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Changed to digital Thermostats, now not working

I am a heating / plumber repair man.

 I changed 2  mercury stats to 2  digital stats.  Client return call one month later to say  pilot was out on  natural gas Cast iron boiler, Burnham 204 nc....  I relit pilot. The two digital stats did not open each of their  zone valves for radiant loops.  But the other zone valve having a old mercury stat opened the n/c zone valve for the upstair fan coil garage unit heater, which turned on the only one pump in the whole system, then the burner came on.  I removed the digital stat , and jumpered   the two wires  ( both  wires have  power ) for the stat and the zone valve opened, and then turned  on  boiler pump ( the only pump in system) . I reinstalled  old mercury stat, works  fine.

I used The WR 1E78 -144 non  progamable stat , single stage, a ok for changing out  mercury stats. followed instructions in package:  (clipped cut  jumper  w905 wire for hot water heating. ) , set stat switch for gas.

 ( Is this system considered multistage systems ? as this stat states in chart that it do not do multistage systems. ( Does  Multistage means two or more boilers ? )

I tried a non progarmable stat  ( HW pro 1000 th111od1000 ) also certified to replace mercury stats , still the same result, only opens zone valve for one cycle, just like the other  WR stat. and after that the call for heat / turn up the stat 4* , several times trying , still it did not open the zone valve. All wire connections on stat : R = black wire, W = white wire. All Batteries were checked and  good 1.5 v.

On  HW stat  set settings to hot water heating as instructions.

I am baffled why the digital stats are not opening the zone valves and the mercury stats are. Do I have to change all the stats to digital, or all to mercury? 


  • billtwocase
    billtwocase Member Posts: 2,385
    what zone valves do you have?

    Taco's can be killers on those
  • Stan W_2
    Stan W_2 Member Posts: 15
    HW zone valves

    Thanks for replying, :  Honeywell n/c zone valves. four wire 1033 with end switches.
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    On the zones that do not

    work with new thermostats if you remove the two wire for heating from the thermostat and jump them out does everything work? if so then perhaps the thermostats may may not be able to fit you particular setup. Many new digital thermostats require an additional circuit for maintaining a charge on the batteries.

    Most of us here would instruct customers that have the old mercury T-87F thermostats to leave them in service. Just my opinion!
  • World Plumber
    World Plumber Member Posts: 389
    too much draw on the contacts

    Check the current draw and compare it to the ratting on the stat. You may need an isolation relay.
  • ChasMan
    ChasMan Member Posts: 459
    edited December 2010
    Digital Smidgital..

    I waited five years to get all digital stats because I refused to do it with two wires. I tried it years ago and Nope.. not a fan of it.

    2 stage means there are two stages of heat like 2 boilers or a 2 stage furnace of aux + regular.. You do not have 2 stage heat so that is not the issue. The stat is rated at 1 amps @ 30vac. That should be enough to open a zone valve at least once.

    In the instructions, on page 4 diagram 2, the jumper is suposed to be left in place between R(H) and RC. Be sure to do that, it could be important.

    The jumper you said you cut seems to widen the temperature swing by about 1/2 a degree so that's not a big deal.

    Choosing GAS or Electric or Oil or whatever shouldn't have any affect in your case either.

    Make sure that the wire you are connecting to the RC or R(H) terminal is indeed the one that comes directly from the transformer. It is possible this thing wants to steal power some how even though it is listed as a battery powered stat who knows whats going on in there. Connect the other wire (The one that goes to the zone valve motor) to the W terminal.

    When the thermostat calls for heat, this will send power from the transformer, through the RC and R(H) terminals, to the W terminal, and then to the Zone Valve motor.

    In a power stealing scenario, its easy to get it mixed up. Don't go by the color of the wires. You can tell if it is power stealing or otherwise messed up by measuring the voltage usualy there will be some present even if the stat is off.

    One other tip, it looks like the relay for the fan is seperate from the Heat relay so you can save some wear and tear by leaving that off.  If you set the thermostat to Gas, in general, it won't fire the fan relay and since you have no fan, thats good. I have mine set to gas even though I have oil heat.

    Power stealing thermostats and zone valves might not get along. There is no mention one way or the other about this in the manual.

    In any case, it sounds like it SHOULD be working, the way you had it. There should be no reason to have ALL digital or ALL mercury stats as they are seperate circuits completely.

    Checkout Procedure:

    Set SYSTEM Switch to HEAT and raise the setpoint well above room temperature. Within a few seconds the thermostat should make a soft click sound. This sound usually indicates the thermostat is operating properly. If the thermostat does not click, try the reset operation listed below.

    Reset Procedure:

    If a voltage spike or static discharge blanks out the display or causes erratic thermostat operation you can reset the thermostat by pressing UP ARROW, DOWN ARROW, and TIME at the same time. This also resets the factory defaults.
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