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Old 01-26-11, 09:07 AM   #1
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Default How to efficiently use a gas furnace/boiler

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.

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Last edited by Piwoslaw; 01-29-11 at 01:18 AM.. Reason: Replaced 'furnace' by 'boiler'
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Old 01-26-11, 12:16 PM   #2
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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.
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Old 01-26-11, 04:45 PM   #3
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Quote:
Originally Posted by Daox View Post
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 60F rise in temperature when it's drawing in 40F air (output: 100F), and a 58F rise in 65F air (output: 123F).
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Old 01-27-11, 01:55 AM   #4
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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.
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Last edited by Piwoslaw; 01-29-11 at 01:19 AM.. Reason: Replaced 'furnace' by 'boiler'
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Old 01-27-11, 02:11 AM   #5
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Quote:
Originally Posted by Piwoslaw View Post
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.
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Old 01-27-11, 02:42 AM   #6
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Originally Posted by strider3700 View Post
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 60-65C, so a smaller burner (17kW instead of 23kW) would still work with no problems.
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Last edited by Piwoslaw; 01-29-11 at 01:31 AM.. Reason: Replaced 'furnace' by 'boiler'
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Old 01-27-11, 06:08 AM   #7
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I searched around and found a EnergySavers.gov page on furnaces and boilers. One of the subpages is about retrofitting and older furnace, and I found this:
Quote:
Derating gas burners

Many boilers and furnaces in today's homes are oversized, particularly if you've upgraded the energy efficiency of your home. It is sometimes possible to reduce the heating capacity of your gas boiler or furnace to make it operate more efficiently by reducing the size of the gas burner orifice, and possibly also the baffles. This is a difficult process that should only be performed by a qualified technician, and in some cases, it could violate local building codes and void manufacturer's warranties. If allowed, though, the modifications should cost less than $100 and can save up to 15% of your fuel costs.
This appears to answer my second question about reducing the size of my burner.
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Old 01-27-11, 02:34 PM   #8
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I have seen that same thing on the energysavers site, Piwoslaw, but I couldn't find anything about how to actually do any of it. Finding anyone around here (southern US) with the expertise to help me is next to impossible. The most knowledgeable boiler tech I have spoken to said he had never heard of putting an outdoor reset on a home boiler. *Doph!* He had never done any energy retrofitting on boilers and tried to convince me that retrofitting for efficiency was a bad idea. The nerve! He did show me how to safely de-rate my burner so that it burns around 100k btu instead of 175k. That, along with setting the aquastat to a lower temperature, seems to have saved some gas, thought I don't know how much. Mine is an old coal boiler converted to natural gas, so it could use the smaller burn chamber, better seals, and a more baffles. There is an advantage of having an 80 year old boiler with a 70 year old gas conversion though; it means that there is no chance of voiding the manufacturer's warranty .
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Old 01-27-11, 05:19 PM   #9
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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

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Old 01-27-11, 06:21 PM   #10
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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.

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