Question: Three Stage Vs Modulating

[beenthere]

You haven't lived in a home with the supplies on the inside walls, and the returns on the outside walls.

Continuous air circulation does improve comfort with either set up.

But with the older register grille placement. To be comfortable it was almost a requirement(or you had to set the temp up high). As the outside walls always remained cold. And would make anyone feel cold that was sitting close to them(increases the bodies radiant heat loss).

That set up always left a wide temp difference between the outside and inside walls.

You can have any type furnace installed that you want.
Part of customer comfort. Is the customer being happy with their choice.

[Ladiesman271]

Furnace only study: http://ees.ead.lbl.gov/bibliography/..._two_stage_gas

Ductwork considerations: http://www.osti.gov/bridge/product.b...osti_id=812518

Both must be read to get a full understanding.

The first link answers the question very well. Two stage furnaces do not save energy as compared with the same single stage model.

The second link about ductwork applies to ducts in unconditioned areas. That information applies to some systems, and not to others. In my specific case the ductwork is all in the conditioned area, so any energy loss would be "lost" into the conditioned area (AKA no loss).

[RoBoTeq]

That study was also done during the time period when supplies were installed on the inside walls, and returns were on the outside walls.

So any method used to extend furnace run time, would improve comfort.

However, contrary to ladies logic, raising the temperature differentual decreases run time. There are also issues with the heat retention of furnaces from this time period that are no longer factored in with cycle rates and temperature swings.

[Ladiesman271]

You haven't lived in a home with the supplies on the inside walls, and the returns on the outside walls.

Continuous air circulation does improve comfort with either set up.

But with the older register grille placement. To be comfortable it was almost a requirement(or you had to set the temp up high). As the outside walls always remained cold. And would make anyone feel cold that was sitting close to them(increases the bodies radiant heat loss).

That set up always left a wide temp difference between the outside and inside walls.

You can have any type furnace installed that you want.
Part of customer comfort. Is the customer being happy with their choice.

Yes, the duct layout in that old study is as you described. They showed floor layout, duct sizes, duct length, outlet location. For purposes of the comfort evaluation, they excluded temperature differences in the horizontal plane in any particular room. The study also ignored temperature of surfaces.

The discussion on this thread as well at that old study are not evaluating different methods of distribution of conditioned air though a duct system. Whether any particular furnace works properly with an existing duct system is a different subject. Whether an existing duct system was even designed and installed properly in the first place is also another subject.

That being said, I realize that the proper ductwork is critical for a "quality" install. However, posters (meaning not you personally) should be clear on what part of the install they are talking about and not jump back and forth between the furnace selection and the duct system.


Anyhow, the conclusion of that old study was that a properly setup single stage furnace single speed fan and a properly setup 2 stage burner single speed fan furnace had similar comfort and operational costs (electric and gas use) with a temp difference of 55*. With a temperature difference of 35*, the 2 stage furnace used more fuel (4% increase).


So, has anything really changed over the years (meaning does a 2 stage furnace cost more to operate than a single stage furnace of the same design)?

[Ladiesman271]

However, contrary to ladies logic, raising the temperature differentual decreases run time. There are also issues with the heat retention of furnaces from this time period that are no longer factored in with cycle rates and temperature swings.

My present furnace is a 1957 install. I adjust the blower temp setting to lower the temperature differential, not to raise it. That old study showed a 15% difference in fuel use with that one adjustment.

Yes, a thick wall heat exchanger takes a while to heat up and to cool down. Even though my furnace has a temperature controlled blower sensor, a time feature would work just as well. About 5 minutes to blower turn on after buner starts, and 5 minutes to cool down after burner stops.

Modern furnaces have adjustable on / off times for the blower somewhere up to two minutes. The new heat exchanges are thinner and a different design, so time is set so you don't burn them out.

Anyhow, how I adjust my old furnace is hardly the issue. I wish I knew what the CFM my old furnace blower is so I can make a duct flow rate comparison (AKA FPM air flow noise level evaluation) with a potential new furnace. The 70k and 90K BTU output furnaces use two different blower speeds.

[homersodyssey]

http://www.achrnews.com/CDA/Articles...00000000475213

It seems ASHRAE likes the York mod.