The following discussion on masonry heater efficiency was the result of a query by
André Fauteux to Skip Hayden and Norbert Senf.

André is the editor and publisher of "La Maison du 21e siècle -
Le magazine de la maison saine", a Québec magazine whose
title translates to "21st Century House - the Healthy Housing
Magazine".

Skip Hayden is Senior Research Scientist and
Deputy Science and Technology Director, Integrated Energy Systems,
CANMET Energy Technology Centre (CETC)
Ottawa, Canada

From: André Fauteux [mailto:info@21esiecle.qc.ca]
Sent: February 5, 2008 12:58 PM
To: Norbert Senf
Cc: Hayden, Skip
Subject: DOE on masonry heaters
 
Hi Norbert and Skip:

Home Power Feb/Mar 2008 quotes DOE's Efficiency and
Renewable Energy Information Center saying that
masonry heaters can reach combustion efficiencies of
90% vs 78-85% for pellet stoves.

Which begs the question:

How do masonry heaters compare with inefficient oil furnaces
in terms of particulate, CO and other emissions?
Quebec's Environment Department discourages heating
exclusively with wood to protect air quality.

Is this justified if we use a masonry heater?

Thanks for your thoughts...
--

André Fauteux, éditeur
La Maison du 21e siècle
Le magazine de la maison saine
2955 Domaine du lac Lucerne
Ste-Adèle Qc  Canada J8B 3K9
Tél./Fax: (450) 228-1555
info@21esiecle.qc.ca
Cours, consultations et archives :  www.21esiecle.qc.ca

From: "Hayden, Skip" <shayden(at)NRCan.gc.ca>
Subject: RE: DOE on masonry heaters
Date: Tue, 5 Feb 2008 16:55

I find that claiming efficiencies of 90% for masonry heaters is exaggerated. 

Firstly they generally run at fair excess air levels, which fixes the sensible heat loss a little to a very large amount above 6%. 
Secondly, the hydrogen content of would results in a fixed loss of nearly 6%. 
Thirdly, 20% moisture wood results in a second latent heat loss of 3%. 
Unless the flue condenses the water vapour AND recovers this latent heat, which is unlikely, a more appropriate MAXIMUM “combustion” efficiency is 85%.  
It is unlikely that masonry heaters have lower particulates than today’s oil furnaces, and even less likely that they have lower CO.
 
Skip

From: Norbert Senf <mheat(at)heatkit.com>
Subject: RE: DOE on masonry heaters
Date: Tue, 5 Feb 2008 19:19:22 -0500

Hi Skip and André:

Nice to hear from you, Skip, it has been a while.

I have been conducting some testing lately, and descriptions are on this page:
http://heatkit.com/html/lopez.htm#current

From the current series of tests on a modified air system and modified ignition (from the side of the pile), we have 18 data points, summarized here:
http://heatkit.com/research/2008/lopezm02d.htm

If we pick a test at the median of the PM range (Test #HK-J11, with 8 tests lower, and 9 tests higher, including deliberate attempts to burn dirty), we get:

• 1.0 g/kg PM (Oregon M-41 (Condar method)).

and efficiencies as follows:   ( OMNI has recently correlated the Condar with EPA Method 5, with very good results, taking into account some limitations with the Condar and the ballpark nature of PM numbers.)

• combustion efficiency: 97.10% (this counts only the chemical losses, CO and HC, using Skip Barnett's Condar spreadsheet formulas)
• heat transfer efficiency: 80.6%
• overall efficiency (= combustion efficiency X heat transfer efficiency) = 78.3%  - this is at the top end of the range, which is 71.9% - 78.3% for this series of 18 test runs so far.
  

Actual efficiency is a bit lower since we are ignoring about 1/2 lb of burning charcoal at the end, assuming that we can have an airtight system and just shut it down.

According to Skip Barnett's Condar calculation method, we get:

• average oxygen: 13.6% (excess air = 180%)
• total latent heat loss (hydrogen + wood moisture): 11.5%
• chemical loss (CO + HC): 2.9%
• CO: 0.13%, or 22.14 g/kg
• dry gas loss: 7.8% (average stack temp = 230F)

It is unfortunate that numbers like 90% keep getting thrown around, because they lower the credibility of those who quote and perpetuate them.

Two common sources for these numbers are as follows:

• "combustion efficiency" numbers get quoted, with no definition, or understanding of what they represent.
• European numbers get quoted. The Europeans tend to define efficiency based on the lower heating value of wood, which ignores the latent (boiling of water) heat losses. So, for example, in Europe you can have condensing gas furnaces with efficiencies higher than 100%.
  
  

As an example, our highest overall efficiency number from our recent test series, is 78.3%. If we add the hydrogen latent heat loss of 6% (using Skip's figure), we get a "European" number of 84.3%. If instead we use the Condar latent heat loss number of 11.5% (which includes the wood moisture), we end up with 89.8%. (Hmm, what are the chances that this is the number that will get quoted ;-)

Skip is right about the excess air. In the mid 1980's, the Finns did some research and said they got the best results with large grates in the firebox. A heater with a huge grate got installed at the first Advanced House demonstration house, around 1992. Energy Mines and Resources (Skip's lab at the time, I believe) did a test on this heater, and got an overall efficiency number of somewhere around 40%, due to the large amount of excess air created by the very large grate.


Best ....... Norbert