On 02/21/2017 08:11 PM, trader_4 wrote:
On Tuesday, February 21, 2017 at 7:34:19 PM UTC-5, Percival P. Cassidy wrote:
On 02/21/2017 11:16 AM, I wrote:

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?

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

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.

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.

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.

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

https://www.amazon.com/Elk-TRG2440-2.../dp/B0007N5LJK

so no wiring-in required.

Perce