01-26-11, 09:07 AM | #1 |
Super Moderator
Join Date: May 2009
Location: Warsaw, Poland
Posts: 964
Thanks: 189
Thanked 111 Times in 87 Posts
|
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.
__________________
Ecorenovation - the bottomless piggy bank that tries to tame the energy hog. Last edited by Piwoslaw; 01-29-11 at 01:18 AM.. Reason: Replaced 'furnace' by 'boiler' |
01-26-11, 12:16 PM | #2 |
Administrator
Join Date: Aug 2008
Location: Germantown, WI
Posts: 5,525
Thanks: 1,162
Thanked 374 Times in 305 Posts
|
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.
__________________
Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. |
01-26-11, 04:45 PM | #3 | |
Journeyman EcoRenovator
Join Date: May 2009
Location: Buffalo, NY
Posts: 344
Thanks: 3
Thanked 17 Times in 15 Posts
|
Quote:
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). |
|
01-27-11, 01:55 AM | #4 |
Super Moderator
Join Date: May 2009
Location: Warsaw, Poland
Posts: 964
Thanks: 189
Thanked 111 Times in 87 Posts
|
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.
__________________
Ecorenovation - the bottomless piggy bank that tries to tame the energy hog. Last edited by Piwoslaw; 01-29-11 at 01:19 AM.. Reason: Replaced 'furnace' by 'boiler' |
01-27-11, 02:11 AM | #5 | |
Master EcoRenovator
Join Date: Dec 2008
Location: Vancouver Island BC
Posts: 745
Thanks: 23
Thanked 37 Times in 30 Posts
|
Quote:
|
|
01-27-11, 02:42 AM | #6 | |
Super Moderator
Join Date: May 2009
Location: Warsaw, Poland
Posts: 964
Thanks: 189
Thanked 111 Times in 87 Posts
|
Quote:
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.
__________________
Ecorenovation - the bottomless piggy bank that tries to tame the energy hog. Last edited by Piwoslaw; 01-29-11 at 01:31 AM.. Reason: Replaced 'furnace' by 'boiler' |
|
01-27-11, 06:08 AM | #7 | |
Super Moderator
Join Date: May 2009
Location: Warsaw, Poland
Posts: 964
Thanks: 189
Thanked 111 Times in 87 Posts
|
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:
__________________
Ecorenovation - the bottomless piggy bank that tries to tame the energy hog. |
|
The Following User Says Thank You to Piwoslaw For This Useful Post: | Daox (01-27-11) |
01-27-11, 02:34 PM | #8 |
Helper EcoRenovator
Join Date: Jan 2010
Location: Little Rock, AR
Posts: 95
Thanks: 16
Thanked 13 Times in 12 Posts
|
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 .
|
01-27-11, 05:19 PM | #9 |
Lurking Renovator
Join Date: Jan 2011
Location: Pa.
Posts: 12
Thanks: 0
Thanked 2 Times in 1 Post
|
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 |
01-27-11, 06:21 PM | #10 |
Journeyman EcoRenovator
Join Date: May 2009
Location: Buffalo, NY
Posts: 344
Thanks: 3
Thanked 17 Times in 15 Posts
|
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. |
|
|