BTU's or steam chest size?

David Hohengasser

I have and HB Smith G210 gas boiler (circa 1978). It is six section with 5 burner tunes. The connected radiation is 343 SFT. The boiler is oversized producing 470 SFT of steam. I will be replacing the boiler in the next few years. This year, I ran a test to determine if 280 SFT of steam will be adequate. I removed the two outer burner tubes to down fire the input. So according to the manual, I have reduced the input from 187,000 BTU to 112,500 BTU. The manual shows it should be producing 280 SFT of steam. All the piping is very well insulated including the wet returns. The boiler has produced enough heat for the entire season, even on -14 degree days and with the reduced amount of steam. I am trying to determine the size on the replacement boiler I should purchase. My question, is the boiler producing 280 SFT of steam based on the BTU input, or does the size of the steam chest, (which is still six section), determine the steam output? As a side note, reducing the input has saved 10% gas usage, based on CCF of gas per degree day.

Mad Dog_2

Interesting experiment

But have you backed this up a combustion test to make sure you are operating in safe limits? Have you clocked the gas meter at the same time? Older boiler relied on a large steam chest. The new header piping with accomplish that if it is done right. I hope you are a full-time hvac or plumber to be trying these things. MaD DOG

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The Boiler Dr.

Another point to consider

By removing any of the burners you have created "cold spots" within the appliance. This will create stress on the heat exchanger and could lead to stress crack failures. Mad Dog is correct in questioning the flue gas concentrations as you have drastically changed the manufactured design

Unknown

Removing burners

I tried that on my W/M C/I H/W boiler as it was grossly oversized.
Also tried a V5155 modulating gas valve, vent dampers & different chimney liners.
Clocked the meter & of course consumption had dropped, but analyzer readings went NUTS.
Long story short, you can`t teach an oversized beast new tricks,, in my case I switched to a modcon.
Different sizes are manufactured complete for a legitimate reason.

Dave

ttekushan_3

on the few occasions

that i have had to do this, I put plugs where the gas orifices are, but ALSO used non-asbestos calcium silicate insulating board to baffle the top of the space between the sections where burners were deactivated. This rigid insulation is the same as the base insulation of the boiler. Access is under the flue collector. Placement of short but long pieces is space between two sections, resting on top of the pins. Insulation sticking too far up into the flue collector from the sections could possibly fall over and block active sections. The flue collector outlet can be narrowed on each side with sheet metal the width of one section.

This is the only way to get a handle on the combustion, and control heat loss through the unused sections. What I think happens if you only disable burners and do nothing else, is that secondary air is diverted uselessly through the unused combustion space, distorting the flame patterns. So excess air is wrong and CO often rises due to unplanned flame impingement, among other things.

In review, the draft on those sections must be stopped also. Disclaimer for the homeowner: don't do this. For any success, you have to be careful, know what you are doing, and be doing so with the aid of a combustion analyzer that you know how to use. You also need the correct materials.

All that being said, I think you can go to the smaller boiler based on your experiment, but choose one with enough water content so you can keep the steam dry and prevent boiler flooding when all the condensate eventually returns to the boiler. A new smith is a good candidate.

BTW, since you only disabled the burners, your savings number is conservative. You will likely be pleasantly surprised when the new boiler saves you more than the 10%.

-Terry

David Hohengasser

Answers

Thank you for the replies. I am not a full time HVAC or Plumber, but my father and brother were, so I am familiar with the trade and do all my own work. Thanks to the great help here on the wall, in 2005 I corrected many problems with my near boiler piping and installed a dropped header, vapostat, main vents, etc. The system works perfectly with no noise. These corrections saved about 20% in fuel. I have placed all the gas usage and degree days since 2000 on a spreadsheet and used that to determine the usage, CCF per degree day. I have not clocked the meter.

I was aware that the removal of the burners would cause cold spots after talking to an engineer at Dunkirk. Since I plan on replacing the boiler I decided to try the test. The results show me that 280 SFT of steam will adequately supply the 343 SFT of radiation. Do you agree? I plan on replacing the two burner tubes for next year as the replacement boiler is not in the budget for this year. This will also confirm the 10% savings if/when the usage goes back up 10%.

bob young

new concept

{ 280 SFT of steam will adequately supply the 343 SFT of radiation. } if what you say is proven & correct , then a lot of pro's , myself included , have been doing things completely wrong. i wonder how the top steam experts will react to this discovery of yours. like gilda radner used to say " its always something ." sounds like a conspirosy to make us install boilers larger than necessary.

The Steam Whisperer (Formerly Boilerpro)

I am not surprised at your results.....

both with the use of an "undersized boiler" and your fuel savings. I have seen this many times before and if the system is properly balanced can work very well. Do you have a one pipe system or two pipe steam (or vapor). This is easiest to do with vapor systems, them two pipe steam and hardest on one pipe....you need small adjustable vents on all the rads. Google "Frank Gifford" and read some of his articles about the subject. I have also recently installed a boiler with his "Gifford Loop" and it really seems to stabilize the boiler water line.

I blocked off 6 out of 9 burners on my old hot water boiler to reduce the input from 270,000 to 90,000 and closed up the tops of the openings between the sections above the missing burners and saw a similiar savings. You get a first hand feel for the wastefulness of oversizing. I have a condensor with a secondary heat recovery heat exhanger now.

Boilerpro

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Steve Garson_2

The EDR capacity of the boiler is based on the assumption of a 33% pickup factor. If the pickup factor is lower, because of less steel, insulated returns and other "not typical" factors, that would alter the published boiler capacity. But if you're not following the specs as a contractor, if it doesn't work, it's your fault.

scrook_3

SF radiation...

If it REALLY is only 280 SF (67,200 BTU/hr) net output (89,376 BTU/hr gross output w/ 1.33 steam pick up factor), about 112,000 BTU/hr input, than either:

1. There's less than 343 SF of connected radiation (e.g. metallic painted radiators full of dust under radiator covers).

2. The radiation is quite over sized (certainly possible) and significantly under vented to compensate, so that the full capacity of the radiation is never used. Are radiators only heated 3/4 across when T-stat's satisfied?

3. The properties of saturated steam have suddenly changed, which seems highly unlikely.

Speaking of possibility 3 though: you're not at high altitude? In Denver 1 PSIg steam's only 13.1 PSIa & 206°F vs. 15.7 PSIa and 215°F at sea level -- so your radiation will appear to be a little smaller, say ~315 vs. 343 SF. (Beware of boiler derating issues too, however, but that's another topic!)

What's current combustion efficiency & is condensation occurring? What's boiler input clock at on the meter? What's Current Therms/CCF of the NG being supplied? -- it's NG not propane, right?

Mad Dog_2

Also, very interesting

Bob's right though, you reallly have to be careful, the analyser is a must. Sometimes you are better off with new smaller gas orifices. Mad Dog

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David Hohengasser

More answers to your questions

The boiler is NG and located in western Massachusetts. I do not know how many BTU's per CCF is supplied by Berkshire Gas. The boiler was adjusted professionally last year with the original 5 burner tubes. I had asked the Dunkirk engineer about using smaller orifices and he said no. He said the burner tubes were designed for 35,000 BTU's, so it would not fire properly. This would be my preferred method to down fire the boiler. Does anyone else have an opinion/experience on changing the orifice size? This is a one pipe system.

There are no radiator covers. The radiation may be oversized as the envelope has been tightened with full insulation, R-38 in the attic and replacement windows. This is a 5 unit apartment building. The thermostat is located in an older gentlemans' apartment and he keeps it about 80 degrees. All the other radiator have Braukmann TRV's to control the temperture and not overheat. The radiators only heat about 3/4 of the way across. Does this mean the actual connected EDR is 3/4 of 343 SFT? The boiler is set to cycle 2-3 times per hour. (2 when it is warmer and 3 when it is colder). This has been done by adjusting the anticipator on the old round Honeywell thermostat. Is this efficient? So the system never really cools down and the built in pick-up factor for the piping in the EDR is not fully used ( I think). Maybe this explains why 280 SFT works.

This is a short term experiment/test and I will be replacing the burner tubes before the next heating season. I want to buy the smallest boiler possible. I didn't know the savings from the oversize boiler would be this significant.