Page 1 of 3 123 LastLast
Results 1 to 13 of 27
  1. #1
    Join Date
    Oct 2009
    Posts
    4

    2-Stage vs 3-Stage

    I am preparing to replace my existing oil furnace with a new gas furnace and had a question on the difference between a 2-stage and 3-stage gas furnace.

    I have been told by two contractors that I should go with a 2-stage gas furnace instead of a 3-stage because the 3-stage cost more and usually goes from low-fire to high-fire and skips the middle stage anyway.

    Is this true? Any recommendations?

    My info is this:
    In Wisconsin
    approx 1700 sq ft
    just updated attic insulation and air sealing last fall

    Thanks!

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,069
    No.

    They go from low to med if needed, and then to high if needed.

    Call other contractors. the ones that told you that, are misleading you.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,164
    Even more important than number of stages is how they are controlled.

    To me, stat control is paramount to comfort. Timers in the board and a single stage thermostat is a considerable compromise.

    In your car, do you turn the heat off in bitter weather? No, it runs non stop on just high enough setting to keep you comfy. So why not have this level of comfort in your house? You can with a good 2 or 3 stage or modulating furnace and controls that will let the furnace run constantly.

    Those installations or furnaces that use board control won't allow constant operation. They'll time to high after about 16 minutes which likely will satisfy the stat and shut things off. Do you turn your car heat hotter as the car gets warm? No you turn it down.

    So a 2 stage or 3 stage furnace controlled by a good multistage thermostat or communicating control will offer the most comfort, 3 stage being better than 2 because it has the ultra low fire capacity.

    Modulating furnaces are the greatest comfort if controlled right. On Rheem's, the matching control provides 13 stages of heat which will raise and fall based on the needs of the house. Nordyne's new Mod uses a communicating control that does the same thing. A-S/Trane's new Mod can use a single stage stat and guess at firing. Use it with their communicating control and it truly modulates based on needs. York's mod uses a single stat stat but has temp sensors in the air flow so it knows a bit more about needs of the house than purely logic. Lennox's mod uses a 2 stage stat to tell it more about the temp in the house. But it won't stage down as heat needs are reduced. Sure doesn't make sense.

  4. #4
    Join Date
    Oct 2009
    Posts
    4
    Ok, this is great! Just the info I am looking for.

    I am glad you brought up a modulating furnace. I am looking at a Carrier 58MVC Infinity furnace. I was under the impression it is a 3-stage furnace, but Carrier's website describes it as "Infinity ICS Modulating Gas Furnace".

    To confuse myself more, the specs on one quote I have has "three stage gas valve" as part of the description of the 58MVC. So, is that three stage description not mean the same?

    Thanks.

  5. #5
    Join Date
    Aug 2009
    Posts
    52
    I am a homeowner pondering the same thing. I have a house in St. Louis. 1200 sq ft, concrete block/brick construction, no insulation in the walls but did air sealing and attic insulation. I've done a lot of research and I have an engineering background.

    My advice to you is get a lot of bids, especially multiple bids for the same system. You'll be amazed at what you learn and the range of $ for the exact same bid. Some people bid tight and get more jobs, others bid high and get less jobs, making more money per job. Load calcs vary quite a bit too.

    Carrier and Bryant sell the same line and have a 3 stage valve, which is significanlty less expensive than a true modulating furance (which has 1% increments). They cost a little more than two stage but to me it's worth it.

    Consider getting the smallest furnace you can and a 3 stage valve. I am planning to go with 60,000 BTU furnace, Carrier With a three stage valve - you get 22, 37, and 57,000 output which gives you a nice low stage output and a great range. Two stage is ok, IMO, but make sure you get the smallest furnace you can. For example, for a while I was being recommended two stage, 80,000 BTU, based on my current system. At 70% for the first stage, the output is still 56,000 - the Manual J for an 8 degree day at my house is 50,000. So the systems were totally oversized. Eventually, several contractors said 60,000 would be just fine.

    Also, watch out for this Infinity/Evolution controller. Some contractors swear by it but if you really analyze it, it really only benefits you for humidity control in the summer (it runs the AC below your set point if the humidity is still too high) No difference in winter temp control. - I've been told this by multiple contractors. It costs a lot and you can always add it later. As long as you have a modern programmable, you are set.They mainly recommend it because it prevents less call back thermostat problems, IMO.

  6. #6
    Quote Originally Posted by Squire View Post
    At 70% for the first stage, the output is still 56,000 - the Manual J for an 8 degree day at my house is 50,000. So the systems were totally oversized. Eventually, several contractors said 60,000 would be just fine.
    8 degrees in F or C? I have a 1550 sq ft house and confirming if a 95% 3 stage 60k BTU furnace is overkill - I live in SF bay area where the weather ranges from 35 to 105 F. But I don't think a 40k BTU furnace is enough though.

    Also, watch out for this Infinity/Evolution controller. Some contractors swear by it but if you really analyze it, it really only benefits you for humidity control in the summer (it runs the AC below your set point if the humidity is still too high) No difference in winter temp control. - I've been told this by multiple contractors. It costs a lot and you can always add it later. As long as you have a modern programmable, you are set.They mainly recommend it because it prevents less call back thermostat problems, IMO.
    I agree with you on this. Many suggested using the Evolution/Infinity control for my Bryant 355CAV but given I don't have humidty issues in the summer time - I don't need it. And I agree I can always add it on later.

  7. #7
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Where are you located in Wisconsin, if your anywhere close to Sheboygan I would be happy to let you have a peek at my furnace which is Carriers Infinity 58MVC, nice piece of equipment worth the money and will give you miles of comfort. Let me know

  8. #8
    Join Date
    Aug 2009
    Posts
    52
    Carrier 58 MVC is a three stage furnace.....being advertised as a "modulating."

    Baldloonie, can you explain the difference between between the ICS controller and a single stage T-stat in regard to temp controle. I know there will be a difference with a large temp difference like when you come home from work. But what about at typical steady stage operation. I've been told ICS can't hold the temperature any tighter than a single stage T-stat. Can you give a concrete example? For example, if the heat load is between first and second stage, how will ICS deal with that compared to a single stage.

    Thanks!

  9. #9
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,164
    The MVC is a 3 stage furnace. It may be that the gas valve is capable of modulating, like Trane's 3 stage was, that they call it modulating. But it does have 3 firing rates.

    The Infinity control is sitting on the wall. It knows what it is set for, it knows what the room temp is, it knows whether you have changed it or not, it can tell if the room suddenly got cold from an open door, it knows if the room is almost to setpoint. So it will tell the 3 stages to do what the house needs. If it is almost to setpoint it will be on the lowest stage. If the room suddenly got cold from an open door or you shoved the temp up, it will fire higher. So you will get the heat you NEED.

    On a single stage stat, all is guessing. Educated guesses but still guesses. All the furnace knows is the stat wants heat. A lot? A little? All it does is look at past cycles and tries to pick a firing rate it thinks you need based on the past. It doesn't know that the last cycle you shoved the heat up. Or a door opened. Or it is getting colder out. Only the Infinity control can tell it that.

    So if someone is spending the bucks for a high end furnace, give it the best controller it can have. The Infinity control also gives you control over fan speed and lots of control over summer dehumidification.

  10. #10
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Bottom line is, the Infinity Controller learns how you heat & cool your home. that's why if you have one it's best to set it and forget it unless you happen to get a radical shift in temp's leave it do it's job.

  11. #11
    Join Date
    Apr 2009
    Location
    Central New Jersey
    Posts
    253
    Quote Originally Posted by Squire View Post
    Also, watch out for this Infinity/Evolution controller. Some contractors swear by it but if you really analyze it, it really only benefits you for humidity control in the summer (it runs the AC below your set point if the humidity is still too high) No difference in winter temp control. - I've been told this by multiple contractors. It costs a lot and you can always add it later. As long as you have a modern programmable, you are set.They mainly recommend it because it prevents less call back thermostat problems, IMO.
    Squire,

    I disagree with your comment the Infinity controller only benefits you in the summer. The controller monitors the outside temperature, so it should be using the outside temperature information to determine how quickly it should move from low to medium to high stage. I think the best feature is measuring the static pressure. It will tell you if your duct work are undersized.

    Be aware you must get the Infinity controller enables you to get a larger rebate from Carrier. I find it surprising contractors would not tell you this.

  12. #12
    Join Date
    Aug 2009
    Posts
    52
    I appreciate the replies. I think the truth lies somewhere in the middle regarding the ICS. With all due respect, I feel like people are generally just regurgitating the marketing instead of giving concrete differences. Like it's "supposed" to do this and that and it "should" do certain things. But I suspect if you had a chart of temp vs time between a single stage stat and the ICS for typical steady state operation, there wouldn't be much difference.

    At the same time, after getting a lot of bids, there are all the same for the 58MVC - whether they include the ICS or not. So the contractors that poo-poo the ICS basically say you don't need the ICS but pay me the same amount anyway.

  13. #13
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    "At the same time, after getting a lot of bids, there are all the same for the 58MVC - whether they include the ICS or not. So the contractors that poo-poo the ICS basically say you don't need the ICS but pay me the same amount anyway"

    Not sure how you come to this conclusion, The Carrier ICS-58MVC is an ICS furnace and there is only one Carrier has, They use to make a 2 stage MVB model but they no longer make this model however some supply house or contractors may have the MVB to sell but there not available any more, and have been replaced with the MVC model furnace.

    If your looking for run times on a 58MVC it's going to differ form home to home as you know no 2 house are the same inside and out therefore it's never going to give you the same reading or run times in any one stage.

    MY Carrier 58MVC during winter run times are first stage approx. 60%-65% of the time running in first stage and about 25% running 2nd stage and for 3 stage is the balance but it only hits 3rd stage when it's bitter cold outside. But when running in 1-stage it's will run all day when necessary unless it's coming out of set-back here it might need to kick into 2nd stage to satifiy the stat in the mornings waking up from a set-back of 64* to bring it up to 67* @ 6am in the morning.

    So if you think people here are just reciting what's in there literature and what they have learned form there sellers, wholesalers or supply house isn't true, it does what it says it does, I have first hand knowledge with this stat.

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event