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two-stage gas valve query

Hello. I've asked a few people this question, and haven't been able to get an answer.
I thought I'd try here.

On a furnace that has two-stage gas valve, what causes the furnace to run at the lower btu stage versus the higher btu stage?

Answers I have received include, "it does it when its colder outside," or, "when there's a higher demand for heat it runs the higher stage."

These don't quite answer my question.

They make me think: how does the furnace know its colder outside? and, colder than what? colder than yesterday? colder than the inside? and, how does the furnace know there's a higher demand for heat?

As far as I know, a thermostat only tells a furnace "start" or "stop."
Do modern thermostats also tell the furnace a demand level in addition to "start"?

One guy said, "oh, well there's a computer in the furnace that does it all." OK. I accept that. But computers are not all-knowing. A computer is simply a decision making entity. It takes inputs and makes decisions. For example, a furnace computer could have an outside air temperature gauge that it reads to know the difference between the outside air and the inside air (assuming it also has an inside air temperature gauge). Do furnaces these days require wires to the outside for temperature sensors?

My question is, what tells the furnace when to activate the lower stage and when to activate the higher stage?



  • Steve MinnichSteve Minnich Posts: 2,312Member
    A furnace with a 2 stage gas valve is connected to a 2 stage thermostat, which have been around a long time. On a call for heat, the "first stage" of the thermostat closes a circuit to the "first stage" of the gas valve (low fire, lower gas pressure). If the the first stage of heat isn't enough to satisfy the thermostat setting, the difference between the thermostat setting and the actual temperature at the thermostat increases. When that difference reaches a certain point (around 2-3 degrees), the "second stage of the thermostat closes a circuit to the second stage of the gas valve (high fire, higher gas Pressure).
    Steve Minnich
    Tell me I can't, and I'll show you I can.
  • IronmanIronman Posts: 4,965Member
    On a two staged gas furnace the thermostat calls the first stage on (low fire). The second is called on by one of two ways, depending upon how the installer set it up: if a two stage thermostat is used, the second stage of the stat calls for the furnace to go to high fire. If a single stage stat is used, the furnace board will activate the second stage based on the length of time of the call, usually 5 or 10 minutes which is adjustable on the board.

    The reason for being able to do it based on time is because a lot of two staged furnaces are installed as a replacement where the thermostat only has one stage or the thermostat cable does not have an extra wire available for the connection of a two staged stat.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    edited January 2015
    Thank you for your replies! If I understand correctly: the lower stage comes on first, and after some predetermined amount of time (or an increase in the difference between t-stat setting and actual temperature) the higher (second) stage comes on. Also, the second stage won't come on if the t-stat is satisfied within the time limit.

    You mentioned "the furnace board will activate the second stage based on the length of time of the call, usually 5 or 10 minutes which is adjustable"

    My existing single-stage furnace is 143000 btu input. It keeps the flames on for 7 to 10 minutes per cycle (when sustaining the t-stat setting). A two-stage whose lower stage is perhaps 75000 btu is not going to bring the actual temperature up to the t-stat setting within the same period (it might take about twice as long, or 14-20 minutes). Does this mean a two-stage furnace in my house would basically be 5-10 mins of not enough heat (slow warming), followed by the second stage coming on (fast warming) - every time the furnace runs?

    If so, would I be just as well off with a single stage?

    I thought the two-stage would be an advantage to me because my existing furnace only requires 50-60 k btu/h in order to sustain my t-stat setting. But now it sounds to me as though the furnace/controls would not let the lower (first) stage run for the 45-60 mins necessary to sustain the t-stat setting (assuming a 110k btu furnace with a 75 k btu lower stage). After 10 mins, it would kick on the higher stage.

    Am I close to being right?
  • SWEISWEI Posts: 7,356Member
    The low stage is almost always fired at 60-70% of the high rate. Almost any space or water heating application will benefit from two stage firing.

    Here's a strange irony: Two stage firing reduces the consequences of oversizing, yet its greatest benefits are derived only by sizing properly.
  • IronmanIronman Posts: 4,965Member
    Why would you have a furnace that's more than twice the size needed? If you need 50-60k btu's, you only need that at design temp (the coldest night of the year). At any other OD temp, you need less btu's (about half that at 35-40* outside). This is the purpose of staging the furnace: to make it match the actual more closely. By having one that's more than twice the necessary size, you've defeated the purpose of staging.

    I would recommend that you have it wired and set so that it can only run in low fire.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • JUGHNEJUGHNE Posts: 5,579Member
    In 35 years I have just about never seen a furnace that was too small for a house. Plenty (most) of return duct too small; sometimes supply duct too small but not often. Bigger fire was always better, (for the installer, he didn't have to spend any time studying the situation as far as heat loss of the building). Not for the homeowner though, he had to pay for overfiring and short cycling for probably the next 25 years; but he felt good that the furnace could heat the house up fast.

    You were concerned about 5-10 minutes of not enough heat for the house; Is that after an overnight set back or each time the furnace fired?

    The Nordyne family (which I happen to use...though other brands operate something like this) of 2 stage furnaces has an "autostaging" feature if you must use a single stage T-stat for operation. The furnace always starts on high fire to stabilize the flame and then goes down to low fire. If your t-stat is 2 stage and only calling for low fire (first stage) to maintain temp then you stay on low, if with 2 stage t-stat the temp continues to drop maybe 2-3 degrees then high fire comes on to catch up. (that happens when you leave the front door open).

    If you use overnight setback say from 65 to 70 in the AM, then both stages would fire to catch temp up because your t-stat is asking for more than that 2-3 degree number.

    If you cannot run enough wires to upgrade to a 2 stage t-stat and must use "autostaging" then the fire starts high for a few seconds, goes to low fire and after 8 or 12 minutes (adjustable) will then go to high fire if the t-stat is still calling for heat. Use a t-stat with adaptive/intelligent recovery and it starts early to hit setpoint at the right time.

    {there are t-stats that will do everything with only 2 or 3 wires, good idea if you have AC & humidifier}

    A 2 stage furnace also gives you an ECM blower motor which has may advantages for comfort over a 1 stage fire with 4 speed blower motor. Variable speed ECM motor will compensate for defective ductwork to a certain degree.

    The first stage should heat your house most of the time. It is human nature that we remember the coldest temps experienced and ingrain that in our heads that that is the norm.
    So don't oversize and don't go cheap on the air filter.
  • ZmanZman Posts: 4,814Member
    I am assuming that you have not yet purchase the new furnace.
    From what you are saying, your existing furnace seems oversized. A furnace that size would usually heat a 4,500+ square foot home.
    I would suggest doing a heat loss calculation on the home then sizing the furnace correctly. This way the furnace will run long efficient cycles with both stages on the coldest day and long efficient cycles cycles with one stage on the typical day.
    The short cycles you have now reduce the overall system efficiency.
    If you can, use a 2 stage t-stat or a model that can stage based on actual duct temperature.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JUGHNEJUGHNE Posts: 5,579Member
    I don't use the coldest night temp of the year. If so here in NE I would be stuck with -29 degrees, and I really remember that as at the time I was a lineman and had to go out that morning. So some reference book suggests either -7 or -10. I use -10, (TO BE SAFE, see oversizing still happens, even though I know better).

    If one uses say -10 design temp and say the house only gets to 68 with full fire because it dipped to -20. Then you go outside to the mail box or go out to hopefully start the truck/car and come back inside that 68 is pretty comfortable. The body will adapt and also there are always more clothes to put on for the 1 or 2 times this happens maybe once a year.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    I have an old single stage furnace, I am investigating replacing it with a two-stage. I am trying to understand how two-stage operates and how it might operate in my home, before I buy.

    I calculated/measured that my existing single stage furnace uses 50-60 k btu to sustain the t-stat temperature in my home. I did this by recording the amount of time the gas is fired each hour - for various t-stat settings and outside temperatures. I am happy to post my measurements and calculations if you would be interested.

    What I found is, my furnace uses more btu's when it is trying to raise the temperature to a different t-stat set point, than it uses when simply sustaining the house at a particular room temperature (a constant t-stat setting).

    I also found that my furnace uses (my house requires) just about the same number of btu's per hour to sustain the temperature as the first (lower) stage of a two-stage furnace. But, in order to be able to take complete advantage of the lower stage, it would need to run for more than 10 minutes - more like 15 or 20 mins.

    Is it realistic to expect that a two-stage furnace would be able to heat my home primarily on the first (lower stage), assuming I keep the house at a constant temperature most of the time?
  • JUGHNEJUGHNE Posts: 5,579Member
    In an ideal system the furnace should run constantly on the coldest outside temp you might see and still keep the house warm. (however I don't count the worst coldest 2 1/2% of the record lows as I mentioned above).
    I believe you said you were in CO? What kind of outside temps are you having for your testing to determine that you need only 50-60 Kbtuh?. And someone here will ask you about your old...and how tight is the construction. The 143,000 BTUH of your furnace, is that input or output, there would be 2 numbers on most nameplates. You need a heat loss/gain done for your house. Do you have AC?

    Up here at about -10 we would expect a furnace should run pretty constant fire as our design temp is -7 to -10 for this area. That is burner on not necessarily just listening to the blower motor run. Somewhere you can research what your design temp should be for the area you live in. In your case elevation would make a big difference. I'm a flatlander and not familiar with mountains etc. (They just block the view and you can't farm them... as some people up here say :)
  • John Mills_5John Mills_5 Posts: 898Member
    I can't stand seeing timers used on 2 stage furnaces. You need 0 thermostat wires these days so the old argument of not enough conductors is over.

    After the furnace runs a while, the house is warming up and rarely do you need MORE heat the longer it runs, you need less. The exception is a bitter morning and the house is warming up.

    When you drive to work, once the heater starts putting out heat you goose it up and as the car gets close to temp, you turn the heat DOWN, not UP. Or your automatic climate control does it for you. The same should be true for your house.

    When I got my first 2 stage furnace it was during a bitter cold spell. I used the existing stat til my 2 stage came in. With the timer the sucker was constantly starting on low, timing to high after 10 minutes then shutting off. With the stat, it rarely ran on high, just a steady, quiet, gentle low for long periods of time.

    With us, it is automatic. Any 2 stage gets a 2 stage stat. If we don't have enough wires we use an IAQ with 2/3 conductor needs or a wireless.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    edited January 2015
    Here is an example of my calcs. I took seven readings over two hours. For this set of readings, the outside temperature was 8F, and the t-stat in my house was set at 64F. I wrote down two numbers: the gas "on" time, and the gas "off" time (time between gas off and gas on).

    Start time: 22:10
    Gas on: 7 Gas off: 15
    Gas on: 7 Gas off: 15
    Gas on: 7 Gas off: 16
    Gas on: 7 Gas off: 14
    Gas on: 8 Gas off: 15
    Gas on: 8 Gas off: 15
    Stop time: 00:24

    I took the average of these.
    Average gas "on" time = 7.3 mins
    Average gas "off" time = 15.0 mins

    I added these together to get an average cycle time of 22.3 mins.

    I divided 60 by 22.3 to get 2.7 cycles per hour, average.

    I multiplied 7.3 by 2.7 to get 19.7 mins of gas used per hour.

    I divided 19.7 by 60 to get a percentage of gas "on" time per hour of .3284 (32.84%).

    My furnace label says 143,000 btu/h input rate (no output rate listed).

    I multiplied 143000 by .3284 to get 46,961 btu per hour being used on average for the duration of the measurements.

    I made similar measurements for two hours when the outside temperature was 32F and the t-stat setting was 72F (usage was 58000), and a set for two hours when the outside temperature was 28F and the t-stat was set to 68F (usage was 48000).
  • JUGHNEJUGHNE Posts: 5,579Member
    You have put a lot of time and thought into your heat loss. You may be interested in this free heat loss program loss calculator...I have not used it but it is mentioned quite often on the Wall. This would give you a very close idea of furnace size needed at your design temp for your area. Your heating contractor should be on top of this. Where are you in CO? There is a table in the IBR guide listing several cities in CO, I would tell you the closest city number. At 8 degrees your furnace only runs 1/3 of the time..Most of CO is 0 degree or above design temp, with the exception of Alamosa, Durango, Leadville and Sterling. All references above refer to boilers & baseboard heat, but heat loss is the same regardless of heat source in your house.

    All I have stated is IMHO, no one here has a dog in the fight.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    "I would suggest doing a heat loss calculation on the home then sizing the furnace correctly."

    We've had one bid for the job so far and the guy came out and spent about an hour measuring everything, counting vents, calculating, and filling out a form. He came up with 112k btu as the heat loss. He proposed a 120k btu furnace.

    I am betting a two-stage 120k would be much less noisy compared with our current furnace: 6 jets running at full-on (23,800 btu each) is pretty noisy!
  • JUGHNEJUGHNE Posts: 5,579Member
    If your old furnace is 143,000 input and 90% efficiency than it puts 128,700 btuh into your house.

    If you have a new 120,000 input of 95% efficiency it puts 114,000 btuh into your house at high fire. If low fire is 60% of high fire than you have an output into your house of 68,400 btuh which alone would heat most houses by itself.

    Yes he downsized the furnace just a little bit, But maybe only because by looking at the old nameplate and he saw that the closest sized new furnace was the 120,000. (And in my book that is the largest furnace--single or two stage-- available for residential use)

    As mentioned above you said that at 8 degrees outside your furnace only runs for 1/3 of the time to keep the house at 64 inside. So without knowing what city you are near, how large your house is and how tight the structure we would have to guess as to what size you need. My gut feeling is that his new furnace is too big and he is sizing by old nameplate.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    edited January 2015
    At best, my old furnace is 80% efficient. Interestingly, 143K x .8 = 114400, and as you say 120k x .95 is 114000. Very similar results. So I'd agree: if my 143k is too big, then a 120k at 95% would also be deemed too big.

    American Standard/Trane offers a 115,000 unit and a 95,000 unit. Based on my measurements, I believe the 95k would work.

    I'd want to know the first stage could be run the majority of the time, so I am interested in the ability of the furnace/t-stat combo to run the first stage longer than 10 mins. I prefer the furnace to run longer at a lower heat level because I think it means it will be less noisy - and I've read that slower heat rise means more comfort (more thorough exchange of the air mass in the house).

    I'm not averse to the 120k, since although my existing furnace may be a bit too big, it doesn't short-cycle (as I understand it). Also, we keep our house quite cool. 68F is as high as we set the t-stat, and we only do that in the evenings on some days. The rest of the time we keep it at 62F, or 64F max. If we leave on vacation it goes to 61F while we are gone.

    We live in Arvada, CO and our house is 2100 sf ranch above a 1600 sf unfinished basement. As I mentioned, the first bidder spent a good while taking measurements and doing calcs by hand to arrive at the 112k btu number - so I don't know how much he may have fudged. We have a rather large set of north facing windows that keep our living room (11' ceiling) quite cold, so erring on the side of a bit too big doesn't bother me as long as we're not short cycling.
  • JUGHNEJUGHNE Posts: 5,579Member
    According to the IBR guide Denver has a design temp of 1 degree. From looking at a map you a close neighbor.

    Equipment is derated by 20% for your 5000' elevation. Your local contractor deals with this everyday. I never have to. But your existing furnace was already derated. And still seems oversized.

    However as far as 2 staging operation goes you want a unit sized small enough that you do get a long run time on low fire but still use the high stage for the few times the ODT approaches that 1 degree. When you were checking run times at 8* ODT you were close to that design temp. IMO The on time should have been longer.

    People who get a variable speed blower and leave the fan switch on during the heating season have noticed improvement in comfort. (That motor slows way down between firing cycles and uses very little electricity )
  • SWEISWEI Posts: 7,356Member
    JUGHNE said:

    Equipment is derated by 20% for your 5000' elevation.

    Natural draft equipment is derated thusly. Induced draft and forced combustion appliances may or may not be derated and if so, by varying amounts. Derating for NG and LPG often differs as well.
  • JUGHNEJUGHNE Posts: 5,579Member
    Thanks for jumping in for this. His elevation corrections are more than 3000' over my head, ;) But his local area contractors are on top of this. His existing equipment is not natural draft, it is either an 80 or 90% plus. But in any event it sounds oversized for the house. Thanks again, as I said I never have to deal with altitude corrections.
  • SWEISWEI Posts: 7,356Member
    I've been working at altitude for 15 years and I'm still learning. There's quite a bit of misinformation out there, and an unfortunate percentage of it emanates from manufacturers.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    edited January 2015
    My existing equipment is 80% at best, circa 1988. It has induced draft and intermittent pilot, that is about it.

    The second bidder came today. He didn't do a load calc, but he did measure the input and output plenums. (He also did not look at the plate showing my furnace is 143k btu). He had a whiz-bang chart that showed for my duct sizes (17x8) the tons should be 3. Meaning, putting a 4 ton unit/system in doesn't do much good since the ducts cannot move enough air volume to support it. Does that sound right?

    Apparently, one 17x8 duct is good for 650 cfm. My furnace is connected in a 'T' fashion in the middle of a long run of 17x8 ducting. Air comes up from the furnace and 650 cfm goes to the left, and 650 cfm goes to the right (if I've explained that correctly), for a total of 1300 cfm. The guy said it was 400 cfm per ton, which is 1300/400 = 3.25T.

    I've looked around to try and find such a duct size to cfm chart, but have not had any luck yet.

    My furnace is Carrier/Bryant model 3958AW048120
    My A/C compressor is model RAKA-042JAZ (3.5 ton)
    My A/C evaporator is model RCBA-4873AS21 (3 to 3.5 ton)
  • JUGHNEJUGHNE Posts: 5,579Member
    So you have a 3 1/2 AC; does it keep your house comfortable, or does it seem to not be working well and leave you feeling uncomfortably "sticky/sweaty" in the house? You mentioned a 4 ton AC. I've always assumed that the Denver area was pretty dry in the summer.

    AC sizing is more critical to comfort than heating sizes are. And smaller is better. Again AC should run continously on hottest day of year. If it cycles much at 95* then it is bigger than it needs.
    AC is more of a de-humidifier than cooler. That is where the comfort comes from. To me a 100* in AZ feels like only 80* in the sweat box of NE.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    These past couple summers, we've only run our A/C in the evening, maybe from 5p-9p or 10p at the latest. It cools the house down, and we don't bother messing with A/C again until the next evening.

    The biggest issue I have with our HVAC system is that it can barely get the temp inside to 70F (22F outside), or down to 75F when it is 80 or 90 outside.

    I have more questions about temperature rise rating of the furnace, should I start a new thread?
  • JUGHNEJUGHNE Posts: 5,579Member
    Are you talking about the actual temp rise of the furnace itself; such as the air temp going in and air temp coming out; which would be the temp rise across the heat exchanger?
    Or you referring to the temperature rise inside your house as the furnace heats?
  • furnaceNewbyfurnaceNewby Posts: 38Member
    Across the heat exchanger. My furnace is rated at 65-95F degrees of temperature rise. I can measure it and it falls within that range.

    One of the new furnaces I am considering is rated 30-60F degrees of temperature rise. Seems like a much colder rating.

    I have ducts that run thru crawl spaces that drop 30F by the time the air comes out the far seems that I'll be insulating those ducts once I get a new furnace.
  • Steve MinnichSteve Minnich Posts: 2,312Member
    Ideally, all ducts should be sealed with mastic and all ducts that run through an unconditioned space should be wrapped with R-8 foil-back insulation.
    Steve Minnich
    Tell me I can't, and I'll show you I can.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    Thanks. I'll be the first to tell you that my heating system is far from ideal! :-)
  • furnaceNewbyfurnaceNewby Posts: 38Member
    Are most new furnaces given only a 30-60 degree rise rating?

    What are the implications for someone like me who is moving away from an existing furnace that has a 65-95 degree rise rating?
  • JUGHNEJUGHNE Posts: 5,579Member
    The brand I use has 35-65 rise for the size you might need. I've put is quite a few furnaces with this rise or lower. I never checked or thought about temp rise compared to the old beasts. The furnaces removed were big hotties like what you have. Almost everyone enjoyed more comfort, although there were other improvements to their HVAC systems, mainly variable speed blower motors which were the main contributor to comfort. The blower may be adjusted to operate the furnace in the upper range of temp rise. Maybe once in a while there was cooler air temp comments but everyone adjusted.
  • furnaceNewbyfurnaceNewby Posts: 38Member
    Is it fair to say that I measured the heat loss of my house at 60k but per hour?

    Does a heat loss calculation determine the size of the furnace needed on the coldest day? And if so, what does it assume for the indoor temperature (using 1F for Denver as the ODT)?

    How does a heat loss calculation differ from measuring the btu/h like I did above? I mean, what different information does the heat loss calculation yield?

    Thanks again!
  • bones5722bones5722 Posts: 1Member
    58mva carrier 2 stage furnace. Up to 16 minutes on low fire, the furnace (through the control board) should go into high fire and high speed blower. Because it is calling for heat for the 16 minutes, the control board makes the switch to high fire and high speed blower. When the thermostat is satisfied and it shuts off, upon the next heating cycle the control board calculates the low fire time and holigh fire time with a formula. So that the new call for heat is on low fire less time before high fire. And so on to a point that when thermostat calls for heat it automatically is on high fire. And high blower speed. If high fire does not occur and blower speeds up check at this time for 24 volts to gas valve. F its not present on high fire it's the control board. If it is present its then the valve.
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