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  1. #1
    Join Date
    Feb 2005
    Posts
    40
    I am getting ready to take a look at a duel fuel system, first time to do so. I have not ever had the experience to work on one before. It is heat pump/ gas furnace system. It belongs to a friend of mines neighbor. I know from conversations that his fuel bill has really gone up since he or someone has installed a new T-stat. I would appreciate very much if anyone could enlighten me in the correct wiring procedure. I haven't called on the home yet and taken a look, don't know as if it is a digital or conventioal T-stat. First system I have heard of here, in Texas.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,617
    Need a heat pump stat. Unless their old stat was a model with built-in dual fuel kit, there should be one mounted to the side of the furnace. All low voltage wires come in there and go to stat, furnace & pump.

  3. #3
    Join Date
    Feb 2005
    Location
    small island in the Pacific Ocean
    Posts
    558
    Need a duel fuel stat, carrier makes a good one. Heat pump and furnace can't operate at the same time.

  4. #4
    Join Date
    Mar 2005
    Posts
    66
    I agree with phosgene, Carrier makes a great duel fuel stat. Best of all it is not to difficult to install. What brand of equipment are you getting ready to work on? It seems to me everybody does dual fuel a little different, and I have asked many manufacturers why? And I have never received a good explanation. It's those engineers we all love to hate.

  5. #5
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,803
    Once you see what is there, if you have trouble yoou can get better answers.
    Without knowing how it wired, what stat is being used, hard to say anything.
    The outdoor unit may be bad, or the stat miss wired.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Apr 2004
    Posts
    7,680
    It happens to the best of us, in fact just today, a partner in crime reminded me of the time he "re-calibrated the conductive cross-junctifier on his Binford 2000. The needlemeyer went spaz..."

    Go look at it, it might be something simple.

    I remember once as a "yoot" I got sent on a service call about 100 miles away to look at a "Centrifigul system with a compressor problem" I got all worked up worrying about how was I gonna know what to do, heck, I'd never even seen a centrifigul. I tried to talk them into sending someone else as I had no idea what I would do when I got there. I sweated bullets 'till I got there and found it was nothing more than the belt had come off the air compressor that supplied air to the pneumantic controls. Phew!

  7. #7
    Join Date
    Feb 2005
    Posts
    40

    duel fuel system

    Thanks for all of the helpfull information. This is what I found. A Goodman HP with a Goodman 80 AFUE furnace for backup heat. It had a Honeywell T-stat that was about 6 to 10 years old. This old T-stat was still in the home owners home, he had saved it for some reason,for what I didn't ask. I checked all of the wiring and it looked to be ok. The old T-stat had a blown/burned diode on the board. No damage to the boards on the HP or furnace. Homeowner had a friend install a new Honeywell RTH7400D T-stat. According to the manuals wiring diagram, it was wired correct. But, when new T-stat was calling for cooling, the HP was delievering heat. When the new T-stat was calling for heat, the gas furnace would come on. The coil on the reversing valve is a 24 volt coil.I Checked the voltage at the control board of the HP condenser unit, when calling for cooling, Nothing. I had noticed that on the old T-stat there was a jumper from Y1 to W1, but there was not a provision for this on the new T-stat, all and all everything was the same on wiring connections. Being late,in the dark of night,and number 4 and 5 lumbars killing my back, I decided to call it quites after reading in the manual of new T-stat that if old T-stat had a connection for Y1 and W1 call a professional contractor. Well not ever having installed a RTH7400D before, I called Honeywell Tech Support. Found out that with this T-stat you have to program the stat and tell it that it is connected to a heat pump. They said the programming takes the place of the jumper between Y1 and W1. I'll return tomorrow and program the little booger. It's hell when you use all your trouble shooting skills and then get blown out of the water by programable T-stat that you've never seen before. Still concerned about the blown/burned diode. Wish I new what caused it to go. Thanks againg for all of the info. I'll post the verdict when the jury comes in. Long story, but I try to be descriptive as much possible. I Know how some get chewed on for lack of info.

  8. #8
    Join Date
    Feb 2005
    Location
    small island in the Pacific Ocean
    Posts
    558
    IMHO a HP stat will not work. You need a specified "dual fuel" thermostat, that will lock out the gas furnace when the heat pump operates. It is not like a heat pump with electric supplemenatal that can operate at the same time. With duelfuel once supplemental or aux comes on it will finish the heating cycle also if it is wired for aux to come on during defrost, aux will finish the heating cycle.

  9. #9
    Join Date
    Feb 2005
    Posts
    40

    dual fuel HP system

    Well, the homeowner was the one that bought the T-stat. Paid good money for it, he said. Manage to get the HP to cool and heat by programming the RTH7400D, to tell HP that it was a stand alone unit with no aux heat. I told the homeowner that to have the system to work like it was suppose to, he would have to have a dedicated dual fuel T-stat or use the HP like it was designed to until it couldn't keep up in the heat mode. He would then have to switch to AUX. heat manually. Said he didn't want to buy another new T-stat right now with the cooling season comming on. I told him really didn't need much heat right now any way. He said he would call me when winter got here again to install a correct dual fuel T-stat. Thanks for all of the help and info.

  10. #10
    Join Date
    Oct 2003
    Location
    Ohio
    Posts
    315
    Is there an outdoor t/stat on the hp? I always recommend a standard 2 stage stat that w2 brings on gas and wire it restricted so if 2nd stage is called for aux completes cycle.Of course I don't know what climate you're in.

  11. #11
    Join Date
    Aug 2001
    Location
    bogart ga.
    Posts
    200

    duel fuel

    rudeman you can also install a bill porter board and use any conventional heat pump t-stat

  12. #12
    Join Date
    Nov 2000
    Location
    Upstate, SC
    Posts
    2,920
    Originally posted by phosgene
    IMHO a HP stat will not work. You need a specified "dual fuel" thermostat, that will lock out the gas furnace when the heat pump operates. It is not like a heat pump with electric supplemenatal that can operate at the same time. With duelfuel once supplemental or aux comes on it will finish the heating cycle also if it is wired for aux to come on during defrost, aux will finish the heating cycle.
    Yeah, you can indeed do it that way, but you can also wire it with an outdoor stat to switch the Y signal from the heatpump to the furnace at about 30 F. In fact, there is several different ways it can be done with a regular heatpump stat and the right controls depending on what you want to do with the system. We see lots of duel fuel around here and rarely are two in a row wired the same way.

    Bobby

  13. #13
    Join Date
    Feb 2005
    Location
    small island in the Pacific Ocean
    Posts
    558
    Yes you are right Bobby there are several ways. It just seemed going by the initial post that the easiest for someone unfamiliar would be the duel fuel stat, less components, let the stat do the work .

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