2 stage A/C thermostat question

[djjsr]

Question for you hvac guys.

I have a 2 stage A/C system that needs a new thermostat. The old thermostat has 4 connections being used as follows:

Terminal Rc - red wire
Terminal G - green wire
Terminal Y1 - blue wire
Terminal Y2 - white wire

My problem is that I can't find a thermostat (locally) that has a Y2 connection for the secondary cooling. So, to keep the A/C running, I used a simple digital thermostat and just connected the blue wire to the Y terminal.

Without the white wire connected, obviously the second compressor doesn't run and only one compressor isn't doing the job.

Can I connect both the white and blue wires to the Y terminal on the new thermostat to get both units running until I find a proper thermostat?

Thanks in advance if you can advise.

 

[LS6 Tommy]

It will work, but it won't be as efficient. I think you'll find it will cool quickly, but doesn't run long enough to remove much humidity. You need a commercial t-stat. Try a local HVAC supply house. I've never bought a stat at a big box store, but I wouldn't be surprised if you can't get a two stage stat there.

Tommy

 

[Troutsqueezer]

There's a number of thermostats at Home Depot and Lowes that will run two stage cooling and three stage heating if you need it. I know, I've bought and used them.

 

[Rockhead261]

Can I connect both the white and blue wires to the Y terminal on the new thermostat to get both units running until I find a proper thermostat?

Yes.

 

[djjsr]

Thanks for the info guys.

I've done a little web searching and found a couple of Honeywell t-stats that will work but I'm looking for cooling only, 2 stage, non-programmable, digital and can't find one.

 

[LS6 Tommy]

Most programmable stats that support cooling will have heat. I don't think you'd want to spend the money to get a stat as specialized as a two stage cooling only digital.

Tommy

 

[brewchief]

Thanks for the info guys.

I've done a little web searching and found a couple of Honeywell t-stats that will work but I'm looking for cooling only, 2 stage, non-programmable, digital and can't find one.

A White Rodgers(Emerson)1F95-1277 should work, it can be configured as a cool only and non programmable T-stat and single or multistage.

 

[djjsr]

You need a commercial t-stat. Try a local HVAC supply house.

I'm quickly finding out how true this is. I went to Menards and Home Depot to see if they had anything at a reasonable price. When I asked which t-stats could handle 2 stage cooling, they looked at me like I was from Mars.

Unfortunately, everything I found was too complicated (and expensive) for what I want. I want simple. Cooling only, digital, non-programmable.

I connected the second wire to the Y terminal and I got the second unit going ok. I'll hit the HVAC supply later this week. I'm in a small town and the closest supply house is about an hour away.

Thanks again for the info.

 

[yeldogt]

What are you talking about "second unit"

If the unit is a Trane and has two compressors in it -- only one runs at a time?

If it is a scroll or recrip compressor then it runs in first stage or second ....not both? One compressor runs in reverse (Bristol/ carrier) the scroll has a second port.

The two stage units require a more complicated thermostat so they operate correctly -- they start out on low and then only go to high when required -- they will also manage humidity running on low with a low fan speed. To work correctly they are matched to a VS blower .. do you have one?

 

[djjsr]

What are you talking about "second unit"

I'm not real familiar with HVAC terminology but there's 2 of these gizmos behind the building. The one on the right runs anytime the thermostat turns on the A/C. The one on the left kicks in when the other one isn't enough to drop the temperature by itself.

They're both plumbed to the coil contraption that gets cold. It's about 6 feet long and 2 feet tall. One of those things cools the top half and the other cools the bottom half.

 

[LS6 Tommy]

Any standard two stage cooling T-stat will run that system.

Tommy

 

[yeldogt]

sorry that is new to me .. I have never seen two condensing units connected to one evaporator

 

[bazar01]

They're both plumbed to the coil contraption that gets cold. It's about 6 feet long and 2 feet tall. One of those things cools the top half and the other cools the bottom half.

Interesting set up.
I would guess you have two ductless split units?
Do you have a pic of the indoor blower unit?

 

[pseudorealityx]

Interesting set up.
I would guess you have two ductless split units?
Do you have a pic of the indoor blower unit?

No, it's a dual circuit standard DX split system. Total tonnage is likely ~10 tons, with (2) 5 ton condensers.

 

[bazar01]

No, it's a dual circuit standard DX split system. Total tonnage is likely ~10 tons, with (2) 5 ton condensers.

Got it.

I wonder how the evaporator coils are arranged and who makes this indoor unit.

 

[LS6 Tommy]

Lots of companies make two stage DX systems. Usually the first stage feeds the bottom half of the coil. Been around longer than I've been on the planet.

Tommy

 

[brewchief]

Lots of companies make two stage DX systems. Usually the first stage feeds the bottom half of the coil. Been around longer than I've been on the planet.

Tommy

The last 10 ton split coil I installed had two circuits and came with wyes to use with a single 10 ton condenser or leave them out to keep the circuits separate using 2 5 ton condensers. This was at least 10-12 years ago and it replaced an almost identical unit. The furnace under it was basically two furnaces in one cabinet, two separate vents, gas valves etc with one large blower. In some cases two separate furnaces are used and a twinning kit is used to make sure both blowers run at the proper times. The last one I did would alternate which condenser would come on first to keep run time equal.

 

[bazar01]

Lots of companies make two stage DX systems. Usually the first stage feeds the bottom half of the coil. Been around longer than I've been on the planet.

Tommy

I am surprised the coils are set up on top of each other. Looks like not a good set up as there will be 50% bypass air. I would prefer a stacked arrangement (one after another)

 

[pseudorealityx]

I am surprised the coils are set up on top of each other. Looks like not a good set up as there will be 50% bypass air. I would prefer a stacked arrangement (one after another)

Face and bypass is actually a pretty good way to cut down on loads, and remove humidity during partial loading. We use it a fair amount, especially for situations where you need significant air changes, but don't want to waste energy with reheat.

Having a stacked arrangement isn't much benefit. Due to the overall size, you would have to space the coils pretty far apart that you would still have considerable bypass.

 

[LS6 Tommy]

Face and bypass is actually a pretty good way to cut down on loads, and remove humidity during partial loading. We use it a fair amount, especially for situations where you need significant air changes, but don't want to waste energy with reheat.

Having a stacked arrangement isn't much benefit. Due to the overall size, you would have to space the coils pretty far apart that you would still have considerable bypass.

^^THIS^^

If you REALLY want to work on a confusing system that looks like it couldn't possibly work efficiently, try an old hot deck/cold deck unit...

Tommy

 

[pseudorealityx]

^^THIS^^

If you REALLY want to work on a confusing system that looks like it couldn't possibly work efficiently, try an old hot deck/cold deck unit...

Tommy

I've only had to replace them, fortunately.