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ptenerelli · Apr 10, 2007, 01:47 PM

I have 2 old Honeywell timer thermostats - on 2 floors - wired to my boiler (we have radiant water heating). Both function poorly and I would like to replace with standard round manual thermostats. Problem - I took one of the timer thermostats off the wall - it has 3 wires coming in - 2 black and a white. The new thermostat only needs 2 wires. Not sure what to do with this.

The old thermostat has a clock - and you can set 2 temperature - a high and a low. As the clock cycles it triggers either the high temp - or the low temp.

Any advice is appreciated.

Thanks,

Paul

hvac1000 · Apr 10, 2007, 09:53 PM

Model number or your old thermostats. I will see if I can find a diagram

ptenerelli · Apr 11, 2007, 10:49 AM

Originally Posted by hvac1000

Model number or your old thermostats. I will see if I can find a diagram

Don't know where to find the model number on the thing - doesn't seem to have one stamped anywhere.

I know it's an old Honeywell Chronotherm.
There are 3 wires in - a black and white wired to 2 terminals on the left side of the wall plate labeled "clock"
And a black wired on the right side.

I can post pic's if this would help.

Thanks,

Paul

labman · Apr 11, 2007, 12:46 PM

Are the old thermostat terminals marked, R, W, and com? If so, use the wires connected to the R and W terminals. Just cap off the third wire. You may want to check the thermostat wiring section of the sticky at the top of the Heating and air conditioning forum.

ptenerelli · Apr 11, 2007, 09:12 PM

Okay - I found the model number. It's a T852A.
Again - only 3 wires - a black and white wired to the clock side - and a black wire on the other side.

Thanks for the help

Originally Posted by hvac1000

Model number or your old thermostats. I will see if I can find a diagram

hvac1000 · Apr 11, 2007, 09:18 PM

Honeywell has no listing for that thermostat. Could it be T8524? If it is the thermostat was made in 1997.

ptenerelli · Apr 11, 2007, 10:37 PM

Nope - it's "Vintage" Honeywell.
Here's a listing on ebay with a photo
http://cgi.ebay.com/HONEYWELL-VINTAG...QQcmdZViewItem

I haven't been able to find it on the Honeywell site as well - it's too old :-)

Originally Posted by hvac1000

Honeywell has no listing for that thermostat. Could it be T8524 ?? If it is the thermostat was made in 1997.

hvac1000 · Apr 12, 2007, 04:20 AM

You will be better served if you trace the wires to where they go on the equiptment. Then look for the two that control the heat.

labman · Apr 12, 2007, 04:59 AM

You never said if the terminals on the back are labeled. If so, it may be easier than sorting out which black wire is which.

ptenerelli · Apr 12, 2007, 08:36 AM

Nope - absolutely nothing is labeled :-)

Originally Posted by labman

You never said if the terminals on the back are labeled. If so, it may be easier than sorting out which black wire is which.

ptenerelli · Apr 12, 2007, 08:39 AM

Okay - will try that.
The wiring in the furnace room is a bit of a mess - there are 4 zones in the house - upstairs, ground floor, basement and garage. The previous owners didn't put much effort into labeling anything.

I know that the clock wires into a transformer. Would it be 1 or 2 wires that control the heat?

Originally Posted by hvac1000

You will be better served if you trace the wires to where they go on the equiptment. Then look for the two that control the heat.

labman · Apr 12, 2007, 06:37 PM

I am sure 2 of the wires go to opposite sides of the transformer, usually thought of as hot and common. The third wire goes back to the zone valve, gas valve, pump relays, etc. Power would flow out the hot wire to the clock and the thermostatic switch. From the clock it would flow back to the other, common, terminal of the transformer. That same terminal will be connected to the gas valve, relay coils, etc.

The zone valves may even switch the boiler on when they open.

ptenerelli · Apr 16, 2007, 02:51 PM

So with all this great information - can anyone suggest a digital thermostat that would work with this system - and how I should wire it?

Thanks,

Paul

JackT · Apr 16, 2007, 06:07 PM

I don't think you'll find a new thermostat to replace the one you removed that will give you two separate set points with just three wires. You can hook up the honeywell round thermostat with a using R & W but that will only give you one set point. Is your new thermostat the Honeywell T87F series or something else?

ptenerelli · Apr 17, 2007, 02:43 PM

Actually - if I can figure out which is the R and which is the W - I would be quite happy with the standard round thermostat.

JackT · Apr 17, 2007, 04:46 PM

What does the thermostat actually control? Does it start/stop a circulator pump, zone valves, or the burner it self. You should be able to use a Honeywell T87F series with the standard subbase that comes with it. Use two of your three wires to terminals R & W and control the device that starts the heating process. If you're controlling a circulator pump there should be a relay box that the pump connects to.

labman · Apr 17, 2007, 08:54 PM

Originally Posted by ptenerelli

So with all this great information - can anyone suggest a digital thermostat that would work with this system - and how I should wire it?

Thanks,

Paul

Any digital thermostat would work, but you still have to identify the function of the 3 wires you have. Connect the wire from the transformer that also connects to any relay coils, gas valve coil, etc. to the com terminal if it has one. Connect the wire that comes straight from the transformer to R terminal. Connect the wire that connects to the gas valve, relays, zone valves, etc. to the W terminal. The thermostat works by switching power from the R terminal to the W terminal when it calls for heat. The com terminal allows the thermostat to draw power for itself from the R terminal.

Apachie58PU · Mar 17, 2009, 06:57 PM

I worked on them for years You could a have line voltage thermostats from what it sounds like the 3 wires are White common to timer motor Black power up to clock and other black is power back to the boiler . You have to look at the boiler wiring and determine which 2 wires each thermostat probable operates a pump. If you are intersted in selling the old stats email me at [email protected] You will probably have to buy a line voltage stat or a fan center which will let the pump run on low voltage Did this for over 30 years. If you want to call me call 407-886-1085 Semper Fi Paul

hvac1000 · Mar 17, 2009, 08:25 PM

Becareful with posting your phone number.

madtech · Oct 15, 2009, 09:38 AM

Hi. I wanted to replace this thermostat and honeywell have no info on the T852A. I finally figured out the wiring. If you look at the wall plate, there is an arrow point up. There are 4 terminals: 2 to the left of the arrow and 2 to the right. 2 words "clock terminals" with an arrow pointing to the left upper terminal. That's the load for the clock. Left lower is the common for the clock. 2 terminal on the right is for the thermostat. If you have baseboard heat only, just take the 2 right side terminal off and use one as R and the other as W (it doesn't matter which wire is load and which is common, the two thermostate just use the 2 wires to complete a circuit). Tape up the 2 left terminal wires (not needed for digital thermostate).