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    Replacing a thermostat (5 Posts)

  • Doug, Valley Stream Doug, Valley Stream @ 2:16 PM
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    Line Volt Thermostat

    I currently have a Honeywell T451A thermostat (120 volt 2 wire on an oil hot water system. The house was built in the 50's and I am looking to replace it with a programable thermostat. Looking on line, I am getting confused, all hot water thermostats are listed as 24 volt. Do I curently have a 'Line Volt' thermostat? Can I use a programable electric baseboard 'Line Volt' thermostat as a replacement?
  • Mark Mentovai Mark Mentovai @ 3:00 PM
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    The T451A is a simple line voltage thermostat. The thermostat is probably just wired in series with the circulator, which takes 120V. It's also possible, but not likely, that the thermostat is actually on a 24 volt circuit now. If you've got a multimeter, probe the thermostat wires to find out. I prefer 24-volt thermostats to line-volt ones. I replaced a pair of original tired old 451s of the same vintage as yours - if not older - and a newer (but junkier) LUX with T8400Bs in my parents' house a couple years ago, and needed relays to run with 24 volts where there had been 120 before. If you've got 120 volts at the thermostat, you can do a direct replacement with a line-voltage thermostat. If you've got 24 volts (plus or minus a wide margin), you can do a direct replacement with a 24V thermostat. (The T451A is a simple switch, so it will work on a 24V circuit even though it's specified as a line-voltage thermostat. If the boiler's ever been replaced or you've had other major work done, the thermostat circuit may have been "converted" to 24V at some point but the T451A left in place.) If you want to use a 24-volt thermostat but are facing a 120 volt circuit, you need a zone relay. This will act as an interface between the 120-volt circuit that the circulator uses, and the 24-volt circuit that you need for the thermostat. If you're not electrically inclined or you don't understand how your boiler is wired, you should not attempt this and instead seek the guidance of a professional. This may become progressively hairier if you've got a dual-limit aquastat (where the boiler provides both heat and hot water) and multiple zones. You (or your hired gun) will use something like a Honeywell RA89A or R845A (single-zone relays, one per zone) or a Taco SR50x relay panel, where x is the number of zones. At last check, Home Depot stocked at least some of these products. If it's any help, take a look at the relay panel I came up with. This is before it's even connected to the system, so keep in mind that there will be even a couple more connections than shown in the picture.
  • Doug, Valley Stream Doug, Valley Stream @ 3:22 PM
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    Thanks, My systen is definitely 120v since I inadvertantly installed a 24v back last winter and fried it (an expencive lesson in 120v and 24v systems). The boiler is original to the house but the the hot water heater was cut out and replaced by a gas hot water heater. I guess that the easiest is to find a 120v programable thermostat and do the direct replacement. Trying to trace the electric back to the boiler is to difficult since the electric is enclosed in a finished basement.
  • Mark Mentovai Mark Mentovai @ 4:13 PM
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    A fried thermostat ought to provide heat!

    Sounds like a classic Long Island system. The one I did for my folks is in Merrick. If you're going to stick with line voltage and you want to go programmable, I'd strongly advise you to get a thermostat that's powered by a battery. Power-stealing or direct-wire line voltage thermostats intended for electric baseboard heat will the expect line to be 240V (your circulator is 120V) and will expect the lower resistance of baseboard (as opposed to the circulator). Your options are a little bit limited, and I can't think of a Honeywell or W-R stat that matches the description. However, the 'Depot does (or at least used to) carry one that will do the trick for you.
  • Mark Mentovai Mark Mentovai @ 4:14 PM
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    A fried thermostat ought to provide heat!

    Sounds like a classic Long Island system. The one I did for my folks is in Merrick. If you're going to stick with line voltage and you want to go programmable, I'd strongly advise you to get a thermostat that's powered by a battery. Power-stealing or direct-wire line voltage thermostats intended for electric baseboard heat will the expect line to be 240V (your circulator is 120V) and will expect the lower resistance of baseboard (as opposed to the circulator). Your options are a little bit limited, and I can't think of a Honeywell or W-R stat that matches the description. However, the 'Depot does (or at least used to) carry one that will do the trick for you.
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