How to efficiently use a gas furnace/boiler

[Piwoslaw]

I read a while back that gas powered boilers are most efficient when working at, or close to, full load.

I remember that our old boiler's burner was either on at max or off, the flame could not be adjusted. The only thing that made this old boiler turn on or off was the temperature of the water in the heating system: when it got too low the boiler would turn on and heat the water at full blast until the temperature on the return rose to a set level. I had to manually adjust the return temperature setting almost every day depending on the outdoor temperature.

New boilers all have an adjustable burner and reduce the flame size once they have warmed up. A room thermostat controls when the boiler cycles on and off, but the flame size depends on the water temperature. If this is set to a high temperature, then the boiler warms the house quickly then shuts off. Of course, after the initial warmup stage the flame gets smaller as the water temperature rises. If the water temperature is set lower, then the flame gets reduced much quicker and it takes longer to heat the house.

So the question is: Does the boiler's efficiency drop when the flame gets smaller?

Also, should I set my boiler so that the water temperature is high or low? In other words, should the house heat up quickly at close to full blast, or should the boiler just barely keep the water at a temperature that keeps the house from cooling down. Does the process of heating water to a higher temperature lower efficiency?

For a moment let's forget that a 100 watt water pump turns on/off with the boiler. Let's also not take into account that our boiler is a little oversized - it is a 23kW model and was installed before the house's walls were insulated with 5cm of styrofoam and the windows replaced with argon filled windows. A few years from now, after I do a few more renovations, I hope to replace it a much smaller condensing-type boiler.

[Daox]

I'd tend to think that as the flame gets smaller, the efficiency would increase. Also, as the temperature of the water decreases, the efficiency would increase.

[RobertSmalls]

Originally Posted by Daox

Also, as the temperature of the water decreases, the efficiency would increase.

True. This isn't immediately intuitive, but it makes sense. The furnace has a heat exchanger to move heat from the combustion gases which go up the flue, to the air that blows through the ducts (or water in the case of a boiler). A furnace causes a bigger rise in air temperature when the air intake temperature is low. Hence, a furnace is more efficient at heating up a cold house than it is at holding a temperature constant.

That's not to suggest you want to cultivate a cold basement or fail to seal your cold air return. To guess at some numbers, maybe your furnace causes a 60°F rise in temperature when it's drawing in 40°F air (output: 100°F), and a 58°F rise in 65°F air (output: 123°F).

[Piwoslaw]

So I'll lower the temperature of the system's water today, that should make the boiler go for longer with a smaller flame (plus the 100W water pump will be on longer, too). I've wondered about replacing the burner with one from a smaller model - that way the boiler would be working at higher load (higher efficiency?) to keep the same temperature. In the spring I'll ask the maintenance guy about that.

A good side of lowering the water temperature in our system is that the radiators and pipes won't expand and contract as much (I can hear them creaking and whining shortly after each time the boiler fires up). This may help keep the 40 year old system from springing a leak or two.

On the other hand, with the system at a lower temperature there will be less heat radiating, so I might be forced to raise the thermostat's temperature a bit to stay comfortable.

[strider3700]

Originally Posted by Piwoslaw

So I'll lower the temperature of the system's water today, that should make the furnace go for longer with a smaller flame (plus the 100W water pump will be on longer, too). I've wondered about replacing the burner with one from a smaller model - that way the furnace would be working at higher load (higher efficiency?) to keep the same temperature. In the spring I'll ask the maintenance guy about that.

A good side of lowering the water temperature in our system is that the radiators and pipes won't expand and contract as much (I can hear them creaking and whining shortly after each time the furnace fires up). This may help keep the 40 year old system from springing a leak or two.

On the other hand, with the system at a lower temperature there will be less heat radiating, so I might be forced to raise the thermostat's temperature a bit to stay comfortable.

do you need a 100W pump? for my solar setup I'm leaning towards something in the 10W or so range. There are tons to choose from.

[Piwoslaw]

Originally Posted by strider3700

do you need a 100W pump? for my solar setup I'm leaning towards something in the 10W or so range. There are tons to choose from.

I've been thinking about how to replace that pump since the first time I looked inside the furnace. Unfortunately, the pump is part of the boiler, plus it is not standard size, so replacing it won't be easy. But I am on the lookout for something smaller, maybe a three speed unit (50/65/80W), so that I can play with the settings. My Grandma's house is very similar to our's and reducing her pump's speed (from 80W to 60W) made it much quieter without effecting how the system works, even though her house uses more energy for heating than our's.

I found the boiler's instruction manual, here is some info:
Thermal power max/min (kW): 23.1/9.2
Efficiency at nominal power (%): 90.2
Efficiency at 30% of nominal power (%): 87.8
Water temperature max/min (°C): 85/34

So the difference in efficiency between full and 30% of nominal power is only 2%. Not much. On the other hand, the boiler has never been set to more than 60-65°C, so a smaller burner (17kW instead of 23kW) would still work with no problems.

[wdrzal]

Derating boilers or furnaces,or seting them up require a anaylsis of the combustion flue gases. You need to find someone with the proper equiptment.The PCA (portable combustion analyzer) directly measures, displays, and stores the following data:
• Room Temperature in °F or °C (Primary Air/Ambient Temperature)
• Flue Gas Oxygen Content in %
• Flue Gas Temperature in °F or °C
• Flue Gas Carbon Monoxide Content (H2 Compensated) in ppm
(For analyzers having a Carbon Monoxide sensor)
• Flue Gas Nitric Oxide content in ppm
(For analyzers having a Nitric Oxide sensor)
• Pressure/Draft in Millibars, Pascals, or Inches of Water Column
(For analyzers having a draft sensor)
• Differential Pressure in Millibars, Pascals, or Inches of Water Column
(For analyzers having a draft sensor)
The PCA will compute, display, and store the following data for any of the
seven standard fuels:
• Combustion Efficiency in %
• Excess Air in %
• Flue Gas Carbon Dioxide Content in %
• Flue Gas Carbon Monoxide Content referenced to Oxygen in ppm
(For analyzers having a Carbon Monoxide sensor)
• Flue Gas Nitric Oxide Content referenced to Oxygen in ppm

I have a Barharach PCA 55 & 65 analyzers. they comes with a wireless printer. It's display is real time and sample info can be stored in memory or printed on site to a wireless infared printer. The NOX (nitrates of oxide) isn't needed for residental set up but used in reports of large commercial equiptment. I cut some of the above/below info from the pdf that can be found here: Sorry I had to remove the link since this is my first post. Go to Bacharach dot com & read about PCA 55 & 65 (the pca 55 doesn't have a NOX sensor,but it can be upgraded to a 65)combustion analyzers for some insight of what is required to do this work properly..... There are pdf manuals there to give you some idea of whats required. I cut & pasted the important measurments and calculations that these combustion analyzers provide to the user.

Combustion flue gas analysis is essential for safe operation. If you look around the Bachrach site or google "combustion analysis" I think you'll be headed in the right direction to understanding the process.

The only down side is they cost about $3,000.00 and the oxygen sensors expire about every 2 years if you use it or not. They cost $175.00, just replaced one. Calibration is another $175.00 so the equiptment is expensive,unfortunately.

The seven standard types of fuels that can be selected are:
• Natural Gas
• Oil #2
• Oil #4
• Oil #6
• Kerosene
• Propane
• Coal
The PCA continuously monitors flue gas exhaust conditions and updates
the above displayed values during a combustion test. If the analyzer is
equipped with an optional pressure sensor, then draft measurements can
be made simultaneously with the combustion test, or made separately.
The analyzer has the capability of storing data that was collected during
a combustion test or draft measurement. The stored data can then at a
later date be either viewed on the PCA’s display, printed using an optional
printer, or downloaded to a computer.

Be Green,but also Be Safe

Walt

[RobertSmalls]

Walt's links:
Bacharach • Products • Combustion Analyzers
Bacharach ? Products ? Combustion Analyzers ? PCA® 3 - Affordable, Best-In-Class Portable Combustion Analyzer

It sounds like you recommend having an HVAC professional help you derate your burner.

[wdrzal]

Originally Posted by RobertSmalls

It sounds like you recommend having an HVAC professional help you derate your burner.

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

[Piwoslaw]

Originally Posted by wdrzal

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.

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.

Originally Posted by wdrzal

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%.

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.

***************************

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'?