Variable vs 2-speed Gas Furnace
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  1. #1
    Join Date
    Dec 2004
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    4
    For those who saw my previous thread, thanks for all the help!. We have ruled out the heat pump because of the cooler circulating air temps. We are going with a new gas furnace. I've had 2 contractors out so far. One says Variable is not good, the other says Variable is great. What are the issues here? My primary concern with the variable is that the circulating air may be too cool. Our recently deceased gas furnace had been working with the blower always running. (Apparantly the mechanism that should cause it to shut down when the flame was off was not functioning.) Anyhow, the point is that the wife was uncomfortable with the room temp air being blown around. I like the idea of keeping the air temp more homogenous in the house by using a variable unit, but it seems to me that the variable would behave like my old always-blowing unit (and hence cool circulating air). What's the real story here?

    Thanks!
    Jesse Bunch

  2. #2
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,914
    The only reason the varriable speed blower would run constantly is if you had the fan switch on the thermostat set to on.
    If you leave it on auto, the fan cycles with the systems calls for heating or cooling. The timing depends on how the jumpers are set on the control board.

    The airflow with the fan switch in the on position is very low, so your wife may not feel the drafts like with your old system that was likely running the fan at full speed between heating cycles.

    I would be very interested in why one of the contractors says varriable speed is not good.....
    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    Dec 2004
    Posts
    4
    Thanks!

    1. Please explain how a variable speed furnace operates vs a 2-speed.

    2. Is there any difference between Trane and American Standard?

    Thanks Again!
    Jesse

  4. #4
    Join Date
    Nov 2002
    Location
    VA
    Posts
    296


    Initial cost of variable speed is typically notably higher. Variable Speed Blowers have a DC motor; two-speed blowers have an AC motor.

    I'm not qualified to debate the vast suite benefits and liabilities of variable verses two speed. Therefore will simply quote my service manager who honestly believes that in our mild winter climate a two-speed furnace may be a wiser "heating" choice (from an accountants viewpoint.)

    Remember the two-speed furnace gas valve will first activate at a low fire rate before stepping up to the higher rate. This does make for quiet operation and in our region these units don't go into high fire very often. The cost of a two-speed furnace isn't significantly greater than the "bargain" single stage thus we strongly recommend them.

    The cost of variable speed however is not trivial, but many HVAC professionals feel it's very good at masking air-distribution shortcomings of slightly “challenged" systems (especially for the AC mode). It is my understanding that the current generation of variable speed blowers are now extremely reliable, but again I trust that some of the more experienced Professionals on this forum can further qualify this inference.

    The "difference between Trane and American Standard" is that American Standard owns Trane... However once the cabinetry is off working on the units is identical. Trane is the only vendor who manufactures it’s own compressors, that allows Trane a very high level of quality control… However for it’s gas furnaces (which are highly rated and very respected) it purchases components from leading suppliers (i.e. Whiter-Rodgers) just as other top vendors.

    Hope this helps…

  5. #5
    Join Date
    Apr 2003
    Location
    Iowa
    Posts
    763
    American Standard bought Trane I think in 84. They have put trane ideas in there furnaces basically the same stuff.
    variable speed uses .05 amps to run where a normal fan uses .6 .... do the homework and you'll find I think at .07 cents should save about $300 a year or more over a regular motor. I think thats the figures I could look it up but don't have time. anyway saves money more comfort yes you'll like it. also comfort R with trane nobody else.
    Nice Choice I think. Customers love them. Pricey but worth it. Two stage can be your choice low for ten min on single stage thermostat or demand with a two stage.
    extended warranties on furnace only can be up to 15 years. lifetime heat exchangers like everyone else...

  6. #6
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Variable speed

    From your post I think you mean the air handler is the thing which is variable speed, am I right? Because the actual gas furnace is available in two speed versions from Trane/American Standard (TAS), and at least one company makes a many-stage gas furnace. But what I own since October is an American Standard variable speed air handler built into a 2-speed gas furnace, which I think is what you are looking at.

    It works very well! In S. Texas where I am, the low speed has been sufficient for all the cold weather so far, and on the low speed you can barely tell when it is operating. I have resorted to taping a thread to some supply registers so it will wave and I can be sure the fan is actually going.

    At the higher speed I expect the airflow to create a certain amount of duct noise in my system, however this is mainly due to *my* supply duct system being of marginal capacity. With full speed airflow, air speeds sometimes exceed the 600-700 fpm maximum speed approved by Manual D (for flex ducts). As long as you don't increase your airflow with the new air handler, expect a "normal" amount of sound with the furnace on high speed -- whatever normal is in your house. But low speed is nearly inaudible, it is so quiet.

    I have the thermostat set to "AUTO" for fan, and your family should have no reason for complaint with this setting.

    A side benefit: when coupled with the better half of the TAS air conditioners (just not the builder grade), this air handler will operate airflow in a sophisticated fashion they call "Comfort-R". The benefit is improved humidity control, get your dealer or sales guy to describe it. They have a graph which helps the customer understand what it is.

    I paid the premium price for the variable speed air handler (furnace) and think this is one luxury product that is well worth it.

    Hope this helps -- P.Student

    [Edited by perpetual_student on 12-09-2004 at 09:10 AM]

  7. #7
    Join Date
    Mar 2002
    Location
    Eufaula OK
    Posts
    4,175
    The VS blower is an expensive option, But it give you saving and comfort for your money.
    The electrical usage is considerable less, often giving you as much as 1 SEER higher on AC and equal saving in heat mode. This will usually pay for the price difference in the life of the system.
    The blower is computer controlled to deliver whatever CFM it is set at for heat and cool, within rated static pressure. And it will automatically change speeds to compensate for minor changes in static pressure.
    It has the ability to accept a humidistat to help control humidity in the summer. Some units have a built in program that will run the blower at a lower speed for the first few minutes of AC to achieve lower humidity without a humidistat.
    The biggest advantage is the ability to run on a very low speed in continues fan. So low your wife will not even know it is on, yet it will help to keep temperatures even through your house, at a very lost cost of operation.
    And last but not least, it ramps up on start and ramps down on stop to give you the proper air temperature of your starting and finishing furnace or AC. In other words, the blower does not run for 3 minutes on high after the furnace or AC has gone off.
    The DC motors are very reliable if installed and maintained correctly.
    Jax

  8. #8
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304

    One more thing to worry about

    I believe it is hazardous in humid climates to run the fan continuously. The pitfall being, when the AC cycle ends the fan moves that humidity right back into the airstream. If you choose this mode of operation, at least pay attention to the humidity in the house. In a dry climate it will be no problem.

    I curse the arrogant Yankees who design our thermostats, who neither know nor care what we need in the Gulf Coast region. After talking with Honeywell about their "circ" air function on the new thermostats, they are in that group too. Minnesota. Sheesh!

    Regards -- P.Student


    [Edited by perpetual_student on 12-09-2004 at 11:58 AM]

  9. #9
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,914
    I have seen a little misuse of the term "2 speed" in a couple of the posts. The correct term would be "2 stage".

    Both the 2 stage and the varriable speed furnaces have 2 stage gas valves, the difference is in the blower. The standard 2 stage furnace uses a conventional blower motor, usually with 5 speed taps. Depending on the manufacturer, it will use as many as 4 different speeds, cool/heat1/heat2/fan.

    The varriable speed furnace uses a DC motor with computer control. The speed of the blower for each mode of operation is set with jumpers on the control board to match the requirements of the application. The motor will ramp the speed up and down depending on the blower timings, mode of operation and changing conditions in the duct system, like dirty filters, wet/dirty coil, etc.

    Here in north Texas, it is very rare for me to sell anything less than a 2 stage furnace, and most of our installations are varriable speed furnaces or air handlers, even if the outdoor unit isn't a top of the line unit.

    Around here, a varriable speed blower will save enough within 2 years to more than cover the difference in cost, and by the time it is out of the 10 year parts and labor warranty, it has saved several times the cost of replacement of the motor or controller if they fail.
    If more government is the answer, then it's a really stupid question.

  10. #10
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
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    2,469
    Variable speed is by far the best deal when it comes to electrical savings, but both variable speed and 2-stage(uses two blower speeds) are 2-stage and can blow cool air out the registers in first stage(less than 100 degrees). They also use 25% more gas in first stage if left at factory settings. This can be field corrected after the installation by properly trained technicians and offer both comfort and savings.
    captain CO

  11. #11
    Join Date
    Aug 2004
    Location
    Canada
    Posts
    30

    how is it that 2 stage use 25% more gas in low stage unless field corrected, I just had a two stage variable speed installed,and they didn't adjust that. Should I be calling them out again to look at it. If the low fire stage is 60% gas usage of high fire, is my unit actually running at 85% of first stage on low fire if not corrected ? I'd like the explanation as to how it is 25% more innefficient on low fire , I don't understand, please explain...yhanks for your reply...

  12. #12
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304

    That's new to me

    Yeah.. Jim.. How can it use more gas on 1st stage when it is a lower BTU output vs the 2nd stage full btu output???

  13. #13
    Join Date
    Aug 2004
    Location
    Canada
    Posts
    30


    Am still waiting for a reply from Jim or anybody else who can qualify what Jim says....

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