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goodman scorched air furnace wiring to start AC this season

archibald tuttle
archibald tuttle Member Posts: 819
got a goodman GMH95 we installed last year and been very happy with it. but in looking for how it is meant to interface with the AC compressor and condensor got 'help' from their tech service that - i could be wrong - doesn't seem right. They don't diagram field wiring very well but were recommending that i just send a parallel connection from the Y terminal on the boiler to the outdoor unit.

this alone will not start the unit of course. it has no transformer itself and 24V contactor. so in theory if i send the Y and the C to the condensor it will start however. the goodman has a 40VA transformer but it doesn't indicate its own onboard load. and just to add to my convenience the GE contactor gives its 40 amp contact rating but not the coil load in amps so i could assess that this additional load is within the range of the onboard electronics for the goodman furnace.

The guy from Goodman told me i wouldn't be adding load to their transformer. I don't believe he can possibly be right. It may be rather that the transformer can readily handle a contactor load of the size expected for a compressor and condensor that would serve the cooling BTU range associated with the sizing of the boiler itself for heating so he wasn't used to such a question or thought it apparent that the contactor load would not overload the transformer, but that's not what he said. And i assume that the logic of the 24V to Y terminal in the Goodman is to run the fan on high without any plenum temperature control but i can't realy find that confirmed in the documentation.

thanks

brian

Comments

  • bill nye_3
    bill nye_3 Member Posts: 307
    Your condenser contactor should have a 24 volt coil , and yes "Y"and "common" are generally used to pull it in. How many wires from your T-stat ? "G" is usually the wire that runs the fan.
  • JUGHNE
    JUGHNE Member Posts: 8,960
    A 40 VA transformer should handle the typical furnace/AC unit.

    You are using the terms of boiler & furnace in your posting; I assume that if you have one of each that the wiring in not interconnected for control by one T-stat??

    For hot air furnace/AC, you need a minimum of 4 conductors between furnace and T-stat. And 2 wires from furnace to AC.
  • archibald tuttle
    archibald tuttle Member Posts: 819
    bill nye said:

    Your condenser contactor should have a 24 volt coil , and yes "Y"and "common" are generally used to pull it in. How many wires from your T-stat ? "G" is usually the wire that runs the fan.

    so i've got 6 wires, no shortage. on the old unit Y pulled a contactor that started the fan via a summer switch that took it off the plenum temp control.

    unfortunately the wiring diagram for the GMH95 shows a G and Y contact but they go into a black box integrated control module so i can't really see the logic of operation. I get G is the fan only call but i would assume Y calls the fan independently. I'm not really aware of any call for cooling that would not call the fan right away , so unless the y terminal doesn't go anywhere in the control module and exists just as a junction for the call to the compressor contactor i wouldn't think you would need G but i'm sure other folks have seen this more than i have.

    i work with scorch air so seldom that i keep dropping back to calling the furnace a boiler as Jughne points out.
  • archibald tuttle
    archibald tuttle Member Posts: 819
    JUGHNE said:


    You are using the terms of boiler & furnace in your posting; I assume that if you have one of each that the wiring in not interconnected for control by one T-stat??

    my bad. when you're a hammer, everything looks like a nail. . . so used to sayin' boiler couldn't get myself out of it even though i started the thread off right. furnace only

    JUGHNE said:


    For hot air furnace/AC, you need a minimum of 4 conductors between furnace and T-stat. And 2 wires from furnace to AC.

    as asked above the minimum 4 is to include G? why doesn't Y call the fan, or put a different way, what does it call on the furnace or is it literally just there as junction for the call to condensor outdoors?

    i know assuming makes an **** out of you and me but there is no G needed for the fan on heat call so I just assumed they would use similar logic on Y for cooling -- the only thing needed for cooling is a call to the fan without the plenum temp. regulator and a call to the condensor so i figured both those calls would get called from Y. the fan wiring would be from Y through the control module and the condensor call would be from the Y (and C) terminals at the boiler two wires to the condensor contactor.

    it just seemed intuitive since the one call to W handles both the burner and the fan for heating that there would be a single call for cooling. i assume if G is the fan call for cooling that it closes on rise with Y?



  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    No. The thermostat does the switching for Y G and W.

    On a cooling call, R becomes Y and G to close the relays for the contactor and the fan. On heating, R becomes W..C is already at the terminal board waiting for whatever ccircuit to make.
  • bill nye_3
    bill nye_3 Member Posts: 307
    "Y" terminal may just be a tie in terminal for the cooling wire from the t-stat to the contactor wire on the condenser. The heating fan often is tied in to a fan-limit switch or a circuit board. Usually a different speed than the cooling mode. On a call for cooling the fan stuff is done in the t-stat ( y & g thing) sorry for the confusion. I know what I want to say but can't seem to type the right words
  • archibald tuttle
    archibald tuttle Member Posts: 819
    that is certainly what everybody is telling me. Y is cooling. G is fan. i guess this leaves me wondering why it takes two calls for cooling, i.e. one for the tower and one for the fan with the same 24V signal running down a single wire to the thermostat and then running back on two wires to the furnace, but a single call is used for heat to call both the burner and the fan.

    i get that the burner and fan are in the same unit and the condensor is independent, so i could quite logically see separating the call to the tower if the tower had its own 24V transformer with the use of an end switch of some sort or extra pair of contacts at the thermostat. but the call is comingled because it uses the 24V from the furnace R & C. so why split that call at the thermostat. why not just run it back on Y and then run a 3" jumper from the furnace Y to furnace G? I guess

    does the Y call actually do anything at the furnace or could i just wire nut the thermostat Y to one side of the condensor contactor (with C fed to the other).

    I guess some applications may want to call the fan independent of either heat or cool for air filtering and circulation so i understood a fan only signal at G. And since that would run independently of the plenum temperature control it could be used to constant run the fan for cooling. But then i'm mystified by what the Y function would accomplish at the furnace.






  • archibald tuttle
    archibald tuttle Member Posts: 819
    bill nye said:

    "Y" terminal may just be a tie in terminal for the cooling wire from the t-stat to the contactor wire on the condenser. The heating fan often is tied in to a fan-limit switch or a circuit board. Usually a different speed than the cooling mode. On a call for cooling the fan stuff is done in the t-stat ( y & g thing) sorry for the confusion. I know what I want to say but can't seem to type the right words

    that makes sense to me if the Y is just a junction although now you have me thinking. most of the furnaces i've dealt with don't have multiple fan speeds, but this one does. I would imagine a call to G is a call for high speed anyway in such circumstances.

    on a heating call i would imagine it might slow down the fan and modulate the fire when the return plenum reaches a certain temp. i haven't looked to see if it has separate temp. sensors for feed and return air streams. but i would think cooling wants the circulating fan on high all the time.
  • JUGHNE
    JUGHNE Member Posts: 8,960
    edited May 2015
    They give you a Y terminal just to make a connection as a wire nut would. The T-stat (with 2 separate switches on a call for cooling) does the calling of Y (AC) and G (fan-high speed). If you tie the Y & G together at furnace or T-stat then if you want fan only you would get AC & fan.

    If you have a boiler does it have it's own T-stat ??
  • unclejohn
    unclejohn Member Posts: 1,751
    If your furnace has a variable speed fan it may most certainly need both a "Y" and "G" input for proper fan speed.
    Bob Bona_4
  • bill nye_3
    bill nye_3 Member Posts: 307
    ouch, my head hurts. Maybe you are overthinking this thing.
    Bob Bona_4
  • archibald tuttle
    archibald tuttle Member Posts: 819
    bill nye said:

    Maybe you are overthinking this thing.

    no question about it.

    thanks everyone for helping me overdo it. i'm off to do as you said.

    get the point that jumping y and g would turn a fan only setting into cooling. and it sounds like Y really is just a junction at the boiler end.

    the reason i'm being hard headed about the control strategy is that on the heat side the control of both the fan and the burner is effected by a single call. obviously from there it is managed by on board furnace wiring including temp. sensors. so i expected it to work the same way, with the cooling signal parsed at the furnace control.

    The modern controls use logic rather than relays to parse the G and W calls to the fan so you don't call heating when the fan is called. In the good old days it could have been relay, or a "summer" switch (i suppose they could have used a diode also as i thing about it), on the furnace so the system i replaced only used the Y.

    the relay at the furnance in the old set up was an extra piece of complication but i also liked it because it cut the loop to the contactor. under the G/Y call the contactor current runs out to the thermostat on a single wire and back on two. sounds like this is conventional so i won't get my knickers in a knot.

    The seal current for the 24V solenoid is around 10VA so well within the transformer capacity. And the extended wire length only results in less than a 1% voltage drop so assuming that the furnace control load is in the same neighborhood or less i see no problems.

    thanks for the practical knowledge to put all my overthinking to rest.

    brian