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Community and Q&A

Temperature Monitoring with a Ducted Minisplit System

aunsafe2015 | Posted in Mechanicals on

Just curious if anybody here has a Fujitsu ducted HVAC unit, either mid-static or multi-position air handler, and has a temperature logger near the thermostat?  If so could you post a screenshot of the temperature logger data during a period of a relatively constant heating or cooling load on the Fujitsu?

For comparison, attached is a picture of a temperature logger for my Trane XV18.  Setpoint is 68 so the thermostat temperature sensor (or the sensor in my temp logger) might be off by a little bit.  But the thing that interests me is that it maintains the temperature within a pretty small range over pretty long time periods if the load is not changing a ton, which I really like.

Don’t have a screenshot immediately available, but my Mitsubishi ductless unit has a much higher spread of about 2-3 degrees, even when subjected to a pretty constant load.

Just curious to see what other systems — especially Fujitsu’s ducted systems — are able to do in terms of maintaining a steady temperature while modulating.

Thanks for any input.  And I suppose I’d be curious to see temperature logger results for just about any system, so owners of non-Mitsubishi’s feel free to post your results as well!

Edit: Oh, and would also be curious to see what happens during defrost cycles.  My Trane’s backup heat strips kick on during defrost, but even so, the temperature will usually drop 0.2 or 0.3 degrees lower than it’s normal modulating range during defrost, and then it will overcompensate by 0.2 or 0.3 degrees when recovering.  The attached picture does not show a defrost cycle.

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  1. bfw577 | | #1

    I log both my mini split supply and return temperatures and room temperatures using an Acurite hub with remote temperature sensors. I also have an Efergy Engage electricity monitor and log the electrical consumption. With those 3 pieces of data you can get a ton on invaluable data on how best to run these units. You can see the performance in real time and you can calculate the real time COP with that data. I highly recommend you get an electricity monitor on your unit. The Efergy Engage I use is around $130 for the hub and 1 sensor. I have 5 sensors total connected on mine.

    Attached is a snapshot of the room temperature and electricity modulation on my 12k Gree Sapphire. The data is from midnight on and those first 3 blips are defrost cycles. I zoomed in on one so you can see the reversing valve operate. Notice the compressor briefly ramps up after the cycle to recover from the few minutes it was defrosting and it locked tight back on the load. ​

    As far as room temperature swings I generally see around a 1 to 1.5 degree swing if there are defrost cycles or big changes in the load like a sudden drop in outdoor temperature. Generally the unit can pretty much lock on a temp and hold it at a steady load. Often times the room temperature is almost a near perfect straight line on the graph. These units have an incredible ability when sized properly to lock on the load and only deliver exactly what's needed. Someone going in and out of the front door bringing groceries wil show up on the electricity chart.

    1. aunsafe2015 | | #2

      Very cool, thanks for posting. Gree's ability to maintain temp close to setpoint sounds like it's quite good, just like Trane's. Is your Gree a ducted unit? If so, off the top of your head do you happen to know the max static pressure it can take?

  2. bfw577 | | #3

    The Gree is a wall unit. I also have a Midea/Carrier Premier floor console unit. The Midea is actually better at holding a temp then the Gree. Its probably because it's a floor console and I have found mine to be much better in heating over the wall unit. I think it's mainly because its down low at the floor level drawing in the coldest air in the room. Wall units are drawing in the warmest air in the room as they are mounted just below ceiling level. There can be big differences in room temp between ground level and ceiling. My floor console is using the return temp 3 ft off the floor while my wall unit is 12 inches below ceiling level. I have to set my wall unit much higher than the floor unit. There is usually a 3 degree offset with the wall unit. So a 73 setpoint will yield a 70 degree temp just above floor level.

    Based on your temp chart which looks like only a .3 deviation on the chart? It would seem a ducted unit properly sized with good returns can maintain a pretty incredible steady temp. Your getting both maximum efficiency and comfort when your unit can dial in at a steady temperature like that.

    Here is a 12 hour snapshot of the return temp on the Midea console. It deviated less than a degree over 12 hours. You can pretty much follow the house load on the graph. Between 1-4 was peak daytime heating. Once it got dark at 4 the inverter ramped up to compensate and then was extremely stable. I often see near steady temps like you do overnight when the load is low as you can see on the graph.

    1. aunsafe2015 | | #4

      Ok wow, your Gree and Midea units do a MUCH better job than my Mitsubishi FH09 wall-mounted ductless unit. I don't have a picture of temperature logging for that unit but it's definitely in the 2-3 degree range. It's in a somewhat challenging environment (a 300 sq ft semi-detached garage that was converted to finished space), and it's lineset is only about 10 feet long, and I've always wondered if that's too short (though Mitsubishi spec does not state any minimum lineset length).

      And yes, my picture is showing a 0.3 degree variation for the Trane XV18 over a 5 hour period. The incredible thing is that it's a 2-ton unit serving about 2000 sq ft that is split into 4 zones, and it can independently maintain all 4 of the zones within a 0.2-0.4 degree range overnight on cold nights when the load is fairly constant, even when the 4 zones are set to different temperatures (e.g., zone 1 = 68, zone 2 = 66, zone 3 = 68, zone 4 = 69). And even when the load isn't constant, the temperature range of any given zone is rarely greater than 1 degree.

      I don't want to sing Trane's praises too much, though, because when the system was only 2.5 years old it developed an evaporator coil leak, and although the coil itself was covered under warranty, labor was $1500 and then I got charged nearly $1000 for new refrigerant... so $2500 to replace the coil "under warranty." lol. But apart from that I've been happy with the Trane and have been impressed with its ability to maintain temperatures extremely close to setpoint.

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