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Community and Q&A

What size of high efficiency furnace should I buy for a 2-storey house?

Greenpeace | Posted in Energy Efficiency and Durability on

Hi, I have difficulties in deciding whether to buy a 80000 btu or 60000 btu 2-stage multi-speed motor furnace.

House is in Zone 7A – cold climate built in 1973. 2 storey rectangular shape, detached
Existing furnace 45 year-old 110,000 btu 70% efficiency, single stage still working fine but have 2 cold spots that could be due to windows
main entrance of house facing NE but the backyard is facing NW with a baseball and a soccer field (no houses in the big open area)
always have people in the house including seniors

using my gas bills to calculate heat load. if I did it correctly, it seems to suggest a required output of 41409 btu/hour; with permissible oversizing by 1.4, it becomes 57973 btu/hour. Same period last year (Dec 2016 to April 2017) suggests higher numbers (46136 btu/hours or 40% oversize 64591 btu/hour). In my calculation, I assumed my old furnace efficiency is 60%. 

Energy audits were done in 2010 and 2018 with air blower tests
2010 report – my home’s design heat loss would be 49003 Btu/hour (14361 Watts) and design cooling load would be 35826 Btu/hour (3 tons). attic insulation upgrade to R50 was done after the energy audit

2018 report – heated floor area 2261.5 sq ft although I calculated the heating area excluding finished basement and garage is 1800 sq ft.
Airtightness: 6.56 air changes per hour at 50 pascals
total 17 windows; mostly two pane energy efficient windows >10 yrs

Amana AMEC96
60000 btu low fire output 40320 btu  high fire output 57,600 btu
80000 btu low fire output 53760 btu  high fire output 76,800 btu
alternatively I may choose Lennox 70000 btu but prefer Amana for better warranty and stainless steel heat exchangers. Supplier did not suggest Rheem which I am also interested.

supplier suggested 80,000 btu claiming I may feel cold with 60000 btu? both sizes are recommended by different retailers but none of them did a heat loss calculation.

Based on the above information, would you suggest 80,000 btu or 60000 btu? what should be the right size for my house? 

Hope to receive some advice or let me know if I miss any other factors to decide.
Thank you.

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Replies

  1. joshdurston | | #1

    Even the smaller furnace you mentioned is oversized if your heat loss calcs are accurate. IMHO, you'd be crazy to get the 80kbtu furnace. Don't make the mistake of thinking maxing out your oversizing is a good thing. Getting a 2 stage furnace where even the low stage is higher than your design heat loss is not ideal. My last house had an oversized two stage furnace, I simply disconnected the second stage. Even first stage had no problem maintaining temp on the coldest nights.

    Ironically you are more likely to "feel cold" with an oversized furnace because it will cycle off faster and have short run cycles. Size it properly (as in just barely big enough) for long cycles and nice even comfort throughout your house with less noticeable cold corners.

    Existing furnace is likely oversized, especially if any sort of updates have been done.
    If it were my house, I would pay someone I trust to do a very accurate heat loss, and I suspect you might get by with 48,000 rather than a 60,000btu/hr but you won't know without a good heat loss calc.

    Smaller furnace means less noise from trying to force a lot of air thru undersized ductwork. Make sure you get an ECM fan.

  2. Expert Member
    Dana Dorsett | | #2

    Do NOT assume that the existing furnace was running 60% efficiency unless it has actually been tested with a combustion analyzer at that efficiency. Hot air furnaces don't tend to degrade in efficiency very much over time, unlike cast iron boilers. The nameplate efficiency of most furnaces in 1973 was 78-80%. The DOE output divided by the DOE input would be the steady state efficiency, even if the AFUE (which includes standby losses such as standing pilot ignition) is a bit lower. Use either AFUE or the DOE out/in ratio, but not some presumed degradation.

    Assuming it's really ~80% efficiency the calculated 41.4K load based on 60% becomes 41.4K s 80/60= 55,200 BTU/hr. If it's really ~70% efficiency as alleged it beoms 41.4K x 70/60= 48.3K.

    The ideal 2-stage furnace would have a low fire output that's below the calculated load, with a high-fire that still exceeds the calculated load, but not by more than 1.4x. It looks like the 60K Amana AMEC96 fills the bill.

    The Manual-J heat load calculations take a WAG at what the system losses will be, but reality can can be either higher or lower than that. A fuel use load calculation already includes those losses. If your Manual-J prior to the insulation upgrades suggested a load of 49K and the measured reality based on fuel use is more like 48-55K even AFTER upgrades it's probably a hint that there are duct imbalances or duct leakage driving a parasitic infiltration load whenever the air handler is running. While that's worth sleuthing out and fixing, it's pretty clear that you'll be fine with the 60K furnace.

    An undersized furnace doesn't make you feel cold- on the contrary, it runs very long warming cycles as the weather gets colder which tends to be MORE comfortable, as long as it's still keeping up with the load. You only feel cold during the hours that the load exceeds capacity by several degrees, not just a couple of degrees, since you'll have a "summer breeze" of warm air constantly coming out of the registers even as it's losing ground bit. Even in the worst-case fuel-use based load calculation based on 80% efficiency the 60K furnace will cover the load more than 99% of the time.

    1. user-4053553 | | #3

      I agree with what you have written.
      I find interesting the last line, one would also have plug loads delivering heat into the house not to mention people.

  3. walta100 | | #4

    If you are getting a 2 stage furnace read the bids carefully as they almost never include a 2 stage thermostat because add that would require replacing the thermostat wires not an easy job. So what they do is set the furnace to run for 20 minutes on low and then kick up to high.

    My gut says the smaller furnace will do fine.

    Walta

    1. user-4053553 | | #5

      How is a thermostat going to do a better job, would it not have a timer built in and switch to second stage if first stage is not enough?

  4. walta100 | | #6

    Without the correct wiring and thermostat you only get to use the low output for 20 minutes before kicks into high and shuts off. Hardly worth spending the money for 2 stage if you are barely going to use it.

    Walta

    1. user-4053553 | | #7

      It should shut off once the thermostat is satisfied by the furnace. Thats a good thing.
      I have a dual stage and its furnace controlled. Its not 20 minutes though.
      What does a thermostat control do for you that your wanting?

      1. joshdurston | | #8

        Thermostats (not just fancy smart thermostats) often have differential or timing setting to defer second stage to when it's needed, or more sophisticated control algorithms. If first stage has run for 45minutes but your within half a degree of set point, then it's better not to stage up, but if you're off set point by 5deg you may want to kick in second stage sooner rather than later.

        1. user-4053553 | | #11

          I know having it furnace controlled its not perfect but its more then adequate. The biggest thing i would want is the ability to customize the number of minutes before step up, something a thermostat controlled can do but as long as its not stupidly quick all the time when its unnecessary i don't see a huge problem with being furnace controlled.
          Much like cars, more features is nicer but they cost more, but no one needs the fanciest features, if your car does not have Ludicrous Speed it does not ruin its ability to get you from A to B.

          BTW 45 minutes is way too long, i would aim for 20 minutes based on my experience.

          1. joshdurston | | #13

            Warning I'm not trying to confuse the OP, but just wanted to clarify to Alan B what I'm trying to say.
            I tend to geek out a bit more than necessary on this (I'm a building controls engineer). My point was that runtime is irrelevant as long as you are maintaining setpoint. 45minutes would be a long time (too long) if the space was actually a couple degrees below setpoint. You would want 2 stage up sooner to avoid comfort issues if you came home to a cold house. Using a PI(D) based control algorithm with a bit of deadband, would allow you to stage quite quickly if you have a huge error off setpoint (maybe 5minutes), more slowly if you're "winning", but would hold a stage indefinitely if it got within the dead band. There really isn't a fixed ideal time to stage up. But, if you are going to ignore temperature and only look at runtime, I agree 20minutes is probably a good compromise. Honeywell thermostats let you specify a cycles per hour in the setup menu, and typically you would have around 3 cph (20minute control period), so if your heat runs more than 20 minutes it basically means that it is at 100% demand. This matches nicely with the 20 minute stage up delay on the a 2 stage furnace.

    2. Greenpeace | | #9

      Thank you all for sharing your expert knowledge. I am actually located in Toronto. Currently I have a very old thermostat with 2 wires. The dealer said he will install a Honeywell digital programmable thermostat for me and also try to add 2 wires to the thermostat (just in case I will also install an air conditioner). He is the only one who offers to do this. In addition, the fan motor he recommends is multi-speed motor, not the variable one. Questions:

      1. are 20 minutes considered too long or too short for low fire to run? Do they have different time settings? and what is the ideal setting?

      2. I saw one video of a dealer in Ottawa who said variable speed motor is going to push X amount of pressure to the furnace whereas a fixed speed motor runs of a fixed amount of velocity. Fixed speed motor is suitable for houses built before 1980 whereas variable fan motor works great in houses built after 1980 because the ductwork of old houses may not be compatible. If this is true, having a fixed speed DCM seems right for my 45 year-old house. He said the energy saving difference is not big. (there is energy savings if I had air conditioning and it is on electricity only, no savings in gas) do I understand it correctly?

      3. If I am going to have a fixed DCM 2 stage furnace, will changing the return duct from 8 x 24" to 10 x 24" necessary to improve the performance of the furnace?

      4. do I really need to insist on a 2 stage thermostat?

      5. One thing I don't understand is two heat exchangers are supposed to share the heating job with a 2 stage furnace, wouldn't the disadvantage of a single stage furnace, that is having the heat exchanger turning on and off frequently also apply to a 2 stage furnace? and is going to shorten the life of both heat exchangers?

      6. Could I simply order 96% single stage furnace since it is also high efficient? But how comes a single stage can also reach that level of efficiency? (without the 2 heat exchangers). could you explain to a layman like me? Since my 46 oversized furnace has not given me any trouble, I am a bit biased towards single stage. But do love to hear favourable feedback on 2 stage furnace in terms of reliability.

      7. As mentioned in my first post, I did two calculations for different periods. The one done for December 2016 to April 2017 results in a higher heat load demand. that's why not 100% confident in my own calculations. If we use that number, will your recommendation as to the right size change?

      Thank you.

      1. user-4053553 | | #12

        I think your person is offering 2 stage or multi speed. This is not a motor type per se (but can be) its a type of furnace.
        I got quotes for a single stage, 2 stage and multi speed. I went with 2 stage since the multispeed is the Cadillac (or Tesla) of furnace types and unnecessary unless you want to blow money.
        An ECM motor can be used on all 3 types, it is not only specific to a furnace type such as 2 stage or multi variable speeds (also called infinitely variable speeds if so designed). On a single stage its a waste of money since that would run at a single max speed only. On 2 stage it runs on two speeds (or 3 depending on the unit and if you install AC) and on multi speed would have a programmed number or infinite number of speeds. If you don't use an ECM motor on 2 stage or multi speed then it will waste loads of electricity because PSC motors use almost as much energy to run at half speed for example as full speed. Get the ECM motor if your going two stage or multi speed, and i recommend 2 stage unless you want to blow money on a multi variable speed.

        "1. are 20 minutes considered too long or too short for low fire to run? Do they have different time settings? and what is the ideal setting?"

        Mine is about 15 minutes and 20 mins would work better in my house IMO (mine is furnace controlled). Funny that i said 20 mins above before i read this or the reply below which i will get to. Having the ability to have it thermostat controlled is a bonus but not necessary. Once its installed set the thermostat where you want it, even a programmable thermostat to save a few percent if you wish then go on with life.

        "2. I saw one video of a dealer in Ottawa who said variable speed motor is going to push X amount of pressure to the furnace whereas a fixed speed motor runs of a fixed amount of velocity. Fixed speed motor is suitable for houses built before 1980 whereas variable fan motor works great in houses built after 1980 because the ductwork of old houses may not be compatible. If this is true, having a fixed speed DCM seems right for my 45 year-old house. He said the energy saving difference is not big. (there is energy savings if I had air conditioning and it is on electricity only, no savings in gas) do I understand it correctly?"

        1980 was not a magic year in housing, though i understand interest rates and inflation were not happy around this time.
        2 stage is the best balance of cost and comfort in any forced air heat house, sized properly.
        The energy savings from 2 stage to multi speed is not huge, a few percent and its more for bragging rights then anything else.
        An example, my old 80% furnace used ~400W for the PSC blower motor. My new furnace uses ~100W stage 1, ~200W stage 2 and ~400W Central air, If i didn';t have an ECM motor it would use ~400W stage 1, ~400W stage 2, ~400W AC. Most (~90%) of my winter heating is done with stage 1, saving me 300W of power times a huge number of hours, i am also not far from Toronto and have a 25K peak load. I don't know how many kW hours i am saving but its quite a bit.

        Duct sizing is a separate issue and you need ducts sized large enough for the fan speed/heat output of the furnace but this is not age dependent, its house dependent. If you use a too large modern furnace for the ducts then it will keep tripping the high limit switch which is to keep the furnace from overheating internally. This shuts down the furnace and depending on model will need to be manually reset like a breaker switch, it leads to premature wear of components meaning more frequent repairs and leads to comfort problems because the furnace shuts down frequently without warning before your house is warmed to the thermostat set point because it overheating. Fixing this means a new furnace or huge ductwork replacement which can meaning tearing down walls or ceilings. Avoid this problem.

        "3. If I am going to have a fixed DCM 2 stage furnace, will changing the return duct from 8 x 24" to 10 x 24" necessary to improve the performance of the furnace?"

        This is hard to answer without seeing your house and depending on airflow volumes which is beyond my experience to calculate.

        "4. do I really need to insist on a 2 stage thermostat?"

        No, but if you can get the wiring done for little extra money for future expansion it wouldn't hurt to do this. An old style single set thermostat is fine, or even a programmable one which is not expensive would save you a few percent in bills if programmed and your on a predictable schedule.

        "5. One thing I don't understand is two heat exchangers are supposed to share the heating job with a 2 stage furnace, wouldn't the disadvantage of a single stage furnace, that is having the heat exchanger turning on and off frequently also apply to a 2 stage furnace? and is going to shorten the life of both heat exchangers?"

        No, it does not work like this. In Canada all furnaces sold have to be high efficiency (FYI for non Canadian posters). The first 80% of efficiency is an old style 80% heat exchanger. The secondary heat exchanger takes it from 80% to 96% or whatever you buy by extracting more heat and condensing the water vapour out of the exhaust getting more heat out of it. All stages use both heat exchangers but differ in volume of gas burned.

        "6. Could I simply order 96% single stage furnace since it is also high efficient? But how comes a single stage can also reach that level of efficiency? (without the 2 heat exchangers). could you explain to a layman like me? Since my 46 oversized furnace has not given me any trouble, I am a bit biased towards single stage. But do love to hear favourable feedback on 2 stage furnace in terms of reliability."

        See my reply to question 5.

        Reliability wise a two stage is not likely to be less reliable except that today furnaces are designed to last 15-25 years instead of 40+ years. Not because they are more efficient but because planned obsolescence and sale price make this a good for business but worse for customer practice.
        If you could buy an 80% in Canada legally it would be about just as reliable as a 96% from the same company. You could import an 80% from the US if you so desire, though i'm not sure how legal this is and as i said it gets you nothing in enhanced furnace life.

        "7. As mentioned in my first post, I did two calculations for different periods. The one done for December 2016 to April 2017 results in a higher heat load demand. that's why not 100% confident in my own calculations. If we use that number, will your recommendation as to the right size change?"

        I'll leave this to other posters to advise on, but there is a few ways to calculate heat load including using old bills and subtracting hot water/other gas uses. If anyone has some links to this that would be great.

  5. joshdurston | | #10

    1. 20 minutes might not be ideal, but definitely works if your thermostat doesn't support multiple stages of heat. Not something you need to overthink unless your the type to geek out about it.
    2.The newer ECM fans basically try and keep the airflow at a setpoint. So if your ductwork is restrictive the fan will ramp up a bit more and possible make more noise, or it can run slower if the ductwork is free flowing. The big energy savings is if you run your fan constantly. The ECM can run the fan at low speed at a fraction of the power of a non-ECM fan. This can add up to more than $100 throughout the course of the year depending on your fan settings.
    3. Your ductwork should support the airflow the required by the furnace. The shape and design is just as important as the size. Generally going bigger is better within reason for ductwork.
    4. A multi stage thermostat is a bonus. I would more concerned about getting 5 or 6 wires between the stat and furnace. You can always upgrade the thermostat later on your own. There are devices to allow a thermostat to run on less wires.
    5.There is only one heat exchanger on a 2 stage furnace (it just fires at different rates).
    6. Your furnace is most efficient when it is firing steadily, a single stage will be oversized a higher percentage of the year since it isn't always -25C. That being said, you would probably be just fine with a single stage furnace (especially if keep the sizing tight). Personally I would get a tightly sized single stage with an ECM fan, before I bought a 2 stage without an ECM fan. Reliability isn't a problem for 2 stage furnaces, they really aren't much more complicated.

    7. You can't really go off a single years consumption since there are warm and cold years, better to calculate accurately. I wouldn't be to concerned about not getting enough heat from the 60,000 btu furnace in your case.

    If you're in Toronto I don't think I'd say it's climate zone 7, more like the cold end of 5 or the warm end of 6.

    1. Expert Member
      Dana Dorsett | | #14

      The 25 year average HDD(base 18C) for Toronto is well under 4000HDD, which puts it firmly in zone 5. Only one year out of the past 20 exceeded 4000HDD, and then only by a small amount, despite a run of 5 years in the mid-1990s that also nudged over that line. (Hello global warming.)

      https://toronto.weatherstats.ca/metrics/hdd.html

      https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/Table_301.3(2)_0.preview.jpg

      1. user-4053553 | | #17

        What are the implications for the OP's sizing?

    2. user-4053553 | | #15

      "1. 20 minutes might not be ideal, but definitely works if your thermostat doesn't support multiple stages of heat. Not something you need to overthink unless your the type to geek out about it."
      Its not perfect but its more then adequate. Too long and you can lose too much ground temperature wise, too short and your defeating the purpose of dual stage.

      "2.The newer ECM fans basically try and keep the airflow at a setpoint. So if your ductwork is restrictive the fan will ramp up a bit more and possible make more noise, or it can run slower if the ductwork is free flowing."
      This is a good point, but as long as filters are changed regularly it should not become a big issue.

      The big energy savings is if you run your fan constantly. The ECM can run the fan at low speed at a fraction of the power of a non-ECM fan. This can add up to more than $100 throughout the course of the year depending on your fan settings.
      I agree and disagree, If i went from ~400W to ~100W ~90% of the time its a good savings. I don;t have exact numbers to work with but there was a Canadian study of two identical custom built test houses some years back and they were able to measure the electrical savings and the compensatory gas increase in going from PSC to ECM motor. Since gas costs ~3c/kWh and electricity from ~15-25c/kWh (TOU pricing) the savings available are large.

      "3. Your ductwork should support the airflow the required by the furnace. The shape and design is just as important as the size. Generally going bigger is better within reason for ductwork."
      I mentioned the high limit switch problem, it may be worth getting a proper technician to determine if there will be a problem or not in the OP's house, especially since he is not replacing a modern furnace with another modern one. Though he is going smaller and not bigger which is a good thing. I went from a 1991 75K 80% efficient to a 40K 95% efficient and had not problems but i know someone who got an 80K and has ducts that are too small (probably started with an older non high limit switch unit) and is constantly hitting the high limit switch, the coldest days are the worst with the furnace running longer then tripping.

      "4. A multi stage thermostat is a bonus. I would more concerned about getting 5 or 6 wires between the stat and furnace. You can always upgrade the thermostat later on your own. There are devices to allow a thermostat to run on less wires."
      I don't know of these devices but future proofing if not pricey is always a good idea.

      "5.There is only one heat exchanger on a 2 stage furnace (it just fires at different rates)."
      In Canada only high efficiency are legal so all new furnaces have two heat exchangers.

      "6. Your furnace is most efficient when it is firing steadily, a single stage will be oversized a higher percentage of the year since it isn't always -25C. That being said, you would probably be just fine with a single stage furnace (especially if keep the sizing tight). Personally I would get a tightly sized single stage with an ECM fan, before I bought a 2 stage without an ECM fan. Reliability isn't a problem for 2 stage furnaces, they really aren't much more complicated."
      If i had to pick between these it would be a hard choice but i chose not to and got a 2 stage ECM motor unit. Was worth the cost.

      "7. You can't really go off a single years consumption since there are warm and cold years, better to calculate accurately. I wouldn't be to concerned about not getting enough heat from the 60,000 btu furnace in your case.

      If you're in Toronto I don't think I'd say it's climate zone 7, more like the cold end of 5 or the warm end of 6."
      I would make sure the sizing is correct but if it is then i agree 60K is the way to go, better comfort from both stages and longer burns will reduce on/of cycling giving longer furnace life and less maintenance over its lifetime.

  6. user-4053553 | | #16

    @Josh
    It won't let me reply to your reply above so i had to do it separately

    "Warning I'm not trying to confuse the OP, but just wanted to clarify to Alan B what I'm trying to say.
    I tend to geek out a bit more than necessary on this (I'm a building controls engineer). My point was that runtime is irrelevant as long as you are maintaining setpoint. 45minutes would be a long time (too long) if the space was actually a couple degrees below setpoint. You would want 2 stage up sooner to avoid comfort issues if you came home to a cold house. Using a PI(D) based control algorithm with a bit of deadband, would allow you to stage quite quickly if you have a huge error off setpoint (maybe 5minutes), more slowly if you're "winning", but would hold a stage indefinitely if it got within the dead band. There really isn't a fixed ideal time to stage up. But, if you are going to ignore temperature and only look at runtime, I agree 20minutes is probably a good compromise. Honeywell thermostats let you specify a cycles per hour in the setup menu, and typically you would have around 3 cph (20minute control period), so if your heat runs more than 20 minutes it basically means that it is at 100% demand. This matches nicely with the 20 minute stage up delay on the a 2 stage furnace."

    I understand the geeking out, in college i had written up a control algorithm for a box that needed precise temperature control with a heat source and a fan and the ways of doing it were quite interesting especially if energy saving was the goal, but i digress.

    Its always nice to get into the complicated features but unless you want to pay more then getting simple effective, reliable and comfortable is the key. I would go with 2 stage over multi speed unless the OP wants the Tesla of furnaces.
    As for stage control mine is about 15 mins, at approximate -5C and colder it goes into 2nd stage. When its borderline it will hit second stage then shut down a minute later. Not at all ideal. But when its say -10C or lower the 2nd stage runs longer. I was able to calculate my house load at 99% (-20C) is 25K but comparing the output at high fire (39k) vs increase over 25K and figuring out if it had stayed at first stage when would it shut down if at all. Coincidentally my house load is almost exactly the first stage. But we did go below the 1% for several weeks and glad i had a larger then 25K furnace. If i had a thermostat that could control stages i could lock out high fire and only enable it at 1% and lower temperature but in the end its a lot of headache for only a little gain.

  7. Greenpeace | | #18

    Thank you very much for kindness in sharing your experience and knowledge. This is really helpful.

    I have recalculated my heat load using gas bills as attached:

    This time I assumed 70% efficiency of my existing furnace and increased the water heater gas consumption from 5% to 20% because the household actually did use more hot water than average home. I simply followed other people's example found in other forum to do my own (not really understanding the formula). The following numbers are copied from others and not sure if I should have my own numbers.

    1. energy content of gas: is 37.5 MJ/m3
    2. Heat Loss at 53HDD/day - how should I calculate for my own?

    The result of recalculation is that I will need a furnace of 46,460.5 btu/hour
    permissible oversizing 1.4 65,044.8 btu/hour

    3. Also based on 2010 Energy Audit Report, the auditor said estimated heat loss was 49003 btu/hour, if permissible oversizing 1.4 it became 68604.2 btu/hour. I increased attic insulation from R25 to R50 after the audit. Assuming it will reduce heat loss by 5%, revised estimated heat loss after attic insulation was 46552.85 btu/hour. With 60,000 BTU furnace, 96% efficiency, oversizing is limited to 24%. (57600/46460), not 40%, is the buffer enough? Is the sizing tight?

    4. based on what you said, it is hard to predict the ideal setting for second heat exchanger to kick in but it makes sense to set at 20 minutes. Does it mean that the primary heat exchanger will not work more than 20 minutes each time, the second heat exchanger will automatically kick in even though it only needs to work for one minute to reach the desired temperature. (21 c) . this is what I mean short-cycling might occur for both exchangers?
    5. my understanding is that 2 stage is for comfort, not for efficiency as single stage does have model of 96% efficiency.
    6. in terms of energy saving, 2 stage furnace may actually increase gas bill because the primary heat exchanger runs longer but it will be compensated by lower electricity costs if we are going to use the fan continuously; particularly in summer time. Do I understand it correctly? We may be talking about $120 per year in electrical bill saving and $100 per year saving for gas bill??

    7. Life span of a single stage HE furnace and 2-stage furnace may be the same. But some technician admitted that repair cost is much lower for a single-stage furnace.

    8. Second heat exchanger is designed to work for -25C?? What temperature of 40320 btu can heat up to?

    Thanks,

    1. user-4053553 | | #20

      "4. based on what you said, it is hard to predict the ideal setting for second heat exchanger to kick in but it makes sense to set at 20 minutes. Does it mean that the primary heat exchanger will not work more than 20 minutes each time, the second heat exchanger will automatically kick in even though it only needs to work for one minute to reach the desired temperature. (21 c) . this is what I mean short-cycling might occur for both exchangers?"
      Your not understanding.
      A high efficiency furnace has two heat exchangers that work all the time. The first 80% of the heat in the exhaust is extracted by the primary heat exchanger.
      The exhaust moves into the secondary exchanger that removes the other 16%. You cannot use one but not the other, its impossible. Think of the inside of the furnace like a snake, the gas is burned and goes into the head and comes out the tail and is sent to outside. The primary heat exchanger is starts at the head and ends near the tail, the secondary starts near the tail and ends at the end of the tail.

      How a dual stage works is at stage 1 the amount of gas burned is less per second. At stage two more gas is burned every second. Again the burned gas moves through both exchangers whenever the furnace is running. When a furnace runs a solenoid allows a set measured amount of gas flow, a vent motor sucks the burning gas through both heat exchangers, the heat is extracted and blown into your house and the exhaust goes out the white pipe to outside. At stage 1 this is a small amount of gas allowed, at stage 2 its the full amount.

      The 20 min thing is unrelated to the heat exchangers. It is an academic discussion that we got off track on about the best algorithm for a two stage furnace. The furnace will already have a plan built into it when it will jump from stage 1 to stage 2. You don't need to worry about this because its set by the manufacturer when the furnace was built and they have been in business for many years.

      "5. my understanding is that 2 stage is for comfort, not for efficiency as single stage does have model of 96% efficiency"
      There will be some efficiency difference but not much. But it does give more comfort. Also the efficiency is not 96%, thats a AFUE rating, its complicated and not really necessary to get into. But you mentioned comfort issues so get the 2 stage with an ECM motor.

      6. in terms of energy saving, 2 stage furnace may actually increase gas bill because the primary heat exchanger runs longer but it will be compensated by lower electricity costs if we are going to use the fan continuously; particularly in summer time. Do I understand it correctly? We may be talking about $120 per year in electrical bill saving and $100 per year saving for gas bill??
      No.
      If you use the fan continuously then an ECM motor saves money.
      If you have a two stage furnace you will still save money by having an ECM motor whether or not you use the fan continuously.

      7. Life span of a single stage HE furnace and 2-stage furnace may be the same. But some technician admitted that repair cost is much lower for a single-stage furnace.
      Don't take what technicians say as gospel. I had a technician tell me i didn't need airflow for the furnace because my 300 sq ft basement had enough combustion air for the whole winter. I think these people learn how to install the thing, not how their products work.

      8. Second heat exchanger is designed to work for -25C?? What temperature of 40320 btu can heat up to?
      If you had a passivhaus 40320 btu would probably kill you with your house above temperatures safe for human habitation. Its all about the heat loss rate, not how much heat a device puts out.
      I don't know where your -25C comes from, it would work at any temperature the furnace can because it does not see outside air, it sees burned natural gas.

  8. Greenpeace | | #19

    see my heat load calculation using gas bills

    does it look right?

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