De-rating gas hot water boilers

Jim Davis_3

Just another note on your boiler. The output of your boiler is based on a calculation versus actually measuring GPM X Delta T X 8.33 X 60. You will be lucky under perfect conditions for the boiler to deliver 72,000 BTUs, therefore it is size correct.

libjim

De-rating gas hot water boilers

We had a new addition put on our home. The boiler the contractor put in was too large . . . our home is 1800 sq. ft. and our heated garage is 800 sq. ft. The new Bryant boiler is rated at input 105,000 BTU/hr, output 84,000 BTU/hr. The estimated BTU required for our home/garage should be about 68,000 BTU/hr. I want to derate the boiler to save heating costs. I was told that it is only cost efficient to derate a boiler by 10%. Should I be reducing the maniford gas pressure of 3.5 inches water by 10%, setting it to 3.15 inches water, OR should the GAS FLOW be reduced by 10%?

Thanks for any input!

Empire_2

Was heat loss performed?

Does not sound out of line. Your total of 2600 sq. ft. what program did you use? Also what area are you in? Much more info needed to formulate an opinion.

Mike T.

Unknown

First question,

are you a professional heating and combustion expert? If not you should not be fooling with gas pressures or derating.

You need to have a pro look at this system and determine exactly what needs to be done. Derating is really not a good idea as boilers are designed to operate at there maximuim firing rate. In addition a combustion analysis needs to be done on the boiler.

Do you get hot water off the boiler, or is it an indirect. In either case the boiler may have been sized to compensate for the domestic load.

Who did this estimated BTU heat loss on the house?

Glen

knocking back pressure -

by 10% - does not decrease input by 10%. It's a lot more complicated than that - please read Timmie's comments. And I must ask - how did you come to the conclusion that the boiler is too large? Do you have BB, radiant, air handler etc??? type of boiler? age of home and building envelope upgrades??? Please view your home as a system - focusing on gas pressure is the very least of the considerations. And as it is stated - it is not an HO doable adjustment. Unless you happen to be a heating professional.

Lurkin' Murkin'

Let me try to help...

Depending on the boiler design and manufacturer's allowances, the safer way is to remove a burner tube (outer) or two, and replace the orifices that are not used with plugs. Lowering the gas pressure is not the safe way. But check with the manufacturer first, and the combustion after.

Bob Forand

Just learned

Went to gas class today and learned that when you change/plug orifices, that you must also look at the chimney size now. It is possible by changing the output that you change the dynamics of your chimney. As everyone has said "there is a lot more to it than that" Good luck

Unknown

No, No, No a thousand times NO!

Do not I repeat do not plug off burners by removing orifices and plugging the manifold.Who on earth is teaching people all this stuff.

Unknown

What gas class was that?

This is not something an instructor should be teaching. We do not do that on gas equipment ever!!!!!!

Glen

State your school -

instructors name and location. Somebody needs to talk to him. Tim was not strong enough in his warning - plugging orifice outlets on a manifold is just plain dumb - unsafe, against any code & manufacturers advice that I am aware of. Our countries may be moving towards a unified gas code - but we should be striving for a unified trades program and curriculum. A common curriculum will not combat foolish advice - but it will help.

Lurkin' Murkin'

Please elaborate...

Some boilers are designed to accomodate lower inputs by plugging orifices and removing certain burners. If this is not a field approved modification, or there are other modifications or considerations required, I apologize for the post. At least we can discuss the whys and why nots, and learn here. If the heat exchanger design and secondary air entrance allow for proper ignition, combustion, and burnt gas distribution, why is this the wrong method? Thanks!

Joannie_15

Thank You, Tim!

I can't tell you how many times people ask to do that on our units.

Thanks for that response. People see us as being overcautious or uncooperative when we tell them that it's NOT a good idea to do that.

Joannie

Laars Heating Systems

Unknown

Even in cast iron boilers...

The baffles are sized to the input, and even the flue collector can be a different vent size.

That they use the same manifold for three different size boilers isn't an indication that the rest of the boiler's parts are identical.

Noel

Lurkin' Murkin'

You're right, Noel. I should have known that. Thanks!

Brendan_4

Noel

So with a Laars minitherm two stage gas valve on low fire, Is this not the same imbalance with regards to the baffles and flue? is it not the same as plugging a burner rack? just wondering thanks.

Unknown

I guess the difference is

Whether it had been designed and tested for a single firing rate, with different components designed for the alternate firing rate; or if it was designed and tested as a multiple firing rate boiler. Most companies offer both fixed rate and modulating appliances. It comes down to the testing and listing of the appliance in question.

Not tested only means not tested. It doesn't infer that other methods will or will not work. It only means that it isn't tested and listed to operate another way. Some of it comes down to passing testing.

Take it only for what it is.

Noel

GLENN_14

NEVER NEVER

I AGREE 100%. NEVER SHOULD YOU UNDER ANY CIRCUMSTANCE PLUG A BURNER AS A MEANS FOR DE RATING... THERE IS SO MUCH MISINFORMATION OUT THERE.... PLEASE NEVER...never... NEVER PLUG A BURNER OFF TO DERATE A SYSTEM

kamney

One of the big problems with plugging orifices in a sectional cast iron boiler is uneven flue pass temperatures and uneven expansion. Bad for nipples and gaskets.

Steamhead (in transit)

LISTEN TO TIM!!

Not sure who's teaching some of this stuff but it's WRONG!

Tim is one of the best in the industry. If he says this is wrong, it's wrong.

We could avoid a lot of this if people didn't install so many oversized boilers......

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JohnNY

Plugging off burners.

*I've never done it*... though I've been tempted.

But for curiosity's sake, why is it ok for an appliance like the Buderus (I think 334X) with 2 separate and independent burner manifold assemblies, yet a single heat exchanger and fixed draft properties, to operate so effectively?

As for how plugging off a burner affects the chimney: please explain how that effect differs from the intermittent firing of a water heater sharing a boiler's flue.
What I mean is, chimneys rarely see fixed usage and there is a pretty wide berth where safely matching chimney size and BTU input is concerned.


This is not to challenge anybody, just legitimate questions.



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tim smith

JohnNY, I have pndered this also

It has always been a rule of thumb per our mechanical codes to not exceed a 4 to 1 sizing of the draft connector to the chimney cross sectional area. It has always been overlooked with the water heaters as long as the main gas user connected to chimney does not exceed this sizing. I have over the years found many old chimney's that don't draft very well when just water heater is on due to this sizing. Just footnote, Tim
Regarding plugging off orifices, years ago we would plug off a orifice in an emergency case if a burner was corroded to a point that it was not safe, as long as cross lighting could still occur and other burners were ok for firing & combustion still ok. This was just for temporary until we could get replacement burner tubes. Even utility would do this back then. Tim

Jim Davis_3

Plugging burners has been done on furnaces for years but each heat exchangers is independent of each other. Boilers is is not recommended because the sections are joined together and cause stress. However I have seen manufactured boilers with two stage burners that fire in half the sections in low fire and the rest in high fire. For years I have recommended the low fire be jumped out immediately. Mistakes have to be manufactured in, they are not allowed to be field fabricated.

As far as flue sizing it would be nice if someone would make up their mind. We stage and modulate boilers in the same size flue all the time. And no boiler was actually tested with the flue it ends up being installed on. If blicking orifices affects heat exchangers and venting, staging and modulating affects it evens more. Commercial jobs have 5, 6, 7, 8, or more appliances sharing one big flue and they all operate independently and have for years. Yet one little water heater and furnace and "oh my!"

bob_46

Orifice

What if you pretend you live in Leadville at 10,000 ft.?

Unknown

A resurrected posting

This is a resurrected posting from January 15, 2008 in case you missed that.

With boilers and furnaces uneven heat distribution due to plugging off burners can result in damage. It has been my experience in servicing thousands of boilers and furnaces that unused sections tend to corrode and deteriorate faster due to accumulation of condensate that typically would have remained vapor and been removed with flue gases. The uneven heat is the cause of this.

On Furnaces it is an ignition problem. Each burner is into an individual section of a typical clamshell heat exchanger. This is a typical flame roll out problem that all furnaces tend to have. That is when all burners are in use. The flame roll out is compounded when you happen to plug of a burner or two.

As for the Buderus with separate burner manifolds into a single heat exchanger this is design specific for that type of operation. We had dual manifold burners in the past used on special heat exchangers designed for that purpose.

As for boilers/furnaces/water heaters into flues together. When they all have pilots then some Delta “T” is created by just the pilots operating. When the call for heat or hot water comes there is a warm chimney, which has a jump on creating draft. With the advent of spark ignition, direct spark ignition and hot surface ignition they are firing into a typically cold flue and wet time is definitely increased. That along with the reduced heat loss up the chimney causes possible condensation problems and poor drafting. When this occurs the water heaters are often not able to create sufficient initial draft when first starting up and have long spill times.

I am totally against water heaters and fan assisted into the same common vent connector. I know the code allows it but it has been my experience that the fan assisted appliance will actually cause flue gases to blow out the draft hood of the natural draft water heater. That along with other problems this creates. I know the NFPA 54 code allows it but bottom line it has to work!!!

NFPA 54 has a 7 times rule which states that the common vent cannot exceed 7 times the square inch area of the smallest flue.

Yes we at the gas company would plug one burner in an overnight emergency. That was as long as the burners being plugged were not on either side of the pilot. In that case we left it shut off until repairs could be made. This was also done when cracked heat exchanger in one section was encountered, that was until the furnace could be replaced. In all cases it had to light off safely.

Plumdog_2

Another matter entirely

De-rating for altitude is not the same as plugging a couple gas orifii to make your heating appliance have less output because it is too big to begin with.