Operating a New Ductless Mini Split

[viyer06]

Hi - I just had a multi zone ductless mini split installed (Carrier brand). 12k BTU in the upstairs master bedroom, and 12k BTU in the dining room on the main floor. We have a 24k condenser outside that has heating and cooling.

I've been playing with this system the first day and I'm new to ductless and trying to figure out if something is wrong with my install.

Here's the issue:
Upstairs theromostat = 68
Mini split set to 64 on the heat only setting.

Downstairs thermostat = 70
Mini split set to 75 on the heat only setting.

The downstairs unit pumps heat in order to get the thermostat to go from 70 to 75. This makes perfect sense.
Now, when I turned on the upstairs unit, the upstairs unit should not be doing anything because the room temp (68) is already higher than the mini split temp (64). However, the upstairs keeps blowing warm air. I'm thinking its because the downstairs is blowing heat to raise the temperature.

Ideally I would expect the downstairs to blow heat and the upstairs to do nothing. When I use only one mini split at a time, it works perfectly, this issue seems to only arise when both units are on at different temperatures. Since this is multi zone, I figured it should be able to operate totally independent of each other. Is this how a mini split is supposed to operate? I checked the wires. and it doesn't seem like anything is crossed at the condenser.

 

[Yankeefarmer]

When you describe the upstairs and downstairs thermostats, are you showing what they indicate the temperature to be, rather than the set point?

One of the principles with most ductless systems is that they tend not to control temperature quite like people are used to with conventional systems. Under some circumstances they will, but more often you need to get used to the idea that the set point is more of a directional goal than a precise set point. In addition, the fan in the wall unit may run even after the goal is met, for more accurate temperature sensing.

From the operation you describe, I suspect that much of the heat coming from your downstairs unit is finding its way upstairs. You may find the situation different when warm temperatures return and you are in cooling mode.

 

[viyer06]

Yea the thermostat is the room temperature (68) and the mini split temp is the set point (64). I didn't know mini split's operate in a directional way instead of at the precise temperature so thats good to know!

It's odd to me that when 1 unit is on, and the temperature of the room is past the set point, the unit shoots out fan air (not warm) but then when I turn on the other unit, which starts heating the downstairs, the upstairs unit starts shooting out warm air (instead of fan air). Its almost like the upstairs unit's pipes are taking some of the warm air from the downstairs unit.

This is my first time with a mini split, it seems very different than conventional hvac solutions. Overall it has been good but testing different temperatures in zones has been complicated

 

[Raisedonadeere]

I have installed several single head minisplits, and two of them could have been dual head with single condenser but I was just not ready to think through how they would operate and how well zone control would work so I used two condensers and considered the redundancy and simpler diagnostics to be OK expenditure.

I have some situations that look more multi head territory coming up and I think it would be very helpful to hear various experiences with the dynamics of multi head units. I have a son in need of a redo of the HVAC in a house he recently moved into.

In particular I am wondering how they would perform in a situations where you would want to cool one area during the day and another area during the night.

 

[Yankeefarmer]

We have a Fujitsu 36k condenser with a 24k head in our many windowed great room and a 12k head in the master bedroom. It’s used primarily for air conditioning, since we have oil fired baseboard heat and a wood stove in the great room. I occasionally run it for heat in spring and fall, and it does a great job.

I suspect our 24k head is slightly undersized for the room it’s in, and the 12k is certainly oversized for the master bedroom needs. I have never noticed any issues with temperature control when only one head is operating. If only the unit in the great room is running, you can occasionally hear what sounds like fluid flow in the master bedroom unit, but the noise is not objectionable. What I like best about the ductless system is its ability to provide humidity control without over cooling the rooms.

 

[justinjoyal]

The 12k in the bedroom is likely oversized, but that’s not what is causing your issue.

Your problem is caused by the ODU letting refrigerant bleed through the upstairs unit’s circuit to keep the pressures within operating range and even if the fan ramps down, it ends up « passively » heating the room past the setpoint.

Some systems can be programmed to stop the fan when the setpoint is reached instead of simply slowing it down to the lowest speed. This helps a lot. Ask your installer/technician if your system can be programmed as such.

Edit: I see user « restaurant mech » gave you the same explaination over at HvacTalk, just in different words.

 

[yeldogt]

With any new/ newish technology it takes a while for even professionals to get "into the weeds" so to speak with operation and installation. it's been my experience ----most pros don't understand how they work.

First -- wall units need air flow through them for the thermistor to know the room temp .... there are ways for some systems to use the remote. Since the head is up higher in the room this can throw off the settings as the temp above 60" starts to rise in heated room with moving air. Turning off the fan in the control setting can make this better or worse depending on the room.

Second: multi-head mini splits with all the lines back to the compressor -- are never "off". They can never restrict all the refrigerant flow to a given head -- the refrigerant flow must continue ... it's just reduced. This is bringing in BTU's even if you don't want it to.

The other items are the compressor and the heads. Each compressor has a turn down ratio. Somewhere there are tables showing the min and max output of the compressor you have ... there will also be a table showing what the heads you picked have .. again both min and max. They can and often are different based on what heads and what compressor is chosen. Manufacturers have extensive tables showing all the possibilities.

Finally -- the room. A proper room load is needed and must be done. This load is then referenced back to the overall (greater)space if only partial. Often you have to look at the greater space when doing partial AC .... IE .. this is what we have when only doing one room .. this is what occurs if we leave doors open ..etc.

What you have is a classic misunderstanding of how they work. Ideally -- multi should never be off and when you have a large mismatch in desired temps what you describe occurs. The head in the colder room has a min output .. the compressor has a mini output. When the warmer room head can no longer use all the compressors BTUs .. they have to go someplace. Also -- in smaller rooms with oversized units ... even the min on a mild day can be too much.

I have done 3 head Mitsubishi systems in two houses. 3 x 12k on a 30k compressor. Yes you can install more head BTU vs compressor .... It's all about using the tables to match the outputs of the heads to the loads in the rooms. With a multi head --- you match the compressor to the overall load needed and then pick the heads .... most pros do it the other way ... not correct. In those houses the units are sized to the spaces and never off .... they zone the space. When you properly zone -- the overall output can drop and that's where the savings come from. When one unit is using more input on one side of the house in the morning sun another does not need it ... when the other needs it ... the first does not.

My new build is an old stone building with a new addition. Because of space (none) and heights we could not cover the whole building with ductwork. Having used and lived with mini splits for many years I understood they were the perfect solution to these problem areas. My initial thought was to use at least one duel system .. but, after I got into the weeds with the manuals and outputs --- went with three separate units. My primary spot was a kitchen where you need some extra capacity. The other two spots were a far flung loft bedroom and a lower entrance area that also included a guest bathroom and dressing area. The loft was out as being part of the duel as it would be rarely used and would need to have refrigerant running to it all the times another head was running. The dressing and bedroom were just too different vs the kitchen operation ... the extra capacity of the kitchen zone makes the minimum compressor output too high for the bathroom/dressing.

 

[yeldogt]

The 12k in the bedroom is likely oversized, but that’s not what is causing your issue.

Your problem is caused by the ODU letting refrigerant bleed through the upstairs unit’s circuit to keep the pressures within operating range and even if the fan ramps down, it ends up « passively » heating the room past the setpoint.

Some systems can be programmed to stop the fan when the setpoint is reached instead of simply slowing it down to the lowest speed. This helps a lot. Ask your installer/technician if your system can be programmed as such.

Edit: I see user « restaurant mech » gave you the same explaination over at HvacTalk, just in different words.

I used to pop on there often years ago .... Still do occasionally. There are many "old school" on that site that do not know much about mini-splits or even VS equipment / zoning. Often giving incorrect advise. It's much better on the mini front now .... There is a guy who does load calculations and I paid him to do a load on my build a few years ago. There are better forums IMO

 

[justinjoyal]

I used to pop on there often years ago .... Still do occasionally. There are many "old school" on that site that do not know much about mini-splits or even VS equipment / zoning. Often giving incorrect advise. It's much better on the mini front now .... There is a guy who does load calculations and I paid him to do a load on my build a few years ago. There are better forums IMO

I noticed a lot of guys over there have very limited knowledge when it comes to mini-split and so they tend to stat away from them and/or even look down on them and anyone who wants to use them !

The mini-split market here is huge and there are reasons for that : ease of installation, low cost, low maintenance, low energy consumption, etc.

We installed ~150 of them in the last year and there was demand for more but there is only so much time in a day...