thermostat installation

Thermostat Installation and You!

  • October 21, 2015
  • By MR COOL

You’re a do-it-yourself kind of person. You’ve done your fair share of home improvements and appliance installation. You know your way around a crescent wrench. You don’t need any help when it comes to thermostat installation.

You’ve got this. You’ll be fine. Everything will be fine. No reason to panic.


Improper Thermostat Installation & How It Can Ruin Everything

Everything could go wrong, but probably not in the way you’re thinking. While it’s possible to mess up the wiring of the thermostat, that’s not the most common installation problem. The most common way people make mistakes during their thermostat installation is all about location, location, location.

Not Too Hot, Not Too Cold

Here are some thing you should keep in mind when you’re installing any programmable thermostat:

  • Do NOT install a thermostat on an exterior wall. If that wall is poorly insulated, your thermostat is going to think it’s either too hot or too cold. You’ll waste energy in summer and winter. Which is bad.
  • Do NOT install a thermostat where there is a steady draft. You don’t want your thermostat directly above or below a vent, since that is going to affect how it perceives interior temperature. Again, this is a recipe for wasting money.
  • Do NOT install a thermostat in direct sunlight. Ideally, you don’t want your thermostat exposed to sunlight at all. This isn’t always possible, so minimize exposure as much as possible.
  • Do NOT install a thermostat behind an obstruction like a door or bookshelf. Even though that spot might meet other thermostat installation guidelines, you’ll come to hate the inconvenience when you want to change the temperature.
  • If you’re installing a smart thermostat, make sure it is in range of your home Wi-Fi network. There’s not much point in paying for smart tech if you can’t use it.

You Can Do It

Yes, it’s possible to incorrectly install your thermostat. However, if you follow the aforementioned guidelines, you should be just fine.

75 thoughts on “Thermostat Installation and You!”

  1. Please help me understand how to program the thermostat on my 18 seer split ac. I can be reached at 954 821 7333.

    Thank you

    1. What are the wires labeled 19rd,20rd,21rd,24rd,22rd and what do they connect to and why no instructions 🤔 when buying this heat strip ? ? ?

    2. Can I put a wired thermostat on my mini split? The mini stat doesn’t read through a Mylar tent to my ac unit and need a wired thermostat.

  2. I’m thinking about purchasing the Advantage 18,000 BTU 1.5 Ton Ductless Mini Split Air Conditioner and Heat Pump (Model # A-18-HP-230A) from Home Depot. Does this model have the USB port to install Wifi up grade?

  3. Would I be able to connect my DIY Enhanced 18,000 BTU 1.5 Ton Ductless Mini-Split Air Conditioner and Heat Pump – 208-230V/60Hz to a third party thermostat?

    1. The Smart Controller may allow for integration with a NEST Thermostat later this year, but at the present time the DIY is not compatible with any third party thermostats.

      1. Hello, was this ever enabled? I’d really like to control my DIY unit with a true thermostat. The built in t-stat is grossly inaccurate compared to 3 thermometers (2 digital,1 non-digital) in the same room. I have tried the manual ‘calibration’ in the app, and it still has issues with keeping the correct temp in the room. thanks!

  4. Is the thermostat in the mr cool advantage units located in the indoor unit head or in the remote control? Also do the advantage units support the hookup of 3rd party thermostats?

  5. I have this unit… 18k BTU 16 SEER MrCool DIY Ductless Heat Pump Split System with WiFi Smart Kit – Wall Mounted. I have the temp set at 63 through the remote with the “follow me” feature. The remote thermostat is never up to 63 – typically between 59-61. Outside temps are around 20-40 degrees. The building is well insulated. The app shows a different temp than the remote. I’m at a loss.

  6. Am I able to wire in an external thermostat into the air handler bypassing the remote? I have a small indoor cannabis garden and cant risk the wifi controller. From the reviews Ive read it drops signal and the how unit has to be reset when that happens. My plants would be dead if that happened

    1. It should be technically possible to wire an external thermostat to an air handler with the right tools and training. However, it is not something we have done, and not something for which we can provide technical support.

  7. How do you install a remote thermostat for the Mr Cool DIY mini split? Is this even possible?

    I’m not wanting a wifi that just sets the temperature on the mini split, but a true remote thermostat, even if it has to be wired.


  8. Hi, I have a universal heat pump mrcool 2/3 ton, I can connect a Honeywell thermostat because the manual said no Y wire to thermostat but conddsnser don’t start…. how you wire a thermostat without a Y wire???

  9. I bought a 2/3 ton condenser and air handler. Why is the thermostat connection so cumbersome? The manual says no Y connection required for the inside unit and for the outside unit no G connection and where exactly on a thermostat should the B be connected since many thermostats out there don’t have B letter inside them. This thermostat connection for both inside and outside unit is the most confusing and non-straightforward that I ve seen. The system is not cooling and does not heat up as the condenser refuses to start just air handler came on and the fan outside too moves slowly. Please assist fully. Thanks

  10. Having an issue where the heat pump works for a while then for some reason it stops producing hot air and power to the thermostat goes off. Not sure what is causing this to happen. I’m not seeing any error codes on my outside unit (MDUO18024036) display.

  11. Where to set the heat/cool dip switches on a mr cool 4/5 ton universal heat putmp for the 4 ton setting,? It has 3 capacity settings

  12. Can your cielo breeze plus be directly wired by the terms on the back of the thermostat to a wall mounted dyi 9k mini split head?

  13. Have mr cool universal thermostat and mrcool 4 ton universal. Unit ducted what setting is right for telling the thermostat what I have

  14. Installed a 18K mini last January. Having problems with the heating on the unit. The air conditioning is working fine but the heating is constantly hunting up and down. The bottom louver is constantly in motion. Acts like the thermostat is reacting too quickly. I’ve called tech twice, left my number but no one ever called back. That was three months ago. Probably something simple but need some direction.

  15. I have purchased a MrCool 5 ton universal split system and MrCool universal thermostat, the wiring diagrams in the air handler manual do not match the wiring diagrams in the thermostat manual, please provide clarification, thanks Scott

  16. I purchased a new package heat pump unit to replace one that had a bad compressor. I hooked up to the existing electric, thermostat and wiring and duct work. I turn the unit on and the LEDS signal normal operation and will run for a few minutes and shows a anti short cycle fault, but a couple second before that it will blink red only which is a low pressure switch fault. Any suggestions?

  17. it seems that many folks are having issues with the thermostat wiring, at least on the universal models. Is it possible to publish a how-to for thermostat?
    I tried to call Tech-Support during last few days and yet to hear back.

    Thank you

    1. MrCool tech-support engineer called me back with the instructions. Apperantly, I’m using an older install document. If you have a challenge with wiring the thermostat, be sure to use the recent install document and refer to diagram 2.71.
      Thank you MrCool…

  18. I have mr cool 10kw auxillary heat strip

    My thermostat has WE (first stage emergency heat)
    W2 second stage energ heat.

    What connects to W1 on my universal air handler ?


    The cooling heating cycles work fine

    1. If your thermostat has both a WE and a W2 terminal, we would suggest using a jumper between those two and then connecting to W1 at the air handler.

  19. How to wite and configure a MrCool Universal Wifi Thermostat for a MRCOOL 2/3 ton universal HeatPump? Please share a diagram and the relevant config information.

    I’ve sent an email via contactUs form and I’ve been trying Tech support for over a week already.
    Please email or call me (the number is on WebSite field of this form.) thank you in advance.

  20. I have a 5 ton universal heat pump with the added heating strip. In the outside unit, it says to run the defrost wire into the indoor unit, but there is no where in the indoor unit that references where to connect the defrost signal wire. Also, the two wiring diagrams between the two units don’t match exactly.

      1. Due to the frequency of this question just in this thread, it seems like an answer here would be beneficial. Or direct us to a solution online. I too have this issue.

  21. I started calling and leaving messages and call back # on the 10th, it’s now the 14th and still no return call!! Unit is Universal 2-3 ton system, installed 6 months ago, heat will come on and run till it satisfies the thermostat, sometimes it will not heat after that, and other times it has heated for 2 days before cycling off. Shutting off breakers for 5 mins resets everything and gets it back going. Sometimes getting h6 (fan speed) code. Assuming I need a control board because fan is free and works fine on reset??

  22. I have a 5 ton universal unit just installed. Installed the nest thermostat and it continues to die. The common wire is run correctly but insufficient power getting to nest to keep it charged. Unit running properly after recharging nest with usb owner supply then dies after 48 hrs.

    1. What was the issue, I just installed the 2-3 ton unit and my nest is dieing after a while but only when the unit is not running for a while.

  23. I’m replacing an old R-22 system with your Universal 3 ton. On thermostat wiring (Honeywell RTH7400/7500 SERIES), would I connect the Y and W1 terminals together? Also, there is no D, defrost option with my 3 ton air handler. Is defrost handled with the B thermostat signal (via air handler control board, from condenser temp. signal)?
    Thanks, LCB

  24. I have thermostat wiring issue that seems to be common in this thread. I have emailed and called already with no response. Is there something else I need to do to be contacted?
    Please advise,
    Thank you.

    1. We apologize greatly for your experience. Would you mind replying to us and sending a reliable contact number so that I can relay it to the service team? The comment with your personal information will not be made public.

  25. Curious if anyone came up with any answers on this. I have the 4/5 ton and having issues as well.

  26. I’m looking for instruction on how to wire up the thermostat there’s no real clear instructions on what wire goes where I have a me cool universal series the heat worked great I go to switch to cool and nothing some clear wiring instructions would be great don’t know why it has to be so secretive on how to correctly place 4 wires

  27. I just installed a 2/3 ton universal heat pump and air handler. Started to hookup the thermostat, manual shows a y wire the units have a D wire instead. Thermostat I am using is a honeywell t5. Please help

  28. The “D” terminals are for connecting a wire from (defrost control in the air handler) to the condenser. It’s not a T-stat circuit. Mrcool changed the Universal terminals. What was “Y” is now “D.” They should have simply added a “D” terminal to the air handler, so the T-stat to condenser only “Y” wire could be conveniently landed from the T-stat to the condenser.
    If you use the online manual (which I did, as the one in the units are miniature manuals–tiny text), you need to be aware that the online PDF manuals were not updated to reflect this change. The tiny manuals that came with the units are correct.
    So, tie the “Y” wire from the T-stat to the wire going to the condenser with a wire nut. And provide a wire from the “D” terminal to the condenser, if the defrost is included (?).

    1. Thank you for posting this!! Luckily my google search wondering what “D” was turned up your reply. Who would’ve thought that the PDF is outdated and the paper manual is up-to-date?! Agreed on the terminal/landing spot. Wirenut/waco it is!

  29. Heat pump not switching on or system not blowing heat. What wire and location does the comm system use to control heat? This is regarding your 2/3 ton 18 seer ducted central system. Why isn’t there a place I can find this info easily? Which controllers are compatible since one isn’t offered by Mr cool?

  30. Does D terminal activate the emergency heat strip? I can’t figure out where to tie the E* wire off my nest learning tstat.

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