My post was made to make DIY guys aware of the combustion process and all the parameters that need to be known.(in the few threads I read it wasn't discused) I wanted to post another link that explained it better but I need a few more posts to do that. Since the thread op mentioned he couldn't find a boiler expert,I thought I would point him in the direction to acquiring the knowdlege to become one himself,or at least make the readers aware of the correct process for setting up furnaces/boilers.

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

We don't talk about safety that much here, but not because we don't care about it. In most cases we just assume that DIY'ing something in a way that would dramatically sacrifice safety is out of the question. Yes, less than safe ideas do come up every now and then, but they usually are either dismissed right away, or it is noted that such a set up should be short term and temporary at best.

Thank you for making sure that safety doesn't get omitted.

I think you misunderstood - the furnace was bought as gas powered (Ariston Genus 23 RI). It is about 10 years old (though the pipes and radiators in the house are much older) and was ~90% efficient when new. Maybe I should use the word "boiler"? I always think of boiler as a big hot water tank, while my furnace is of the flow through design. Anyway, it will be inspected by a professional when heating season is over and that's who I plan to ask about reducing burner size.

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.

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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'?

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.

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.

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

I think there may be two reasons why condensation should be avoided in an exchanger that wasn't constructed as condesation type:

• The condensate is acidic, which may cause problems with corrosion,
• When the gasses condense into a liquid they suddenly take up much less volume. This may effect the pressure of the exhaust, slowing it down before it escapes the chimney. Furnaces and boilers with a closed combustion chamber (including condensation models) usually have a fan that sucks the exhaust out of the chamber. My model relies on natural air flow to remove the exhaust gasses, so I'd rather not fool around with that.

That being said, it has crossed my mind to wrap the return around the exhasut flue to pull a few more calories out of the heat, but space restrictions are against me.

I'll start looking for a smart and helpful technician.

Sorry about that. I've replaced 'furnace' with 'boiler' in my previous posts. Thanks:)

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

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):
The only way to satisfy both of these is to superinsulate your house and replace all radiators with hydronic floors. That way you can have both lower system temperature and shorter running time.

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.