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Lennox furnace question

I work at a Lennox dealer and have over 25 years experience in the Hvac service buisness but have only worked here for 6 years. That 5 ton blower would come from the factory on high speed (5ton 2,000 cfm's) and unless a tech would of change the speeds you would of had too much air flow. This can be a problem because a higher eff. A/C such as yours cools well but won't de-humidify as well as a 10.00 seer and with too much air flow, that creates an even less A/C de-humidification situation. another reason would be the furnaces cabinet would be wider and the coil wouldn't of fit on there as well.
All of this aside, I'm sure your 4 ton drive furnace uses less electricity than the 5 ton one.
IMO, ask for an extended warranty!!!!!!!!
And buy their maint. agreement and have the system maintained!!!!!!
Trust me it'll be worth the money
10 yr. Parts and labor extended warranty should cost around $400.00-$600.00

Comments

  • Jim_116
    Jim_116 Member Posts: 4
    Lennox furnace question

    Thanks in advance for any help. Recently built a 3100 sqft house. The original HVAC proposal called for a Lennox G51MP-60C-090 furnace with a Lennox 13ACD-048 AC unit to be installed. The correct AC unit was installed but the furnace installed was a Lennox G51MP-48C-090. The furnaces have the same BTU output and efficiency but the CFM for the 48 is less and the 48 is rated for 3-4 tons where the 60 is rated for 4-5 tons of cooling. My question is what effect will this have on my heating and cooling? I figure less airflow equals poorer performance but have no HVAC knowledge. I have had numerous problems with the builder and subs and figured this is one more way they were cutting corners. I plan to bring this up with my builder and HVAC contractor but would like to have upfront information to assist in my discussion and understanding of the 2 furnaces. Thanks again for any information and assistance.
  • Uni R_2
    Uni R_2 Member Posts: 589
    You missed something...

    What are the heat gain calculations for the house?
  • don_185
    don_185 Member Posts: 312
    It sound

    to me they did a proper job.Why install a 5 ton blower when your condenser is only rated for 4 ton?

    Any more airflow other then 1600 cfm will give you a warmer coil in the summer, which is not what you want if you're in a area that has high humidity and low ambient temps.

    Any other issue maybe it would be best to takeup with your hvac contractor.
    I know if it were me I would want a direct phone call and not have to hear this guy on the internet said... before you called me.
  • Jim_116
    Jim_116 Member Posts: 4


    Thanks for the reply to my original question.

    Part of my problem is that one furnace (48C) is rated 3-4 tons of cooling and the other furnace (60C) is rated 4-5 tons of cooling. Both of these furnaces could support the 4 tons of cooling that I currently have. So, when you say why use a 5 ton fan for a 4 ton unit, I wonder why use a 3 ton fan for a 4 ton unit?

    I was wondering why if the original written proposal said they would be putting the 60C with the 4 ton AC unit why I wound up with the 48C. I was looking for possible pros/cons to why this would be from experienced professionals. I do plan to follow-up with my HVAC contractor but wanted to make sure I had a little information before I took it to them.

    I don't plan on saying I "heard on the internet" just wanted professional thoughts on the possible explanations for the use of a furnace model that only differs on CFM.

    I have had a HORRIBLE experience with almost everyone associated with the construction of my house and have heard too many BS explanations to cover 1/2 A$$ work.

    Thanks again for any information that you can offer, it is appreciated.
  • Jim_116
    Jim_116 Member Posts: 4


    > What are the heat gain calculations for the

    > house?




    I don't have this information but plan to get it from my HVAC contractor when I talk to them.
  • Steverino
    Steverino Member Posts: 140
    Lennox Question

    The G51MP-48C-090 and the G51MP-60C-090 have identical input and output Btuh. The difference is the amount of cooling that can be accomplished by the respective units.
    The G51MP-48C can deliver between 950 and 2180 cfm,the G51MP-60C can deliver between 1440 and 2840 cfm so either furnace is capable of delivering the 1600 cfm required for 4 ton of cooling.
  • S Ebels
    S Ebels Member Posts: 2,322
    CFM

    Aside from the fact that you did not get what was originally proposed, the air floe should be adequate. That is assuming that the duct system, the correctness of which is much more difficult for you to determine, is up to snuff.

    The blower on the 48C is rated up to 1,600 CFM which will do all your AC will demand, the 660C has a blower taht will do up to 2,000 CFM which is more than you need. Both are multi speed motors which will deliver a variety of air flows.

    I'd have some serious issues with your guy if he didn't provide what was quoted.
  • don_185
    don_185 Member Posts: 312
    Just ask

    them if your contract cover a performace test..if so ask them to show the test sheets.Then you would know if all is up to par.If they can not do so bring in someone who can and then set down at the table with your findings.

    Just so you understand that most builders are not willin to pay for anything other then btus in a box.
    So all the extra that needs to be done to dail it in if you will are, EXTRAS.

    I'm sure you properly are aware of this by now.

    All one needs to do is holdup a finger on each hand and slowly bring them together untill you have about a 1nch gap between the two.Thats little portion is all we get from the
    budget to install a hvac system.Not much is it?

    Good luck, with yours.







  • Jim_116
    Jim_116 Member Posts: 4


    > The G51MP-48C-090 and the G51MP-60C-090 have

    > identical input and output Btuh. The difference

    > is the amount of cooling that can be accomplished

    > by the respective units. The G51MP-48C can

    > deliver between 950 and 2180 cfm,the G51MP-60C

    > can deliver between 1440 and 2840 cfm so either

    > furnace is capable of delivering the 1600 cfm

    > required for 4 ton of cooling.




    Thanks for your reply. This was the same information that I had found, which made me wonder why the different installation. What effect would this have on the life of blower motor since the wheel of the 48C is smaller than the 60C requiring more RPMs to produce the same 1600 CFM? I would think it would be better to have the larger blower wheel.
  • Dr Pepper
    Dr Pepper Member Posts: 38


    > I work at a Lennox dealer and have over 25 years

    > experience in the Hvac service buisness but have

    > only worked here for 6 years. That 5 ton blower

    > would come from the factory on high speed (5ton

    > 2,000 cfm's) and unless a tech would of change

    > the speeds you would of had too much air flow.

    > This can be a problem because a higher eff. A/C

    > such as yours cools well but won't de-humidify as

    > well as a 10.00 seer and with too much air flow,

    > that creates an even less A/C de-humidification

    > situation. another reason would be the furnaces

    > cabinet would be wider and the coil wouldn't of

    > fit on there as well. All of this aside, I'm

    > sure your 4 ton drive furnace uses less

    > electricity than the 5 ton one. IMO, ask for an

    > extended warranty!!!!!!!! And buy their maint.

    > agreement and have the system

    > maintained!!!!!! Trust me it'll be worth the

    > money 10 yr. Parts and labor extended warranty

    > should cost around $400.00-$600.00



  • Dr Pepper
    Dr Pepper Member Posts: 38
    Your forgetting something...

    Next to the heat calcs you also need to know pressure drop through the ductwork. If all the ducting is undersized he may need the 5 ton blower to move the air needed for a 4 ton job. Just like a pump the blower has a curve and physics will always win over marketing. If the duct is undersize it will also be far from a quiet, plesant system to live with. This should have been designed and ductilator calculations should be available to the owner. I know because I do commercial HVAC for an engineering firm.
  • GW
    GW Member Posts: 4,691
    finally

    I was wondering if someone would chime in about the duct system being a major factor!

    Dr. Pepper, you know more than the average dude! But you already knew that.

    At this point, who cares what the heat loss/gains are, the system is installed, and if the furnace and ac can't deliver the energy it is converting from gas or electricity, you did not get what you paid for.

    Even if Jim DID have the heat loss calcs, so what? Does he have a blower door test and an infrared scan? Most likely -no. So, let's deal with the original question.

    Jim, if your 4 ton drive can move the proper cfm through the furnace/cool, that's achievement #1. If the duct system can deliver it to the house to properly heat/cool, that's achievement #2. We can't have #2 without #1, but we could have #1 without #2. bla bla, now I'm getting off track.

    As far as air flow, it takes more energy to operate a 5 ton drive than a 4 ton drive, that should be common sense. However, as mentioned, there are different "taps" on the motor or control board that allow the tech to adjust the amount of juice going to the motor. So, conventional wisdom says a 5 ton on lets say "medium high speed" might move the same air as a 4 ton on "high speed". Does it make sense that these two examples use the same amount of electricity? It takes work to move a given volume of air, so a bigger motor simply works a little less than a smaller motor, but energy is equal.

    Basically, if your duct system stinks you have more problems than you care to process, Jim. You may want to consider getting a 3rd party guy to check the static pressure. BTW, was this unit used to heat the building during construction? If so, a big strike against you, the end user. You could ask the current heating guy to do a static, but I bet you a dollar and a donut he doesn't know how... I guess 90% of the heat guys out there don't know how, yet it's the foundation of proper air flow. Rules of thumb are just that. Do you want a "guess" or do you want "facts"?

    You might spend a couple of hundred to have a guy give you a quick "look over". But that's peanuts compared to the costs of a new home, isn't it?


    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
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    Wilson Services, Inc
    Northampton, MA
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  • Techman
    Techman Member Posts: 2,144
    Morning All!

    > I was wondering if someone would chime in about

    > the duct system being a major factor!

    >

    > Dr.

    > Pepper, you know more than the average dude! But

    > you already knew that.

    >

    > At this point, who

    > cares what the heat loss/gains are, the system is

    > installed, and if the furnace and ac can't

    > deliver the energy it is converting from gas or

    > electricity, you did not get what you paid

    > for.

    >

    > Even if Jim DID have the heat loss calcs,

    > so what? Does he have a blower door test and an

    > infrared scan? Most likely -no. So, let's deal

    > with the original question.

    >

    > Jim, if your 4 ton

    > drive can move the proper cfm through the

    > furnace/cool, that's achievement #1. If the duct

    > system can deliver it to the house to properly

    > heat/cool, that's achievement #2. We can't have

    > #2 without #1, but we could have #1 without #2.

    > bla bla, now I'm getting off track.

    >

    > As far as

    > air flow, it takes more energy to operate a 5 ton

    > drive than a 4 ton drive, that should be common

    > sense. However, as mentioned, there are different

    > "taps" on the motor or control board that allow

    > the tech to adjust the amount of juice going to

    > the motor. So, conventional wisdom says a 5 ton

    > on lets say "medium high speed" might move the

    > same air as a 4 ton on "high speed". Does it make

    > sense that these two examples use the same amount

    > of electricity? It takes work to move a given

    > volume of air, so a bigger motor simply works a

    > little less than a smaller motor, but energy is

    > equal.

    >

    > Basically, if your duct system stinks

    > you have more problems than you care to process,

    > Jim. You may want to consider getting a 3rd party

    > guy to check the static pressure. BTW, was this

    > unit used to heat the building during

    > construction? If so, a big strike against you,

    > the end user. You could ask the current heating

    > guy to do a static, but I bet you a dollar and a

    > donut he doesn't know how... I guess 90% of the

    > heat guys out there don't know how, yet it's the

    > foundation of proper air flow. Rules of thumb are

    > just that. Do you want a "guess" or do you want

    > "facts"?

    >

    > You might spend a couple of hundred

    > to have a guy give you a quick "look over". But

    > that's peanuts compared to the costs of a new

    > home, isn't it?

    >

    > _A

    > HREF="http://www.heatinghelp.com/getListed.cfm?id=

    > 368&Step=30"_To Learn More About This

    > Professional, Click Here to Visit Their Ad in

    > "Find A Professional"_/A_



  • Techman
    Techman Member Posts: 2,144
    Morning All!

    Jim,it could be as simple as no one checking ! The warehouse guy, counter guy,the pick-up guy, the installer guy, the start-up guy,contractor,if so ,no big deal.Just so long as ,both A/h's are the same ,just different sizes.With both blowers set at 1600CFM they will use the same ammount of electricity
  • GW
    GW Member Posts: 4,691
    uhhh

    "if" the static pressure is high, the 4 ton drive, even "set" for 1600 cfm, will not move 1600 cfm, and the power will be LESS. As anyone with a little bit of knowledge knows, the fan speed settings are directly related to the static pressure the motor "sees". This chart is found in the service manual. This is basic air flow dynamics.

    An ECM motor has a different set of rules.

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  • rucomfy
    rucomfy Member Posts: 43
    how come..???

    ECM motors still must overcome the installed duct system static pressure. The only difference is that the motor will ramp up to overcome the static load, up to it's max rating.

    The ECM motor still lives by the same rules as any other motor and systems should be designed by Manual J, D and T in that order. The only difference between an ECM motor and a PSC motor is the power factor and electrical efficiency.

    An undersized duct system with a ECM will share the same poor operation symptoms as any other motor.
    Such as noise, increased duct loss, increased duct velocity and decreased filter capability due to excessive velocity.

    Please do not think that an ECM motor will solve any duct design flaws.
  • don_156
    don_156 Member Posts: 87
    How true!

    Ever seem a ecm motor with a clogged filter in the works.

    That motor is screaming and ramp at highest it will go and
    yet their is still very little air coming out of the duct.

    Funny how those motor will only put out what they take in.

    Great post Allan!
  • Techman
    Techman Member Posts: 2,144
    O.K.

    I agree w/ the "if" stuff concerning duct sizing and static pressure and fan's and ECM 's ,BUT the thread is about two different MN#'s .Let's not hang the "innocent until proven guilty" contractor! That contractor could be me ,or you!
  • Can we stay on track?

    ECM vs PSC is another thread(I like both). It seems if the contractor sizes the duct properly and does the proper heat gain/loss calcs the 4 ton should be fine. Me myself see too often a duct system system that is too small for the blower resulting in high velocity noise, drapes that move and "where did the cat go? Last I knew it was by the R/A grill." The best indicator of proper blower performance is your temp rise or fall based on existing conditions. I know it's a rule of thumb but if it's not right, it usually doesn't take long to figure out why.

    Darin
  • GW
    GW Member Posts: 4,691


    Now that we're really off track here (BTW Jim, you have a PSC motor, I don't know why we're gonna have a lesson on ECMs here, but I love a pleasant debate!), we have a disconnect between what we know and what we type. Of course the ecm can overcome crappy duct design, are you serious? "How crappy" is a different topic entirely.

    Obviously if you cram 1600 cfm down a trunk line that rule of thumb says it "should" based on the old .10 rule of thumb/Duct-U-Lator, and in reality the crappy duct system is running a .2, or whatever number you want to dream up, you're velocity will be higher.

    PSC: If you have a TESP of .9", there is no way the 4 ton drive will move 1600 cfm. Can we agree on this? It's a mechanical law, even Lennox obeys this, and types it in their manuals (sorry, I lost my DaveNet password, or I'd happily tell you all what it would move for cfm).

    ECM: If you have a TESP of .9" with an ECM, it will indeed move the 1600, and not even complain.

    Something tells me there's still some confusion.

    There are gross misconceptions about power consumption regarding ECMs, but now we're really off the beaten path.







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  • GW
    GW Member Posts: 4,691
    On Track

    I just mentioned some stuff about PSC (Jim has this), and wanted to make an omission with ECMs, and out came all this stuff... I'm sorry; I'll try to play nice.

    The bottom line is NO ONE on the list can say "hey Jim, don't sweat it, your 4 ton is just as good as the 5 ton unit". Anyone saying this has no idea what they're talking about without knowing all sorts of other real life conditions.


    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
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  • Techman
    Techman Member Posts: 2,144
    Jim

    Please, keep us up to date on how this situation turns out,I think all of us would like that!
This discussion has been closed.