Can reducing the GPH of a nozzle be compensated by increased PSI??

DonnieOil

I have a Weil McLain series 68 model p468 boiler with a Beckett AFG burner. The manufacturer states that it should use a 1.25 GPH x 80 A nozzle. I have no problem with that but I do not get a good burn and start up until the PSI on the pump is at 125psi which is really running the furnace at 1.40 - 1.43 GPH at the nozzle. Would it be wise to use a 1.10 GPH nozzle at 125 PSI so that I get to a 1.25 GPH burn rate. Just thinking about fuel efficiency but a bit concerned that I do not want to go against the manufacturers specs. Why would the PSI need to go to 125 and not be at 100? Also is there any correlation between the air flow (band / shutter) and the oil pump PSI setting? Beckett has the OEM settings at shutter 10 (full open) and band set to 0 (full closed). After the seasonal check up and adjustments the shutter is at 10 and the band slightly opened 1/8 inch. Could the air flow be too great which causes the PSI to be higher at the nozzle. Any comments and discussion will be much appreciated.

Jamie Hall

The flow rate from the nozzle varies -- almost linearly -- with the pressure. However, many nozzles and setups produce the best flame pattern at a higher pressure than the nominal 100 psi test pressure.

The amount of fuel burned sets the heat input -- and, all else equal (it never is, quite) -- the heat output of your boiler. You want a nozzle with a decent flame pattern for your boiler which produces the heat output to your structure. Whatever works in that regard.

Now the fuel delivered by the nozzle needs the correct amount of air to burn completely. That is set by the shutter and band and draught -- and the manufacturer's settings are an approximate starting point.

The only way to achieve the correct combination is to use a nozzle and pressure which deliver a good flame pattern for the boiler and provide the heat output needed, and set the air and draught using a combustion analyser. There is no other way to do it.

EBEBRATT-Ed

The mfg may rate that boiler at 1.25 nozzle that is also pushing the boiler pretty hard.

My advise would be to fire it at 1.00-1.10gph

Nozzles are rated at 100 psi. Most modern burners run at an increase in pump pressure because the burner manufacturers finally figured out what Iron Fireman new back in the late 50s. They ran 300psi on their commercial burners. It took everyone else years to catch on. An increase in pump pressure increases the oil flow rate of a given nozzle and makes the oil spray out in smaller droplet size which gives better combustion.

So I would install a 1.00 gph nozzle and leave the oil pressure at 125 psi.

This will give you a firing rate of 1.10 gph

You shouldn't over fire the boiler as you are now doing

Also you must use combustion test equipment

STEVEusaPA

Higher pump pressure/smaller nozzle is a fairly standard practice to better atomize the oil resulting in a cleaner burn.
Sometimes you may also have to change the angle, too.
The only way to know is establishing proper over-fire draft, and setting the combustion air with a smoke test, and seeing the results on an analyzer.
You could improve light off with more modern primary control that has pre-purge-you'll need a delay valve for your fuel pump.
You're probably aware that boiler is pretty old, most likely oversized, and probably delivering an overall system efficiency of about 60%.

HVACNUT

Everything that "@Jamie Hall" said, and, unless the AFG was OEM spec'd for the 468, those numbers can go out the window as the starting point for combustion tests.
Is the Beckett chassis blue and have that centrifugal damper?
The OEM came with an F6 head to fire 1.25 GPH @ 100 psi.

There will be better atomization at a higher pump pressure, assuming everything else is correct.

And it doesn't need to be max fired unless the heat loss of the space needs all those BTU's. I have many 468 WM boilers firing a .85 80A @ 140 psi.

My own next generation WM Gold 3 section is rated at .95 GPH input. I know, the Cobbler, blah blah blah.
With a Riello burner, I'm firing a .60 nozzle at 150 psi for .74 GPH.
Longer cycles while maintaining condensate protection. Burns clean and purrs like a kitten.

But a pro with a combustion analyzer is needed.

DonnieOil

Thanks for the info. To answer some questions presented and some additional info.

The Beckett burner is blue and is OEM spec'd to the boiler with the centrifugal damper and F6 head. The boiler is setup for 2 zone hot water baseboard heat. The house is built 1890, no insulation (not even the Attic), 1700 SQ FT living space and is drafty as hell. Actually just replaced some rotted porch decking and the decking comes right into the house under the 1st floor wall. When I removed the decking I could see a 1 1/4 inch space right into the basement so that is basically 30 feet of opening where the floorboards don't actually make it to the back of the wall. Will be fixing that this fall and insulating the attic. Also there are times when the temp drops to 10-15 degrees during the winter and the boiler is always on and firing every 15 minutes for hours but the heat in the house can't get above 68.

It seems that the lowering of the nozzle gph to 1.00 - 1.10 at 125psi is the way to go but will this affect the heating on those really cold days?

Jamie Hall

The question as to whether running a 1.00 gph nozzle at 125 psi will affect your heating on the cold days can't be answered directly. As @EBEBRATT-Ed pointed out, that nozzle and pressure combination will give you a 1.10 gph fuel input rate. That translates to about 150,000 BTUh gross input. At 80% boiler efficiency -- which it can only achieve if you get a high quality tech in there to set the boiler up -- that would give you 120,000 BTUh output. Is that enough to heat the house? Dunno. Perhaps more to the point, there isn't enough information here to make even an informed guess. Is it enough to heat all your radiation? Dunno that, either.

EBEBRATT-Ed

And that is about as low as you can go with the F6 head

HVACNUT

> @EBEBRATT-Ed said:
> And that is about as low as you can go with the F6 head

A company I worked for years ago installed the dreaded Peerless JOT-3 a lot.
We would downfire them .75 at 100 psi with the F6 head and used the low fire baffle.

But nobody knows the heat loss, so I guess we're stuck.

DonnieOil

I will do a heat loss calc and posted it here later this week.