On Dec 16, 8:50*pm, Home Guy wrote:
wrote:
How much it changes, ie if you screw around with an old 75%
efficient furnace and turn the burners down, will it raise
the efficiency from 75 to 76, or 75 to 80, however is an
important part.
You are assuming it's substantial. *I'm not so sure.

Based on where I've turned the regulator dial on my home furnace, and by
judging the resulting flame height, I think I'm easily running at about
1/2 the full-throttle fuel flow. *My furnace is a lennox g8-120-1,
manufactured October 1976. *Specs:

0 - 2500 ft altitude:
120,000 btu input
96,000 bonnet BTU
orifice size 39
temp rise 70 - 100
1/4 hp motor (?)
3 1/4 inch pulley

The furnace is about 22" wide, 28" deep, and about 4.5 ft high. *AC
coils mounted in plenum above the furnace.

I changed the pulley to 4" a few years ago to get more airflow (higher
fan RPM). *Interesting that it says it's supposed to have 1/4 hp motor.
The higher altitude spec says 1/3 hp, and higher static pressure (.75 vs
.2 inches WC). *I am at a lower altitude (850 ft above sea level).

We've had a few nights now where the over-night temp has gone down to
10F, daytime high 15F, and my furnace is still cycling (not 100% duty
cycle). *I have the thermostat set for 71F (electronic thermostat, not
mechanical, 2 degrees hysteresis). *I haven't started my humidifier (I
don't always run it in the winter). * *Current indoor humidity is 20%..
Today's low and high temp is 13F / 22F.

Bottom line is that it's amazing how much you can turn down the burners
and still keep the house at the desired temperature, even given these
low outside temps.

Which unfortunately says nothing about whether the furnace is more
efficient
at the lower firing rate or by how much. However if you can fire it
at that
reduced rate and keep the house at the desired temp on the coldest
days
it does say the furnace was oversized to begin with.

Any differences in furnace efficiency in heating at full firing rate
versus 70% would be so
small that you'd have to have instrumentation to figure it out.
Note that HVAC
and manufacturing folks claims for two stage "efficiency" are really
based on:

less cycling of the furnace, so less heat loss when it shuts down,
etc

more even heat distribution, due to the blower running longer,
possibly
resulting in feeling more comfortable at a slightly lower tem.



But the problem here aside from the obvious practical problems of
doing it, is that you can't undo it on the fly.

?

Turning the control knob on the regulator is pretty-much on the fly. *If
you mean that my thermostat can't do it, well I don't think that's
necessary. *For the next 3 months I'm looking at a pretty constant
outdoor climate in terms of temperature, so constant regulation of the
furnace burner output is not necessary.


So, essentially you have it throttled back in Fall and Spring and on
full
during winter. That's part of what a two stage furnace would do.
Difference
is that with a two stage furnace and thermostat, if you're out of the
house
in Oct and set it back to 60, it will fire at the higher rate. Even
the 2 stage
without a 2 stage thermostat will start at low, then move to high
after an
adjustable period, eg 5-12mins.



If you can turn the burners down so that it fires at 70% for
greater efficiency, then one of two things must exist:

1 - The furnace will now be unable to heat the house to normal
temp on the coldest days

2 - The furnace was oversized all those years and you now have
downsized it permanently.

I don't know what's normal for a 2000 sq foot (including basement)
2-story house, brick sided first floor, aluminum sided second story (no
insulation under the siding) in terms of furnace BTU. *My climate zone
is basically Detroit / Cleveland / Buffalo / Toronto.

I think that the HVAC industry was probably targeting a 25% furnace duty
cycle back in the mid-1970's, so a 120k BTU furnace was spec'd back then
for a house like this.

25% cycles when? on the coldest days? From all you've said, sounds
like it's oversized.


Those modern two stage or variable burner furnaces can change the
firing % on the fly.

I really don't think that a fully controllable / variable burner is
necessary or is cost efficient for the average home. *Two stage -
maybe. *Two stage aftermarket electronic thermostats are insanely priced
compared to single stage. *I bought a 2-stage programmable thermostat
for a leased office about 10 years ago (to replace a mechanical
thermostat) and I think it cost $300 at the time, compared to $50 - $70
for a single-stage "home" version.

I can buy a top of the line Honeywell VisionPro 2 stage Tstat for
$100-125.
Or a cheaper prgrammable one for $60. The delta for getting two
stages
on a good thermostat is maybe $25




Another thing strikes me here. * You keep pointing out how you
can buy cheap HVAC online and you apparently believe you could
do installs yourself or pay someone on the cheap to do part of
it for you.

I would install it myself, because I belong to the school of "it's not
done right unless you do it yourself", and there's a lot of stuff I do
for myself (car restoration, welding, plumbing, wood-working, concrete
work, etc).

With the current incentives, ie Fed 30% credit, nat gas utility
credit, electric credit, state credit, I can get $3,000 off
the cost of a new system. * That means doing it your way, you
could have a whole new high efficiency 95% furnace and AC
system for $1000.

A new furnace might cost me $1500 max to buy, a 30% rebate of that is
max $450.

Are there no gas company or state rebates/credits available? Here in
NJ
there are and they would chop another $600 off that price. From what
I've seen posted here, there frequently are similar credits available
in
other states. That could make the cost of that new furnace $450.





It's not the money that would motivate me one way or another. *What
would really cut my energy bills is putting a 2" layer foam-board
insulation around my second-story and getting new windows. *I'll let
everyone else pay through the nose for new furnaces so that they will
use less natural gas resulting in a continued depression for natural gas
prices which will mean that people with inefficient furnaces will
continue to pay less anyways.

Hard to see how you would not recover the cost in a reasonable time of
getting a furnace that is:

A - Sized correctly
B - 95% efficient compared to your current whatever, say 75%

Using your own numbers, ie $1500 cost, Fed Tax credit, no other
rebates,
it's $1050. IF you only cut your fuel costs by $200 a year, that's a
fast
payback. With any other rebates/credits it could be down to a few
years.



Don't you think that's a good value proposition, with a good
payback period?

I've got several other home-projects in mind for the next few years.
When I'm ready to alter my air-handling system to include the concepts
I've talked about here, that's when I'll take a good look at my furnace
and decide if I'll buy a new one (I probably will). *But I'll still
install it myself, and if that means I'll have to buy it on the internet
because the local hvac dealers won't sell it to me unless they install
it, then so be it.