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Replacing 9 old in-floor heating 2 wire thermostats

Canadian_EnergyMiser | Posted in General Questions on

We have in-floor heating with 9 zones. Each zone is controlled by an in-room older thermostat with no electricity or batteries, ie a simple basic thermostat where you set the desired temp by hand ie a simple on/off switch basically that trigger a basement based loop valve to open or close.

I’d like to replace this with something more modern and accurate (as I find the temp settings often imprecise) but do not want to bring a wire to each of the 9 thermostats (and cut drywall, remud and tape & paint afterwards x 9) as most modern thermostats require a 3 wire feed ie one with electricity.

Suggested options here ie likely best battery based thermostat system, or perhaps wireless from one central area?

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  1. Expert Member
    BILL WICHERS | | #1

    Some people have used the CT100 Zwave thermostats from Thermostat Company of America.

    There is another potential option for you though. Waaay back I was a cable installed (datacom cabling, not cable TV), so I've fished a LOT of wire. Most smart thermostats require at least three wires to operate, so if you don't want to deal with batteries you'll need to run a new wire -- but it might not be that bad! I see two options, which I'll describe after I describe a basic wire fishing and hiding technique.

    Drill a hole behind your thermostat of at least about 3/4" or so. If you have a two-piece baseboard (base and quarter round, for example), remove the 1/4 round from the front. If you only have a one-piece baseboard, remove the section under your thermostat. Drill another hole behind the baseboard and DIRECTLY under the thermostat. Use some nylon twine (the cheap 1/16" stuff, you want lightweight and flexible) with a weight (fishing sinkers work well, but I've used just about anything from a bolt or nut to some leftover solid copper wire folded over). Use the weight to drop a string down from the hole behind the thermostat to the hold behind the baseboard. There will usually be a small gap between the bottom of the baseboard and the floor, sometimes this is between the edge of the carpet and the wall. There is almost always enough space to hide a small wire here. Run the wire along this gap until you get to....

    1- A place you can fish the wire the next part of the way to wherever your valves are. This can be a corner where you can drill straight down and hide the hole, or the back of a closet. Anywhere you can hide a hole where it won't be seen. Get the wire down to the area with the valves and connect it as needed to run the thermostat.

    2- under a power outlet. If you get a 24 volt AC "wall wart" type plug-in power supply, you can use this to power your thermostat from any nearby power outlet. You can run 2-wire thermostat cable from the wall wart to the thermostat.

    Once you've done 1 or 2, just put your baseboard back together carefully so you don't damage the wire. When you're done, the wire is hidden out of sight, and so are the holes, so you don't have to do any drywall work at all. I've fished wires like this many times.

    Since you really only need to get power to those thermostats, you could potentially use one power supply to run multiple thermostats, tapping off the same wire as you go. This might be easier in terms of wire fishing depending on your floorplan. You can use the small gray or blue wirenuts to make these connections and just hide them in the hole behind the baseboard since it's a low voltage system.

    If you run new wire only for power, the existing 2 wire cable becomes your control cable. Be careful to connect the thermostats all the same way so that you don't short out your power supply with a backward connection somewhere.

    If you're creative, you can fish low voltage cables all over a house and hide the access holes behind baseboards, crown molding, and alongside receptacle and switch boxes (but remember that your low voltage cable can NOT enter those boxes!). There are even drill bits with long, flexible shafts that let you drill through studs inside a wall (these take some practice to use without damaging things though so be careful). Wire fishing is all about being clever and patient, and a lot of it is done by feel. You rarely have to actually open up a wall any more than a small hole though. Most of my wire fishing work was done with nylon string, fishing weights, a stiff wire coathanger (to grab the string), and a set of fish sticks like the "creepzit" from labor saving devices, or later the Greenlee glow-in-the-dark fiberglass rods.

    Don't waste time with an electrician's fish tape. Those are only really good for pulling wire in conduit. I maybe fished a wire with one once in a wall 30+ years ago, and that was the last time.


    1. Canadian_EnergyMiser | | #2



      Let me check.

      Follow on question: if I buy a 24V 1.5 amp transformer which of the two wires do I connect to the new thermostat? Or will most new ones requires 4 wires, ie 2 for power and 2 for the control?

      P.S.: Battery based thermostats too expensive to change batteries every 2 years ?

      1. Expert Member
        BILL WICHERS | | #3

        The transformer itself will only need two wires for power. Electricity works like a circle or loop, it comes out on one wire, does it's work, then goes back on the other wire. Any extra wires are for control, what we call "signaling" when we're using fancy words for job security :-)

        As long as the transformer is isolated (no other connections back to the system), you can connect the wires either way BUT you ALWAYS need to connect them the SAME way on ALL the thermostats if you're sharing one transformer among multiple thermostats. One of the transformer wires would go to the "C" terminal on the thermostats, the other would go to the R terminal on the thermostat, which is the powered terminal of a 2 wire thermostat. The W terminal then signals heat, which the thermostat does by connecting the R and W terminals together with an internal relay (usually).

        What you need to make sure you don't do is connect the transformer in such a way that the thermostat either shorts it out when it calls for heat, or connects it in series or parallel with the power supply of your existing heating system. That's the reason why you have to keep the wiring consistent everywhere. With the transformer isolated from the "other transformer" in the heating system, you'll be OK, but you need to make sure there is only a SINGLE connection (i.e. all the R terminals, but never a W terminal) linking the two transformers together. With only one connection in common between the two, there is no loop, so no current can flow. That's what you want -- no current flow BETWEEN transformers.

        I don't like battery systems because the batteries need replacing, usually when I don't have any spares. I also don't like the need for batteries anyway since it seems wasteful to me. Many of those battery powered thermostats talk about battery replacements every so many months, not years, so you end up going through a lot of batteries over time. If this were my project, I'd spend a few days fishing in new wire so that I never had to worry about batteries. If I were running new home runs (a separate run from the heating system out to each thermostat), I'd run at least 5 conductor cable too for some future proofing. That would allow smart thermostats and air conditioning if you ever do any system changes.


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