modulating vs two-stage
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  1. #1
    Join Date
    Jun 2007
    Location
    Lake Forest Park, WA
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    35

    modulating vs two-stage

    Reading and reading and reading... Want to make the best decision for me, so will try to keep questions simple.

    For starters, this is a heating furnace only. No heat pump, no AC, so please don't offer that info. Too many other threads are hard to decipher the heating info because they're outweighed by SEER and tonnage and cooling coils, and...and...and...well, you get the point.

    I had never heard of modulating furnaces until this forum. Initial reading sounds like they're the greatest thing since sliced bread....almost. Then I start reading about:

    - blower rates may not be sufficient.
    - you can/can't run setbacks on mods.
    - you need/don't need special tstats.
    - mods are/are not quieter than two-stage.

    So, should I only consider a modulating furnace or not?

    Okay, gentlemen....start yer engines!

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,884
    I am only familiar with Rheem's Mod where the thermostat determines firing rate and the blower varies to maintain temp rise.

    Never had an issue with blower speed
    The Mod stat is programmable
    With Rheem's you do need the special stat. York lets their board decide firing rate regardless of house needs so you can use a single stage stat
    Rheem's Mod is as quiet or quieter than most 2 stage 90s.

    The chart below shows how Rheem's Mod output can virtually follow the heat loss of the house below freezing where a 2 stage will have lots of cycles.


  3. #3
    Join Date
    Jun 2007
    Location
    Lake Forest Park, WA
    Posts
    35
    I found that chart searching the forums in my research, but I don't understand what I'm looking at. WGFD and MVP must make sense to HVAC pros, but don't tell me which line is two-stage or modulating.

    Can you explain it better to this poor layman?

  4. #4
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584
    WGFD-Rheem mod furnace
    MVP-2 stage Carrier
    "Everyday above ground, is a good day".
    "But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,884
    Yea, the chart is comparing the Rheem Mod (using an old WeatherKing model # WGFD which is a RGFD in Rheem brand) to the Carrier MVP 2 stage furnace. Actually it is a comparison to ANY 2 stage of the same size, the originator just used the figures from Carrier's site. The Carrier is 65% output on low, A-S/Trane is 60%, most others 70%.

    The heat loss of the house is in purple. That line represents heat lost at varying outdoor temps. Right around 35 out, the loss of the house will be around the same BTU figure as the RGFD-09 furnace puts out on lowest fire. As the outdoor temp drops and the heat loss increases, the Mod slowly increases its output almost perfectly matching the needs of the house. Meanwhile, the 2 stage keeps cycling which is less efficient and definitely less comfy. At 0, the 2 stage furnace would start running constantly on low fire and occasionally need to cycle to high when you hit -10 or -20 out. Obviously in a very cold climate.

    A more accurate comparo would be a slightly larger house and comparing the Rheem 105K model to Carrier's 100K. But this is the chart I had to work with. The other comparison would just have a lower constant run point for the Mod.

    The Mod can't do near what the Infinity control can do with blower speeds, dehumidification and readouts on the stat. But it can provide heating comfort the Infinity and others can't. For a cold climate, it's best. In a steamy climate or if you just want to play with the control, the Infinity is best.

  6. #6
    Join Date
    Sep 2004
    Location
    Memphis
    Posts
    2,502
    A few years ago I switched out my Carrier 80% single stage furnace to a Rheem 90% modulating furnace. At the same time I switched my upstairs to a regular Rheem super quiet 80 furnace. I personally cant stand that mod. At the same time I redid all my ductwork. Summertime is fine, the A/C cools it off good. Wintertime though, the airflow is so week that some rooms are just nonexistent. I keep the thermostat on 74-75 just to be comfortable. And before with that Carrier, we'd always keep it around 71. The upstairs system works great, but that mod is a disappointment to me. I will say this though, even with that thing running all the time & me still feeling cold in here, the utility bills are very reasonable.

    I honestly wish I had just put a 90% single stage furnace in.

    I've screwed with those toggle switches & still have some rooms with just a breath of airflow. And no vents are strong enough that I feel like I can damper em down any.
    Life is like a jar of jalapenos. What you do today might burn your ass tomorrow.

  7. #7
    Join Date
    Aug 2007
    Posts
    24

    I have heard the same thing...

    ...from my Premier Rheem Team Contractor who told me on Saturday that I risk "challanges" in heating my master bath that is above the barely insulated garage and farthest away from the furnace.

    He also notified me that to get the approximate 1000 CFM of my current single stage 25 year old blower with heating and cooling off but the fan on, he could only provide about 800 CFM with the mod furnace outside of the cooling cycle with the fan on. He said he could "fake" the furnace out to make it think it was in the cooling cycle (when it would not be with only the fan on) and that is how he could reach 1000 CFM it he needed to.

    The key to keep in mind is that each home is different. Each dealer is different. I can easily see how a bad install could give a mod furnace a bad name. There is alot of options that a Rheem Team contractor can work with if they decide they want to. At least that is what my contractor was able to demonstrate to me with the different configs that he and his peers have recently been involved with.

    Having said all this, I am glad that I looked at it as it helped me better understand the challanges of my two story colonial. I now know that I will ask a few more questions regarding the two stage options to see how the low heat stage will or will not handle the same challanges of the home.

    I count myself lucky to have found a Rheem Team contractor that could talk to alot of installs, as well as alot of pros and cons of the mod furnace itself. Another Rheem dealer was not as able (or willing) to talk about these concerns.
    Good luck.

  8. #8
    Join Date
    Feb 2005
    Location
    cincinnati ohio
    Posts
    2,020

    two stage

    The orginal ideal behind a 2 stage furnace is a comfort thing . A drafty house and a furnace that cycles alot ( on - off - on - off ) gives you the hot and cold feeling . A two stage should run longer and give you a more even heat . Im not sure if the mod furnace provides this comfort of a two stage . mabe someone else has a better ideal on the difference between the two .
    My avatar is a picture of a Goodman Silencer .....These were commonly used in Goodman country ....Photos by hvac tech ( PaysonHVAC )

  9. #9
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584

    Thumbs up 2~stage furnace* Carrier

    I will tell you about my experience with 2 stage furnaces and I would not be able to say anything about the Rheem modulating furnace because I have never installed one or worked on one as of yet.

    I sold American Standard /Trane equipment for several years and there 2 stage furnace are great even the 80%.

    I would have to say that being that I now sell alot of Carrier Infinity systems and mostly 80% models, the performance and even temperatures on the homes I have sold and started systems up in are great, the clients have nothing but , great things to say about how much better the comfort level is vs, there old furnaces.I have sold only a few 90% furnaces here in Texas so I can't say much in the way of pro's-con's on them. The only issues I have with 90% models is the drain system could stand some improvement.
    "Everyday above ground, is a good day".
    "But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

  10. #10
    Join Date
    Aug 2007
    Posts
    24

    mod versus 2-stage...

    Based on the pros that have come to visit the big difference is:

    two stage runs at 65% and 100% with a low heat output and a high heat output. I did not realize that it is good to look at both as the low heat output versus your low range of your load calc will give you an idea as to how oversized you are.

    Rheem's mod can run as a three stage at 40%, 65% and 100% (I believe these values are accurate). The 40% stage is where my challange could be due to the amount of btu's that are being pushed to the most tough location in my 2-story colonial (versus the low stage of 65% on the 2-stage models).

    You can also run the Rheem as a quazi-modulating furnace if you go to a 2-stage stat instead of the modulating stat. But then you lose some of your modulating capability.

    The most "advanced" stage is the fully modulating stat with the mod furnace.
    In theory, you may even notice a mod furnace running longer than a 2-stage as it most likely will strive to keep an even heat (according to the stats to within a half of degree of the temp on the stat, I believe). This is not bad and is by design. Remember, the low stage on the Rheem runs at 40% versus the low stage of 65% on the 2-stage models, so you are using less btu's and heating to a more constant delta. I would say that the on-off on-off functionality of a 2-stage is much reduced because the mod furnace can by very definition "modulate" in 5% increments (I believe).

    So you have the best efficiency in terms of using only the btu's that is really needed to keep you at your desired comfort level. It should also give you the most even heat if you understand how it works (running more often at a lower heating stage adjusting in 5% increments to keep your temp consistent) and your home design works in your favor.

    The pros that work on these everyday can translate all this into pro speak and can make it sound real pretty, but I am pretty sure I got the jist of it.

    To me, the idea makes a lot of sense. The home owner needs to make sure the installer is very comforatble installing these and understanding any limitations that have been seen (ie, running 3 stages or quazi-modulating instead of fully modulating to respond to home owner concerns after the fact).

    At least, that was the advice given to me. The one thing I haven't done yet is ask for and talk to references of mod furnace users with at least one winter's heat cycle and a 2 story colonial to see what the customer feels comfort wise after the fact.

  11. #11
    Join Date
    Nov 2005
    Posts
    1,350
    MOD VS. 2STAGE I am only familiar with the York mod 95. The biggest advantage of the mod furnace is the comfort, 65 stages. Second would be the energy savings, and rebates, up to $1500 in some areas. Third would be noise, first hand just way quieter with the vs motor. With the York no special stats are needed, and you can uses any set back stat you want to.

    You will have to look at your bids and see if the cost difference is worth it to you.
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  12. #12
    Join Date
    Jan 2006
    Location
    Lilburn, Georgia
    Posts
    52
    Another "ho" here with only the best comments to say about the rheem mod. The duct work on the first floor where we have it installed is very standard. Only a small laundry room is at an extreme end of the house. The heating level is consistent in all rooms. There is almost no noise from the system.

    The only complaint regarding the mod I have received from my family is the noise and the amount of airflow when the AC kicks on - suddenly they are aware of mechanics that were all but invisible during heating season.

    Probably the only negative is that it's efficiency is only 90 or 92 % (sorry can't recall exactly) whereas other furnaces may be higher. Given the chance to repeat the purchase I'd only consider a heatpump in it's place. In both instances good duct design is a must.

    Ah yes - can also say that a 3m filter from HD fits snuggly and is a little challenging on the unit - and I plan to put a honeywell version of the Trane Clean Effects on at some point in the future. The throwaway filter is fine. I have the fancy Trane filter on the 2nd floor and think it is one of the better solutions available - although many have said that solutions by other manufacturers are also good.

  13. #13
    Join Date
    Jun 2007
    Location
    Lake Forest Park, WA
    Posts
    35
    I appreciate all the comments. If I hadn't learned anything about mod furnaces, it would have been a no-brainer; the Carrier 2-stage. But since I do now know about the mods, I have to decide between the Rheem and York dealers. The Rheem dealer has a good reputation, but the salesman (a nice guy) has not made me feel good about them.

    Not hearing anything from the York folks. They all on vacation?

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