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<div class="moz-cite-prefix">Marc and Crispin,<br>
This is a useful document for assessing boiler/furnace
performance, especially when they are on all the time. Seasonal
space heating efficiencies need to take into account standby
losses.<br>
The masonry heater folks are grappling with this. Can we cap that
chimney in the off cycle? Thats frowned upon. What is the true
cost of that chimney? My daughter needs a new way to heat her
home. There are interuptable electricty rates which together with
mass thermal storage for off peak power rates allow for
chimneyless space heating. (Still need some air change is humans
are involved) Competing combustion based systems require venting
where the effects of barometric draft dampers and their kin are
often left out of the sales literature. <br>
<br>
Alex<br>
<br>
Gleaned by google;<br>
<br>
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<h2>Heating Efficiency</h2>
<p class="MsoNormal" style="text-align:center;line-height:11.0pt;
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align="center"><u><span
style="font-size:14.0pt;mso-bidi-font-size:10.0pt"><o:p><span
style="text-decoration: none"> </span></o:p></span></u></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">High
efficiency heating systems have become popular because they
save fuel and, one
would suppose, save money. But what do they mean when they
talk about
efficiency? What is furnace efficiency? Actually, there are
two ways to measure
it -steady state and seasonal. <o:p></o:p></span></p>
<h3>Steady State Efficiency </h3>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">Steady
state efficiency refers to how much usable heat is<span
style="mso-spacerun:yes"> </span>created when a furnace is
running.
Conventional gas and oil furnaces have steady state
efficiencies of roughly
eighty percent. When the furnace is on, .twenty .percent of<span
style="mso-spacerun:yes"> </span>the heat that is
generated goes up the
chimney while the remaining eighty<span
style="mso-spacerun:yes">
</span>percent is transferred through a component called a
heat exchanger and stays
in the house. This is<span style="mso-spacerun:yes"> </span>the
heat that
travels through the ductwork and ends up coming out of the
registers in each
room. But even in the dead of winter, furnaces are not on all
of the time. They
cycle on as the thermostat calls for heat, and off when the
thermostat is
satisfied. During start-up and cool down, the furnace is not
operating as
efficiently.<span style="mso-spacerun:yes"> </span>Also,
when the furnace is
not on, heat from inside the house escapes up the chimney flue
the same way
that heat would escape from an unused fireplace, if the damper
was left open. <o:p></o:p></span></p>
<h3>Seasonal Efficiency </h3>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">If
you add these off-cycle losses to the steady state losses,<span
style="mso-spacerun:yes"> </span>you end up with the
seasonal efficiency.
Seasonal efficiencies for conventional gas and oil furnaces
are about sixty to
sixty-five percent. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">Therefore,
if you buy a thousand dollars worth of gas or oil over the
course of a winter
and you have a conventional furnace, only about $600 to $650
worth of<span style="mso-spacerun:yes"> </span>the fuel will
be used to heat your house and
rest will be wasted.<span style="mso-spacerun:yes"> </span>But
what about mid
and high efficiency furnaces? How efficient are they, and what
makes them so
efficient? <o:p></o:p></span></p>
<h3>Mid-efficiency Furnaces </h3>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">Mid-efficiency
furnaces
have a seasonal efficiency of about eighty percent. They
achieve this
by cutting off-cycle losses. Mid- efficiency gas furnaces do
not have a
continuously running pilot. It<span style="mso-spacerun:yes">
</span>is shut
off when the furnace is idle. Also, to prevent heat from
escaping up the
chimney when the furnace is not on, some manufacturers install
a motorized
damper in the exhaust flue to close it during idle periods.
(Other
manufacturers use other approaches to accomplish the same
thing.) </span><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt;font-family:"Times
New Roman";
mso-bidi-font-family:Arial"><span style="mso-spacerun:yes"> </span><o:p></o:p></span></p>
<p class="MsoNormal"><b style="mso-bidi-font-weight:normal"><u><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">High-efficiency
Furnaces</span></u></b><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">High
efficiency furnaces employ similar techniques to reduce
on-cycle losses, but go
further to improve the steady state efficiency. Instead of
having one heat<span style="mso-spacerun:yes"> </span>exchanger,
most have two or even three heat
exchangers to extract more heat from the burning fuel. High
efficiency systems
can be 95 percent efficient.<span style="mso-spacerun:yes"> </span>Because
high
efficiency systems are more complex, they cost more -typically<span
style="mso-spacerun:yes"> </span>$1,000 to $1,500 more
than a conventional
furnace. If you look at fuel costs only, the house that cost
$1,000 to heat
with a conventional furnace should cost only $650 or so to
heat with a high
efficiency system, yielding a savings of $350 or so a year. In
a few years, the
system would have paid for itself. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">Some
models of high efficiency furnaces, however, have had
reliability problems and
virtually all high efficiency systems require more costly
maintenance which
eats into the savings. Therefore, if you are considering a
high efficiency
system, on a retrofit basis, or <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">as
an option on a new home, it is best to speak to a reliable
heating contractor
to discuss .the pros and cons of various models and the
estimated increase in
maintenance costs. <o:p></o:p></span></p>
<p class="MsoNormal"><b style="mso-bidi-font-weight:normal"><span
style="font-size:12.0pt;mso-bidi-font-size:10.0pt">Source :
Dwelling View
Engineers 2000<o:p></o:p></span></b></p>
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<br>
On 15/10/2012 3:04 PM, Marc Pare wrote:<br>
</div>
<blockquote
cite="mid:CAPJQZbx-x3MYc_Xc==MeLfomK5+510EJv1apvU_8BBcxX8vjDg@mail.gmail.com"
type="cite">
<div>Sure thing, Crispin.</div>
<div><br>
</div>
<div>Crispin help me set up an interesting efficiency calculation
the last few weeks, and the results may be of general interest
to the list.</div>
<div><br>
</div>
<div>I have been trying to quantify the efficiency of the direct
combustion of rice hull for heating applications. This should
have some interest in stove design since sometimes stoves are
used for such applications e.g. space heating.</div>
<div><br>
</div>
<div>The big question was how efficient is direct combustion of
rice hull when it is powered by natural draft?</div>
<div><br>
</div>
<div>The tricky part of answering this question is that the amount
of excess air (the amount of air pulled into the system that
isn't used for combustion) has a great effect on the overall
efficiency. It's too hard to measure the outlet velocity of
chimney. Plus, you would need an accurate measurement of fuel
consumption. Finally, it's not really feasible to measure the
temperature of the products of combustion.</div>
<div><br>
</div>
<div>Luckily, you don't need any of these things. By leaning on
the chemistry of the combustion reaction, you can calculate the
overall "efficiency as a heater" with just a gas concentration
reading (O2 or CO2) and an outlet temperature.</div>
<div><br>
</div>
<div>In other words, with just a combustion analyzer, you can see
how much heat energy was available for your system. This is
exactly the number you need to compare between options for
energy sources.</div>
<div><br>
</div>
<div>The details of the calculation with this method are in the
attached document -- BS845, a British boiler efficiency
standard.</div>
<div><br>
</div>
<div>The results from direct combustion of rice hull are really
fascinating. I attached a plot of the measured efficiency.</div>
<div><br>
</div>
<div>To get most of the energy out of the fuel in the traditional
process, the combustion is done in phases. </div>
<div>Phase 1 (5-10 minutes): A pile of raw hull is dumped on an
inclined grate. It roars up into flame. Reaches > 1000 C</div>
<div>Phase 2 (10-20 minutes): The rice husk turns to char. It's
still red hot and still burning. This proceeds slowly and the
temperature gradually decreases to < 500 C</div>
<div><br>
</div>
<div>The char is scraped off of the grate. It hasn't combusted all
the way to ash.</div>
<div><br>
</div>
<div>What's interesting about this is that the efficiency of
"Phase 2" drops considerably. This isn't the case with other
fuels I have seen reports of. I plugged in some of the data from
an assessment in India of charcoal kilns, and even with similar
excess air values, the efficiency doesn't drop as low as rice
hull.</div>
<div><br>
</div>
<div>What I think is responsible is that the particular chemical
structure of rice hull (high ash, a silica skeleton) acts as a
brake on the "char reactions" (I'm not sure of the technical
term for this). You have to strike a balance between getting the
energy out of the rice hull char and running at low efficiencies
in your process.</div>
<div>I'm going to take a deeper look into this area some time
soon. </div>
<div><br>
</div>
<div>Any others have thoughts to weigh in?</div>
<div><br>
</div>
Marc Paré<br>
B.S. Mechanical Engineering<br>
Georgia Institute of Technology | Université de Technologie de
Compiègne<br>
<br>
my cv, etc. | <a moz-do-not-send="true"
href="http://notwandering.com" target="_blank">http://notwandering.com</a><br>
<br>
<br>
<div class="gmail_quote">On Sun, Oct 14, 2012 at 11:19 PM, Crispin
Pemberton-Pigott <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:crispinpigott@gmail.com" target="_blank">crispinpigott@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
Dear Marc<br>
<br>
Would you like to describe some of your recent work on using
rice hull to fire bricks in Vietnam?<br>
<br>
It is an interesting application and he methods you used are
of general interest.<br>
<br>
Thanks<br>
Crispin stuck in Beijing<br>
<br>
<br>
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