Confusion about wiring for dual-fuel system with VPro IAQ
I recently had the following Trane system installed:
XV95 2-stage natural gas furnace (100KBtu)
XL16i 2-stage heat pump (4 ton)
In case it matters: Coil 4TXCD050BC3HCA
Vision Pro IAQ thermostat
My contractor had experience with the IAQ stat, but not with using the DHM terminals to slow the blower in dehumidification. When I asked him about it, however, he was perfectly willing to read through the install instructions and do it that way.
We spent a bit of time scratch our heads over the instructions, though. The installer's guide for the furnace has a section on adding a humidistat. It clearly calls for cutting the on-board R to BK jumper, which he did. The instructions also *seem* to say that you should jumper YLO to O. That didn't make any sense to him, and it doesn't make any sense to me, and he didn't do it. The wording is a bit ambiguous: "On single speed cooling only/ non-heat pump systems, jumper Y to O for proper operation... For two compressor or two speed systems, jumper YLO to O."
I'm wondering if the second sentence should read, "For two compressor NON HEAT PUMP systems ...". Can anyone advise on this?
There also seem to be contradictory instructions regarding the Y1/Y2 wiring for the heat pump. The installer's guide for the furnace shows the Y1 terminal of the tstat wired to the Y1 terminal of the furnace and the Y1 terminal of the heat pump. Similarly, Y2 on tstat goes to Y2 on furnace and Y2 on heat pump. This makes perfect sense to me and is the way he wired it up.
But the installer's guide for the heat pump says to wire Y1 on the tstat to Y2 on the furnace, rather than Y1. It shows Y2 on the tstat wired to BK on the furnace. Oddly, it shows the Y1 terminal of the furnace, with nothing hooked up to it. The instructions also say to cut the R to BK jumper, and add a R to O jumper.
Certainly this method would have the desired effect of slowing the blower to 80% in stage 1 as compared to stage 2. But what's the point of having Y1 and Y2 terminals on the furnace, then? The table of blower speeds in the manual shows different CFMs for stage 1 vs stage 2 in heating mode, but only shows one value for CFM in cooling mode. I assume "heating mode" is aux heat in my setup, and "cooling mode" really means either type of heat pump operation (Y1/Y2 energized). Am I right? Is it the case that Y1 and Y2 on the furnace produce the same fan speed???!!!?
If the wiring needs to be changed to correspond to what's shown in the heat pump manual, it seems I'd have to abandon the idea of using the DHM terminals of the IAQ entirely. Yes?
BTW: One obvious response to my post would be to ask me if I notice a airflow difference between stage1 and stage2 heat pump operation. I'm going to try to answer that question this weekend, BUT.... I'd be shocked if there isn't exactly one right answer to this question, so my observations are really irrelevant.
Any help is appreciated.
Here are a couple of good links. Its top secret information that Trane doesnt want anyone to know about. If they did, they would have included it in the installation manual. This topic has been discussed many times on this board, and you will get several different answers.
My brain hurts....
Thanks, that does shed some light on the issue. The document you reference certainly implies that Y1 and Y2 produce two different fan speeds. Does anybody know what the speed reduction of Y1 is? Trane documents pretty well that disconnecting BK produces a 20% reduction, but I can't find any info about Y1 compared to Y2.
Originally Posted by nashent1
I'll try to search the archives here as you suggest. My initial attempts didn't produce much -- perhaps I need to figure out the right search terms.
I find it mind-boggling that there isn't *one* official answer from Trane about how these two pieces of Trane equipment should be wired up. Has anyone tried to extract such an official answer from them? Any success?
As a homeowner, I had this same problem with my install. There are 3 different diagrams if you count the furnace, iaq and hp diagrams.
I learned from Mark B. (I think) that the HP install diagram on page 5 of the installer's guide is the one to follow.
Mark or someone else will need to comment on the reduction speeds.
I have basically the same system provided by the same contractor earlier this year. I have a question about your success/issues with the IAQ install and setup. Can you please contact me via the e-mail in my profile?
A clear answer emerges...
Well, you solved my forum searching problem... I just looked through all *your* posts and quickly found multiple threads addressing this issue. It seems pretty clear, from the multiple times this issue has come up, that my system is wired wrong. My attempt to distinguish airflow and fan sounds in stage 1 vs stage 2 confirms this -- I hear and feel no change at all, even listening right next to the furnace.
Originally Posted by markf57
I guess I should call my contractor out and have him change it. It'd be easy to do myself, but since he's warrantying his work I won't mess with it.
My feeling is that I won't be able to use the DHM terminals of the IAQ and will have to rely on Comfort-R (which isn't a disaster, of course...) Can any pro suggest a wiring scheme that would still support using the DHM feature of the IAQ?
The systematic approach...
I've been attempting to infer a set of fan-speed rules based on these documents, other posts on this site, and a bit of logic and common sense (which may be a mistake....)
Originally Posted by nashent1
The basic observation is that the labels on the furnace air-handler (other than W1/W2) really don't imply any particular function... they are all just ways of adjusting blower speed. They have names chosen by the manufacturer to conveniently correspond to the tstat and OD unit signals you *might* be hooking up to them.... but then again, you might wind up having to do it in a way that completely ignores the traditional meaning of the labels. It might be better if the manufacturer just gave up on labeling them Ylo/Y/O/Bk and chose labels that represent what's really going on.
These rules are written from the perspective of a Trane two-stage variable-speed gas furnace coupled to a Trane 2-stage heat pump, using a tstat that can control every stage. That doesn't mean none of this applies to other cases, but that's what I was thinking about when I put it together.
1) W1 and W2 produce fan speeds appropriate to the furnace's two stages. You can't wire this wrong, 'cause the furnace really truly knows the right speeds.
2) If O is off ("heating mode"), Y and Ylo produce identical fan speeds (100%.
3) If O is on ("cooling mode"), Ylo produces 50% fan.
4) If G is on but Ylo/Y/W1/W2 are not, produces 50% fan.
5) If O is on but Bk is off, whatever speed Ylo/Y/W1/W2 would normally produce is reduced by 20%. This does not apply to fan-only (G alone). If O is off or Bk is on, the standard speeds described above are used.
[EDIT: Actually, I have no clue if #5 applies to W1/W2. It wouldn't make sense for it to.]
Take this with extreme caution. In particular, I'm very uncertain about #3, though I'll present my reasoning below.
I inferred #2 from the current observed behavior of my system, which is currently operating in heat-pump mode but doesn't exhibit fan speed changes between Ylo and Y.
I infer #3 from forum posts that indicate that dual-compressor systems such as the XL19 really run at 50% capacity in stage 1, and really should use Ylo/Y. Conversely, my XL16 runs (I gather) at roughly 80% in stage 1, which is why the recommended wiring uses the Bk terminal to adjust by 20%.
Comments appreciated, particularly if I got something wrong. Feel free to make fun of my excessive geekiness. Be glad I didn't go with my original version, which was a truth-table for all 32 combinations.
You have come close with your analogy of how each terminal works. I have posted several times that the IAQ was not designed to be wired to the 16i as most people anticipate.
The IAQ is a good Tstat with benifits that allows the relay board to control options added to the equipment and control sequences of the equipment.
The 16i is unique in its wiring but if the installer follows page 5 and uses the Comfort R set up in the wiring diagram you will not have any problems.
The furnace is designed to be used with a single stage compressor, a two compressor system (19i) or a 2 stage single compressor (16i).
The IAQ stat has a remote relay board that has a N/O or N/C relay that is controled by the humidity thermistor built into the Tstat. On a single stage compressor and a variable speed furnace or air handler B/K jumper is removed and the N/C relay is wired in series. When the humidity rises the relay opens and removes 24 volts from the B/K terminal. When this happens the V/S module will reduce the fan speed by 20% of the fan speed selected by the dip-switches.
On the 16i first stage compressor is wired through the Y terminal and the second stage is wired through the B/K terminal. While in first stage B/K does not see a 24 volt signal so the fan runs at 80%. When the unit switches to 2nd stage of the compressor the Tstat sends 24 volts to the B/K terminal and now the fan will run at 100%.
The misconception with the IAQ is that it will control the fan speed in first and second stage to control humidity and it will not on a 16i.
You can wire it and select Comfort R and use the ramp profile for dehumidification, and both heating and cooling while in the heat pump mode. When the fossil fuel is on, the control board will run either W1 or W2 fan speed.
The "O" terminal has to see 24 volts to tell the V/S module to follow the ramp profile of the Comfort R. or a dehumidify comand given through a humidistat through B/K.
While you can't wire an XL16i system to lower the blower speed for dehumidification, the IAQ thermostat can still do some dehumidification by overcooling by up to 3º when more humidity control is needed.
Thanks to both of you for confirming my conclusions. I was pretty sure I had it figured out, but I'm very glad to hear from people who have experience with these exact models. I will ask my contractor to modify my wiring per page 5 of the HP installer's guide.
I just realized last night that my current wiring, while merely annoying in heat mode (higher fan speed than necessary), could actually be damaging in cooling mode. In cooling mode, the O terminal will be energized, so in stage 1 I'll get 50% fan instead of the required 80% (since my contractor wired tstat Y1/Y2/O to furnace Y1/Y2/O). I don't know if running the fan that slow would actually freeze up the coil, but it certainly doesn't sound healthy.
Just make sure that he wires it according to the diagram on page 5 and follows all of the directions for Comfort R.
Originally Posted by dbb1
After he has wired it correctly he needs to check operation in both stages and adjust the charge in second stage and then compare the pressures and temperatures to your service facts.
Verify all of your other install data: refrigerant line size 3/8th and 7/8th maximum virtical lift 25 feet and maximum linear length of 85 feet. Outdoor sensor location and seperate wiring from the control wires.
When the unit comes on in both the heating and cooling mode you will get one Comfort R ramp profile per heat or cool call. During the furnace W1 and W2 call the ramp profile will be cancelled.
Any pic's ?
Ok, I have a really crazy proposal. I have no intention of doing this, it is a thought experiment for the pros. I'll start with a question:
Originally Posted by mark beiser
The XL16 stage1 requires 80% airflow. If it were possible, presumably the low-speed dehumidify setting would be an additional 20% off of that, or 64% total. Would it be acceptable to run at 50% total? Is it too low?
If 50% is OK, here's a wiring diagram that accomplishes the goal:
DH2----------Y2 and O
Y2-----------Y1 and Bk
Please forgive the horrible ASCII art. I know this wiring looks truly bizarre, but here's the chart of resulting fan speeds:
IAQ wants............ G Y1 Y2 O Bk Fan
Stage 1, no dehumid... 1 0 1 1 0 80%
Stage 1, dehumid...... 1 0 0 0 0 50%
Stage 2, no dehumid... 1 1 1 1 1 100%
Stage 2, dehumid...... 1 1 0 0 1 100%
This will only dehumidify in stage 1, which is when you mostly need it anyway. You could use it with or without Comfort-R, I suspect. Comfort-R would not activate in either stage when there's a call for dehumidification (because O turns off), which is not a problem in stage 2 and very important in stage 1.