Oil furnace efficiency

JimPettit

Can a lower gal/hour nozzle with same degree and shape reduce oil consumption or to rephrase - should a lower gal/hr nozzle be installed at what point is it not efficient and not advisable ?? What else can be done - yes regular maintenance but stepping it up a notch ...? Thanks !

JUGHNE

Just a disclaimer to tell you that I haven't worked on oil for over 30 years.....I am sure a lot has changed since......FWIW:

I used to downsize the nozzle on every furnace by one step every 2 years until I got down to the minimum size by nameplate.
Usually the homeowner was unaware of this and I anticipated someone to call about lack of heat.....never happened. They were all grossly oversized. Some went down 3 sizes to hit the bottom.
In my mind it was more efficient by reducing short cycling.
Also I kept track of the sizes and eventually changed out all to NG or (regretfully) heat pumps.
The history of nozzle sizes helped size the new replacement units.

Leonard

My oil furnace supplies heat and DHW, (tankless coil in boiler water jacket)

I was at 1.36 or 1.24 GPH nozzle and had oil guy go down to either ~ .9 or .95 GPH.

After that I noticed shower ran cooler when thermostat called for heat, not bad but not what I wanted. So next year I had him up it to 1.1, all the hot water I want now. Recently it was VERY cold outside -10 deg, still had all the heat and DHW I wanted. Lowest ~ 100 year historical temp here is -15 to -20

Shower head is NOT the wimpy 1.1 GPM type, it's an old not restricted flow type. Hot water line has an old 5 GPH restrictor on it, but without shower head attached shower spout flows 3 GPM.

DZoro

Most are very much over sized. I too have reduced nozzles ect... Just keep in mind it MUST be done properly and with analyzers, and stay within specs of the system. If you do all this you will be much more comfortable with your wallet while sitting in your chair.

STEVEusaPA

It's hard for that internal coil to keep up with a high flow shower head, especially when the incoming cold water from the street drops in temperature after extended cold spells.
You either have to fire the boiler as rated (most efficient) or keep the boiler temperature up if you down fire (least efficient).
As mentioned, you have to check combustion if you down fire (actually always), and you'll probably notice when you fire under the rating on the boiler plate, your efficiency goes down as well as your stack temperature. You don't want to condense in the flue or the boiler.
Combustion expert @Jim Davis has commented before about down firing and why it's not necessarily a good idea.
You may also want to have the coil checked/flushed and make sure your mixing valve is working properly.

JUGHNE

All of the ones I downsized were forced air.
All were belt drive. Temp rise was not checked by me or anyone in those days. People were just glad to have someone work on oil at all, not have to be hitting the reset button. Never realized the danger in that.....I have held the inspection door shut with my boot on occasion....don't try this at home. Or don't throw a lighted newspaper in a slightly flooded chamber to get heat in a freezing house after the ignition transformer failed and non available for 2 days....then turn the burner. Don't try this at all.
These old beasts were quite durable and ran pretty hot anyway. The HO and I both knew they were going to go away in a few years.

But, yes with more efficient burners with less going up the flue that stack temp should be checked as you downfire. Just advice from a guy who doesn't do oil.

lchmb

If you'd like to see a good investment add an indirect if possible. Put a mixing valve on it, run the tank at 140 and mix out. Keep the boiler at least warm (110) and enjoy all the hw you want with some nice savings behind it..

DZoro

@Leonard If you have a boiler and especially with domestic. Down sizing must be done carefully if at all. There is a lot more variables with a boiler compared to forced air.

EBEBRATT-Ed

In most cases you can downfire 1 nozzle size without a problem. Your stack temperature will decrease and your efficiency will increase. A combustion test should certainly be done. With a tankless coil you may reduce the available dhw

SeymourCates

@JimPettit


A typical oil fired three section CI boiler will have a .85 nozzle and deliver 110K net.

When installed in a typical 2000 square foot residence with two zones and 50 feet of baseboard (low mass) per zone, a single zone can deliver only 25K BTUH.

When controlled by a typical aquastat with a 10F or 15F differential, it takes no more than 90 seconds for the excess 85K to heat the boiler by the differential. Then the boiler shuts down and the process endlessly repeats itself. This is the height of inefficiency. It cannot get any worse (unless, of course, a fool installed a four section boiler into the same residence).

We successfully downfire all of these machines to a .55 nozzle and achieve 84% combustion efficiency with actual stack above 325F. When configured with an aquastat that offers a 30 degree differential, the boiler will run for almost six minutes when a single zone calls. This remains not an ideal situation but is superior to the factory setup in all possible ways.

EBEBRATT-Ed

@SeymourCates their is much opposition to down firing on this forum sometimes.

In my view down firing promotes better efficiency (lower stack temp) longer run times, less start and stops=a cleaner boiler and less ware & tear on the equipment. As long as the burner end cone is In the range of the new smaller nozzle and combustion testing is done it's a winner IMHO

captainco

It has been an industry theory forever that the lower the flue temperature the higher the efficiency. This is most likely because of the fictitious combustion analyzers calculations. Burners heat boiler sections and heat exchangers not buildings. Equipment has a certain mass that needs to be heated. There is radiant transfer and convective transfer and to maximize these we need the biggest flame and the most combustion gases.

Big flames or little flames don't matter if the combustion is not set up correctly. Underfiring is most commonly used to make equipment soot up slower versus fixing the problem. That is the excuse I have heard hundreds of times. Leaning out a car engine because you drive slow won't save you on gas. The weight of the car doesn't change.

Might get away with firing 10% below equipment rating with minimal losses but more than that efficiency takes a tumble.

SeymourCates

@EBEBRATT-Ed

Your view is correct. If one can maintain efficiency while reducing fuel and air flow simultaneously, the BTU/unit time is reduced thereby increasing the run time.

We both realize that an oil fired boiler with a net rating of 90K (per factory) is absolutely ridiculous in a structure that has a heatloss of 45K with two zones, Such a machine will short cycle itself to death

I find it interesting that there are folks that refuse to agree that a 12K minimum fire mod-con is not acceptable on a zoned system without the use of a buffer tank, but utilizing a 90K boiler (net) on the same system is perfectly OK and nothing needs to be done about it. The boiler barely gets out of condensing range when it satisfies the 'stat for one zone (except in extreme cold).

We downfire the machine to a .55 nozzle and utilize some timer relays to purge the boiler after a call. We also utilize a Johnson Controls A321 on the indirect to provide a 30F differential. Fuel savings with all of these changes is on the order of 18%. It would be impossible for anyone to believe that the A321 and the timer relays are the sole benefit here.

Paul48

"Equipment has a certain mass that needs to be heated".

There's one point that's not being stated. And that is, regardless of the mass, applying less heat reduces output ie: a mod/con boiler. The boiler manufacturer has tested to meet one set of criteria.........This much heat, applied in this way, will produce this much heat at this level of effiency. I would imagine if those criteria were for a 100k boiler and they discovered they could only sell 90k boilers, they would simply change the criteria, and not the boiler.

captainco

Mod Cons are all by themselves. They are low mass and have heat exchangers that have a much higher thermal conductivity than the heat exchangers on oil equipment. Not sure but they are 4 to 6 times greater in exchanging heat.

In the end how the combustion is set plays a major part in the efficiency. Poor set-up in high fire can be less efficient than good set up in low fire. Set up the same, high fire will win most of the time.

If we could measure exact amount of oil used, exact cfm or gpm and Delta T, we would know the real answer. Believe it or not, this has never been done when it comes to the actual efficiency rating and certification of equipment.

Jamie Hall

A couple of thoughts (as usual?). First, it should be painfully obvious that the boiler should be chosen to match the heating requirements of the rest of the system -- although how that is determined depends on the rest of the system. Doesn't seem to happen all the time, but...

That said, most boilers that I've seen or read about will have a range of firing rates within which the manufacturer has determined that they will be reasonably efficient. Some -- like a 10 to 1 modulating boiler -- are astonishingly wide. Some not so much. Usually the manufacturer -- particularly for oil firing -- has also determined which nozzle(s) work best at particular firing rates -- angle, geometry, pattern, etc. The objective of the exercise is to ensure that the flame geometry and hot gas aerodynamics are correct. This does border on rocket science! Using nozzles of other characteristics may or may not produce acceptable results -- but you are being the test pilot, and it is disconcerting to have the company's tech. rep. say "You did what?" when you call with a question.

And of course going well outside the manufacturer's range -- say putting a 0.5 gph nozzle in a 5 section Weil-McLain -- can have slightly absurd results...

STEVEusaPA

Yes Jamie, but changing nozzles is completely testable, if someone wants to put in the time. As a matter of fact, my first Oil Heat Technician's Manual (many moons ago-this one has a lot of input from FireDragon) outlines how to perform a 'nozzle substitution test' to determine best firing rate, nozzle size and spray pattern. This was more for when people were putting in conversion burners and were kind of on their own. You can also tweak the head adjustment.
In my experience, I noticed a few things. On oil boilers/furnaces that show multiple firing rates, the lowest firing rate is (technically) the 'most efficient'. But as Jim referred to earlier, only because of the way the analyzer calculates efficiency. CO2 goes down, and stack temperature drops (sometimes too low) so it reads 'more efficient'.
I just fire them as recommended, get the right draft, true zero smoke, add in air as recommended for that 'cushion', and the efficiency is the efficiency. Only if the analyzer shows me high CO, very high stack temperature, or high excess air do I need to be concerned and investigate further.
Another issue with dropping the nozzle size, is in some instances you now need to change the head or turbulator (Beckett) or add a low fire baffle (also Beckett).
I don't believe anyone 'saves money' dropping the nozzle that low.
Like others said, and @Jamie Hall put a fine point on:
"...it should be painfully obvious that the boiler should be chosen to match the heating requirements of the rest of the system -- although how that is determined depends on the rest of the system..."

EBEBRATT-Ed

I don't agree with the Co2 automatically going down when down firing. Dropping the nozzle 1 size is seldom a problem as long as the burner has the correct parts.

Years ago there were no "factory package" boilers and burners. You went and bought a boiler and then bought a burner and made it work.

And yes, as @STEVEusaPA mentioned we ran a "nozzle application test" which was described in Charlie Burkharts book (the name is escaping me). Back then you put a burner in and had no idea which nozzle to use , you found out by testing and trial and error.

Furthermore the nozzles recommended by the manufacturer for a package unit is the place to start.......not always the right answer. On the commercial side I have had more than a few that had the recommended nozzles that wouldn't work, have called the burner manufacturer in (Carlin & Power Flame) and changes were made. Buderus boilers that wouldn't work with the factory packaged burner, baffles had to be pulled out of the boilers to be able to fire them.

Anyone thinking the factory is right 100% of the time is living in a dream world.

In my opinion, residential equipment is probably tested better than commercial regarding combustion issues. People live in houses where lousy operation and smells are noticed immediately and the shear volume of equipment sold in residential work requires this attention.

I could tell story after story of commercial problems where the factory was wrong and inadequate testing. I was a factory rep for a burner manufacturer that was one of the worst violators of his and saw the problems first hand (one of the most popular burners sold)

My life has been spent in commercial boiler rooms. It has made me skeptical of anything the "factory" says. It's a place you HAVE to start at, try it their way first.....then you may have to change things.

I know I am ranting and most of this doesn't apply to residential but I have seen plenty of jobs that wouldn't work that were factory perfect.

JimPettit

Great comments All - Thanks !