Replies

1. 
   [email protected] | May 03, 2011 04:48pm | #1

   Probably

   I'm just guessing, that the thermostat you have is a 24-Volt unit.  Get a multi meter and check if it isn't printed on the unit. 

   If so you should be able to use almost any 24-volt thermostat. 

1. 
   Daft_Craftsman | May 03, 2011 04:58pm | #2

   Yes - 24V AC, 60Hz

   It is 24V AC.  Then I wonder about wiring a new thermostat.  As I said, it's actually wired with telephone connections.  Do I just cut the telephone connection off of the thermostat end, strip the wired and apply to the new thermostat?  I'm not certain how to determine which wires get connected to the new thermostat.

1. 
   [email protected] | May 03, 2011 07:19pm | #3

   Have you opened the thromostat?

   I'd suggest opening up your thermostat and check to see what wiring scheme it has internally.   Check to see what wire on the phone cord is doing what inside. 

   I'm just guessing, but you should have: a red wire, which is the 24-Volt "hot"; a brown, white or black which is the 24-volt "nuetral"; and, a white or green which is the heat call. 

   Regardless, you need to determine what wires on the phone jack do what. 

   Once you know that, I'd recommend you go to Radio Shack and get a female phone jack and build a base to mount the thermostat to, that you can insert and crimp the required wires into.  This is just because I have never had much luck stripping phone cord wiring.   

1. 
   Daft_Craftsman | May 03, 2011 08:09pm | #5

   Yep, opened thermostat.

   OK, opened the thermostat.  I tracked down the model number and found this schematic.  I've attached the Operation manual.  I have a Microtemp MTD 3999.  It does not have a floor sensor or remote switch. 

   I like your idea about the phone jack connection, but since I opened the thermostat, I now know that the thermostat has terminal screws that I can loosen to get the wiring free to connect to a new thermostat.

   The more I look at this thing, the more I want a different one.  To get the unit open, you have to remove the temp setting dial to remove a hidden screw.  To put the dial back on, you have to know what the room temperature is at that given time.  Not a good system!

   Very helpful so far.  What do you think now?

   
2. 
   DanH | May 03, 2011 08:00pm | #4

   If it's a standard 24V 2-wire thermostat then basically any standard thermostat can be used.

   However, some digital thermostats do not work and play well with hydronic systems because they can't deal with the slow response of a hydronic system.  So be sure to examine the package for any message such as "Not recommended for hydronic heating systems."

   The other problem you may encounter is that you might not have a 2-wire setup.  I believe some hydronic systems need a thermostat with an extra set of contacts (and a 3rd or 4th wire) so that it can operate the burner and the pump motor separately.

1. 
   Daft_Craftsman | May 03, 2011 08:12pm | #6

   Not a two wire

   It doesn't appear to be a 2 wire to me.  It doesn't operate a burner either though.  It only controls the pump.  The pump sends the water throught the system which includes a seperate coil that runs through my water heater.

1. 
   DanH | May 03, 2011 08:22pm | #7

   Actually, I believe you could replace it with a standard unit, but you'd give up the floor sensor function.  Basically, cap off all the wires except those going to 2 and 3, and wire the new stat to those two wires.

1. 
   Daft_Craftsman | May 03, 2011 09:56pm | #8

   Maybe?

   I don't have the floor sensor, so that's not a concern for me.

   Could you explain the circuit by wiring only to terminals 2 and 3?  Terminal 2 is neutral and 3 is part of the dry contact?

1. 
   DanH | May 03, 2011 10:07pm | #9

   I apparently misspoke -- it would be the wires on 3 and 4 that you would use.

2. 
   Daft_Craftsman | May 03, 2011 10:21pm | #10

   I see

   Makes sense now.  Thanks for the help.  Much appreciated.