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  1. #1
    Join Date
    Dec 2003
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    PLEASE - Help Me Understand How Dual Fuel Heat Pumps Really Work

    PLEASE - Help Me Understand How Dual Fuel Heat Pumps Really Work

    I really need some technical insight / help into how and why Dual Fuel Heat Pumps work...

    First some background:
    We own a 1,600 sq ft lake cottage 1 hour north of Raleigh, NC. This location is near the NC / VA border. While not routine, it's very coming to get a stretch of 3-5 days of low teens / low 30's temps. When this happens the house is unbearably cold / uncomfortable with our current 3 ton heat pump. This heat pump is nearly 20 years old and we're considering options for replacement.

    Dual Fuel Heat Pump Questions:

    1-My first question is related to the when / how the triggering temperature is set on the gas back-up furnace. Is this pre-set by the manufacturer or can this triggering temp be set by the home owner on the thermostat?

    2-My next question is related to rated BTU heating capacity. Most 3 ton heat pumps are in the 24K range of heating output... yet a gas furnace back-up is typically in the 60K btu range... Is this beause as the temperature drops, more BTU's are reacquired? Does this mean that a 24K BTU rated heat pump may work just fine at temps between 40 - 60 degrees, but will struggle when temps plummet? Is this when the gas back-up furnace would typically kick in?

    3-Lastly, are there any important "cons" to a dual fuel heat pump I should consider?

  2. #2
    Join Date
    Sep 2007
    Location
    State College, PA
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    I will try to answer best I can:

    Q1. There will most likely be an outdoor sensor that records the outdoor temperature. If you have a smart thermostat, the outdoor temperature will come from the internet after you enter your zip code. Then when the temperature reaches the point where it is set up to switch from heat pump to the backup heat source it will turn off the heat pump and the backup heat source with take over.

    Q2. A 3 ton heat pump will produce approximately 36,000 BTU at around 60 degrees. As the temperature drops outside the amount of BTU's drop off but the house actually requires more BTU's. Thus there is the backup heat source (i.e. Dual Fuel).

    Q3. The only con I could think of is if your back up fuel is propane and you run out. (I have seen that happen)

    Now I have a question... What do you currently have as a backup heat source for you existing heat pump? All heat pumps need to have a backup heat source since its performance will degrade as the temperature drops. If you don't have a fossil fuel, normally you would have electric heat strips.

    Hope that helps somewhat.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  3. #3
    Join Date
    Feb 2010
    Location
    Metro Atlanta, GA
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    Most heat pumps in the south are sized for the air conditioning load, so if 3 tons is your cooling load you get a 3 ton heat pump.

    For dual fuel your heat pump lockout temp can be set by your contractor based on your unit's performance at XX outdoor temps in your area.

    As far as furnace sizing is concerned. If you need 3 tons of cooling you're going to need a furnace capable of 3 tons of airflow so most manufacturers will pair that to a 60K BTU or similar furnace just for ease of production. That doesn't mean you need the 60K, it's just what you're going to get.

  4. #4
    Join Date
    Dec 2007
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    Heat pumps MAINTAIN a temp and are not good, past a specific temp related to the equipment's ability, at making up for setback.

    A fuel burning or electric indoor unit can utilize a setback.

    When using a heat pump, set it at your desired temp and leave it alone!!! Any setback will require the indoor unit to run usually resulting in a less efficient cost of operation.

    What is your indoor unit now?
    What thermostat are you using now?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
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    1,600 SQ FT., NORTH of Raleigh NC close to VA border

    1. Determine the Heat Loss using MANUAL J methodology at Design Temperature / 19'F
    2. Select Heat Pump Size based on Heat Gains and Losses

    3. Determine $ / kw-hr
    4. Determine $/ Therm Natural Gas

    5. Review Heat Pump Performance at 17'F and 47'F
    6. Review THERMAL BALANCE POINT
    7. Review Operating Costs for Heat Pump, Furnace and Hybrid

    Built ~ 1990 - Example
    Attached Images Attached Images  
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    Last edited by dan sw fl; 09-13-2019 at 06:16 AM.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Sep 2007
    Location
    State College, PA
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    Quote Originally Posted by dan sw fl View Post
    1,600 SQ FT., NORTH of Raleigh NC close to VA border

    1. Determine the Heat Loss using MANUAL J methodology at Design Temperature / 19'F
    2. Select Heat Pump Size based on Heat Gains and Losses

    3. Determine $ / kw-hr
    4. Determine $/ Therm Natural Gas

    5. Review Heat Pump Performance at 17'F and 47'F
    6. Review THERMAL BALANCE POINT
    7. Review Operating Costs for Heat Pump, Furnace and Hybrid

    Built ~ 1990 - Example
    Holy Moley Dan,

    Where did you get that spread sheet? Care to share?
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
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    Quote Originally Posted by joemach View Post

    Holy Moley Dan,

    Where did you get that spread sheet?
    Care to share?
    The goal of EXCEL sheet that I am developing
    is to address THERMAL AND Operating Economics BALANCE POINTS.

    I have not Fully Tested the Operating Economics portion to-date
    regarding the use of hybrid operating costs.

    I Don't Care-to-Share my Full Proprietary Document.
    It has taken me A While to make it Useful and Versatile.
    One needs an appropriately selected " Local " TEMPERATURE BIN DATA and selected Heat Pump Performance Characteristics
    to develop an applicable TIME-WEIGHTED Average C.O.P. for operating cost analysis.

    HP Performance Characteristics = C.O.P. from ~0'F to 55'F and heat output
    Generic HP at ~16 SEER is used.

    I have yet to add a TEMP BIN DATA locale in the proximity of State College PA so it will have to added.
    I could share with A STATE COLLEGE PA associate
    a slightly different SIMPLIFIED version with a Lot of Locked cells without including the Hybrid economics.

    The locked cells essentially make it easier to use.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #8
    Join Date
    Dec 2003
    Posts
    29
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    Thread Starter
    Thanks Dan! I'll give this a go!

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