Y is shorted to W on both thermostat and the cable. Something tells me that I don't really want the aux heat (not that I have any) to kick in every time the compressor starts... I remember BamaCracker was telling that this is the way it is usually done with the heat pumps, but why?

The switchover valve - currently, O is not connected, and B has to be closed to put the unit into heating mode. Does it always work like that - either O or B is connected and has to be closed to put the unit into cooling/heating mode, or there are units that require O to be closed for cooling, or B to be closed for heating?

What is the acceptable voltage drop on the closed contacts? I'm talking about solid state switch drop, so no heat will be dissipated, and there's no resistance. About 2.5V OK?

__________________
--vt

Yep, Bama is correct.

Some manufacturers control the switchover valve - aka the reversing valve - so that when the valve is energized, the unit is in cooling mode. Others control this valve such that when the valve is energized, the unit is in heat mode.

If you have supplemental heat, it would be wired to the stats W2 connection or some similar nomenclature.

Jumpering W and Y is common for stats not specifically designed for heatpumps, but will only work for a unit such as yours. (Whenever there's a call for heat on the W wire, the jumper turns the compressor 'Y' on). This obviously would not work for a unit where the reversing valve would need to be de-energized for heat mode.

[Edited by gbfromsd on 04-23-2002 at 02:16 AM]

gbfromsd:

O/B wiring makes sense. Would I be right if I say that the tstat that controls both O and B will work with both types of units?

Y to W on fail to cool makes sense, except for one question - if I have aux heat connected, how does the tstat command it to switch on? Or they are jumped together only if there's no aux heat?
__________________
--vt

Your aux heat should be on the stat's W2 or some similar connection.

gbfromsd:

OK, getting closer. Please be patient with me I'm just trying to understand the logic behind wiring together W and Y.

The tstat doesn't have W2, does it mean it is unable to control the aux heat, or does it mean that if I unjump W and connect it to the aux heat, it will control it?
__________________
--vt

Well, if ya unjump W and Y - then you'll have no Y ergo no heat.
What kinda unit is this anyways?
Heat pump with what kinda aux or supplemental heat?

Ok, I'm getting it. For stats with no O or B, you can use the W for the reversing valve, but only for units where the reversing valve is energized in heating mode. Since the compressor must also run in heating mode in a heat pump - the jumper between Y and W is needed since the T'stat will only energize W on a call for heat.

gbfromsd:

OK, here's my logic.

Switchover valve controls whether the unit is in heating or cooling mode.

Fan switch (G) controls the fan regardless of the mode.

Compressor switch (Y) controls the compressor regardless of the mode.

(and your next message has just arrived)

OK, now I see the light: so the Y gets closed only in cooling mode, and W only in heating mode? That makes sense.

So the generic connector should look like:

O cooling
B heating
Y compressor cooling
W compressor heating
G fan

and the reason that Y is shorted to W is that the tstat closes one of them, depending on the mode, and the compressor should run in both modes. The jumper takes care of that.

Damn, that's convoluted... Thanks for the explanation.

PS: I should've mentioned where this is coming from: http://hvac-talk.com/vbb/showthread.php?threadid=9220

__________________
--vt

Bitchin!

[Edited by gbfromsd on 04-23-2002 at 02:52 AM]

gbfromsd:

Not to show off, but just to verify the deduction:

and the reason for a separate call to W is that so I can connect the tstat to something other than heat pump.

and if I have a multistage heatpump, W2 will be shorted to Y2.

Correct?
__________________
--vt

I suppose, never seen that, but if you paid that much for a dual compressor unit, why not get a stat made for it?

gbfromsd:

Right now, the equipment is 30 year old Goettl, living the last few months of its life and serving as a guinea pig.

Next one - probably Trane XL1600.

Currently, there's no supplemental heat, though I'm thinking about it - winter 2000/2001 was unusually cold for this location, and the house felt pretty bad.

The reason I'm so paranoid about these little details is I will not get it working right until I grok the abstraction, which I didn't yet. The thermostat connector is a last piece in the "diy zoning" puzzle - search this site. So yes, you have a point, I have made a stat for it
__________________
--vt

VT, if, as you say, you have no supplemental heat, then the installer was able to save a few dollars by installing a "single stage" tstat. As you dedueced, this stat has the ability to work a switchover valve (O&B) but no supplemental heat (W2, or X).

The real problem here is the lack of uniformity between manufacturers. For instance, some manufacturers use E for supplemental heat, some use X and some use W2 or W3. Trane actually uses (or used to use) several of these. As you also figured out, some manufacturers default to heat and switch to cool (O) and some manufacturers default to cool and switch to heat (B).

Now, for a fundemental understanding, the thermostat is just a switch. What it switches is determined by what you connect to it, and in what sequence or order. IFF you wanted supplemental heat from your existing stat, connect the wire that controls supplemental heat to the W terminal. But, you'd have to disconnect it each spring, because when Y energizes in cooling, it will close the supplemental heat switch also.

If you want, you can use the switches in your thermostat to switch the TV on and off. It's just dry contacts.

But, it sounds as if you have a pretty good handle on the situation at this time. I hope I haven't confused the issue.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

BamaCracker:

I guess I'm just looking for something that doesn't exist, namely, a generic connector I can implement once and connect to any device later. I remember Trane came forward with an initiative to standardize the connector (the name was something like "Trane Unified Thermostat Interface"), but don't know what happened to it afterwards.

You know what my pipe dream is (one of, that is)?

I wonder how difficult it is to provide the unit with a simple, generic and universal connector like RS-232, USB, Ethernet, 1-Wire - take your pick. Incompatibility problems solved once and forever, at least at this level.

But NO, they have to go their own ways.

Oh well. Mark my words, they'll get there sooner or later.

As for dry contacts, I got that part back then, but was busy ironing out the wrinkles out of the rest of the system and didn't get my hands on actually connecting it - and now started having second thoughts about using triacs to close the contacts. They're going to have up to 2.5V voltage drop across them when closed, I wonder if that's going to impact the unit - do those contacts just energize relays or something more complicated happens?

__________________
--vt

Pretty much they just energize relays. If you have a good strong transformer, putting out 25 or 26 volts, three of four triacs aren't going to kill you. But, if you start out with 22V and then drop 2 or 2.5, you may chatter some of those contacts, which has a tendency to kill them. Especially when you have numerous triacs and relays pullnig current from that transformer. You can solve this problem one of two ways, either multiple transformers, or one big ole, kick a$$ bad boy to do the job. Say a 150VA. That way, you're not likely to ever drop the voltage enough to hurt it.
__________________
There are no stupid questions. . .just a lot of inquisitive idiots.

OK, I hope I see the picture.

Triacs don't consume power, and the voltage drop across them is different from the voltage drop across the resistor - in the latter case, you will also have the current reduction, but with the triac it is just it - voltage drop without loss of current. This is an artifact caused by its structure - semiconductor is a nonlinear thing.

I guess I'll just give it a try and see if it goes up in smoke
__________________
--vt