01-27-11, 09:09 PM | #11 | |
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My only reason to be here is to share/aquire knowdlege freely. BTW the op mentioned he had a coal boiler converted to gas. The combustion efficiency can be as low as 40% to 60%. thats means he's loosing 40 to 60 percent of the heat out the flue. Thats not good when modern boiler efficiency for gas is around 90%. A slight change in the fuel /air ratio can drop combustion efficiency 10% or even 20%. A burner properly burning the fuel can have 0 to no more than 5 ppm CO output. Just a slight change in the fuel or air can send carbon monoxide readings to 500,1000 or more ppm and that can't be preceived without the proper equiptment. Thats incomplete combustion. Not only is it dangerious but wastes fuel also. All furnaces/boilers also require "Excess air" 5% to 15% for oil and 5% to 10% for gas. Now excess air is just as is sounds,air above what complete combution requires........so why is it needed,well atmospheric conditions change. It's essential for safe operation. As your excess air percentages goes higer,your combustion efficiency drops. So there are trade offs. But no one wants to sacrifce their safety for a couple percentage points of efficiency. Walt |
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01-28-11, 05:24 AM | #12 | ||
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Quote:
Thank you for making sure that safety doesn't get omitted. Quote:
EDIT: I've replaced 'furnace' with 'boiler' in my previous posts. As for "excess air", the boiler is in the basement which is more than drafty - there is a fist-sized hole in the wall for fresh air, plus the garage next to the basement has cracks in the door that you can easily see through. I have no plans to seal up the drafts until the boiler is replaced with higher efficiency model with a closed combustion chamber in a few years. *************************** I posted the original questions on a local forum and someone told me that the difference between heating high and fast vs low and slow won't be measureable, but didn't say which is better in theory. Also, it was remarked that reducing the burner size won't help much without reducing the size of the exchanger. I answered that in many cases (though I'm not sure if this also true for gas boilers) a larger heat exchanger improves efficiency, so reducing the burner would be similar to having a smaller model with a larger exchanger. The answer was that a heat exchanger that is too large may cause condensation, which is bad for normal gas boilers (but good for condensing models). I have yet to find any hard evidence on how heat exchanger size effects boiler efficiency. When does 'larger' become 'too large'?
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01-28-11, 07:02 AM | #13 |
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If a larger heat exchanger is causing condensing you are definitely pulling more energy from the exhaust which is exactly what you want. I think having the pro resize the system is probably the best way to go as he can size it appropriately. You may not even get close to 'too large' based on his calculations.
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01-28-11, 10:05 AM | #14 |
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Piwoslaw-
I think Walt was replying to me - I have the coal conversion boiler. "Boiler" is used to describe a heating system that heats water while a "furnace" heats air. Walt- Thanks for your response. Thanks for pointing out the risks of derating the boiler. I have a couple of CO monitors in the house (one near the boiler and the other near the wood stove). When I calculate energy costs, I assume about 50% efficiency for my coal conversion which seems about right. Instead of replacing the boiler (which needs to be done) I opted to first insulate and weatherize which cut the heating bills by a third to a half. Going from R-1 in the attic (just the plaster and lathe) to R-20 made a huge difference (ROI of about 2 years). The estimates I got for replacement were about $6000, a greater than 10 year ROI, and since I won't have the house much longer I couldn't afford to do it. The combustion analyzer is a nifty (if expensive) toy. I guess it is a portable version of what the DMV uses to check my car exhaust every year. I'd likely have to find someone who works on commercial boilers to help with that, but I'm not well connected to the commercial boiler community. Probably the best thing that I could do at this point is remove all of the old, cracked furnace cement and replace it with refractory cement to better seal the combustion chamber and set it back to it's original burn settings. |
01-28-11, 12:23 PM | #15 |
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Hi Ben
Yep it was you I was refering too about the coal to gas conversion. I orginally wanted to post a link to another pdf file that actually gave a good explanation of combustion analysis. A lot of information like many guys here seek falls under copyrighted material and is hard to come buy on forums or even the net in general. I wish I could freely digitize and post a library built over 30 years. There is another condition known as "cold firing" . The internal flame temperature inside the heat exchanger is not hot enough for complete combustion. In coal or fuel oil this produces a noticable increase in soot. But gas is so clean you don't have the soot as a indicator,only the increased CO. Also its the stack temperature heat that induces the draft,cold fire a boiler and the draft can be reduced to a point where ventilation of flue gases is a issue in some flues depenging on size and configuration.. Walt |
01-29-11, 01:50 AM | #16 | |
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Quote:
I'll start looking for a smart and helpful technician. Sorry about that. I've replaced 'furnace' with 'boiler' in my previous posts. Thanks
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01-31-11, 10:25 PM | #17 |
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Buring any fossil fuel produces Co2 and H20 (water) during the combustion process.
Here read this about combustion analysis. http://www.tsi.com/uploadedFiles/Pro...ic-2980175.pdf |
06-13-11, 12:18 PM | #18 | ||
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I found a page on boiler efficiency:
How a boiler works – boiler efficiency It has a few tips, of which two seem contradictory (to me at least): Quote:
Quote:
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06-13-11, 01:50 PM | #19 |
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I don't think that short run time is desireable. If your boiler/furnace can modulate itself down to match the BTU loss of the house, thats great. It is using just enough fuel to maintain the temperature you want.
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