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  1. #1
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    What is a modulating furnace?

    I'm inclined to think of an ordinary (single stage) gas furnace as one which when operating is able to burn gas and produce heat at one rate. Implementation wise, I think of a single burner "stage" that is simply turned on for awhile and then turned off for awhile.

    I'm inclined to extend that definition to multistage gas furnaces, and believe that a N-stage gas furnace is one which when operating can burn gas and produce heat at N different rates and while running can switch between those rates as deemed necessary by some kind of logic. Implementation wise, I think of multiple burner stages that can be independently turned on and off. In practice I imagine (residential) N-stage gas furnaces to have but a tiny number of stages and thus be limited to a tiny number of output rates.

    I'm inclined to think of a modulating furnace as one which can output at MANY different rates because it [also] has a gas solenoid valve with a wide range of settings.

    I think, based on my own perhaps flawed "definition", that modulating > N-stage > ordinary single stage because I think a furnace which can better match output to demand has some advantage when in comes to providing comfortable regulation of temperature. More specifically, I imagine a 2-stage furnace that can output at only 36000 or 56000 vs a modulating furnace that can output at many different levels between and including 36000 and 56000. In a home that is experiencing a 44000 BTU/Hr heat loss. My "modulating furnace" is theoretically able to output at or close to the 44000 BTU/Hr required. It is able to meet demand without creating more hot feeling air and/or noise than is necessary. Whereas the 2-stage furnace will, for a time at least, output at an output level that is significantly greater than necessary. It will for a time exceed demand and create unnecessarily hot feeling air and more noise than is necessary.

    Is my thinking sound? Am I overlooking something, such as the ability of some multistage furnaces to duty cycle furnace stages so as to achieve intermediary levels of heat?

    I realize that when it comes to regulation and providing comfortable conditioning, there is more to it than just the "raw" output capabilities of a furnace. However, I am hoping that my basic view of a modulating furnace isn't fundamentally flawed. Comments? Thanks.

  2. #2
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    too tired ,I am going modulate my body into bed in small increments. From a sitting position, to a standing position, back to a lying down position, to a dream position, I am going to modulate.

  3. #3
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    May 2008
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    Portland, OR
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    Your explanation of a modulating furnace is one of the better ones I've seen. You are correct, that's exactly what a modulating furnace does. Think of them as 70+ stage furnaces.

  4. #4
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    A single stage furnace has zero range and thus has zero capability to modulate inside a range. Even a two stage furnace with proper controls can effectively modulate to any output inside its range. I would submit that the number of steps greater than one is relatively unimportant and that total range is a more important factor. Modulation then depends on implementation.

    The only furnaces that I know of that meet this implementation standard would be the Infinity two and three stage models. They appear to be the only furnaces available that can modulate their output based on the exact conditions inside the house, going beyond a crude on/off call, to modulating exact outputs based on rate of change of temperature. They do this despite n stages less than infinity.

    The major advantage of multi-stage furnaces where n stages is greater than two is a greatly expanded output range, allowing longer run times in theory. Typical two stages have a range of 30 percent where higher n number stage furnaces have double that range. That has nothing to do with the semantics of the phrase modulating, but everything to do with operating potential given good enough controls.

  5. #5
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    Actually. The Rheem/Ruud mod(13 steps of modulation) that has been out for about 13 years longer then the Infinity(3 steps), and can modulate up and down and match the house heat loss far far better then the Infinity can.

    The York/UPG mod, with 100 steps of modulation. Only mods down after the heat call ends(new control communicating should correct that soon).

    2 stage do not modulate. They are at 1 of 2 fixed outputs. But when controlled by a thermostat. They will or can be cycled up and down as often as needed during a heat call. Depending on how the thermostat is set up.

    Noise is only an issue. If the duct work is undersized for the furnace.
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  6. #6
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    Quote Originally Posted by beenthere View Post
    Actually. The Rheem/Ruud mod(13 steps of modulation) that has been out for about 13 years longer then the Infinity(3 steps), and can modulate up and down and match the house heat loss far far better then the Infinity can.
    In theory yes. How does it ramp down as it approaches the set point? All it knows is an on/off call.

    The York/UPG mod, with 100 steps of modulation. Only mods down after the heat call ends(new control communicating should correct that soon).
    Good to know. I had thought that it ramps down based on information learned from prior runs. Any idea on the delay for the new stat?

    2 stage do not modulate. They are at 1 of 2 fixed outputs. But when controlled by a thermostat. They will or can be cycled up and down as often as needed during a heat call. Depending on how the thermostat is set up.
    I would call that modulating. How is that different from what a furnace with more stages is or does?

  7. #7
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    Quote Originally Posted by commerce48 View Post
    A single stage furnace has zero range and thus has zero capability to modulate inside a range. Even a two stage furnace with proper controls can effectively modulate to any output inside its range.
    Range of what? Your second sentence implies that you are speaking of output BTU/Hr. Range implies some degree of continuity and you seem to suggest that a 2-stage furnace with only two firing rates... 36000 & 56000 BTU/Hr... can adjust its output to anything in between such as 44000 BTU/Hr. I don't think a 2-stage furnace can do that. However, both the 2-stage AND the single stage can cheat in the same basic fashion. They both can utilize the firing rates they have and time the usage in such a way that they produce a non-constant output that totals 44000 BTU over the course of an hour. For example, an 88,000 BTU/Hr single stage could "fire on for 1/2 hr, fire off for 1/2 hr". However, that is very different from a constant rate. Conceptually, if they could turn their stages on/off quickly, they could average 44000 BTU/Hr over much shorter periods. For example, that 88,000 BTU/Hr single stage could "fire on for 2 seconds, fire off for 2 seconds". However, I haven't read anything to suggest that such rapid switching is actually used.

    Quote Originally Posted by commerce48 View Post
    I would submit that the number of steps greater than one is relatively unimportant and that total range is a more important factor
    Total range as in max fire rate minus min fire rate? If so, that would seem to suggest that a two-stage with firing rates of 1 & 56000 BTU/Hr would be better than one that fires at 36000 & 56000 BTU/Hr. I must be missing something.

    Quote Originally Posted by commerce48 View Post
    Modulation then depends on implementation. The only furnaces that I know of that meet this implementation standard would be the Infinity two and three stage models. They appear to be the only furnaces available that can modulate their output based on the exact conditions inside the house, going beyond a crude on/off call, to modulating exact outputs based on rate of change of temperature. They do this despite n stages less than infinity.
    I'd certainly agree that a modern system shouldn't be hamstrung by an archaic architecture. It seems to me that *something* should be using indoor temperature as an input to the furnace output control equation. I noticed from that Infinity Control manual excerpt that you can set things up so the furnace controls staging or the Infinity Control controls staging. That reinforces an idea in my head... that some smarts are built into the furnace so that it can be connected to a dumb controller and some smarts are built into the controller so that it can be connected to a dumb furnace. Thinking about that I came to wonder whether there might be room for confusion regarding indoor temperature monitoring...

    I've seen various remarks to the effect that the Infinity *furnace* adjusts its output based on indoor temp readings but other high end *furnaces* can't do that. If other systems are like the Infinity system where either the furnace or controller can run the show, is it possible that [some] other manufacturers simply decided to leave indoor temperature based logic out of their furnace and only build it into their controller? Were such the case, the statements I've seen made would be correct AND the other high end systems wouldn't be lacking a seemingly important ability.

  8. #8
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    Quote Originally Posted by commerce48 View Post
    In theory yes. How does it ramp down as it approaches the set point? All it knows is an on/off call.

    Rheem has been using a DDC control on that furnace for longer then the Infinity has been out. So it knows more then on and off.



    Good to know. I had thought that it ramps down based on information learned from prior runs. Any idea on the delay for the new stat?

    Its suppose to be out now. But I haven't seen it yet. So it may or may not be.



    I would call that modulating. How is that different from what a furnace with more stages is or does?
    No. 2 stage is high or low. Not modulating. Same with 2 stage compressors. they are not modulating.

    No manufacturer that I know of, has or does call their 2 stage furnaces mod furnaces.
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  9. #9
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    The Rheem mod stat, adjust its input based on indoor temp. And has a return air temp sensor and a supply air temp sensor. To maintain a constant temp rise so that you don't have varying air temps coming out of your registers as it adjusts it input.
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  10. #10
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    Quote Originally Posted by beenthere View Post
    No. 2 stage is high or low. Not modulating. Same with 2 stage compressors. they are not modulating.

    No manufacturer that I know of, has or does call their 2 stage furnaces mod furnaces.
    Staging modulation is switching between any two stages without turning off. That applies to 2, 3, 15, and 60 stages equally if they have such control ability. Ideally both up and down.

    Time based modulation would be turning on and off (referred to in RandomNick's post). While this might indeed achieve good comfort, it is not very sophisticated and harms efficiency and equipment life. Nordyne refers to their IQ unit as modulating down to 15 percent, but below 50 percent it is time modulation, not gas stage modulation.

    Depending on manufacturer's marketing use of the word modulate as a good definition is sketchy. If that were so, no one could argue against Carrier's three stage as modulating as some of their marketing says it is (I currently agree, but have thought it overwrought marketing in the past and others will undoubtably agree).

  11. #11
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    Quote Originally Posted by beenthere View Post
    The Rheem mod stat, adjust its input based on indoor temp. And has a return air temp sensor and a supply air temp sensor. To maintain a constant temp rise so that you don't have varying air temps coming out of your registers as it adjusts it input.
    OK, I'm moving the Rheem up in my personal list of best modulators that read the actual indoor temp and respond appropriately. That list now includes the Infinity and the Rheem. The York may bump the Rheem back down when their controller is released.

    Do you know how the Trane works?

  12. #12
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    No. 2 stage is NOT modulating.

    Don't try to make your definition the industries definition. Because it is not. And there are 2 stage furnaces that can not go back to first stage once they go to second stage.

    1 stage
    2 stage
    3 stage
    Modulating.
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  13. #13
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    Quote Originally Posted by beenthere View Post
    No. 2 stage is NOT modulating.

    Don't try to make your definition the industries definition. Because it is not. And there are 2 stage furnaces that can not go back to first stage once they go to second stage.
    Actually, my definition is basically what the dictionary says. But I do think a good refinement for this industry would be up/down capability, not just one way. The industry does not have a definition. Modulating was a term that they came up with when they moved past two stages. Carrier uses it for their three stage. The term works well for inverter based equipment, but for some reason variable was what is often used instead for that equipment.

    Sure, there are two stage furnaces that cannot modulate down. Like so called modulators that also cannot modulate down. That is why I said if they had the control ability.

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