Trane dual stage heat but single stage AC control 2 stage AC now? control question

PeteA · 5:04AM

I have a question that's right up your alley as far as controls goes but maybe the overall AC piece if there is an issue you're not sure of someone else can chime in and say why not to do it.

My sons house has an older but in great shape furnace Trane XL90 TUX100R series but his much older and not in as great shape R22 Carrier condenser unit sprung a leak and rather than repair it we just opted to get a new evap coil and condenser and lineset to go to R410a system and not worry about cleaning out the old R22 stuff for reuse. The new condenser is a York modulating AC only unit HMCG2 series. I never considered I would have any issues until today when I was wiring everything up. I saw that the condenser has 3 wires C,Y,and an S1 wire terminal that goes to the y/y2 terminal on the furnace. Problem is the Trane only has a SINGLE (UGH) stage cooling control board, not a dual stage like the heat side.

The question I have for you on the control side is if I move the G wire from the terminal on the nest thermostat over to the y1 terminal (assuming G on the furnace is typically a lower speed on the fan) and y1 over to the y2 (assuming this would be the typical single stage AC fan speed). I would then look to connect the original Y at the furnace now on the G and the condenser y onto the G then to simulate a second stage I would connect the s1 from the condenser to the Y terminal on the furnace with the original Y (now Y2) from the Nest.

I know I should have looked at this before getting everything assembled (just waiting on lines to be brazed, vacuum and pressure tests now) but when I looked into his furnace and saw 2 stage and how immaculate it was I didn't even realize it was just 2 stage for the heat and 1 stage for the AC. I figured maybe something like I described above may give him some of the benefit of the higher efficiency condenser until the furnace does crap out then we will make sure to get a much better unit to make everything work together as drawn on the diagram. Otherwise I'll see if there is a way in the settings or DIP switches to keep the condenser running in a single stage mode.

Thanks

Pete

109A_5 · 9:09AM

In the manual S1 and SI seem to be the same, however I not sure if that is so. Two S1 modes ? Dip switch SW5-3. Looks like the S1 may be an output from the condenser unit that controls the blower speed by monitoring system conditions or just time. In which case the Y2 from the NEST may confuse things.

ratio · 11:03AM

If the air handler is only one stage cooling, that means it only has one fan speed for cooling.

To understand two stage cooling, you've got to realize that it's not y1=cooling, y2=more cooling—you get a better understanding if you think of it as y2=cooling, y1=less cooling. Many two stage furnaces will have the cooling terminals labeled Y1 & Y/Y2, Y1 for low airflow during stage 1 operation, Y/Y2 for non-staged or high stage cooling.

Simplest solution is to wire the condenser as single stage, but there might be a cleverer solution. If the furnace has a separate G call fan-only speed that's independent of the heating and cooling speeds, you may be able to adjust that speed to the correct airflow for low cool, then it's just a matter of adding enough controls (probably a single relay) to pass only a G call to the furnace in low cool mode.

PeteA · 1:28PM

https://forum.heatinghelp.com/discussion/comment/1861575#Comment_1861575

Thanks @ratio

Yes it does have a G on the furnace control board which is what I was thinking in my overly wordy original post may give me that "less cool" and "cooling" modes. Your cleverer solution above is exactly what I'm trying to figure out if it may work. I assumed that the condenser is just looking for a signal to call for the "low cool" and that it gets it from the thermostat, so that's why I was thinking that even though I only have the single stage cooling mode on the furnace (Y) if the fan speed is slightly lower on the fan only (G) does that mimic the way that a 2 stage furnace that has a dedicated (Y and Y/Y2) terminal works. The condenser does not seem like it is communicating digitally to a furnace for the feedback to operate based on the bottom wiring diagram @109A_5 posted above so I believe its just the 24v signal that tells it, fan inside is on low or fan inside is on High.

ratio · 3:53PM

If the fan-only side can be adjusted to give the proper airflow, that side of the equation shouldn't be an issue. I'm not in a position to look up the schematic of that outdoor unit right now, but judging by what @109A_5 posted above, the S1 terminal is an output that forces the indoor unit into the low cool fan speed. I don't know if that means there are other wires going to the outdoor unit, or if the outdoor unit decides to go into high cooling on its own & thus forces the indoor unit into high speed fan (or, more accurately, stops forcing the indoor unit into low speed.)

PeteA · 4:03PM

Thanks @ratio I see what you mean as far as the wire only comes off of the outside unit and goes directly to the y/y2 position in the wiring diagram with no other wires on that position. So I see what you mean that it is possible that it's just an output from the condenser to energize the 24v on the furnace board and activate that next stage.

What you see in the bottom diagram from @109A_5 is accurate, the unit outside only gets 3 wires from outside to the inside furnace control board.

109A_5 · 6:14PM

I fat fingered something when I was composing my original post. At that time I could not edit it or add another post. And it only seems to be with this post, very odd… If I can I will add this to my post later so it makes sense to others. I PMed the @PeteA with much of this below.

Not sure what that mode Dip switch SW5-3 does , maybe switches between ECM motor control (newer way) OR utilizing a relay on the furnace's control board switching the motor speed (old way).

Anyway maybe this wiring is what you were saying in your post (image below), I got a bit confused it was late. Depending on what the terminal S1 actually does (logically) the G and the Y wires (diagram below) may need to be switched at the furnace.

Not sure with the OEM drawing above S1 to Y/Y2 who is the signal source S1 or Y/Y2, which is the input and which is the output ? If it is the furnace is the output, it is probably timed by the control board, if the signal source is the modulating condenser unit it may work from the line temperatures to determine the system load and modulate its self and then the evaporator blower fan accordingly. Which to me is what the ACC STD ECM drawing above implies. However in this case you would be working with the ACC VS ECM diagram which I modified below. There may be other ways.

Trane dual stage heat but single stage AC control 2 stage AC now? control question

Comments

Categories