Is my boiler too big?

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  #1  
Old 01-11-14, 07:13 AM
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Is my boiler too big?

I bought a house that has a hand fed coal fired hot water boiler. It is a 1957 American Standard Severn model 21. I'm not sure what the btu rating is but data from Beacon boiler reference book is, fire box 18" X 21". The fire box is 14.25" from grate to bottom of loading door. Rating for water is 725 at heat emission of 150 btu/hr. So I think that puts the boiler at 108750 btu/hr but I'm not sure if that's right.
The boiler gravity feeds to 13 cast iron 4 tube radiators, The 2 header pipes are 2 1/2" with 1" feeds to each downstairs radiator and 3/4" feeds to the upstairs.
Based on Cast Iron Radiator Heating Capacity tables at 170 btu/hr per section the radiation totals are, First floor 297 sq. ft. or 50490 btu. Second floor 247 sq. ft. or 41990 btu for a total of 544 sq. ft. or 92480 btu.

At 8 degrees F outside, 72 inside the boiler water temp runs around 107 degrees and the ash pit draft regulator is basically closed, the over fire draft is about half open to burn off the volatiles from the bituminous coal I'm burning. I've gone over the boiler and sealed all air leaks and made tight seals for the doors. Doing this improved the situation but wasn't enough.

My problem is, as the outside temperature rises I have little room to reduce the fire so the house gets very warm. Even closing the ash pit draft damper, opening the the over fire draft wide open, forcing the barometric damper wide open and closing the manual pipe damper as much as possible to check the fire is not enough.
I believe that with all the energy saving upgrades over the years the boiler is now to large for the building heat loss.

I was thinking of reducing the max btu of the boiler by making the fire box smaller. As a trial I thought of covering one or more of the 5 grates with fire bricks to effectively reduce the open area. If it makes an improvement, I would build a permanent wall out of a suitable material.
Think it will work? Worth trying? Thoughts?
Anyone know if there a formula to calculate the max btu/hr based on fire box size and open area?
 
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  #2  
Old 01-11-14, 11:39 AM
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Here are pics of the grates...There are 5 of them.
 
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  #3  
Old 01-11-14, 01:49 PM
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I have some experience with large, coal-fired power boilers, but my primary experience with hand-fired residential boilers was carrying out ash and clinkers as a kid.

There are two ways to cut back the heat output that I'm aware of. First, try to anticipate the next day's outside temperature, and start reducing the feeding of coal.

You can also bank the fire by moving the fuel over to the side of the grates, piling it up, and reducing the inlet air damper - meanwhile, crack open some windows. Our old late 1940s coal burner had a chain that could be operated from the upstairs to adjust the damper.

Hand-fired boilers have a slow response time. Stokers, a little faster. Oil, natural, gas, and propane boilers are much more responsive. Electric heat is obviously very controllable.

If your beast of a boiler has been there since 1957, can you contact the former owners for their experience and suggestions?

At this point, I wouldn't worry too much about boiler efficiency or whether your boiler is too large (which it probably is). It is what it is.
 
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Old 01-11-14, 01:51 PM
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Rating for water is 725 at heat emission of 150 btu/hr.
Not sure what this means? Is 725 the EDR rating?

Your system is still GRAVITY?

boiler water temp runs around 107 degrees
Really? that low? How cool is the return?

Have you measured the flue gas temperature?

Cast Iron Radiator Heating Capacity tables at 170 btu/hr per section
Not at 107F it's not!

opening the the over fire draft wide open, forcing the barometric damper wide open and closing the manual pipe damper as much as possible to check the fire is not enough.
Sounds to me like a recipe for caking coal tar onto the inside of the chimney... I think you're heading for trouble.

When was the last time the chimney was inspected?

I can't really comment on the fire brick idea... just don't know.
 
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Old 01-11-14, 03:11 PM
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Most of the old gravity coal systems were designed for 100f - 120f water temp. Beyond that the windows would start going up.
To determine boiler size do a heat loss.
Btw most use 150 btu's for calculating hot water output from the old cast iron rads since boilers today are cold start. The 170 number was when them boilers were maintaining water temperature.
 
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Old 01-11-14, 03:59 PM
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@gilmorrie
I did the ash clinker carrying thing as a kid too.
The draft is only open 1mm, at 8 degrees F outside, 72 inside the boiler water temp around 107 degrees. Not much room to close it.
 
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Old 01-11-14, 04:20 PM
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I'd try banking the fire and opening windows. I've heard of covering the banked fire with sand, but I don't remember it at home. You don't want the fire to go completely out because of the time require to relight it.

The number of people that show up currently on this forum with modern hand-firing experience is between slim and none. I assume that it is more common in your area of Canada?
 
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Old 01-11-14, 04:25 PM
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Not sure what this means? Is 725 the EDR rating?
Your system is still GRAVITY?
725 is the EDR rating
Yep, still gravity, it's pumped for free...no pumps or controls to fail.
Not at 107F it's not!
The table is at 170 btu/hr
http://www.columbiaheatingsupply.com...ty%20Guide.pdf
When was the last time the chimney was inspected?
I burn wood in the boiler during milder weather, I've cleaned it 3 times this season, no creosote little soot. Creosote seems to collect on the inner surfaces of the firebox where it flakes off and burns.
Really? that low? How cool is the return?
Have you measured the flue gas temperature?
Never measured return, it's just a little cooler to the touch than the supply.
Flue gas temp was 315 at 8 deg F outside, 107 degree boiler water. It goes much lower when I idle the fire.
Sounds to me like a recipe for caking coal tar onto the inside of the chimney... I think you're heading for trouble.
No signs of a problem in the heat exchanger or chimney, yet anyway. The methodology I use to is as detailed in the original instructions for the boiler.
 

Last edited by imaddicted2u; 01-11-14 at 04:42 PM.
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Old 01-11-14, 04:36 PM
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Btw most use 150 btu's for calculating hot water output from the old cast iron rads since boilers today are cold start. The 170 number was when them boilers were maintaining water temperature.
It was just convenient to use the values from table.
To determine boiler size do a heat loss.
Do you mean to determine the right size boiler or the size of the current boiler? Not sure how to do either.
 
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Old 01-11-14, 04:44 PM
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deleted - posted by accident. ==================================================
 

Last edited by gilmorrie; 01-11-14 at 05:03 PM.
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Old 01-11-14, 04:48 PM
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The table is at 170 btu/hr
As mentioned above (#2) and as the following chart shows a radiator's heat output is
directly related to its temperature:
Right...

At say 110F AWT you are talking about THIRTY BTUH/SF EDR.
 
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Old 01-11-14, 05:08 PM
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Right...

At say 110F AWT you are talking about THIRTY BTUH/SF EDR.
Thanks missed that.
I used 30 in my spreadsheet calcs instead of 170...Do these numbers make sense? Seems awfully low. Do they equate to boiler output?
First floor = 8910 btu/hr
Second floor = 7410 btu/hr
Total for house = 16320 btu/hr
 
  #13  
Old 01-11-14, 05:19 PM
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I'd try banking the fire and opening windows. I've heard of covering the banked fire with sand, but I don't remember it at home. You don't want the fire to go completely out because of the time require to relight it.
I've tried to bank the fire didn't seem to help. I'll try again now that I have all the tramp air sealed. The only place the fire can get air now is through the dampers. Control is much better but I can't turn it down enough. Even when the fire is checked the coal continues to smolder and produce heat. That's why I think reducing the size of the firebox might be the answer.

The number of people that show up currently on this forum with modern hand-firing experience is between slim and none. I assume that it is more common in your area of Canada?
Not nearly as many of us burning coal now, since all the mines closed. Coal has become more difficult to get and the quality is lower. The coal I have is not screened so is everything from huge lumps to dust. Coal is being dug, by enterprising individuals, from near the surface.
 
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Old 01-11-14, 05:49 PM
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Control is much better but I can't turn it down enough. Even when the fire is checked the coal continues to smolder and produce heat. That's why I think reducing the size of the firebox might be the answer.
How can a smaller firebox put out much less heat than the heat content of the coal you are shoveling into it? Cut back on the stoking rate and bank the fire when it's too warm inside. And open windows, when all else fails.

I can't turn down enough.
I don't think you will, until you try to follow some of the suggestions that have been offered.

Part of the problem is that you bought an old house with a hand-fired coal boiler. Such heating systems have been replaced hereabouts in the U.S. with natural gas, propane, or fuel-oil decades ago. 65 years ago our town was 100% coal heat. By 50 years-ago, it was zero-percent coal heat. Nobody now, even out in rural areas, where natural gas is unavailable, burns coal. Even coal-fired electric power plants in the Midwest are shutting down. Coal heating is a major pain and it is no longer cheap. How much are you paying per MBtu?

Is electric power in Canada cheap enough to be an alternative?
 
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Old 01-11-14, 06:06 PM
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How can a smaller firebox put out much less heat the than the heat content of the coal you are shoveling into it? Cut back on the stoking rate and bank the fire when it's too warm inside. And open windows, when all else fails.
I'm pretty sure the firing rate of a coal fired stove or boiler is based on the floor area of the firebox and the open area of the grates. I haven't found the formula yet.
Is electric power in Canada cheap enough to be an alternative?
Electric heat costs more than oil for heat here.
I have an oil boiler too.
This is my house:
 
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Old 01-11-14, 06:11 PM
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How much are you paying per MBtu?
Coal is $160 a ton delivered so around $8.33 Mbtu, Electric heat costs $41.76 Mbtu and oil $40.48 Mbtu
 

Last edited by imaddicted2u; 01-11-14 at 06:29 PM.
  #17  
Old 01-11-14, 07:00 PM
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I'm pretty sure the firing rate of a coal fired stove or boiler is based on the floor area of the firebox and the open area of the grates.
The firing rate is the gross Btu/hr input released by the fuel, which is proportional to the fuel feeding rate. The net Btu/hr delivered to the circulating water is the boiler input multiplied by the boiler efficiency, maybe in your case, 50% or less. (For a gas-fired, non-condensing boiler, the maximum efficiency is 80%, due to stack losses.

If the boiler's firing rate, i.e., the fuel input, is decreased, the stack temps and losses should be lower and the boiler efficiency higher.
 
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Old 01-11-14, 07:35 PM
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The firing rate is the gross Btu/hr input released by the fuel, which is proportional to the fuel feeding rate. The net Btu/hr delivered to the circulating water is the boiler input multiplied by the boiler efficiency, maybe in your case, 50% or less. (For a gas-fired, non-condensing boiler, the maximum efficiency is 80%, due to stack losses.

If the boiler's firing rate, i.e., the fuel input, is decreased, the stack temps and losses should be lower and the boiler efficiency higher.
Agreed, but doesn't coal burn better and longer with a deep coal bed. Which is part of my reasoning behind making the firebox smaller. At the same time it would achieve the lower fuel input you are suggesting. Anytime I've thrown in a few shovels full it burned up quickly.
 
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Old 01-12-14, 01:35 AM
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Yes, your boiler is too large and yes, if you reduce the grate area you can burn less fuel. The smaller the fire the less heat released to the boiler.
...but doesn't coal burn better and longer with a deep coal bed
Depends upon the coal. Some coal, like Pennsylvania anthracite will burn in a deep bed but some sub bituminous coal will barely burn at at all no matter what you do.
 
  #20  
Old 01-12-14, 12:22 PM
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Heat loss calc'd

First floor = 19142 btu/hr
Second floor = 19532 btu/hr
Basement = 17400 btu.hr

Total for dwelling = 56074 btu/hr
 
  #21  
Old 01-12-14, 11:48 PM
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Burning wood in the boiler this morning.
Boiler water=95F Flue temp=320F Outside temp=35.6F Inside temp=73.5F
An hour and a half later:
Boiler water=98F Flue temp=310F Outside temp=35.6F Inside temp=74.4F
 

Last edited by imaddicted2u; 01-13-14 at 01:29 AM.
  #22  
Old 01-13-14, 12:44 AM
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Corrected:
Heat loss calc'd

First floor = 19142 btu/hr, 1180 Square feet, 16.2 btu/sq ft
Second floor = 19532 btu/hr, 1050 Square feet, 18.6 btu/sq ft
Basement = 15825 btu.hr, 1050 Square feet, 15.0 btu/sq ft

Total for dwelling = 54499 btu/hr, 3280 Square feet, 16.6 btu/sq ft
 
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