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  1. #1
    Join Date
    Feb 2008
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    30

    How does a modulating furnace modulate?

    I am looking to upgrade my HVAC and going through the usual research between single stage, 2-stage, and modulating furnaces. I have read some of the threads here on this already.

    I have also looked at the thermostat connection schematics for the York 9C series furnaces and see that generally it just takes a single stage thermostat "w" wire connected to the Modulating Furnace control board W terminal.

    So all the board ever sees is just a typical call for heat. How then does the furnace decide at what blower speed and flame % to run? Is it just a timed thing or are there other inputs and sensors in the furnace? I really like the idea of extended lower % running for comfort but wonder just how it works and if it is adjustable to any extent?

  2. #2
    Join Date
    Feb 2007
    Posts
    999
    As your post's title is generic in nature, I'll comment on the Rheem modulating furnace, based on my experience.

    Simply stated, modulation is controlled by their modulating thermostat. The stat senses the room temperature, and based on the set point, creates a signal (V signal) which causes the gas valve to open to the required flow (BTUs).

    The control board will set the airflow to provide (maintain) a proper temperature rise. When starting up 'cold', based upon the original temperature difference (previous paragraph), the thermostat will select a heat level designed to rapidly address the temperature difference.

    As room temperature rises, the stat will rapidly respond by lowering the heat output of the furnace in stages, until eventually, an equilibrium is reached with the house and its contents. Once that point is reached, the furnace will continue to operate at that level and can respond to increases or decreases in heating demand.

    Other brands operate by pre-programmed software, some of which may be based on prior heating cycles.

    Likewise, some brands differ in the way they modulate (up/down).

    Amp

  3. #3
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,916
    I know little about residential equipment, but the modulating burners that I've worked on control based on air temperature.

    Airflow across the heater remains constant and a special type of gas valve regulates more or less gas into the burner to keep a constant air temperature.

  4. #4
    Join Date
    Feb 2008
    Posts
    30
    Thanks for the input amp....

    I am hoping that some of these modulating furnaces would work w/o a proprietary thermostat. I have a Wifi enabled 3M thermostat that does 1 or 2 stages and I would like to keep that with any new system I get. It lets me control my system remotely over the Internet via PC or smartphone.

    I was looking at a York but not committed to any brand. I just happened to find the install manual for it and was trying to understand how the modulation works since it does have connect diagrams for single stage t-stats and there don't appear to be any specific modulating t-stats listed.

    I guess I could see how such a system would "learn" over time based on past heating cycles. But I wonder how it would work in a situation where heat was setback to say 60 during the day, and then upped to 70 in the evening. If it just gets a basic call for heat what would it do then? That is one situation you would want it to go full blast but how would the system know?

    I guess I could get the dealer out for a quote and try to get a more definite answers, but I'd rather skip the sales pitches until I understand my options better.

  5. #5
    Join Date
    Jun 2006
    Posts
    96
    Our Tekmar control system monitors outside temperature and insider temperature as well as the rate of temperature drop and temperature rise. From this, it calculates how may BTU's our Weil McLain boiler it needs to heat the water in our hydronic system to a specific temperature and keep it there during the heating cycle.

    I have to say that the system works brilliantly. Not only does it save gas, but we don't have severe peaks and valleys in indoor air temperature. We have nice even heat in our home including during the shoulder heating seasons in fall and spring. Our old boiler was either on 100% or off. This let to overheating of the rooms especially in the shoulder seasons.

  6. #6
    Join Date
    Jun 2006
    Posts
    96
    Quote Originally Posted by raylo View Post
    Thanks for the input amp....

    I am hoping that some of these modulating furnaces would work w/o a proprietary thermostat. I have a Wifi enabled 3M thermostat that does 1 or 2 stages and I would like to keep that with any new system I get. It lets me control my system remotely over the Internet via PC or smartphone.

    I was looking at a York but not committed to any brand. I just happened to find the install manual for it and was trying to understand how the modulation works since it does have connect diagrams for single stage t-stats and there don't appear to be any specific modulating t-stats listed.

    I guess I could see how such a system would "learn" over time based on past heating cycles. But I wonder how it would work in a situation where heat was setback to say 60 during the day, and then upped to 70 in the evening. If it just gets a basic call for heat what would it do then? That is one situation you would want it to go full blast but how would the system know?

    I guess I could get the dealer out for a quote and try to get a more definite answers, but I'd rather skip the sales pitches until I understand my options better.
    I think most setback thermostats learn how long it will take at a given outside (and inside temperature) to get the house back to the desired temperature. Our old Honeywell thermostat did this 20 years ago in our old house. The only time it was "out" was when it was -35C for a week at night and then the temperatures rose to -20C suddenly (overnight).

  7. #7
    Join Date
    Feb 2008
    Posts
    30
    My stat has a time adjust feature but i find it easier just to set the actual time I want the thing to kick on than to have a time when I want the temp to be X. It just isn't that critical.

    But that isn't the question. The question is: what does a modulating furnace like a York 9PYC series model that is connected to a single stage t-stat base its decisions to select and change blower and burner % for the start of and during any given heat cycle?

    Quote Originally Posted by kayjh View Post
    I think most setback thermostats learn how long it will take at a given outside (and inside temperature) to get the house back to the desired temperature. Our old Honeywell thermostat did this 20 years ago in our old house. The only time it was "out" was when it was -35C for a week at night and then the temperatures rose to -20C suddenly (overnight).

  8. #8
    I cant say about york but i know about carrier cva 2stage variable speed they can run on a plan ol tstat and it will control the speeds on time its built into the logic on the control board

  9. #9
    Join Date
    Sep 2002
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    I don't know
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    York's furnace has a algorithm which works based on runtime and previous cycles.

    I believe that if the required output is above 50% or something like that, the furnace continues to run (output drops down though) after the t-stat is satisfied.

    York's literature specifically states to set the cph parameter to 6. (3 is normal for high eff furnaces)

    Lennox's older mod (G71) uses a 2 stage t-stat; output slowly increases when there's a call for second stage, but doesn't drop back down when the W2 call ceases.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,586
    Rheem uses a special stat, communicating or otherwise, to tell the furnace what the HOUSE NEEDS. Then the gas valve modulates between 40% and 100% as needed. It will stage up & down as the heat needs of the house changes. The blower is variable speed and varies with the firing rate to maintain 50 or 65 temp rise.

    Trane & Nordyne use communicating stats as well to tell the furnace what level to fire at.

  11. #11
    Join Date
    Jun 2005
    Location
    Winston-Salem NC
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    1,133
    Quote Originally Posted by kayjh View Post
    I think most setback thermostats learn how long it will take at a given outside (and inside temperature) to get the house back to the desired temperature. Our old Honeywell thermostat did this 20 years ago in our old house. The only time it was "out" was when it was -35C for a week at night and then the temperatures rose to -20C suddenly (overnight).
    Where may I ask?
    that's -31 degrees F

  12. #12
    Join Date
    Feb 2008
    Posts
    30
    What is the "cph parameter". I don't see that term in the literature.

    Quote Originally Posted by amd View Post
    York's furnace has a algorithm which works based on runtime and previous cycles.

    York's literature specifically states to set the cph parameter to 6. (3 is normal for high eff furnaces)

  13. #13
    Join Date
    Feb 2008
    Posts
    30
    Those communicating systems seem like they would produce the most accurate response to need. But to do that they have to use the proprietary thermostats. Maybe they also have an algorhythmic mode for use with a generic t-stat? When I get my new system I really would like to keep my existing 3M wifi enabled t-stat so any systems that absolutely require a special t-stat are out.

    Quote Originally Posted by BaldLoonie View Post
    Rheem uses a special stat, communicating or otherwise, to tell the furnace what the HOUSE NEEDS. Then the gas valve modulates between 40% and 100% as needed. It will stage up & down as the heat needs of the house changes. The blower is variable speed and varies with the firing rate to maintain 50 or 65 temp rise.

    Trane & Nordyne use communicating stats as well to tell the furnace what level to fire at.

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