can I regulate steam boiler...

r. perry

Is there any danger in trying to regulate an over-sized steam boiler by throttling back the gas using the gas shutoff valve as a regulator?  Will I throw the combustion mix way off?  Will it create a carbon monoxide danger?  Will it create a "dirty burn"?  I've closed off the gas valve by about one-half and the flame still looks "good", no yellow, etc., but without a variety of testing equipment, I'm just guessing at all of it.  I first started doing this when skimming the boiler, to keep the boiler from steaming and discovered that if I throttle back the gas, the entire system works so much better, which has led me to believe the boiler is too big, has far more capacity than needed.  THANKS!! in advance for any info anyone cares to offer.  

Charlie from wmass

Bad idea

Hire a pro and do it correctly. I have seen this done and it does not end well.

Tim McElwain

It is never a good idea

to reduce the input to a design boiler system. The combustion efficiency is directly affected. The throttling of the gas valve can also cause delayed ignition.



If you are planning on replacing the boiler, size the boiler to the heat loss of the house or square feet of radiation otherwise leave the present settings alone. Have a combustion analysis done to insure safe and efficient operation of the existing system.

Tim McElwain

It is never a good idea

to reduce the input to a design boiler system. The combustion efficiency is directly affected. The throttling of the gas valve can also cause delayed ignition.



If you are planning on replacing the boiler, size the boiler to the heat loss of the house or square feet of radiation otherwise leave the present settings alone. Have a combustion analysis done to insure safe and efficient operation of the existing system.

r. perry

regulating boiler....

Thanks for all the responses and sharing your professional opinions.  Very grateful and most appreciated.  Again, Thanks men. 

nicholas bonham-carter

modulating burner for steam

the manufacturer of your boiler may have as an option, a high-low-high burner. this would usually be controlled by a vaporstat to keep the pressure at just a few ounces, by switching the burner back and forth between high and low. typically, the boiler would start in high, and when the primary cut-out pressure had been obtained, the burner would switch to low fire.at least this would be a setup approved by the company who made the boiler.

for instance, peerless list a "mod-u-pac" option to do that, and i am sure others do as well--nbc

alcraig

I'm curious...

I've read a lot on the Wall about high-low or modulated gas burners setups, and some custom setups by pros like Gerry Gill.

From what I researched, I understand that a gas burner adjusts it's firing rate by the amount of gas pressure, but whenever the pressure is adjusted, doesn't the air setting need tuning to make combustion efficiency?

For example, say the high rate is setup and the air settings are properly set for good CO/CO2...what happens to those settings when the pressure drops to low fire? How are the air settings matching this change?



Al

Mark Eatherton

Not usually on atmospheric boilers...

If it were a commercial multipass boiler, they DO adjust the air with the fuel.



Residential atmospheric boilers may drop the gas pressure, but that usually ends up causing excess air in the combustion process which drops the combustion efficiency.



I've never been a big fan of reducing the gas pressure on these boilers, and have seen quite a few copper fin tube boilers that were done that way and died an early unnecessary death due to it.



Cast iron is probably less susceptible to the corrosive tendencies of copper fin tube, but they are still subject to the principles of combustion, that dictate an optimum air to fuel ration, and when you can't adjust the air, it upsets the combustion process.



I have reduced the gas usage on buildings by 20% just by getting rid of the modutrol gas valve.



Sounds like a good idea on the outside, but causes inefficiency in reality.



ME

Charlie from wmass

we have down fired boilers

But we use other methods than dropping pressure on the larger end of the atmospheric boilers we have due to budgets of the building owners down fired boilers when the boiler was grossly over sized. We do this by changing the gas train.

jim_72

Change the train

Charlie could you explain under what example  you have had success by changing the train .If I had a over sized  5 section boiler with a 8 tube train and rather then plug a orifice chose to install a 6 tube train from the same manufacture's 4 section do I not face the same CO/combustion shortcomings as just plugging and orifice.Thanks Jim   

Tim McElwain

Make sure in this discussion

we distinguish between commercial industrial applications and residential. The boilers used in commercial applications many times have a very sophisticated "gas train" with "mod Motors" and double block and bleed valves. This allows variables in firing rate, although I am not always convinced based on combustion testing that they are as efficient as some would think.



Atmospheric residential boilers and furnaces which have a fixed combustion chamber with fixed air openings do not lend themselves to run very efficiently at reduced (down firing ) setups. As was already mentioned you have excessive "excess air" on the low end which lowers efficiency. Two stage setups on most systems do not work well unless the boiler or furnace external piping and duct work are set up correctly.



What some are doing on steam boilers is a different animal. It has recently been shown on many modern two stage systems that firing the boiler or furnace initially on high fire and then going to a low fire if there is not a high fire demand is a better way to go. You establish some delta T and create some drafting in the flue which when then a drop to low fire occurs will give better combustion. Keep in mind these two stage systems on steam are being set up on new boilers which tend to make steam must faster. They are also using the pressuretrol (vaporstat) to control low fire if I am not mistaken.



Now lets us consider the modern designed Mod/Con boiler well to quote my friend Jim Davis when Ii recently ask him this question he states,



"The fact that these new boilers are low mass, drastically improves their operating and stand-by efficiency.  Most that I have tested run with very good O2 readings, 3% to 5%. Concerned that their flue temperature can be cooler than the water temperature they are operating because they are just a little too underfired.  They seem to hold these readings when they modulate which makes their modulation not as inefficient as furnaces.  They still lose radiant heat transfer by reducing the flame size but because they maintain slower vent motors their losses are 10% to 20% versus 30% to 40%.  Even running bad they would save 25% to 30% over any cast iron boiler."



In other words when a piece of equipment is designed from the ground up to operate at varying inputs it can be a more efficient way to go.

 



I am not a big fan of plugging off burners or changing orifices to reduce input on designed residential gas boilers or furnaces.

 

Charlie from wmass

In a word Jim No

This being a public forum information can be miss used when people know enough to be dangerous. I am simply saying it can be done to a certain degree of function. These were done back in the dark ages when gas boilers were still tuned by eye and we did not test them for efficiency. But they saved fuel and ran better as to steam production than before we got there.