Hybrid Car – More Fun with Less Gas

Are replacement windows ever worth it? or What's wrong with my math? - Page 3

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Posted by Paul M. Eldridge on May 12, 2008, 3:29 am
On Sun, 11 May 2008 14:59:31 -0400, "Kitep"

Hi Kitep,

I've only skimmed through your calculations, but there appear to be
two key factors you've overlooked.  One is the operating efficiency of
your gas furnace or boiler; to account for this, multiple the number
of BTUs per CCF by the AFUE rating of your heating system -- if, for
example, your furnace's AFUE is 80 per cent say, then your true cost
per MM BTUs is 1.25 times higher than what is shown here.  In
addition, your cooling costs per BTU are likely to be very different
from those of your heating, so they really should be treated

Secondly, your simple payback assumes no increase in natural gas
prices over the next ten to fifteen years and it's safe to say the
cost of natural gas (and, likewise, the electricity you use to run
your CAC) will significantly outpace the rate of inflation going
forward; in fact, the Nymex Henry Hub futures price has more than
doubled in just the past eight months alone.

Other potential considerations include 1) increased comfort (low-e
coatings do make a noticeable difference), 2) reduced uv fading (helps
minimize the damage done to furniture, drapery, rugs, etc.), 3)
reduced sweating and related mould and mildew problems (aluminum clad
frames can be somewhat problematic in this regard) and 4) lower air
infiltration as you seal any gaps and leaks that exist with respect to
your current windows.

You might also ask yourself if these new windows will add to your
home's curb appeal and make the interior spaces more attractive.
Lastly, new windows and doors can increase your home's resale value
and this could be an important consideration should you decide to sell
at some future point.


Posted by Kitep on May 12, 2008, 4:22 am

Good catch.  I'm not sure what my natural gas furnace operates at, but since
it vents through the wall instead of through the roof, by code it must be
93% or better (I think).

So 93% -> 1.075 multiplier, and my $.25/year should really be $.94/year,
payback time = 14.5 years.  That's a year better than before, but still not
enough to make it worthwhile.

True enough.  I also didn't put in the opportunity costs.  I think like PV,
it's not worth it yet, but maybe later as energy costs keep increasing --  
and I assume window technology gets better as well, the longer I wait.

I wish I knew a way to test this.  My windows definately feel cold in the
winter.  I've even referred to them as "cold radiators".  I know with walls,
that if an outside wall feels colder than an inside wall, it means you don't
have enough insulation.  I was hoping the same was true with windows, ie
that double or triple glaze, low-e, argon filled, would stop them from
feeling cold.

My windows look fine.  However they have a built in screen that rolls up
into the frame when closed, and on most of windows this doesn't work too
well.  It'd be a resell point, but since I don't open my windows very often
it doesn't bother me that much.

Posted by daestrom on May 12, 2008, 10:11 pm

My low-e windows don't feel as 'cold' to stand next to as the one
sliding-glass door with non-low-e glass (or so it seems to me).  The easy
answer for radiant losses is simply thick 'insulated' drapes that you close
at night.


Posted by Kitep on May 12, 2008, 11:32 pm

The difference if feel could simply be caused by the difference in size.
Larger areas of glass are obviously going to feel colder than smaller areas.

Posted by Paul M. Eldridge on May 12, 2008, 11:55 pm
 On Mon, 12 May 2008 19:32:47 -0400, "Kitep"

Hi Kitep,

Actually, it's a little more than just that.  The glass temperature is
several degrees warmer -- potentially as much as twenty or thirty
degrees warmer in fact.

http://www.askthebuilder.com/B97_Low_E_Glass_-_Savings_and_Comfort.shtml .

I notice a big difference in terms of comfort.  With regular windows
you can feel the heat literally being sucked out of your body on a
cold winter's day; far less so in the case of low-e/argon.


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