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

Percival P. Cassidy has brought this to us :
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

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. :')

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

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

On Tuesday, February 21, 2017 at 5:07:04 PM UTC-5, wrote:

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

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

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

On 02/21/2017 05:06 PM, wrote:

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?

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.

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 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.

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?

This gets complicated, but should be possible.

Perce

On 02/21/2017 05:09 PM, wrote:
On Tuesday, February 21, 2017 at 5:07:04 PM UTC-5, wrote:

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

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

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.

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

On Tuesday, February 21, 2017 at 7:14:45 PM UTC-5, Percival P. Cassidy wrote:
On 02/21/2017 05:06 PM, wrote:

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?

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.

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 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.

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?

This gets complicated, but should be possible.

Perce

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?

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

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.

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.

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

On Tue, 21 Feb 2017 17:11:41 -0800 (PST)
trader_4 wrote:

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.

So no one else can either...do keep up there in the rear.

On 2/21/2017 7:18 PM, Percival P. Cassidy wrote:
On 02/21/2017 05:06 PM, wrote:

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?

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.

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 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.

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?

This gets complicated, but should be possible.

Perce


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?

On Wednesday, February 22, 2017 at 8:39:49 AM UTC-5, Peter wrote:
On 2/21/2017 7:18 PM, Percival P. Cassidy wrote:
On 02/21/2017 05:06 PM, wrote:

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?

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.

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 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.

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?

This gets complicated, but should be possible.

Perce



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 way round, the phasing will change... so make sure it is set the right way and can't be changed.

I had an idea, if you connect the new transformer in series though a car brake light bulb, if there is any problem, the bulb will light and burn out protecting everything else. If the connections are correct, the current will be low and the bulb will not light or be very dim. I'd keep the bulb in 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

[snip]

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?

[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
http://notstupid.us/

"The state (the U.S. Constitution) has not the right to leave every man
free to profess and embrace whatever religion he may desire." [Pope Pius
IX]

On 02/21/2017 07:11 PM, trader_4 wrote:

[snip]

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?

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

I've played around with my
fan switch a few times over decades, but never found a real use for it.

Have you tried using cooling with G disconnected?

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.

[snip]

--
Mark Lloyd
http://notstupid.us/

"The state (the U.S. Constitution) has not the right to leave every man
free to profess and embrace whatever religion he may desire." [Pope Pius
IX]

On 02/22/2017 07:15 AM, burfordTjustice wrote:
On Tue, 21 Feb 2017 17:11:41 -0800 (PST)
trader_4 wrote:

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.

So no one else can either...do keep up there in the rear.


I used to know someone who ran the fan all the time.

Also, as in my other reply, not using the fan switch isn't always the
same as having the wire disconnected.

--
Mark Lloyd
http://notstupid.us/

"The state (the U.S. Constitution) has not the right to leave every man
free to profess and embrace whatever religion he may desire." [Pope Pius
IX]

On 02/22/2017 10:54 AM, wrote:

[snip]

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 way round, the phasing will change... so make sure it is set the right way and can't be changed.

I had an idea, if you connect the new transformer in series though a car brake light bulb, if there is any problem, the bulb will light and burn out protecting everything else. If the connections are correct, the current will be low and the bulb will not light or be very dim. I'd keep the bulb in 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


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

--
Mark Lloyd
http://notstupid.us/

"The state (the U.S. Constitution) has not the right to leave every man
free to profess and embrace whatever religion he may desire." [Pope Pius
IX]

On 02/22/2017 07:55 PM, Mark Lloyd wrote:

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?

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

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

I've played around with my
fan switch a few times over decades, but never found a real use for it.

Have you tried using cooling with G disconnected?

No.

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.

[snip]

On 02/23/2017 08:59 AM, Percival P. Cassidy wrote:
On 02/22/2017 07:55 PM, Mark Lloyd wrote:

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?

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

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

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]

--
Mark Lloyd
http://notstupid.us/

"The certainty with which a religious belief is held is usually in
direct proportion to its absurdity." [Rev. Donald Morgan]

posted for all of us...




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

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

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

--
Tekkie

On Saturday, February 25, 2017 at 3:31:57 PM UTC-5, Tekkie® wrote:
posted for all of us...




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

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

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

--
Tekkie

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.

On 02/23/2017 09:59 AM, I wrote:

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?

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

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

I've played around with my
fan switch a few times over decades, but never found a real use for it.

Have you tried using cooling with G disconnected?

No.

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.

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