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Community and Q&A

Minisplits and thermostat: replacing the remote controls

fcserei | Posted in Mechanicals on

I’ve recently built my house in Delaware. It is  a 2 story 1300sf, better than code insulation hose, but nothing extreme, except it has a lots of glass ( 800 sf, u 0.24). It house is in a forest between mature trees. It is completely shaded in the summer, but lots of solar gain in the winter.

Heating and coling is with 2 ducted mini split units with very short ducts.
They work very well, but as others noted regarding mini splits they need constant fiddling with the controls to get the best result.
The bundled IR remotes/wired remote units are units are not really up to their task, not to mention they have confusing user interfaces and they are unsightly. Currently I use the wired remotes with the remote temp sensor, but still to keep the house comfortable it needs more manual intervention than I prefer.

Mostly it means I have to manually turn the units on/off via the remote. 

In the summer, when I mostly need to control the humidity I use the dry mode for this, but if I don’t watch the unit and turn off in time it will undershoot the temperature and turns the house into a freezer. The cool mode does not have the necessary humidity removal capacity, it reaches the set temp quickly, then just blows the air around as the humidity rises.

In the winter, the heat with auto fan mode is mostly working fine, except when the sun comes out during the day and the unit is not able to adjust quickly. It reduces the compressor load, and the fan switches to high when you just want the whole thing to turn off. Also if the unit is around the set temp and the fan is blowing at auto setting (while you would want the unit off at this stage), at sunset it does not start to ramp up the heat until the inside temp drops several degrees. If I turn the unit off/on, it immediately starts the heat.
I’d like to have a just set and forget thermostat, which only shows the actual temp/humidity, the set temp and have up/down controls only for the set temp. I’d like to hide everything else. I know about the Cielo Breeze and similar, just I heard contrary opinions about the built in sensors, I don’t need the complexity and I don’t want to include the “cloud”.

So here is my thought: Most of the mini split units have an extra remote on/off wire connection somewhere on the main board. What if I connect a simple relay type thermostat/humidistat ( programmable or not) to this terminal to turn the unit on/off according to the actual temp/humidity at the thermostat. The mini split then does the heating/cooling according to the season.
Twice a year, when I would adjust my regular thermostat from heat to cool or vice versa, I also set the working mode of the mini split. I mean in the winter if my preferred indoor temp is arouns 70deg, I set the mini split to 75 deg, heating,  auto fan. In the summer I set the target temp 76deg, the mini split to 70deg, dry, low fan, and hide all the mini split controllers, remotes until winter. 

I know it sounds like I want to introduce cycling to the system, but it seems with the proper selection of the mini split set temp and the hysteresis of the regular thermostat, the setup will just turn the split units on/off when they are outside of their comfort zone anyway.

I think it can resolve a lots of comfort and conveninece issues.

Any thought about viability, efficiency hit, durability problems?

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  1. walta100 | | #1

    To my ear it sounds like you have the best set of factory controls available.

    If your plan is to buy thermostat kits and use another brands thermostat to control your minis understand you will give up some efficiency. The thermostat will have 3 options off, low and max and your thermostat will cycle between the three and maintain the set temp. The way I see it you will give up dry mode and the thermostat will only be working to maintain optimal temp without any regard for humidity.

    I the one cloud based systems power graph I have seen made it clear its ability to modulate the compressors speed rudimentary at best that control was made by the same manufacturer as the mini. If you mix brands you are less likely to get fine control.

    If you want to control humidity without over cooling your choices will be to overcool and reheat or run a dehumidifier.

    It would expected that new building materials will be moist and will dry a lot in the first year and sum in the second. So things may work better over time.

    Have you consider moving the remote sensors into a sunny or less sunny location?

    How sure are you that the mini is programmed to use the remote sensors? It would be easy to connect them but not change the program option.


  2. joshdurston | | #2

    I can see why your struggling with humidity with all that shading.
    I would run only one of your units on the lowest fan speed. This will force that unit to work harder and probably remove more moisture than running both. At low loads the discharge temperature is sometimes above the dewpoint (meaning no dehumidification). As it loads up the discharge temp drops and the humidity removal rate really increases.

    I wish you could increase the min output in cooling mode. Going above 400cfm/ton you're not going to get much latent capacity.

    This might be controversial but I will partially block my return (on my wall mount with some newspaper) until my supply temperature drops to somewhere between 10-12C. This increases the latent ratio. I believe this is fine. since it gets down to 7-8C unblocked when cooling at higher loads.

    You could use something like the Sensibo ($120 right now), you could program it to turn the unit on when the room temp exceeds 76F, at a setpoint of 70-72F. This will ensure the unit modulates to a higher output. And then disable the unit when the temp drops below 75F. You can also create rules based on humidity.
    I've never used one, but the climate react features look interesting. You could use it to stop dry mode before it overcools the place.

  3. fcserei | | #3

    Current thermostat locations are fine, they reads the proper temperatures and the indor units are using those measurements.
    As I said the whole setup works well most of the time, but needs too many manual inventions during a sudden external change or during a very humid but mild day or night. I want to automate these interactions. Also I don't like that they are blowing air even when they are not heating or cooling at all, and dont use the thermistor at the return air intake because of the wired remote. I played with the internal dip switches to change this with no result.

    I understand that there might be an eficiency tradeoff with the extra thermostat control, mostly because the mini split have a slightly different target temp set than the desired room temp. So basically it is working harder than it should to reach the set temp. But there should be an efficiency gain also, because the mini split is turned off completely as soon as it reaches the desired room temp/humidity instead of just rolling back and still blowing air around.

    The Sensibo climate react mode and the Cielo comfy mode is doing something similar but with more options and much more complexity. ( As you may guess I'm not a big fan of IoT).
    I am looking for a simple automated hardware to turn the mini split on when the room temerature/humidity calls for it, and let the mini split software to figure out how to get there, then turn it off.

  4. Expert Member
    Akos | | #4

    You can get the thermostat interface module for your ducted unit and use standard multi-stage thermostat.

    The Mitsubishi module lets you configure the temperature setpoint on low stage. It will pretty much run the same as setting say 72F(or whatever temperature you have the module configured to) on your remote, so you get good modulation and will run on full tilt on the second stage.

    If you can get a thermostat that has multiple fan speed outputs, you can set it the units to low flow rate during the shoulder season to get a bit better de-humidification.

    An option is also to use to use the unit on the upper floor for de-humidification and the one on the lower floor for a bit of heat. This would add some sensible load to the upper unit and prevent the house from getting over cooled with only a small energy penalty.

  5. willymo | | #5

    In shoulder season/hi humidity here in coastal RI I sometimes run one unit on dry and the other on heat. For not much $$ reduces the humidity.

  6. cldlhd | | #6

    We live in a similar area of the country and I'm experiencing the same issues. I live just north east of Philadelphia. I've had my Mitsubishi mini split wall units since 2017. My house is one story with three bedrooms. It was built in the early 1950s but during renovations I replace all the sheathing with zip system did a lot of air sealing and insulating. Two of the bedrooms have closed cell spray foam in the walls the rest of the house has fiberglass batts. The attic has fiberglass with blown in insulation on top of that. Each bedroom has its own wall unit, one 6,000 BTU and two 9,000 BTUs. The main living area has one 15,000 BTU unit. I don't get the issue out of that wall unit but with the bedrooms, which have the room outdoor compressor, if I set the temperature to 70°, it reaches that temperature too quickly and the humidity kicks back up. I'm of the understanding that what's happening is the fan continually runs so the moisture that's left on the coil in the wall unit gets redistributed into the room which gives me a musty smell. If I put it in dry mode it goes away but like you if I leave it that way the bedrooms end up feeling like a meat freezer. I've tried putting the fan in whisper mode so it takes longer to reach temperature but that hasn't done too much.
    I've recently tried a Cielo Breeze Plus in the master bedroom, it has comfy humidity mode where I can set it to automatically switch to dry mode at a certain humidity level then switch back to cool once the humidity gets low enough. The thing is it still gets colder than we would like before the humidity gets down to where we would like it but it's not as bad compared to if I put it in dry mode and go to sleep for the night and then wake up in the morning and it's freezing.

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