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Backfeeding 'G'

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Tom_80
Tom_80 Member Posts: 22
For T-87 stats, and others, an isolated "G" subase is available. Check with your supply house and they should be able to give you part #. I think Q539A1147 has the isolated "G" terminal. Double check this though.

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  • MD Shunk
    MD Shunk Member Posts: 14
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    Have a customer with a straight a/c air handler that has a hydronic coil added on the end. (This coil is heated by an outdoor wood furnace and associated circulator). This system was newly installed, and they want the fan to come on when the circulator for the heat is running. No problem, I though. The circulator relay had a extra set of contacts, so I also have the contactor sending power to the AHU to energize "G" to run the fan. It seems that the stat has some sort of internal connection between "G" and "Y" when the fan is in "AUTO" mode. When you backfeed "G", the compressor kicks on... no good. I am told by the supply house that most stats have "Y" and "G" internally connected when the fan switch is in the "AUTO" setting. Since they said "most", I wonder if the readers of this site are aware of a stat that will permit me to backfeed "G" and have the stat not also kick on the compressor (because of an internal Y-G connection)?? If not, I guess I'll just retrofit a fan center relay that will have "W" pull in the coil of the fan center relay to keep the compressor from running when "W" is energized.

    Is there a more "textbook" approach to this problem?
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
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    Honeywell FocusPro or VisionPro Thermostats...

    will make it one step easier. You can set the thermostat to call for fan (G) with a call for heat (W) in the installer setup menu. Saves the trouble of the extra contacts wiring, and will keep your "Y" and "G" terminals isolated.
  • MD Shunk
    MD Shunk Member Posts: 14
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    Thanks for the info on the Focus Pro, etc. I have seperate heating and cooling XFMR's, so which "R" connects with G when you make G energized on a call for heat (Rc or Rh?)? I'd need the cooling transformer to energize G, and not the heating transformer, since the cooling XFMR is in the AHU.
  • Unknown
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    you need to isolate the xfmrs...

    So you would need a "Dual feed " t'stat that has terminals "RC" and "RH". Your heat xfmr power goes to rh and your cool goes to rc. Then, on a call for cool the compressor and fan will be energized through y and g. For your heat you need to wire your coils for the circ relay and another fan relay in parallel. (I like to use a delay for the fan) So they will both be energized on a call for heat. THEN you wire your fan relay contacts in parallel with the other fan relay or to a different motor lead if you want different fan speeds for heat vs cool and have a multi speed motor. Generally it's high speed for cool, low speed for heat. OR instead of an additional fan relay you can install a "strap on" aquastat on the HW line near the coil and when it senses it is warming up it will switch the fan on. Again it's contacts are in parallel with the cooling fan relay and will feed either the same lead or a different lead just like the second fan relay. The dual feed t'stat may have a jumper between rc and rh. You would remove the jumper, this gives you 2 separate circuits through the stat. RC-Y-G for cool and RH-W for heat. Ask more questions if you don't understand. I'm trying to be clear but don't know if I am. Y and G will ALWAYS backfeed on any t'stat to the best of my knowledge.
  • Jerry Valentine
    Jerry Valentine Member Posts: 15
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    How to stop \"y\"

  • Jerry Valentine
    Jerry Valentine Member Posts: 15
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    How to stop \"y\"

    What we do is to add a double pole relay to the air handler with a 110v coil. We use the circulator voltage (110v) to energize the coil. One set of the contacts is a normally open set that closes -- we use this to start the fan using red and green in the air handler. The other set of contacts is a normally closed set that opens -- we run "y" through this. Doesn't affect cooling at all. We've been doing it for years. If you want a delay on the fan so you don't blow cold air add an aquastat to the hydronic coil and wire red (or green) through the aquastat which would be set at what ever temperature you wanted the fan to start. Really a simple fix.

    Jerry
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
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    I'm not 100% sure, but I am almost positive that "G" is powered from the "Rc" terminal, which would be the cooling transformer.
  • MD Shunk
    MD Shunk Member Posts: 14
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    > I'm not 100% sure, but I am almost positive that

    > "G" is powered from the "Rc" terminal, which

    > would be the cooling transformer.



    I might pick up a visionPro and focusPro at the supply house, set them up on the bench, and see whether Rc or Rh energizes G on a call for heat (when you have that option set in the installer menu). Then I'll know. If it doesn't work out, I'll just have a couple neat stats for truck stock, for the next person who seems like they're a gadget geek and wants or needs a new stat. :)
  • MD Shunk
    MD Shunk Member Posts: 14
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    I like the ideal of an aquastat for a time delay until the coil is heated up, but these are PEX lines. Does a strap-on Aquastat work very well with PEX lines? I might have to do it with a regular, ordinary delay timer. I have plenty of anti-short cycle timers for a/c that should work well.
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,557
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    The Q539J subbase that energizes the G terminal on a call for heat or cool.
  • MD Shunk
    MD Shunk Member Posts: 14
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    Thanks for all the good input so far.

    I've just been informed that what I thought was straight air is actually a heat pump, which they also want to use, with automatic changeover (??) somehow to the wood furnace. I'm seeing an outdoor thermostat in the future, perhaps. I have no idea what they intend to use for heat during the heat pump defrost cycle, since there is no electric heat in the AHU.

    Some people are just weird.
  • Unknown
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    Hmmm,,,

    It looks like I misread this. For some reason I thought you had two xfmrs. Apparently not?

    There are digital stats out there that have a "gas/electric" switch on them that will energize the g on both heat and cool when set in the "elec" position. Look at the Robert Shaw 9600 series. In tyhe "gas" position they will NOT energize G and rely on the furnace to control the fan. Now, for this heatpump thing. I'd just shut it off at the coldest outdoor temp I could get away with without it needing to defrost. Do they burn the wood boiler continously or would they have to go fire it up when the heatpump shut down?

    Maybe you could set it up so that when the heatpump shuts down it initiates a little spark under some tinder in the preloaded firebox of the boiler. ;)
  • MD Shunk
    MD Shunk Member Posts: 14
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    Yes, there are two transformers. One for the heating, which basically turns on the circulators. The other is in the AHU, and does the fan relay and control power for the condensing unit.

    I think I have resolved to use an outdoor stat to do the changeover from heat pump to wood furnace at a particular outdoor temp. I'm just not sure how to handle heating during the heat pump's defrost cycle, since there is no electric heat in the AHU, and the wood furnace may not be fired up until later on in the heating season.
  • Unknown
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    OK,,,

    So I guess you DO need the isolation T'stat and the digitals with the gas elec switch won't work for you..

    Then, I wouldn't want the heatpump to ever go into defrost. You need to shut it down before it gets cold enough outside to require a defrost. I'm guessing 38* but you may need to fiddle with that one way or another to dial it in closer. That's how Carrier (anyways) handles combining fuel based heat with a heatpump. Below a certain outdoor temp the heatpump shuts off and the fuel based kicks in. Much easier when it's oil or gas. Wood sounds like it'd be a little tricky.
  • MD Shunk
    MD Shunk Member Posts: 14
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    I guess wood is a little trickier, in that you need to keep it manually fueled and lit. They do have the combustion air controlled by a solenoid activated air shutter, much like coal furnaces that more people might be familiar with. Matter of fact, a wood furnace is functionally just like an old coal furnace. At least that's how I look at them, but I'm from coal country.

    Yes, I suppose if I can get them to set the outdoor stat at 40 degrees or so, that would take care of any defrost concerns I had.
  • hot_rod
    hot_rod Member Posts: 22,157
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    The only solid fueld solution

    to that puzzle would be a biomass (pellet)boiler. I installed a biomass Zener boiler last year to back up a heat pump. The ignitor and inducer fan would have the pellets lit in under 15 minutes. Depending on the load, the boiler would be 180 within 20- 30 minutes at no load.

    With a regular OWF they would need to idle the beast, which unfortunatly is the least efficient way to operate one. Expect 20- 30% efficiency from OWF when the sit idle or short cycle without loads.

    It's very hard to build an automatic OWF (wood fired)start control :) And you can dump a lot of time and parts into an automatic switching attempt.

    At some point hiring a "fire tender" like a neighbor, may be cheaper.

    Insulated buffer storage can work with wood fired heat, but at a large insulated storage capacity & cost

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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