Newsgroups: sci.energy
From: John De Armond
Subject: Re: increasing home gas-fired furnace efficiency
Message-ID: <h28s92=@dixie.com>
Date: Tue, 23 Feb 93 08:06:55 GMT

babb@sciences.sdsu.edu (J. Babb) writes:

>Hi all,
> I've got a gas fired furnace in my home (Rheem, I think). I've already
>bought and installed a smart thermostat. Now I'm interested in increasing
>the efficiency of the furnace. Here's what I've done so far:

>I don't want to replace pilot light with auto-igniter, it would be an
>expensive retro-fit on a 17 year old furnace and I like the fact that if
>the pilot goes out the pilot gas valve will shut off. Besides I shut of the
>gas during the spring, summer, and fall, so I'm not sure that the expense
>and risk of auto-igniter could justify the retro-fit.

The auto-ignitor retro-fits aren't very expensive if you install them
yourself.  I've seen them sitting on the shelf of my friend's HVAC
wholesale warehouse for under $100.  These systems are probably
safer than the pilot systems in that the main gas will not turn on until
the pilot is on and turns off when it disappears.  You'd be surprised at
how much gas an older pilot light uses - I sure was.

>Also, a few years ago, I saw these aluminum radiator/fins that you put on
>the exhaust flue, these apparently recover some heat and re-radiate it
>within the house (even if it's just the attic). I can't remember who made
>them or how much they were. Anybody got ideas for a substitute? Assuming I
>got them, how about silicon thermal compond between the flue and fins? Is
>there a max temp for that goop? Will it run when heated?

You don't really want to do that.  If you remove enough heat that the
temperature of the flue gas reaches its dew point at some point in the
flue run, you will get condensate that will run back down the pipe
and perhaps into the furnace.  This stuff is very corrosive and will rust
out your firebox in short order.  Even if it does not rust the firebox,
wherever the water collects, it will rust, resulting in holes in the
flue.  Depending on where, this could be a carbon monoxide hazard.
Condensing furnaces have stainless steel burners, are designed to keep
condensate out of the firebox and use induced draft fans.

If you decide to go ahead, you MUST instrument the flue as it emerges
from the house and make sure the temperature stays above 300 degrees F
or so.

John