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Home > Archive > Heating and air conditioning > March 2006 > Oversizing a condensing furnace
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Oversizing a condensing furnace
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| Does this make sense?
Rec'd this reply from a a government home energy specialist:
"Thank you for your inquiry. Slightly oversizing a condensing gas furnace
allows for a shorter duration of furnace run time and therefore longer idle
periods between calls for heat. During idle periods, the secondary heat
exchanger metal surface is permitted to cool further and more (latent) heat
can be extracted from the flue gas during the transient start-up phase, with
the condensed water being removed to the drain and the latent heat gain
secured to the furnace. The efficiency gain may be a couple of per cent. In
the case of conventional furnaces, they do not have a secondary heat
exchanger to remove latent heat from the flue gas so there will not be an
efficiency gain. In fact, it may reduce efficiency since the furnace will
not be operating at a steady state condition which gives the highest
efficiency. Although there is some condensing at start-up, the water remains
in the furnace exchanger and is varpourized as the furnace continues to heat
up, and the latent heat is returned to the flue gas. Thank you for your
inquiry. Slightly oversizing a condensing gas furnace allows for a shorter
duration of furnace run time and therefore longer idle periods between calls
for heat. During idle periods, the secondary heat exchanger metal surface is
permitted to cool further and more (latent) heat can be extracted from the
flue gas during the transient start-up phase, with the condensed water being
removed to the drain and the latent heat gain secured to the furnace. The
efficiency gain may be a couple of per cent. In the case of conventional
furnaces, they do not have a secondary heat exchanger to remove latent heat
from the flue gas so there will not be an efficiency gain. In fact, it may
reduce efficiency since the furnace will not be operating at a steady state
condition which gives the highest efficiency. Although there is some
condensing at start-up, the water remains in the furnace exchanger and is
varpourized as the furnace continues to heat up, and the latent heat is
returned to the flue gas. "
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| On Sun, 26 Mar 2006 08:17:25 GMT, "Fred" <Fred@zzz.net> wrote:
>Does this make sense?
>
>Rec'd this reply from a a government home energy specialist:
>
>"Thank you for your inquiry. Slightly oversizing a condensing gas furnace
>allows for a shorter duration of furnace run time and therefore longer idle
>periods between calls for heat.
The problem is your heat loss is not going to change. You will still
loose X amount of btus per hour that will need to be replenished to
maintain temperature.
The other problem is you will end up using more fuel. This is because
even though its a 90% furnace you still have 10percent of waste. 10 %
of 50K is less then 10 % of 100K.
>During idle periods, the secondary heat exchanger metal surface is permitted
>to cool further and more (latent) heat can be extracted from the flue gas during
>the transient start-up phase, with the condensed water being removed to the
>drain and the latent heat gain secured to the furnace. The efficiency gain may
>be a couple of per cent.
This is an explanation on how a condensing furnace works.
>In the case of conventional furnaces, they do not have a secondary heat
>exchanger to remove latent heat from the flue gas so there will not be an
>efficiency gain.
No kidding. :0
I always ask one simple question to people considering a heating or ac
upgrade. How much money do you want to keep funneling to the arabs?
The answer should be, and often is, the as least as possible. The way
to do it is to spend a little more and buy the best shit there is and
have it properly installed. Thats it. Theres no black magic.
>In fact, it may reduce efficiency since the furnace will
>not be operating at a steady state condition which gives the highest
>efficiency.
This statement implies that a properly sized furnace wont reach SSC
before satisfying the demand. That is total bunk. Typical
residential furnaces reach SSC in a very short time and Id have to see
some data supporting this oversizing is good theory.
>Although there is some condensing at start-up, the water remains
>in the furnace exchanger and is varpourized as the furnace continues to heat
>up, and the latent heat is returned to the flue gas. Thank you for your
>inquiry.
Who writes this shit?
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| Al Moran 2006-03-26, 10:21 am |
| On Sun, 26 Mar 2006 08:17:25 GMT, "Fred" <Fred@zzz.net> wrote:
>Does this make sense?
>
>Rec'd this reply from a a government home energy specialist:
>
>"Thank you for your inquiry. Slightly oversizing a condensing gas furnace
>allows for a shorter duration of furnace run time and therefore longer idle
>periods between calls for heat. During idle periods, the secondary heat
>exchanger metal surface is permitted to cool further and more (latent) heat
>can be extracted from the flue gas during the transient start-up phase, with
>the condensed water being removed to the drain and the latent heat gain
>secured to the furnace. The efficiency gain may be a couple of per cent. In
>the case of conventional furnaces, they do not have a secondary heat
>exchanger to remove latent heat from the flue gas so there will not be an
>efficiency gain. In fact, it may reduce efficiency since the furnace will
>not be operating at a steady state condition which gives the highest
>efficiency. Although there is some condensing at start-up, the water remains
>in the furnace exchanger and is varpourized as the furnace continues to heat
>up, and the latent heat is returned to the flue gas. Thank you for your
>inquiry. Slightly oversizing a condensing gas furnace allows for a shorter
>duration of furnace run time and therefore longer idle periods between calls
>for heat. During idle periods, the secondary heat exchanger metal surface is
>permitted to cool further and more (latent) heat can be extracted from the
>flue gas during the transient start-up phase, with the condensed water being
>removed to the drain and the latent heat gain secured to the furnace. The
>efficiency gain may be a couple of per cent. In the case of conventional
>furnaces, they do not have a secondary heat exchanger to remove latent heat
>from the flue gas so there will not be an efficiency gain. In fact, it may
>reduce efficiency since the furnace will not be operating at a steady state
>condition which gives the highest efficiency. Although there is some
>condensing at start-up, the water remains in the furnace exchanger and is
>varpourized as the furnace continues to heat up, and the latent heat is
>returned to the flue gas. "
It makes perfect sense that a goverment entiity would send an email
with the sme information in it twice.
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| Tekkie® 2006-03-26, 10:21 pm |
| Al Moran posted for all of us...
I don't top post - see either inline or at bottom.
> On Sun, 26 Mar 2006 08:17:25 GMT, "Fred" <Fred@zzz.net> wrote:
>
>
> It makes perfect sense that a goverment entiity would send an email
> with the sme information in it twice.
>
And that Flintstone would post it that way. Dyslexic?
--
My boss said I was dumb and apathetic.
I said I don't know and I don't care...
Tekkie
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| Murdentech 2006-03-27, 9:21 am |
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"Fred" <Fred@zzz.net> wrote in message news:p6sVf.5189$Ph4.2973@edtnps90...
> Does this make sense?
>
No, in fact it is total crap.
> Rec'd this reply from a a government home energy specialist:
>
> "Thank you for your inquiry. Slightly oversizing a condensing gas furnace
> allows for a shorter duration of furnace run time and therefore longer
> idle periods between calls for heat. During idle periods, the secondary
> heat exchanger metal surface is permitted to cool further and more
> (latent) heat can be extracted from the flue gas during the transient
> start-up phase, with the condensed water being removed to the drain and
> the latent heat gain secured to the furnace. The efficiency gain may be a
> couple of per cent. In the case of conventional furnaces, they do not have
> a secondary heat exchanger to remove latent heat from the flue gas so
> there will not be an efficiency gain. In fact, it may reduce efficiency
> since the furnace will not be operating at a steady state condition which
> gives the highest efficiency. Although there is some condensing at
> start-up, the water remains in the furnace exchanger and is varpourized as
> the furnace continues to heat up, and the latent heat is returned to the
> flue gas. Thank you for your inquiry.
>
Never oversize a furnace, it simply wastes fuel and decreases efficiency.
The idle time is dictated by heat loss of the structure. An oversized
furnace will short cycle but the idle time will remain constant as the
building looses heat at a steady rate in direct correlation with the ambient
temperature.
The argument of continued condensing heat release during off cycle would be
true of any size furnace. However, this residual heat gain will remain in
the plenum and most likely lost to the equipment room or basement, never
reaching the conditioned space.
As Mike points out in his reply, flue loss is a percentage regardless of
equipment efficiency. Even a on 70% efficient equipment of say 100k btu, 30%
of 100k is less than 30% of 150k going up the flue.
Furthermore, an oversized furnace will not reach Steady State during the
average run cycle. Even a properly sized , single stage furnace (rated for
maximum heat load) does not reach steady state in milder weather... thus the
development of multi stage equipment. All furnaces are oversized 60% of the
heating season, let's not oversize them 100% of the time.
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| B-Hate-Me 2006-03-28, 2:21 pm |
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"Fred" <Fred@zzz.net> wrote in message news:p6sVf.5189$Ph4.2973@edtnps90...
> Does this make sense?
<<SNIP>>
Some states (MA for one) require a manual J on every
new install. Oversizing would not be allowed in these states.
If you want to "gimmick" a couple of percent out of your
warm air furnace, set the fan to come on at call for heat.
Most people find this uncomfortable....That what we sell.
Comfort.
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