HVAC question: Honeywell Wi-Fi thermostat

[Percival P. Cassidy]

I want to install a Honeywell RTH8580WF Wi-Fi thermostat in place of an existing Honeywell thermostat, but there is no C wire connected to the existing thermostat (which may explain why we have to keep replacing batteries in it) and no "Spare" wire that could be connected to C at the HVAC unit and at the thermostat -- even if I could get at the furnace: these are rented premises.

I've read that it's possible to use an external 24V transformer, but Honyewell tech support just told me in a chat that that's possible only for 2-wire heat-only systems.; is that true? There are posts and YouTube videos that seem to suggest otherwise. This is a 4-wire gas and A/C system.

Advice?

Perce

[Naturous]

Percival P. Cassidy has brought this to us :

Interesting I run across this subject at this time. I had the igniter R&R'd on My heater yesterday. I too have a honeywell thermostat that I thought was giving up, but turned out to be the igniter. $175.00 later My heater works again. :')

[makolber]

You may be able to connect a new external 24V transformer to the new thermostat terminals R and C. Connect the existing R wire to the R terminal as well.

The thermostat will draw power for itself from the R and C connections to the new transformer.

I give no guarantees that this is right, find some other confirmation.

The phasing of the new transformer relative to the old one __may__ be very important. Connecting wrong may do damage.

This gets complicated, but should be possible.

m

[makolber]

Much simpler and less risky solution.....

Can you hook up a WiFi controlled table lamp, placed under the thermostat. When you want the heat turned down, turn the light on. etc.

m

[Percival P. Cassidy]

That is what I had in mind. I have the thermostat sitting on the bench at present powered from the 24V transformer connected to the R and C terminals but not to the HVAC unit. That seems to work fine. With the Wi-Fi hotspot two rooms away, the thermostat shows a decent signal strength and I can change settings from my smartphone.

I thought of that. Am I right in assuming that I would connect one wire from the transformer to either the C or R terminal, leaving the other wire from the transformer disconnected, then check for zero Volts between that unconnected wire and some other terminal (which one?) on the thermostat rather than 48 Volts?

Perce

[Percival P. Cassidy]

That might work in some setups but not ours. The purpose of the Wi-Fi thermostat is to override the default schedule for a building that is used only a few hours each week (and not always the same hours on the same days each week) without having to remember to reprogram the thermostat for the next event before leaving each time. So, typically, raise the temp from 50F (to keep the pipes from freezing) to 69F, or lower the temp to 72F.

Perce

[trader_4]

That test may work to get it phased right. You'd connect one end of the additional transformer to the R wire, then test for 0 or 48 volts between the other transformer lead and the W wire. R is one side of the furnace transformer, W is the call for heat wire. That's where it gets tricky. With an old funace that W wire would go through a relay coil to the other side of the furnace transformer. In that case, you should see 0 or 48 volts, telling you the phase. However with a modern furnace with an electronics control board, who knows? It could just be an input, in which case you may not see your expected 48 or zero. Even so, you might see some difference, from which you could infer the phasing. Or you might smoke the input to the control board in the furnace, I suppose.

I'd think powering it off the additional transformer would probably work, after you have the phase right, but then the manufacturer says no, so IDK.

There is also the issue of this being a rental, apparently you also don't have access to the furnace. So, what happens if it blows up the electronics board on the furnace or AC?

[Percival P. Cassidy]

I have now gained access to the HVAC unit, an ancient Singer, and can see the C terminal, but the 4-conductor wire bundle to the thermostat (there's a separate 2-wire to the outside compressor unit) goes up through the ceiling of the utility room and is either plastered around or caulked around, so there is no easy way to use it to pull a 5-wire bundle to replace it -- and who knows whether it's stapled to the joists in the roof space?

There is a G wire connected to both the furnace and the Tstat, which I understand could be repurposed as a C wire but at the "cost" of no longer being able to control the fan on its own. But they also say about checking with the local building code, so I guess that's what I need to do.

Perce

[trader_4]

I doubt local building code, which would be the electrical inspector, would give a rat's ass if you gave up the fan wire on a thermostat. Do you ever even use the fan by itself? I've played around with my fan switch a few times over decades, but never found a real use for it. Re-purposing that would be my first solution, assuming you're OK with any liability if the landlord claims you screwed up his furnace, etc.

Adding the additional transformer, in a rental property you don't own, if you tried to do that here in NJ, it's illegal because you'd have to wire it in and you can't do that on a property that you don't own or on a property that you rent out, unless you're a licensed electrician. Come to think of it, I'm not sure what the official position is on doing thermostat wiring, though I'm sure lots of people do it in properties they don't own.

[Percival P. Cassidy]

We've hardly ever used the fan on its own. I may be the only person that's ever done it; I doubt too many people know it's an option. We have a ceiling fan anyway, if we need to move the air around.

The 24V transformer I have is a plug-in one:

so no wiring-in required.

Perce

[Peter]

Given the price of the thermostat and the risk of damaging or destroying either the thermostat or the furnace control board (even more expensive), perhaps a few phone calls to reputable, experienced HVAC contractors in your area are worth the investment, even if it involves a house visit by one of them?

[makolber]

If your ability level is such that you understand what I meant by checking the phasing, and you get the phasing right, then it should work.

Note, if it is a wall wart type transformer, and you plug it in the other w ay round, the phasing will change... so make sure it is set the right way a nd can't be changed.

I had an idea, if you connect the new transformer in series though a car br ake light bulb, if there is any problem, the bulb will light and burn out p rotecting everything else. If the connections are correct, the current wil l be low and the bulb will not light or be very dim. I'd keep the bulb i n the circuit. I'm talking about connecting the bulb on the 24V side, not the primary. good luck... let us know how it goes

m

[Mark Lloyd]

[snip]

[snip]

I remember a 2-piece electronic thermostat that solved that problem. One piece was in the room with the HVAC unit, and the other (with the temp sensor and controls) on the wall. The 2 were connected with 4 wires (a power pair and a data pair) like USB. However, I don't know anything about where you'd get one.

[Mark Lloyd]

[snip]

On some systems, Y is connected only to the outdoor unit, and needs the G wire to run the fan.

Have you tried using cooling with G disconnected?

[snip]

[Mark Lloyd]

[snip]

Would the thermostat work with DC? Then there'd be no phase problem.

[Percival P. Cassidy]

Honeywell does have "official" instructions for repurposing the G wire, and they say to jumper G and Y at the HVAC unit.

No.

[Mark Lloyd]

That ought to work, as long as Y is available at the HVAC unit. At the unit I was talking about earlier, Y ran within a foot so connecting it to G should be doable.

BTW, I did add an electronic thermostat to that unit, using a wire from C.

[snip]

[Tekkie®]

[e-mail] posted for all of us...

I don't understand what phasing would do with an AC transformer?

[trader_4]

ermostat terminals R and C. Connect the existing R wire to the R termina l as well.

to the new transformer.

ery important. Connecting wrong may do damage.

It wouldn't if there was only ONE transformer. What PPC wants to do is power a new wifi thermostat that requires power, with another transformer that's in addition to the existing HVAC one. That's because to get power at the thermostat you need a common return wire to the HVAC transformer and his wiring does not have that wire or a spare one. In that case, phasing probably matters. At least I would not attempt it without knowing they are in phase.

An alternate approach, re-purposing the fan wire was suggested and that would be my solution.

[Percival P. Cassidy]

I re-purposed the G wire and jumpered G to Y at the HVAC unit, and all is fine. Of course, the new "smart" thermostat is still learning how far ahead of time it needs to turn on the heat for it to be at the desired temperature at the right time.

It makes life much easier. And I can grant access to the thermostat settings for other users; e.g., someone may have a previously unscheduled meeting and can set the temp. to be comfortable by the time the meeting starts.

Perce