Question about derating a gas fired, atmospheric steam boiler. Again.

JohnNY

Hello all,
I've got a client with a 300% oversized Weil-McLain EG boiler. I've done some shady stuff in the past with plugging off burners or swapping out orifices but I can't say for sure either have been much help in the long run.
My thought is that with steam the EDR output is based on the size of the heat exchanger which of course doesn't change with fuel intake, and the energy content of the water which has a definite target that has to be met in order to make steam, but the speed at which that target is reached is not largely relevant. Did I get that wrong?
I may be able to reduce or control the short cycling that's going on but there is a real problem with getting condensate back from the risers and branches that I'm blaming on the boiler overwhelming the system at every run cycle.
That being the case, can anyone tell me there is any benefit to derating a steam boiler? I'm hesitant to suggest it if I can't monitor the boiler myself on the regular.
Many thanks,
John Cataneo

Jamie Hall

@JohnNY , I'm tempted to say "if you don't know the answer, there's not much hope that anyone else does either", but...

Working only from hands on with power boilers -- steam engines -- they are routinely "derated". There is a minimum, of course -- different for each boiler design -- below which they simply won't steam, but above that minimum it is normal to fire them just hard enough to maintain whatever steaming rate (and pressure, in power boilers!) is needed for the task immediately to hand.

Now they are usually designed so that the burners -- whether coal or oil or gas -- provides a reasonably even distribution of heat in the fire box and the heat exchangers, but it would seem to me that if you manage to do that you would be fine.

The biggest trick -- other than that -- with power boilers is to ensure that the air/fuel ratios are at least reasonable (there's a reason why "make less smoke": was a single flag hoist in the old Navy!) under widely and often frequently or abruptly varying loads, but that shouldn't be much of a problem in heating work.

I would think in the instance you mention that making sure you had even heat distribution in such a wildly oversized boiler would be the biggest problem -- and probably set a lower limit on how far you could derate it.

But... it's not a steam engine, so I'm not going to say I know what I'm talking about.

JohnNY

Hi @Jamie Hall
Agreed that there is surely a low limit to derating and I would imagine that would be defined by the point where the heat introduced to the exchanger is equal to or less than the energy absorbed by it. This would lead to condensing. Of course I intend to avoid that at all costs.
But this:

Jamie Hall said:

Working only from hands on with power boilers -- steam engines -- they are routinely "derated". There is a minimum, of course -- different for each boiler design -- below which they simply won't steam, but above that minimum it is normal to fire them just hard enough to maintain whatever steaming rate (and pressure, in power boilers!) is needed for the task immediately to hand.

I think that's really my question. Once my heat exchanger is heated to the point it's making steam, will it make more or less steam if more or less fuel is burned (per minute/hour/etc)? It seems like it wouldn't matter since my HX surface area is fixed.

Hap_Hazzard

If you're removing burners, you need to consider how it affects flame impingement. I've been looking at two Peerless boilers, the 63-04 and the 63-04L, which differ only in the number of burners. The L model has 4 burners, one per section, while the 64-04 has one extra burner, so the flames from each burner can't envelop the sections as efficiently, resulting in a slightly lower AFUE rating (82.4 vs. 82.6). Not a huge difference, but scaled up to a much bigger boiler, the difference could be significant. Then again, depending on how many burners and sections you're working with, you might be able to position the burners more strategically than they are now.

Jamie Hall

Again, speaking from a wealth of no hands on knowledge -- always risky -- but in a power boiler, as it might be steam engine, so long as at least some of the heat exchanger is hot (in a steam locomotive it's the crown sheet) it will steam. Keep in mind that any metal which can "see" the flame will be well above steaming temperature on the flame side, and how much -- if at all -- it will steam depends a lot on the circulation in the boiler. Quite right about avoiding condensation, though. In a power boiler, this avoided by using feedwater heaters -- not practical for what you're up to -- so I suspect that one of the possible limits for derating would be stack temperature.

Honestly if I were doing it, I'd quite literally fiddle with it, trying to make sure I got reasonably even burning (and no flame impingement as @Hap_Hazzard said!) and keep the breach above say... 240 or 250 or so. It would be rather fun to play with -- but I think you're being a test pilot here.

delcrossv

It's an energy balancing problem. On the one hand you need to put in enough energy to boil the water in the boiler but any additional energy you supply goes to Boiling more water at a faster rate. So you have 2"floors" one floor is you have to put enough energy to steam and the second floor is including your thermal losses you have to put in enough energy to not condense the exhaust gases

If it were me, I'd prop open the relief valve and keep turning it down to see the minimum fire to get steam (continuous flow out the relief at no pressure- if the steam production keeps climbing, you can turn it down more) Then check the flue temp to see how cool it is. If it's above condensing, that's the floor and you can crank it up form there. If the flue gas is too cool, you'll have to crank it up (even with more steam) to get above condensing. 

I'd imagine to make the system work, you'll have to crank it up above minimum to make steam. After you've figured out the CFM of gas at the output you want, then I'd mess with shutters and maybe orifices to get the mixture as good as you can. 

That's my guess on this. I've asked the same question here recently in a  different context.

Lance

As a conversion burner licensed Master plumber, HVAC and gas fitter, my training took me into the realms of designing combustion systems and heat ex-changers. Understanding the physics of different fuels with heat and air flow through a combustion device also helped me build a duel fuel methane / hydrogen gas furnace. Many factors need to be considered before modifying a combustion device. Legally you become solely at risk for anything that goes wrong. Lets say you know your stuff. And you can tweak a problem system to fix or make it better. One rule I follow is there is only so much can be done before all can go bad. So I ask, will my modifications make it light reliably, burn safely, shutoff safely? Will the manufacture have anything to add or help in the process? It pays to ask them! Will I achieve my goal without the laws of physics proving me wrong? There are modifications that can safely be made like fuel rate adjustments, venting adjustments, but they need to be carefully measured & documented. Beyond this we can make sizing adjustments. But again I am reminded, we can be too big or too small and ignore the Goldilocks size at our peril or at the point we will exceed boundaries. Now some by error will make incorrect application of equipment. And we all know this can cause expensive problems. The real solution is to properly size and operate, but clients choose to spend less and we say, "sure I can try and make adjustments." Reducing capacity to minimum design will always reduce recovery rate only if combustion rate does not affect system design parameters. Below this minimum combustion design runs the risk of destroying the equipment. The way we can do it depends solely on the equipment's design. Plug this, cap off that, reduce this pressure, change this all create changes that imbalance a very carefully balanced system. Draft pressures, fuel pressures and barometric conditions all play a role. And remember when you design at the edge, it does not take much in changing of conditions to make it all go wrong. There is a lot more to this business than most realize. People trust us to not may devices kill us or destroy property. And from what I have seen it is a miracle many lived without tragic results of bad work.

SlamDunk

Sounds like a job for a modulating burner.

cross_skier

If it's a new boiler can sections of the boiler be removed and a smaller manifold be used to make the boiler "equivalent" to a smaller boiler?

EBEBRATT-Ed

At 300% oversized I doubt any changes will make much of a difference.

The ones I have played with that wan't 3.5" on the manifold I have set some down to 3" and they seem to run ok. That's a 14% reduction in pressure. Don't know what that translates to in gas input

GW

I will do a hard pass on that one, if things take a turn, gas pressure drop, the boiler could potentially burn poorly. If your spouse or sibling or BFF is a good defense attorney, that eases the pain a little bit. Captain CO probably has an opinion- he’s been in some court rooms. When you mess around with the original equipment, it’s all on you

motoguy128

The Hydrotherm I have in my home is rated at 175k and 200k with the same burner, castings all parts except different orifices.  I actually just ordered the smaller orifices since I have so many radiators on TRVs I’d like to slow things down.  

I’ve tried 2 stage gas valves, 1.7” on low fire for approx 70% firing rate.  It worked fine. But I didn’t put a combustion analyzer on it.  I suspect the efficiency drops.  

If you downforce with the same size flue passages and burner tubes you’ll end up with more primary and secondary air.  So economy will drop.   You’d almost have to block burners and block off flue passages both to make it like a smaller boiler expect with excess water volume.  

But even with all that, with the liability aside,  a drop to 1/3 output is a stretch.  

   

Steamhead

What model EG?

JohnNY

Steamhead said:

What model EG?

Hi, Frank. Happy holidays, my friend. It's an EGH-85 (875 sq. ft. EDR). The system EDR is 268 sq. ft.

Steamhead

Back at you. Some of those EG-series boilers have two different ratings for essentially the same block, but not the EGH-85. So, just make sure your analyzer is giving you good numbers.

dopey27177

You can derate the boiler, here is what will happen.
1. to much water to heat to produce steam at a quantity that is needed.
2. Insufficient heat in the flue gases to reach a proper stack temperature.
3. Soot depositing on the fire sides of the boiler.

300% is grossly over sized, in my experience 25% derate is about all you can do safely.

As to removing sections>
1. Need to buy a new cabinet.
2. Need o buy a new rear section for proper sized breaching.
3. Need to replace all the near boiler piping.
4. Need to buy new gas manifold.

Excess labor to jury rig that boiler.

Cheaper to install proper sized boiler, and if you have storage you may be able to sell or install that monster some where else.

Jake

PMJ

Big boiler running too long every time and "overwhelms the system".

You need a control that never lets it run a cycle long enough to do that. More, shorter, controlled pulses will tame that big boiler at next to no cost.

Doesn't take that many more cycles, really. But the standard control has no chance here.

ethicalpaul

I like your suggestion, PMJ, but I think it would depend on just how far into a cycle the current boiler can get. @JohnNY is it able to get steam into the radiators before it starts to cut out?

I've never seen a 300% oversized boiler in person so I honestly don't know. I imagine with very good main venting you'd be able to let it fill at least the mains before it cut out the first time.

But I wonder how much steam can get into the radiators after that before cutout. Might have to have larger-than-normal venting on the radiators?

PMJ

ethicalpaul said:

I like your suggestion, PMJ, but I think it would depend on just how far into a cycle the current boiler can get. @JohnNY is it able to get steam into the radiators before it starts to cut out?

I've never seen a 300% oversized boiler in person so I honestly don't know. I imagine with very good main venting you'd be able to let it fill at least the mains before it cut out the first time.

But I wonder how much steam can get into the radiators after that before cutout. Might have to have larger-than-normal venting on the radiators?

You are right - if the boiler is really cutting out on significant pressure before anything even gets to the radiators. I doubt it even at 300% but admit I don't know for sure. Cold pipes put a large dent in boiler capacity. Hot pipes deliver steam fast. My system doesn't even think about showing more than 2 inches water at the header until radiators start getting full. I have at least double the boiler needed on design day.

I am sure the concept of deviating at all from standard control is rarely even considered. There are a lot of oversized installations out there where this will work for sure.

JohnNY

The boiler fills a few nearby radiators and then shuts down on pressure, then short cycles back to running, blows some condensate out of the heated radiators' air vents, runs for about 2.5 minutes, then shuts down on pressure again. This is not going to be fixed by a control setting. I would just be trading one problem for another.

pecmsg

In buildings there's a saying................."You can't fix bad design with controls"

The only real solution is a properly sized boiler.

ChrisJ

Can that EG be "converted" to a smaller boiler by removing sections and swapping sheet metal and the burner manifold, tray and draft hood?

I know it's not fun, but it could be an option when boilers can't be had etc.

PMJ

pecmsg said:

In buildings there's a saying................."You can't fix bad design with controls"

The only real solution is a properly sized boiler.

And running a steam boiler on high until the thermostat is satisfied sucks as a control strategy which boiler size won't fix.

ChrisJ

PMJ

I will point out one more time that these systems all originally had big boilers running in the middle of their capacities with the flame modulated in real time based on the actual demand. Because the boilers were big, a relatively small change in fire produced a larger percentage change relative to the piping, hence providing significant adjustment from relatively small damper changes.

To these systems we now attach one speed fire (high) burning straight until the tstat is satisfied with no regard whatsoever to the current demand. From a control perspective that is undeniably a giant step backwards, and one which has astonished me as an engineer from my first real contact with steam heat 30 years ago.

This observation remains steadfastly unpopular here. By all means continue your quest for the perfect single speed boiler.

neilc

JohnNY said:

The boiler fills a few nearby radiators and then shuts down on pressure, then short cycles back to running, blows some condensate out of the heated radiators' air vents, runs for about 2.5 minutes, then shuts down on pressure again. This is not going to be fixed by a control setting. I would just be trading one problem for another.

doesn't this point at main venting being inadequate then ?
even at 300% over ?
clear the mains before the rads can start,
and pipe in a temp or pressure sensor at end of main to trigger the lock out time,

JohnNY

neilc said:

doesn't this point at main venting being inadequate then ?
even at 300% over ?
clear the mains before the rads can start,
and pipe in a temp or pressure sensor at end of main to trigger the lock out time,

@neilc At 300% output, the steam trying to squeeze through pipes sized for ⅓ the volume is enough to meet the pressuretrol setting even if the 30' main were to terminate in an open ended 1" nipple.

PMJ said:

By all means continue your quest for the perfect single speed boiler.

You seem to have convinced yourself you're speaking to a room full of uninitiated fools.
You may be right.
So, with all due respect, you're an engineer and I'm just a lowly plumber and maybe you can explain this to me like I'm not the sharpest bulb in the tree. The point of my post is not that I'm looking for advice on how to fix this condition. I'm pretty comfortable with what has to be done here to produce a quality result in the long term. What I'm looking for is technical confirmation that derating an atmospheric, natural draft, gas-fired steam boiler (do you have experience with these?) will produce a significantly less volume of steam per hr/min/etc than running it as delivered by the manufacturer. My thoughts are that with the heat exchanger surface area being a fixed value, and the values at which water changes state also being fixed, then what would it mater if we turned down the manifold pressure? It would just take longer to make steam, wouldn't it? Am I missing something?

Let me throw this into the mix while we're here....
I clocked the meter with the boiler running. The boiler is only taking in about 250 MBH (from 349 MBH). So one of the 3 plumbers who came before me already tried to solve this issue by dialing down the gas valve, but that didn't seem to help. And this is why I'm asking; because I might have tried that in this case even though we're so wildly out of whack here.

SlamDunk

I qualify as an uniniated fool. Is a modulating burner out of the question? A two stage burner? You could fire it using water temp as your process variable and the thermostat as your on-off switch. I won't solve this for you because I know there are things I dont know. Just spit balling.

PMJ

@JohnNY,

You supplied all the derogatory terms for what you suppose I think of you guys and assigned them to me. You didn't find them in my post. I'm sorry if you insist on receiving what I did say that way. Yes, I do find it frustrating that the same basic control has existed for 60 years plus now and I get told without fail by someone that it has nothing to do with this and the only solution to all problems is a perfectly sized boiler. I see lots of effort going into new boilers and boiler modifications. Control solutions are dismissed out of hand pretty much without fail. I find that unfortunate and frankly depressing. The heat can be so much nicer. A balanced system for flow is always required. A specific boiler size is not. Lots of extra can easily be handled and is not by default "bad design".


On your topic, last I clocked my boiler I figured it was downfired at least 25% from the plate maybe 30% just by using the regulator. I found it that way and left it alone. It is natural draft gas fired atmospheric. Even with that running 50% of the time at -20F outside it will easily heat my place. So I think your thoughts are correct on that. The flame can be adjusted for the reduced pressure and the capacity of the boiler is thereby reduced. Mine has run that way this entire time. So I think you can take away 30% of that boiler easily. If it were mine I sure would try doing the rest with timing. I would never let it hit pressure one time.

jumper

As much as I admire other guys I agree with PMJ. Old boilers were much bigger physically. So why can't a lesser burner fire this boiler? Better a multistage burner where all tubes fire to raise steam but then only some tubes if you can make one. Even if not not OP's idea is worth a try.

PMJ said:

I will point out one more time that these systems all originally had big boilers running in the middle of their capacities with the flame modulated in real time based on the actual demand. Because the boilers were big, a relatively small change in fire produced a larger percentage change relative to the piping, hence providing significant adjustment from relatively small damper changes.

To these systems we now attach one speed fire (high) burning straight until the tstat is satisfied with no regard whatsoever to the current demand. From a control perspective that is undeniably a giant step backwards, and one which has astonished me as an engineer from my first real contact with steam heat 30 years ago.

This observation remains steadfastly unpopular here. By all means continue your quest for the perfect single speed boiler.

JohnNY

This is the least valuable thread in the history of the internet.

ChrisJ

JohnNY said:

This is the least valuable thread in the history of the internet.

I promise you it's far from that.

  

EBEBRATT-Ed

The biggest problem that is being missed that the manufactures ships the boiler to fire a certain way. The boiler is tested to operate and work that way by the various testing labs......so they say.Altering that puts a contractor in a truckload of liability

I don't consider myself a wuss so I have altered some equipment in my day to make it work

But some on this forum that are not contractors and not doing this for a living are free to monkey with their boilers as they see fit. They are under no liability except for what they do in their own home. They are not 'offering their services to the public" and don't have to take out permits or carry liability insurance for the work they do.

Big difference

If all that was requires here was a 20% reduction in boiler capacity fine downfire it and take your chances.

We are talking about a boiler that is 300% oversized.

PMJ

EBEBRATT-Ed said:

The biggest problem that is being missed that the manufactures ships the boiler to fire a certain way. The boiler is tested to operate and work that way by the various testing labs......so they say.Altering that puts a contractor in a truckload of liability I don't consider myself a wuss so I have altered some equipment in my day to make it work But some on this forum that are not contractors and not doing this for a living are free to monkey with their boilers as they see fit. They are under no liability except for what they do in their own home. They are not 'offering their services to the public" and don't have to take out permits or carry liability insurance for the work they do. Big difference If all that was requires here was a 20% reduction in boiler capacity fine downfire it and take your chances. We are talking about a boiler that is 300% oversized.

Right. I do appreciate the legal liability. I wouldn't physically change the boiler. When you turn it on and off is a whole lot safer way to go. 

neilc

JohnNY said:

This is the least valuable thread in the history of the internet.

wait till I tell you to pipe a 3x larger main, and a train whistle honking vent, at it's end,
#smiling now

JohnNY

PMJ said:

EBEBRATT-Ed said:

The biggest problem that is being missed that the manufactures ships the boiler to fire a certain way. The boiler is tested to operate and work that way by the various testing labs......so they say.Altering that puts a contractor in a truckload of liability

I don't consider myself a wuss so I have altered some equipment in my day to make it work

But some on this forum that are not contractors and not doing this for a living are free to monkey with their boilers as they see fit. They are under no liability except for what they do in their own home. They are not 'offering their services to the public" and don't have to take out permits or carry liability insurance for the work they do.

Big difference

If all that was requires here was a 20% reduction in boiler capacity fine downfire it and take your chances.

We are talking about a boiler that is 300% oversized.

Right. I do appreciate the legal liability. I wouldn't physically change the boiler. When you turn it on and off is a whole lot safer way to go. 

So, the short cycling and pulses of too-much-steam-for-the-piping don't bother you? I'm just asking because when this boiler turns on, the air vents sound like leaf blowers and look like lawn sprinklers.
When a boiler overwhelms a piping system it's not easy to be around. Bad stuff happens. As it is, this 349 MBH boiler is down fired to 250MBH and it's still a short-cycling nightmare. Adding a vaporstat will be the opposite of helpful with that short cycling but I'm going to suggest it anyway just to test another issue I think may exist with a slow, buried return pipe. If I go any lower on my gas pressure I'm going to start having gas wander out of my burner tubes and I'll start condensing my flue gases. I'm not doing that.
Yes, I could engineer some comprehensive atmospheric burner system that fires only some of the tubes at a time based on steam pressure or heat exchanger temperature, etc, but why would I do that? The theory of available options vs the safety of the family and longevity of the device lead me down an admittedly well-beaten path.

PMJ

The short cycling usually referred to occurs after the system is fully warm and has hit pressure one time already. The burner turns off and basically immediately refires into a system still at cut in pressure single digit minutes after the end of the last burn. That I can see causing a lot of noisy vents.

 The control I would be recommending would not have allowed any pressure to develop in the first place, and by the size of the boiler here would not allow another firing within 20 minutes of the end of the previous one. The reheating of the mains required after 20 minutes off would consume much of the boiler capacity and be a very different  start up situation than rapid fire short cycles on pressure.

Most people are thinking about what to do after pressure is already reached.  Think about never allowing it in the first place. Pressure isn't ever needed and was never part of the original plan.

JohnNY

@PMJ Well, then, what's the control you're talking about?
But keep in mind, this system hits the pressuretrol setting before the mains are full of steam. Giant boiler. Badly piped header. Too small pipes. It doesn't take more than that.

PMJ

Sounds like a lot of issues. If the header pipes are so small that steam can't even get out of the boiler without hitting pressure the situation is pretty extreme. How much pressure is required to heat and fill a cold main? Maybe that much in the header has to be allowed without a stop then until you get steam everywhere.

What you describe is like partially closing the main valve over my boiler. That would make pressure in my header for sure on every firing. Not much choice in that case but to allow the pressure. It would be required to feed the rest of the system. How much is it really on a cold fill?

Hap_Hazzard

JohnNY said:

…maybe you can explain this to me like I'm not the sharpest bulb in the tree.

Mr. Cataneo, sir, you mix a mean metaphor. I love it!

ethicalpaul

I think you know this, but you keep asking it, and no one is answering it, so I will answer it.

A derated (down fired) valve and/or burner will definitely result in less steam production and will help your oversized situation. I think you already said your boiler is currently derated. I'm confident you'd see the cycling would be even worse if you restored its rating.

I know you know there are limits to the derating and that the combustion should be analyzed after any change to the gas pressure or burner situation.