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Green wire / EL terminal?

David Nadle
David Nadle Member Posts: 624
My Honeywell Visionpro works off batteries or will run off the transformer if you supply 24V common on a third wire. When under 24V power you can set a mode where the electroluminescent backlight remains on constantly. I kind of like it that way, so I don't have to walk over and touch it to see the temp or setting. Anyway, that's my guess.


  • Coal to oil conversion

    Here is what I'm being asked on another forum:

    "On a panel on a colum next to the boiler is a Honeywell Type R845A control box.
    The primary thermostat goes into this box. There are two terminals labeled T - red wire to one, white wire to the other. There's third terminal next to those two marked EL. The green wire from the current Detroit thermostat is connected to that one."

    I am not familiar with this green wire to EL terminal. I'm told this is a gravity type coal fired HW system converted in the 50's to oil and circulators. Can anyone fill me in on this green wire phenomenon?
  • bruce_21
    bruce_21 Member Posts: 241

    This green wire is a common to the thermostat to provide it with a return path for its 24V power to likely run a clock in an ancient incarnation of a setback type thermostat. The red and white are the hot in and out to and from the stat to call the circulator or whatever is connected to the line voltage side of the R845A relay.
  • bill nye_3
    bill nye_3 Member Posts: 307
    Series 10

    In the 1950's they had series 10 controls and the thermostats had 3 wires. I don't remember EL terminal. Maybe the wire is green because the 3 wire thermostat wire had a green wire? It could have been red or blue or black or whatever.

    Was it 845? or 145? relay. Sorry, just guessing w/o seeing it.
  • 845 as far as I know.

    Thanks all

    The T'stat has a G terminal. and it is hooked to the EL terminal on the 845. He says the circ comes on when you jump it to R but he's a bit sketchy on what he's doing so I'm not sure he's checked it correctly. It's just a friend and all he wants to do is put a new t'stat on it. I've never heard of a G terminal on a heat only hot water t'stat. He didn't mention it being a setback but I asked. Waiting for a reply from him.
  • JJ_4
    JJ_4 Member Posts: 146

    G = Fan (forced air), but it could be used for a pump in a HW system. Likewise, W = heat, R = hot to contact for heat, Rc = hot to contact for cool (usually jumpered off of R), Y = cooling (Y1, Y2 for 2 stage).
  • Mike Irwin
    Mike Irwin Member Posts: 2
    The set up at home...

    Hi, Mark, thanks for bringing this forward for me...

    OK, here are the basics...

    1. House is a 1903 Queen Anne victorian. Originally built with coal-fired gravity hot air, but that didn't work out and by about 1914 or so (judging by style of radiators and fixtures which I've matched to American Standard books of th time) had been converted to coal fired gravity hot water.

    2. Grandparents buy the house in 1943, and in 1954-56 Grandpa (mechanical engineer in charge of boiler systems at the American Viscose plant in town) and my Dad convert it to oil fired circulated hot water with a tankless hot water coil. At that time it's split into two zones, one for the front of the house, one for the rear of the house (what I'll call the "main" loop).

    3. Boiler is Burnham Jubilee, thermostat in question on the main loop is a Detroit with Red, White, and Green terminals.

    That thermostat is wired into the R845A control box -- Red to T, White to T, and Green to EL.

    When I took the old Detroit off in prep. for replacing it with the new Honeywell, touching the red and white together kicked the boiler on.

    Touching the red and green together kicked the circulator on.

    What I do NOT know, however, is if tying the red and white together without the green in the loop would result in the circulator coming on at any time during the burner cycle.

    4. The thermostat for the front loop of the house is TWO wire, not three, and feeds into the Honeywell Triple Aquastat Relay, L8142A, C, J on the front of the boiler.

    5. A third zone, for the kitchen, was added in 2003. It uses a 2-wire thermostat and feeds into an Argo Industries controller which is mounted on the column under the R845. It works the way it's supposed to, so I think we can ignore that.

    6. The main loop thermostat is a Detroit, probably from the 1950s. It's probably the original thermostat to the system, it's the one I remember being there from when I was a small child.

    I didn't pay enough attention to this thermostat, but the wording on it makes me think that it MIGHT have some sort of mechanical delay system that allows it to turn the circulator (green wire). It's definitely not a setback thermostat. It has a single thumb dial at the top to control the temperature.

    What I DO know for certain is that this thermostat will, when calling for heat, kick the circulator on first. It will run for awhile, and if the heat demand isn't satisfied, the burner will fire.

    The reason I want to replace it is that 1) it's terribly out of calibration (thumbwheel at 62 results in 70+ in the main loop rooms) and I want to be able to program it to drop the temperature back in the house at night so Mom isn't paying to maintain the themperature in the main loop while she's in bed.

    I got a Honeywell thermostat from Lowes (danged if I can remember the model, but it's a multipurpose for hot water, steam, and nothing in the directions prepared me for this kind of set up given that the green wire is supposed to be the fan wire. I hooked the thermostat up to see what would happen, and it fired just fine, except for the green wire into the fan control.

    Thinking about this some more, though, I think I have at least a possible solution where I can wire the new thermostat in and make it usable...

    Wire the red wire from the boiler into the red terminal.

    Wire the green wire from the boiler into the WHITE terminal.

    Cap off the white wire.

    I THINK that that set up would give me what I'm looking to do...

    The thermostat would call for heat, and instead of sending a signal down the white wire to the burner to kick on it will send a signal to the circulator to kick on.

    This will (I think) result in heated water being pulled from the boiler. As the temperature in the boiler drops, the aquastat will kick the burner on to maintain temperature in the boiler.

    Does anyone see any problem with that plan?

    I obviously don't want to run the risk of damaging Mom's boiler, but I'd like to be able to give her just a little more control and possible cost savings in heating this old barn of hers.

    If worse comes to worse, I try to recalibrate the thumbwheel on the existing Detroit to make it match up a bit better and I return the Honewelly electronic to Lowes.
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