boiler firing rate for efficiency

zett

I'm just not seeing the forest for the trees and could use the wisdom of the wall...

I'm wondering how to set the firing rate.

I'm the facility manager at a small school and have a Peerless Cast Iron boiler that has a potentiometer on the burner. I can set the burner to high or low fire or anything in between. I've set it so the water in the glass is not bouncing wildly or that there is no water dropping into the steam side of the glass. This is as high as I dare go. This will get the pressure to 1.5 pounds in about 3 to 4 minutes. If I set it lower, I assume I will use less gas, but it takes 2 or 3 times as long to reach the cut out pressure. Either will keep the building warm just fine.

My question is which setting is more efficient in the long run?
Does the higher fire use more gas or the longer low fire?

I'm attaching a photo of the firing rate per hour (i've attached a sensor to the gas valve, so it's actually showing the number of times the valve has opened.) You can see how the firing rate was lowered after 1pm.

Any help appreciated,
John

nicholas bonham-carter

What is the make of the burner? Normally the burner would be set up with combustion testing equipment to make sure the exhaust is within certain parameters for efficiency.
Ideally, the capacity of all the radiators should match the steam generation of the boiler, and maybe the burner could thus be adjusted to make this happen.
1.5 psi should normally be the upper limit on pressure for the system, which would be limited by the pressuretrol.—NBC

EBEBRATT-Ed

If the burner has a potentiometer then the boiler also has (or should have) a Honeywell L91A or b modulating pressure controller on it. This is a 3 wire control with the connections marked R,W,B this is what we call a "full modulation" burner. Power Flame? Webster? Industrial Combustion?

When adjusted correctly this control will modulate the burner to maintain the steam pressure you want without having to adjust the manual potentiometer.

Depending on the burner you have the wiring is a little different. Some burners have a manual-auto switch and you can select between the pressure control or the potentiometer to modulate the burner.

If the burner on high fire bounces the water level too much then you have 1 of two problems:

1. The boiler water is dirty and needs skimming
2. or the burner isn't set up right and could be over firing

I would set the steam pressure high limit or operating pressure control to 5 psi. Then use the modulating control to modulate the burner at 1 1/2-2 psi.

This will reduce burner cycling and maintain the pressure you want

zett

The burner is a Gordon-Piatt and I believe we do have a Honeywell L91 modulating pressure controller.

The burner may be overfiring as the water is not dirty.

I did adjust the operating pressure control to 5lb with cutout at 1lb and put the burner on automatic with the potentiometer at low setting.

The pressure seems to be holding at 1.5 now!
Looking forward to longer cycles now.
Any other suggestions?

Thanks!

EBEBRATT-Ed

Sounds like it's working. You can get a bouncing water level even if the water looks clean it could have oil etc on the surface but if it isn't surging leave it be

Looks like you have a manual auto switch on the panel next to the pot.

Gordon Piatt is out of business. If you need burner parts there is a company in Ohio (I think) that has some parts. You can Google it.

zett

Appreciate the tips!

Dave in QCA

Yes. What EBEBRATT-Ed said. I had skipped some of the replies, was going to type a detailed answer, but looking back, I see that Ed already covered it. Only thing I would add, is that proper adjustment of the air/gas mixture for proper efficiency is done at both ends of the modulation range. I should be covered in your burner manual.

Mike_Sheppard

If you need Gordon Piatt parts, call Cici Boiler Boiler Rooms and ask for Chris Dasher. Tell him Mike Sheppard referred you.

Here is their website:
https://gordonpiattparts.com

That burner has a manual/auto switch. It should be set to auto. The modulating pressuretrol should be set lower than your operating control and will automatically modulate the boiler between low and high fire. In auto mode on that burner, the potentiometer can also be used to limit high fire, down to a maximum of 50%. This helps during low demand times of the year. I believe they have writing there around the manual/auto switch that explains this.

mikeg2015

Generally you get a little less efficiency on low fire, with cast iron boilers but on variable induced draft, it’s probably not as big difference. I think the reason is less infrared transfer from the smaller flames.

Combustion analysis would confirm this. And as mentioned,d important that it’s tuned on both ends, and they might check it in the middle too and need ot make sure any mechanical linkage doesn’t have a lot of slop.

EBEBRATT-Ed

In general the flame on low fire may be slightly less efficient than on high fire but the stack temperature is also reduced on low fire so it balances out

ethicalpaul

Plus low fire by definition uses less fuel so I never understand why people get concerned about a lower efficiency number during down firing.

mikeg2015

If does use more fuel if it runs longer. If you downfire 50% and it loses efficiency, then it has to run >2x longer to produce the same BTU’s.

A 200k boiler @ 80% efficiency delivery 160k per hour. If heat loss is 100k, it will run 37.5 minutes per hours and consume 1.25 Therms. if downfired to 140k, if effeciency is 78%, It runs for 55 minutes per hours and consumes 1.28 Therms. However, standby time is only 5 minutes vs. 23 minutes.

That being said, there are some savings to reducing standby losses due to longer run times. But that depends on if the boiler is adjacent to the conditioned space. But in this case, I’d suspect if might be a wash, so the comfort of longer run times presents and advantage. On a 1 pipe steam system you reduce the volume of oxygen ladened air being drawn in and out.

Dave in QCA

Don't forget that longer runs also reduce the starts and stops. This will reduce the number of prepurge and postpurge cycles which fan force a chunk of heat right up the chimney.

EBEBRATT-Ed

And down firing also always lowers the stack temperature (is has to) which always raises the efficiency as long as the Co2& 02 #s don't fall off the table

Mike_Sheppard

Don’t forget jacket losses. Jacket losses stay the same whether it’s at low fire or high fire.

https://heatinghelp.com/systems-help-center/are-modulating-burners-really-worthwhile/

The Steam Whisperer

Jacket losses are nearly constant for most boilers in the mid season. I've seen several organizations say that running at high fire is much more efficient because the jacket losses are much higher at low fire. They base this on calculations that assume that as soon as the boiler shuts off, it goes instantly cold.....They have never actually seen a boiler operate.
Now losses from the draft hood into a room on boilers with stack dampers are a different story.

ethicalpaul

mikeg2015 said:

If does use more fuel if it runs longer. If you downfire 50% and it loses efficiency, then it has to run >2x longer to produce the same BTU’s.

A 200k boiler @ 80% efficiency delivery 160k per hour. If heat loss is 100k, it will run 37.5 minutes per hours and consume 1.25 Therms. if downfired to 140k, if effeciency is 78%, It runs for 55 minutes per hours and consumes 1.28 Therms. However, standby time is only 5 minutes vs. 23 minutes.

That being said, there are some savings to reducing standby losses due to longer run times. But that depends on if the boiler is adjacent to the conditioned space. But in this case, I’d suspect if might be a wash, so the comfort of longer run times presents and advantage. On a 1 pipe steam system you reduce the volume of oxygen ladened air being drawn in and out.

Thanks for your nice reply. My comment came from a place of thinking where the boiler was oversized and the down firing would be sought as a relief for some of the issues that can cause--I would never advocate it just on its own.

mikeg2015

EBEBRATT-Ed said:

And down firing also always lowers the stack temperature (is has to) which always raises the efficiency as long as the Co2& 02 #s don't fall off the table

With and atmospheric, on low fire you get more excess air, so stack temp is lower, but O2 goes up because secondary air does not reduce proportionally. Ultimately, air velocity tends to stay similar due to convection. Air mixing in the tubes does stay constant, so air/fuel ratio is probably nearly the same I’d expect.

If you could drop the steam temp or in a hot water boiler, the water temp, the draft could drop slightly and you might gain economy or at least maintain it.

Leonard

Just for comparison in summer my 60 year old cast iron uses ~ .9 gallon of oil per day to stay hot to make DHW ( DHW coil in water jacket). Nossel is downsized to 1.1 GPH

No motorized exhaust dampers

2 old people, two warm/hot 10 minute showers per day, 3GPM

zett

Thanks for all the info!
I will definitely get a combustion analysis and have it tuned on both ends. I'm feeling like I'm finally getting a handle on this.

We have a Tekmar and it is doing it's job.
I am including a spreadsheet of the other day.

As the outdoor temps rise, the boiler is running at 4 oz for longer cycles, the rads are warm, not hot and the classrooms are just fine.

We have a building (1913) with single pipe and a 1923 wing with 2 pipe, both heated with same boiler. The boiler was downsized when it was installed in 2000. the engineer did not do a heatloss, but counted rads. 40,000 sq ft, 2 floors with longest runs about 150 feet.