Tstat Question

[dgbrown]

Install is gas (hot water) heat with separate central A/C. 2 wire from the valve control box from the heating system and 4 wire from the a/c units to the stats. Can I use the common from the 24v A/C system to power the tstat ?

Big thanks in advance !

[dgbrown]

Bumping this up .... Does it matter which common is used ? A/C or Furnace ?

[diamonddesign]

I let David one of my guys do most thermostats (I've done my share) but my undestanding is that you don't have a common if you only have 4 total? If you are thinking Red that really isn't the common and it is the one you use the jumper on and power steal with.

However, we've had great luck with only 4 wires with power stealing and the jumper.

Are you saying you have 6 wires, 4 from AC and 2 from heat?

[diamonddesign]

Also, photos of the wires would help. Note the symbols they are hooked up to on the current thermostat as well as they may be labeled or have letters like W1 W2, etc.

[dgbrown]

I have red and white from the furnace, and red, white, yellow, and blue from the A/C.

furnace - R -> Rh, W -> W1

A/C - R -> Rc, W -> W1, Y -> Y1, Blue is unused

I was thinking of taking the blue and using it for a common off the A/C 24v transformer.

[dgbrown]

Just an fyi. I was setup to powersteal but the furnace was continually running (new furnace and valve control) - likely too sensitive for PS.

[diamonddesign]

Did you have the resistor installed that came with the C4 thermostat? Thats what its for (to help with sensitivity on power stealling). If you don't have one I have a bunch I can send you one. I bought a 10 pack on ebay for like $3 shipped.

[dgbrown]

Resistor was installed - no love.

Here's what I tried first -

1) Disabled power stealing on the tstat

2) Picked up the common on the spare blue wire from the AC unit, and wired it into TS/C

RESULT: No power

Here's what I tried second -

3) Swapped the red wire coming from the AC unit from RC to RH

RESULT: Tstat powers up without power stealing. Heat will obviously not come on.

It seems the C4 Tstat will only work in a non-power stealing mode using the RH, TS/C (Common) from the heating system. Correct ?

Is there a way to get this to work based on the facts at hand ?

BTW - Thanks in advance for any help on this Sunday afternoon !

[dgbrown]

Bumping this up .... any help is appreciated !

[seth_j]

Are there two different transformers in this system?

[dgbrown]

Yes. One 24v in the valve control box in the basement for the heating system and one 24v in the attic for the AC system.

So the one thought/question : can I wire rh (from the heat 24v) and rc (from the AV 24v) respectively and use a jumper between the two ?

Thanks in advance Seth.

[seth_j]

I think you can but do not jumper them... both would be separate systems..

furnace - R -> Rh, W -> W1, You'll need a Common here to TS/C terminal

A/C - R -> Rc, Y -> Y1, no G wire? for Fan?? Probably need to have that. W is not needed here.

[dgbrown]

I have no spare wire from the furnace to carry a common. I do have a common coming from the AC transformer - but the stat will not power up with the RC (from the AC) and Common (from the AC). If I move the red wire (from the AC) to RH and leave the common (from the AC) - the stat powers up. Will the stat not get power from RC/Common combination?

Is there a danger/risk in jumpering as I previously postulated ? RH (to furnace) and RC (to AC) with a jumper between them ? The transformers is just looking for a completed circuit between the respective power source (the red from the mechanical) and the relay wire (either W1, G, or Y1, or common) - no ?

BTW, I do have G (forgot to document that above).

[dsharbani]

power separately from a transformer, you can not use ac 24v since it uses comon for heat only.

[seth_j]

Is there no way to get another (more) wire down to the furnace?

[seth_j]

The only way I can think to make this work is if you disconnect the transformer from downstairs and put everything powered on one transformer. There should be enough wiring to do this.... you might want to bring in a professional (HVAC tech) though.

[dgbrown]

There's always a way to get additional wiring Just trying to avoid the headache.

So based on what I'm hearing, powering the stat specifically requires a common from the furnace (heat) RH/Common - RC/common is not an option.

I just dropped a jumper between RH (to furnace) and RC (to AC) at the stat. And left the common from the AC in place. All seems to be working.

Is there a danger/risk in running this way ?

[seth_j]

I doubt there is danger. I don't play in AC land too often and its been a long time since I have. Worse thing that could happen is you blow one of the transformers...

This is all still low voltage.

[dgbrown]

Thanks for staying with me on this Seth - Much appreciated./Dave

[dgbrown]

Interesting enough, after some more digging - post #15 here: http://forum.micasaverde.com/index.php/topic,786.15.html seems to indicate wayne-dalton recommended powering their stat with two hot leads (and a common) one hot from the furnace to RH and a second hot lead to RH from an external24v transformer (common coming from the the external 24v transformer as well.). My setup from post #17 above effectively achieves the same.

Again - just to wrap this up for anyone cruising tstat threads. This configuration involves two 24v volt transformers in geographically different locations in the home - one in the basement for Heat (2 wires up to the stat) and one in the attic for AC (5 wires down to the same stat). Common from AC in attic to Tstat TS/C, RH from furnace (valve controller) in the basement to Tstat, RC from AC in attic to Tstat, jumper from RH to RC at TStat.

Follow the standard directions for W, Y and G.

This configuration apparently works.

If anyone knows of a downside/risk specific to the C4 stat I would be interested in hearing about it.

Updated: To complete doc

[thegreatheed]

The control4 tstat powers from RH and TS/C.

RH and RC are different terminals, to allow you to control different hvac units, but assuming you can power from the heating unit. Since we have to power from the AC, we need to isolate the different transformers with a relay.

Here's how to wire it.

rh/rc jumpered > r from ac

y1 > y from ac

g > (might need fan control on the ac, might not, but this terminal is fan control)

ts/c > blue from ac to it's common

get a relay, triggered by 24v ac, that will switch 24v ac.

w1 and ts/c > trigger side of the relay

2 wires from heat > switch side of the relay

[dgbrown]

thegreatheed - First a big thanks. Excellent detail. Two points:

1) Can we consider the detailed wiring in your post #21 as option two for this scenario or is this the ONLY approved/supported way to wire this up ?

2) Reference the wiring detail in my post #20, any downside or risks associated with this configuration ?

Just looking to nail down pro's and cons, etc. While improving, specifics surrounding tstat installation are still kind of sparse.

Dave

[thegreatheed]

with only 2 wires from the heating, it's transformer is basically useless to the control4 tstat. we can't draw voltage across 2 wires, while also simultaneously using them as a relay for control.

also, again, we don't want to cross voltage between the transformers, so, ya, as i explained in 21 is really your best solution. (not the only, there's rarely an 'only' solution in the integration field).

[dgbrown]

^ So I've done a considerable amount of research and haven;t arrived at an answer for myself. So if you can tolerate me being perhaps a little thick - Can you explain why : "we don't want to cross voltage between the transformers" ?

[thegreatheed]

because there is potential to harm the transformers. unfortunately, i'm no electrician or electrical engineer so i may not be able to explain the 'why' very well.