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Thread: 95% efficiency unit vs 80% efficiency unit

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  1. #1
    DIY Junior Member
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    Default 95% efficiency unit vs 80% efficiency unit

    Hi folks,

    Just a general question for you. Is it true, that the 95% efficiency furnace will produce warmer heat coming out of the registery than a unit that only does 80% efficiency?

    In my previous home, I had a 95% efficiency unit installed and the air coming out of the registery was very hot. Since we moved, this unit is only 80% efficiency, is air coming out is only luke warm....

    please let me know

    thanks

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    DIY Senior Member zl700's Avatar
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    The two are not related in fact, high efficiency furnaces typically have a lesser temp rise. The differences you are feeling is ducting distribution and fan speeds.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If you also have central air conditioning, the fan speed may be set to promote that which typically wants a higher flow rate than heating. It doesn't always change based on the mode, you have to do that yourself. Many times, you can adjust the fan speed to be slower which allows the air to absorb more heat as it goes through the heat exchanger. You must keep it above the minimum for the system's heat output or you risk overheating the exchanger. The manual should show you your valid options.
    Jim DeBruycker
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    General Engineering Contractor ballvalve's Avatar
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    If the ducts and fan speed were the same you would get air theoretically 15% warmer. You are putting 15% of your heat in the garbage can in this house.

    If our GOV banned 80% rigs, the cost of the 95% units would drop dramatically.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The efficiency of the unit has NOTHING to do with how hot the exiting air is...it is all dependent on the heat exchanger design, the fan speed, and the distance and state of the ductwork before it hits the register. If the ducts run through unheated space and are uninsulated, that may be the primary reason the exit temp is cooler. Have you actually measured the temperature at the register? Just like a cooling breeze in the summer feels cool, it will feel cool in the winter, too if you can't slow it down a bit.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member zl700's Avatar
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    Ball valve

    Actually if you compare similar size furnace from same manufacturer you will quickly notice higher recommended temp rise on 80%ers in order to maintain higher flue temps to avoid condensation of ht exh and flue. Thus 90%ers which there is a desire to condense have lower temp rise allowances meaning more cfm's across ht exh, resulting in cooler discharge temps.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

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    yes, the 80% out puts temp at the register at 131, returns 72 when themostat is set at 74 (winter). The temp rise is 59, as this is when fan speed is set at med-low.

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    General Engineering Contractor ballvalve's Avatar
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    Quote Originally Posted by jadnashua View Post
    The efficiency of the unit has NOTHING to do with how hot the exiting air is...it is all dependent on the heat exchanger design, the fan speed, and the distance and state of the ductwork before it hits the register. If the ducts run through unheated space and are uninsulated, that may be the primary reason the exit temp is cooler. Have you actually measured the temperature at the register? Just like a cooling breeze in the summer feels cool, it will feel cool in the winter, too if you can't slow it down a bit.
    I think thats what I said. In reverse. Anyway, its nuts to toss 15% to 20% of our precious natural gas. This is ONE place we need some regulations to make the MFG's go all condensing.

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