The System

I have a relatively simple system. It starts out with a standard, run-of-the-mill Rheem gas forced air setup with central air. One transformer in the furnace goes to one thermostat like normal. I recently upgraded the wire to 18/5 so that a C wire now exists in the system. Everything with the heat and A/C works as it should. However, during the heating season this system is used only as a backup "emergency" source of heat.

That's because I have added a hydronic radiant floor as the new primary heat source which is a lot more comfortable and efficient. The relevant part of this is the NIX circulation pump controller which has its own transformer tied to its own thermostat. It also works as it should.

My goal

What I want is to simplify the system to one thermostat if that is possible. At negative outdoor temps the radiant floor cannot hold 70 degrees indoors because I have so much glass in the home (bay windows and patio doors) (and yes, this is a "double-tube" install). It would be great if a thermostat would see this condition (say, a delta of 5-8 degrees) and kick on the gas forced air furnace when the radiant floor cannot keep up.

Here's what I've tried

To get this to work, I have been under the impression that what I needed was a "2 stage" thermostat. Accordingly, I purchased and installed the Honeywell RTH6360D1002. FYI, this thermostat "clicks" every time it does something. I've hooked the NIX radiant transformer to Rh and the furnace transformer to Rc, the NIX controller's W to W1 and the furnace's W to W2. The other G, C, and Y wires are as per usual.

This results in the A/C working fine (I assume the Rc from the furnace is being connected to Y and G). This also results in the radiant floor working fine (I assume Rh from the NIX controller is being connected to W1).

However, the 2nd stage is not turning on the forced air furnace. In a 75 degree room, I have set the thermostat to its max of 90 degrees. The radiant floor comes on almost immediately. After 2 minutes or so of the radiant floor being on, I hear the Honeywell "click" once again. I assume this is the thermostat realizing there is a 15 degree delta and is turning on the second stage. I assume this is Rh from the radiant NIX controller being connected to W2 (furnace).

Is it a no-no to cross transformers like that? Can the 24v from the radiant's NIX transformer be used to signal the W in the furnace (via W2 in the thermostat)? What is the solution? Can I just wire differently? Is there a solution at all, or must I continue running two thermostats, one for each system? If this Honeywell is not right, do you have a recommendation for a thermostat that will accomplish this? Thank you.

3 Answers 3


Comment, not an answer (but it might be): [I am new and won't let me 'comment' until '50' reputation; but it DOES let me 'answer', go figure...]

Correct, it is a No-no to cross transformer supplies (at the R's). Most equipment now have one side of the 24VAC transformer physically attached to ground. This will lead to having both (or potentially 3, if AC has it's own) 'crossed' in parallel. Both transformers could get hot if not precisely the same voltage, and would actually be a 48V short, if their outputs happen to be 180° out of phase; 50/50 chance!). And not safe in case of a future replacement of a transformer, or unit. I believe it should be isolated for serviceability in the future.

My opinion is one system should be put on a 24VAC relay. I would put the primary (hydronic) control on the relay output contacts (isolating that one's transformer), and run the thermostat and the furnace on the furnaces transformer. (this way future AC changout would not present any issues, and the R and Rc feature on the thermostat can be split for any AC needing that).

I think any standard small 24VAC relay with say 3A contacts will be adequate. The hydronic W probably draws less that 1.6A, possibly near zero.

To wire the relay so the coil is on the furnace 'side' and the contacts are on the hydronic side, would go like this: The relay will need a connection for the 'ground-side' of the coil, probably the 'blue' transformer wire of the furnace transformer. The furnace C is probably available at your thermostat too, (blue?) depending on where you choose to place the relay. The thermostat W1 would go to the coil 'hot'. The contacts are wired to the 'red to white' hydronic cable, to 'make' the connection for the hydronic W signal.

Since the thermostat has an LCD display, it likely has both 24VAC supply wires at the thermostat location, and won't need to run an added wire. You will need 2-wire red-white to go to the hydronic, in case there is only one white there now.

The R from your hydronic would normally go to your thermostat R, but now will go to one of the relay contacts. The hydronic W would normally go to your thermostat W, or W1 on dual stage; but now will go to the other relay contact.

The relay coil is now going to be connected to the thermostat W, and the 'ground' for the coil will be connected to the C wire (blue?) from your FURNACE, NOT the hydronic. Relay is now a 'go-between', receiving the output of the thermostat W, and closing a pair of contacts that are connect to the cable from your hydronic. Connect R-Rh with jumper, and the furnace is powering everything now, (except the hydronics is running on it's own transformer).

Coil is powered from the furnace side; contacts connect only to the hydronic side.

I think I described that correctly. Any mistakes please correct me, anyone. This is how I would do it, if I was doing it. I can be more specific if you need, but would need more details of your layout.

Mr K L AC technician


Ok, I got your system/situation figured out; as to why your furnce is currently not functioning correctly: You must have the Rc-Rh jumper removed (a good thing), and your hydronic (or your furnace) does not have a ground on it's transformer. Apparently one or both of the transformers are direct wired to the controls, without using a frame ground (providing a 'return' path). [This is only a problem when one unit is trying to activate something 'outside' of itself]. Or (if you find both transformers are grounded, then) one of the units itself is not grounded good, or not grounded at all.

So the Rh from the hydronic's transformer has no voltage (to 'ground') relative to the furnace's circuitry. The furnace transformer is powering the AC fine, but the thermostat is using the hydronic's transformer to power the hydronic's heat (works fine), AND the furnace's heat (which has no path for the 'return' current back to the transformer).

You can confirm this by checking the AC voltage from Rh (hydronics power) to C from the furnace. (this may be different when 'loaded', ie it may show 24VAC until W2 demand kicks in and tries to draw actual current).

I think you could theoretically do this without a relay, but you will need the 'ground' side of both transformers tied. Check if each unit has one side of it's transformer grounded to the frame, and if the 120V ground to the building is intact/present. A (blue?) wire from one unit to the other would insure direct current return, instead of depending on earth ground (and it is not proper to use ground to conduct current, even low voltage AFAIK).

I also don't know if the hydronics transformer is rated to also power additional loads like this, or if the 48VAC issue can damage these controller boards. The relay is probably the only right way to do this. It also makes sure that ANY future replacement of either unit (or the thermostat) will not have to be customized to work right... If you use a relay, you won't have to worry about the grounds, or a return path between units. (It makes both units' power circuits totally independent).

First, I would test your furnace alone to insure nothing has been damaged, and then try the relay setup.

Set it up the normal way: Temporarily unhook the hydronics, put the furnace on W1, jumper Rh-Rc, and test it. I am confident it will work normally again. [it's the transformers' grounding and/or lack of return path that prevented the W2 from functioning].

Oh, With the relay you will have Rh-Rc jumpered, the furnace transformer will power the furnace stage 2 heat, the thermostat, the relay, and the AC cycle. The hydronics transformer will only power itself, (totally isolated), and is only controlled by the relay.

Mr K L

Your post was clear, my head was not. LOL I misunderstood, and missed some things that were clearly stated in your post. (also while in 'edit post' mode, I can't see the OP to refer to)

  • Yes, there is central air in the Rheem furnace. No, the hydronic NIX controller does NOT provide anticipation; it relies on the anticipation setting in the thermostat for that. Yes, the hydronic side is a simple 2 wire red-white combination. Thank you for your answer. I will refer back to it several times I am sure as I wire up whatever the solution is. Commented Sep 4, 2020 at 17:38
  • Not done this before; I just understand electronics extremely well. Relays draw little coil current, this might not give any 'anticipation'. I can help sizing a relay or adding a resistor across the coil to give the expected current draw. It would depend on how your thermostat works (if it has an amp setting or is done by time delay (if done by time then won't matter). Depending on your layout it might be easiest to put the relay at the furnace, and run the hydronic pair there. Or behind the thermostat in the wall? I don't know if there would be any code issues there. Lots of ideas... Mr K L Commented Sep 5, 2020 at 2:14
  • Something similar to this might work well, and be easy to mount: amazon.com/Emerson-380-Relay-Volt-Coil/dp/B000LDCPQS/… Commented Sep 5, 2020 at 2:15
  • Yeah, a bog-standard 24VAC fan relay will do the job here -- this is indeed an answer, BTW, but would be improved with a bit of condensing :) Commented Sep 5, 2020 at 3:33
  • Thanks @troubleshooter - checkmark issued. I finally got everything put together and the relay is kicking on/off correctly. Sadly, I am not getting the 5-8 degree delta I want from the second stage and so I may after all this go back to a 2 thermostat system for maximum control. Time will tell. I am going to run this way for a while and see what I think. Commented Oct 17, 2020 at 7:42

I have almost the exact same setup as you (hydronic floor heat and air handler for backup heat) and mine works with one tstat just fine. So while you aren't asking for too much from "a" thermostat, you may be asking too much from "that" thermostat. I looked at the specs and didn't see anything about 2nd stage heat or aux heat, it might be there but I couldn't find it. On my tstat it's call "aux heat". I think 2nd stage heat is for a 2 stage furnace or heat pump. When the more economical first stage can't keep up or you've turned up the heat a lot, the 2nd stage kicks in on the same device.

I think Aux Heat means a different heating device. Ed Beal could probably contribute more here.

Anyway, I run a Honeywell RTH8500D. And it does the job you want. I didn't hook it up, so can't give you the specifics on that. But what you want can be done and this is the tstat that works for me.

Honeywell RTH8500D

  • Thanks for this answer; its nice to know someone out there has the same setup and has it working with one thermostat. I've now had time to look at the wiring schematics for this Honeywell RTH8500D and it doesn't look any different than my new RTH6360 really. I don't see any place to hook up a second R for heating (essentially Rh1 and Rh2) along with Rc. I think that is what is needed for control of two systems. My idea was to hack this thermostat's second stage somehow to get the needed effect. Who is this Ed Beal person? Commented Sep 3, 2020 at 5:58
  • Ed is a frequent contributor here and an expert in HVAC. Hopefully he'll chime in here. Thinking more about your problem...you might need a relay connected to the aux heat or stage 2 terminal and the contacts of the relay connected to the furnace, it would then look like a thermostat to the furnace. Also, your desired delta T of 5-8 degrees might not be attainable. My tstat kicks in aux heat if setpoint is off even a couple of degrees, (it doesn't do that during the "recovery" period unless it's recovering too slowly). I have not found a way to change that. Commented Sep 3, 2020 at 14:46
  • Thank you George. I am actively working this problem and will keep this question active / checked. I am also thinking about electrical workarounds, but a relay is a new idea. I am thinking that if a connection between Rh and W2 is seen, that I somehow change that to connect Rc (furnace) to W2. Not yet sure how to do that but I am gonna explore the idea with a breadboard. Commented Sep 4, 2020 at 17:32


Here is a 24v relay that can solve the problem

Hide it in the wall behind the thermostat

Use relay from c and w1 to r and w of the radiant floor heat

Use the w2 for furnace and have furnace control fan not the thermostat.

  • While I have no trouble reading this, someone less familiar with relays might not quite grasp as well which way around (coil-side vs. contact-side) the relay goes, perhaps you could add a bit more about that to your answer? Commented Aug 28, 2023 at 1:28

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