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  1. #1
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    Lightbulb Time-of-Day Fuel Selection for Dual-Fuel systems

    I am looking at changing both of our A/C split systems to Dual-Fuel, zoned systems.

    From an equipment point of view, each system would be zoned and consist of a Variable-Speed heat pump coupled to a modulating, variable-speed gas furnace. Equipment from Lennox (XP20/SLP98), Bryant (288BNV/987MA), and American Standard (Platinum 18/ZV) can all achieve the HVAC needs of our house and quality HVAC-professionals are available for each manufacturer.

    As an automation engineer, I have been more focused on the control systems and have delayed our upgrade for 3 years ... they are getting better. The main concern with Dual-Fuel here in Southern California is the cost of electricity which changes dramatically based on the time-of-day (e.g. $0.17/kWh at night up to $0.49/kWh during peak day time). With solar and net metering it is extremely beneficial to heat using a Heat Pump at night and a gas furnace during the day (if needed). Obviously, the Heat Pump would need to run during the day for cooling.

    Besides integrating the systems into a whole house automation system, does anyone know of a system that can accomplish this now?

    For example, I noticed that the new Bryant Evolution Connex controls have single entries for cost per kWh and BTU but our rates change dramatically by time-of-day (some rate schedules even change rates based on weekday vs. weekend). Also, the Bryant control only uses this information for reporting purposes and not fuel efficiency choice decisions.

    Now that most manufacturers have gone to WiFi upgradeable controls I am not as reluctant to buy a system and hope they add the feature in the future. Three years ago, United Technologies engineering staff told me they were working on it and that I would have to throw out $1,000s in t-stats once the feature was added since they could not be upgraded in the field. Not a reasonable option ...

    Your thoughts?

  2. #2
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    What is the question?

  3. #3
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    Thread Starter
    Quote Originally Posted by timjimbob View Post
    What is the question?
    Is there any (control) system that can select between Heat Pump or Gas Furnace based on what Time-of-Day it is?

    For example, if it is between midnight and 5 a.m. then use the Heat Pump for heating if possible (but still fall back to gas furnace if more BTUs are required).

    Similarly, only use the gas furnace for heating between 12 noon and 6 pm (even if the heat pump could satisfy the demand).

    Thanks.

  4. #4
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    Many dual fuel stats can use an outdoor temperature sensor to select between gas or HP operation, Gas being selected at the lower temperature range. Most likely a thermister. You can insert a resistor in place of the thermister to falsely drive the system into the mode you want with a time clock switching between the two resistors.
    If you design controls, this is a piece of cake.

  5. #5
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    Quote Originally Posted by timjimbob View Post
    Many dual fuel stats can use an outdoor temperature sensor to select between gas or HP operation, Gas being selected at the lower temperature range. Most likely a thermister. You can insert a resistor in place of the thermister to falsely drive the system into the mode you want with a time clock switching between the two resistors.
    If you design controls, this is a piece of cake.
    Thanks. I was thinking that something like this would be possible ... my only concern was that the Heat Pump is needed for Cooling at any time of day. If the resistor would just be prevent the heat pump from heating cycles that would work great.

  6. #6
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    The dual fuel controls that I have used in residential applications are all strictly temperature controlled. As you suggest, it would be possible to use wifi to change from "heat" (fuel determined by ODT) to "emergency heat" (fossil fuel use). However that will require your input each day.
    I suspect most applications are only concerned with ODT and for that reason only places such as California, which I see is your location, and other variable fuel rate areas would have a need for time of day controls.
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  7. #7
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    Quote Originally Posted by pragsdale View Post
    Thanks. I was thinking that something like this would be possible ... my only concern was that the Heat Pump is needed for Cooling at any time of day. If the resistor would just be prevent the heat pump from heating cycles that would work great.
    If your stat is set to COOL, I don't think it would prevent the HP from operating.

    Get the outdoor sensor and measure the two values. One at 60 degrees and one at 20 degrees. Substitute resistors with the same values and it will work.

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