to ponder while staring out a windsheild.

"If a smaller flame is better than why don't we just heat with a candle??" I belive that I said this to a customer one time.

Two stage and modulating equipment use considerably more energy than single stage equipment. What if we modulate the flame size AND the mass size? 8-6-4 V8, 4-3-2 boiler sections.

Even at many factory settings oil equipment runs under 70% efficiency regardless of what it may be rated, and drops lower at less firing rates. Most gas furnaces barely make 60% efficiency even though they are rated at 90%+.

"Combustion analyzers almost never calculate efficiencies anywhere close to accurate!!!"
What readings are we missing on our combustion analyzers?

Live/Learn

I'm confused!!

Hi wallies, Now I'm really confused . I seem to be hearing two different opinions about my question. With fuel running over $3.00 per we have to get the most "bang for our buck". I've been know to squeeze a nickel so hard as to get the buffalo to have a BM. Is there a difference between effiency and economical operation? I can see the logic of both trains of thought but I'm more interested in saving fuel than effiency numbers. I hope more people weigh into this discussion as I feel it's quite important due to the high cost of fuel oil. Thanks in advance, Live/Learn

zeke

fuzzy math

I'm having trouble with a 500,000 BTU heat load and 4+ hours to satisfy it. How do you get that?
Do you mean 500,000 BTUH, or 50,000 BTUH ?

By my calculus you can't satisfy that load with a 1 gph nozzle.

zeke

Why don't you run your own test and use each nazzle for, say a week. I've been tempted to do it myself.
You have to put a clock into the burner circuit that will give you the total "on" time of the burner in each case; then you find the number of the degree days over that period.
You simply take the no. of deg days and divide by the product of nozzle gph* elapsed time; this will give you degree days per gallon in each case, or DD/( nozzleGPH*HRS)
The winner wull be the higher number.

Jamie Hall

Have I missed something?

everyone has good points -- especially the oil at three plus per! But... one thing I haven't seen is that there is a minimum firing rate for any particular steam boiler which is set by the boiler size itself, and another minimum firing rate for any steam system, set by the size of the system. Go below that, and things get very chilly.

Dave_4

Soot Monkey?

Do you have any data at all to support your contention that boilers should all shoot the largest nozzle possible - regardless of load?

Using your style of thought in reverse: "Using a 1.25 GPH nozzle in a boiler rated at that maximum input in a home that requires 50,000 BTU's on a design day is like killing flies with a sledge hammer"!

You cannot overturn the principals of heating physics with wishful thinking.

If we use your assumption(s), we'd all cook this Thursday's turkey in a 500 degree oven!

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Steamhead (in transit)

Ideally

the boiler's rated output at maximum rated input (firing rate) should be pretty close to what the system (on steam) or building (on hot-water) needs. But some installers oversize everything. Also, it's not always possible to get an exact match of boiler rating to system rating.

In the latter case, only some fine-tuning is needed. In the former, you might not be able to down-fire enough to avoid short-cycling without getting into flue condensation.

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John Abbott

Smaller nozzle

definitly go with the smaller nozzle as long as you don't have extended run times below 140 degrees you will save money. As someone mentioned a perfectly sized boiler will run continuously on the coldest day of the year.Hard to achieve but something to shoot for.Oil burns most effcently with a hot combustion chamber and the longer your cycles the more efficent you make your equipment.The gentleman who advised you to use a larger nozzle is mistaken in my opinion and is contrary to all my experience and training.

sootmonkey

run time

I think all of as are talking about different aspects of effecency.

The ideal heating system will be fired at its rated input and the btus would be going out to the load, just as fast as they are transfered to the transfer medium.

I have a Burnham V73 with a Beckett AFG burner. It is rated for a .85 nozzle @ 140 psi.
I have tested it with that nozzle and with a .75 and a .65 nozzle.
Yes, I can get lower stack temps, but not enough to make up for the longer run time. (Electic power consumption)
If you follow my post above, I used an example of a 2% increase in effency by down firing.
In real life, using the same boiler, at the same tempture , I was not able to get a two percent increase in effency.

"Yes, you want long run times, No you do not want long run times."
"Yes", ideally the unit would never shut down and all the btus it produced would be needed and used. Not likely in the real world.
"No", you want to produce the btus that are needed quickly and shut the electric power off to the burner.

As a test, take your own boiler, fire it at its rated limit and take a combustion test. Note the boiler temp, co2, co, and stack temp. Do the same tests with the next two or three smaller nozzles. Post your results here.

Not much that you can do for a boiler that is oversized for even the largest load that it will see.

just a monkey chewin on a bannanna

Live/Learn

Return water below 140 degress.

My return water chills my system down to about 130 degees when the circ. first comes on . It takes about 6-7 minutes to reach 140 with the circ. running and another 25 minutes to hit it's hi limit of 190. After that it's 5 mins off and 6-7 minutes on untill home is up to temp. This scenario happens after a 7 degree set back overnight from 60 to 67 degrees. This is a hot water monoflo fin type system. Live/Learn

Constantin

Interesting...

Would you mind reconciling your viewpoint with published data from various manufacturers? I ask since there are multiple manufacturers out there that offer variable-output equipment in various flavors.

For instance, Smith's series 8 boilers come in a high and low configuration where the higher-input version always has a lower AFUE rating. Is this an issue with the way the AFUE is tested for? I seem to recall similar ratings with Dunkirks and other boilers but I don't have the time to look them all up.

On the gas side, I distinctly remember the thermal efficiency of the Vitodens going up as the firing rate goes down (i.e. as it modulates to low fire). Perhaps the heat exchanger has something to do with that behavior vs. the behavior you normally encounter?

Any additional insights would be appreciated. Cheers!

zeke

For what it is worth, I just did a comparison of two nozzles, one a .75gph ( mnufacturers spec) and a .65GPH that I changed out last Feb. With the .75,from Nov to Feb, I calculted an average K factor (degree days/gal) of 5.50 and with the .65 for the rest of the heating season I calculated about 6.03 which looks like an 8% improvement in efficiency.

DISCLAIMER

This is just my experience with one furnace and does claim anything. Just some food for thought.

Live/Learn

Hi there Zeke!

Zeke are you saying that the .65 is more economical to run or the other way around? I also found the consumption to be pretty close as I did about the same thing last winter and the winter before. The only reason I changed back from the .85 from last year was due to the real short run times. I figured I'd increase the run times more with the .65 which I did. So that's why I posted the question of what's more important for fuel economy less fuel per hour or less time burner runs? Thanks for your interest Zeke Live/Learn

Dave_4

Dear SM,

Here's a few things that you may have forgotten to take into consideration:

1) The electrical ratings of the motors and transformers you rightfully are concerned with have two ratings. One, the amount of current consumed while running - this is what you see on the motor data plate. The other, the true load spike that occurs every time you start the motor; typically four times the "run" amps!

2) True efficiency is the result of steady-state operating combustion efficiency, which you have considered (and can measure with combustion analyzing equipment) AND: "off-period" standby losses - which you have not measured - or included as part of your calculation. "True efficieny" is also the result of: Standby losses; super poor efficiency for the first 2 minutes of every cycle you would have us increase the number of; stack losses during off cycle something your plan would increase the duration of as well; overcoming the thermal mass of the boiler and the water therein - BEFORE any heat goes into the structure we are trying to heat. The frequency of that event will be much greater with short runs and long off-periods - than the long burn time we suggest.

3) You suggest there are different aspects to efficiency? This is true, but not for our purpose in this thread. The contention that a smaller nozle and longer run-times is less efficient than a larger nozzle and shorter run times is not debatable. Whether or not a smaller nozzle and longer run-time could create excessive condensing and therefore become a destructive force IS debatable; but that is NOT the debate,

Or is it?

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Mitch_4

the biggest thing to remember IMHO

the output needs to closely match the load. I was at a seminar here and the tech explained it this way.

Every time the boiler/furnace fires it has to heat the heat exchanger material before that heat is transfered to the medium to be dispersed to the home. If you run non stop for 30 minutes, versus 6 five minute cycles, you will use the same amount of fuel, but more is deliverd to the structure as the heat exchanger is only heated once, not 6 times. so it becomes more ECONOMICAL to have a longer runtime. The electrical consumption is negligible.(starting motors / pumps draws inrush current increasing wear and power) Less cycles is less wear and tear on components as well.

This is particularly true he said with condensing appliances. They do not reach condensing mode until 2-3 minutes of the cycle and it is when they condense that they achieve their efficiency, so in one 30 minute run, the equipment is in condensing mode for 27 minutes, versus 12 on a 6 cycle scenario. (assuming 3 minutes to start condensing) (Thanks to Mike at Olsen for this explanation)

By lowering the nozzle / picking the right size equipment, you are making the operation more economical and less expensive. It is not about how many BTU's are used, but where they get placed, we want them IN the house, not heating metal, or going up the chimney.

Mitch

zeke

accurate testing

In doing this test, you have to be very careful to get the degree days over the period very accurately and then perform the DD/gal calculation.By just using usage without regard to average outside conditions, you can't get an accurate measure.

The gal usage is easy, just from the delivery numbers. However, to get the degree days, you must go to the weather bureau data for your area. And by the way, since deg days are measured from 65 degrees, you really should adjust that number for your actual thermostat setting. The adjustment is the difference between it and 65 degrees multiplied by the number of days you are testing; i.e. for example if the setpoint is 70 deg, and the test is taken over 50 days, then you add (70-65)*50=250 to your total Deg day number to get the modified deg days for the test.

I forgot. I got the .65 nozzle to be better than the .75.
Also, I forgot the NOT in my disclaimer in my previous post. above.

I just looked at your OP and the answer to run times depends on the load at the time and the nozzle sizes. If your .65GPM nozzle is the size just needed to sustain say a load of 0 degrees outside temp running continuously, then if you check the run time at say 30 degrees and your indoor setpoint is 70, then the ratio of the "on" times for the two nozzles would be

[.85-.65(70-30)/(70-0)]/[.65-.65(70-30)/(70-0)]= 1.7

zeke

quote

"Longer run cycles means the equipment isn't delivering enough heat to handle the load. 80% of the winter all equipment is oversized. Except for mechanical deficiencies of certain equipment all equipment is designed to deliver maximum efficiency at maximum mechanical input. Underfiring heating equipment is no different than undercharging an air conditioner.

In the last 28 years, 100% of equipment that has been brought up to its highest firing rate has shown a decrease in fuel usage, much of this usage was audited by utilities and in house energy engineers.

The only way underfiring could save energy would be that the firing rate was set up incorrectly.

When the campfire gets smaller you don't stay warm no matter how long you sit there.

If anyone actually checked they would find that the flue temperature of an appliance keeps rising the longer it runs which means more heat is going up the chimney."
------------------------------------------------------------------

Jim,

You made a bunch of statements (above quote) which are at odds with my thinking . I would appreciate your explanations if you may.

Paragraph 1:
I always thought that if you had an oversized boiler then by definition a smaller nozzle may be able to satisfy the load and still have enough left to heat up the boiler.

paragraph2: Please provide documentation. I never heard of this.

Paragraph 3: Documentation please.

Paragraph 4: I don't see what a campfire has to do with heating a house

Paragraph 5: Taken out of order. On this one, you can't get unlimited temperatures, since it comes to an equilibrium temperature in all cases. However, In fact I find that the smaller nozzle results in lower flue temps but the longer run times may partially make your case. I just don't know.

zeke

Mitch_4

comments

para 5...if the heat distribution system is "perfectly" matched (Theory I know, and has little to do with reality) the water / airflow should be scrubbing heat off the exchanger at its trasfer rate fromthe flue gasses. Increasing stack temps during a cycle mean that medium flow across heat exchanger si not optimal.

As to the campfire comment..nothing to do with heating a structure as the loss is 100% and all heat is radiant from the flame..

Put that campfire in an enclosure. putting every scrap of wood ina single fire, light it and see how long you stay warm. (assuming the structure remains intact.

Now same structure, small fire, that is fed steadily, you will have heat longer, because you conserve fuel and use only what is required...2nd structure will stay warmer longer.

by your comments any and every structure would save more money oversizing...

Mitch

Mike E_2

I doubt that "100% of equipment that has been brought up to its highest firing rate has shown a decrease in fuel usage" especially since the complete opposite happened at my house over the past couple of years.

I have a Vaillant F100-50HE oil boiler. OEM 1.25 nozzle @ 140 psi on a Beckett AFII. 1.5 gph firing rate. Rather oversized for my house after weather sealing updates done the previous year. 27.6% excess air, 464 F flue temp, 4.8% O2, 12.0% CO2, 122ppm CO

2005-2006 heating season. Nov. 12 - May 18. 640 gal used, 5895 degree days. This works out to .108567 gal/dd

Summer of 2006 changed to a .75 nozzle @ 150 psi on a Carlin EZ1. 0.92 gph firing rate. I took out the baffle plates so that the flue temperature stayed above 350 F. 38.2% excess air, 375 F flue temp, 6.1% O2, 11.0% CO2, 149 ppm CO.

2006-2007 heating season Oct 23 - May 12. 600 gal used, 6294 degree days. This works out to .095329 gal/dd. Less oil used with more degree days. There were no insulation/weather sealing changes to the house in this time, so the only place the reduced usage came from is the reduced firing rate.

If you convert usage to the same base degree days of 6000, the reduced firing rate would consume 572 gallons, and the maximum firing rate would consume 651 gallons.

Charlie Masone

On Peerless Website they list different firing rates for the WBV boiler and the corresponding AFUE

Using the WBV3 as an example

.60---87.5
.85---86.2
1.10---84.5