Does mini split fan speed have any effect on efficiency?
I am logging data on 2 12k mini splits. I installed electricity monitors and return and supply temperature probes. I also have 6 room sensors montoring space temperature throughout the house.
With the fan set to high I get way better even heat distribution throughout the house. They also seem to run better with more stable readings. My 6 room monitors easily confirmed this. When in the auto setting it seems to keep the fan to a absolute minimum. There are many references in the manual about not blowing air fast so people don’t feel the wind chill effect, etc. My floor console unit actually says it blows air out the top for the first 15 mins then opens the lower outlet to not make people cold.
Is there any efficiency gain by moving more air across the evaporator? Do the manufacturers put a dent on efficiency over wind chill comfort?
Both units at max capacity seem to target the below duct temps.
Fan auto 130-140 degrees at max capacity. They both seem to target 136 degrees for some reason as they almost always run dead on that number.
Fan High 105-115. Both seem to target 108/109.
There is a definitely a downside with more fan noise and wind chill effect with the fan on high. Anyone else notice any difference between running on high or auto?
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As you note, higher fan speed reduces heat exchanger temperature (when heating). This does improve efficiency, up to some point where fan power becomes a larger issue.
Refrigeration cycle efficiency is highly dependent on the temperature difference, evaporator to condenser. Less is better.
I just realized I can put my temp probe on the evaporator return line and compare the return temp with fan speeds. I would assume a lower refrigerant return temp to the outside unit would equal more heat extracted from the evaporator?
Don't have an answer for your direct question but am curious:
>I am logging data on 2 12k mini splits. I installed electricity monitors and return and supply temperature probes. I also have 6 room sensors montoring space temperature throughout the house.
What are you using to capture and log data? "Temp probe" ... do those have IR temp sensors at the units in addition to ambient temp sensors? I didn't think ambient temperatur sensors were a good fit for finding spot temperature, like what's coming out of a minisplit vent. And for the electricity usage, what did you go with? clamp on CT to each head unit's power supply? I have a multi-split so cannot do each head unit's power AFAIK because the power supply runs to the central external unit, unless I turn on each head independently.
I got an IoTAWatt, but haven't hooked it up yet (which is a shame! but other projects took priority right now).
I am using a Efergy engage electricity monitoring hub with CT sensors on each minisplit circuit. For temperature I am using an acurite weather station with a separate temperature probe. The return sensor is right next to the unit attached on the side intake right where the consoles sensor is. The return sensor reading is usuallt within a degree of the set point.
Ingenius use of a weather station's sensors :)
Ty for sharing.
I found a department of energy study that found COP increased 60% with the fan on high. This prettt much confirms my findings. Run your fan on high.
At least with my Fujitsu wall mount connected to their wired thermostat, it's frustrating in auto mode because it will actually let the room go below the set point without kicking in to high by multiple degrees. If I switch to high fan, it immediately starts to blast full speed, so it's not like the unit is maxed out or anything.
At what point does running the fan high use enough power to negate the benefits?
Bill
The fans are very efficient resin packed brushless DC motors. I think mine uses around 25-50 watts so it costs practically nothing to run.
Very interesting info.
Also makes yet another case for not oversizing heads as they will never automatically run on high fan.
An oversized head would allow for the same heat transfer with a lower fan speed than would a smaller head with a higher fan speed. The efficiency aspect is related to the temperature difference, not the fan speed itself.
Running more air over the high side of the system (indoor coil in heating and outdoor coil in cooling) should improve compression ratio. The biggest electrical load in a ductless system is the Compressor. The compressor doesn’t have to work as hard with a better compression ratio. So it makes sense that it would run more efficiently. And substantially so.
Its interesting that manufacturers seem to be so concerned over the wind chill effect, that they put such a huge dent on efficiency by keeping the fan speeds low. But they are doing other things as well.
On my floor console unit I can direct air out the top, button, or both at the same time. I always have it set to both open but its overridden by the software so "air doesnt come in contact with people". See the attached picture of the manua below. Having both open doubles the the supply surface area and I am sure significantly increases efficiency. It also heats significantly better with the bottom outlet open. So it's not just fan speeds that manufacturers are altering over efficiency.
I attached a FLIR shot of how much better the heat gets thrown out along the floor with the lower outlet open. I am currently looking into possibly removing the flap or something to try and keep it open permanently. The software seems to only keep it open at much higher speeds.
> The compressor doesn’t have to work as hard with a better compression ratio
Agreed - note that a "better" compression ratio is a lower ratio.
You need to know and understand what the heat transfer coefficient (h) is and how it works.
Q = - h A (T2 -T1)
Tom, care to illuminate it for us? I mean, I know what h is, but how does it affect this issue?
Bill
Bill you state you know what h is , so why do you need an explanation?
Tom thanks for your generous help.
I did some further testing and have also discovered there is actually a significant electrical savings in running the fan on high. Compressor load is reduced by about 25 percent. Here is a graph of the unit on auto fan for 20 mins, then switched to high fan for 20. Notice the drop in load on the graph when the fan was switched to high.
I'm not following the conclusion of "drop in load". The graph is electric power in Kwhs over time? And is less that .5 kwh, then than 2kwh, but over a time period greater than 20mins followed by 20mins. You're saying the 1st time period is on auto fan and the 2nd time period is on high fan speed, but the high fan speed is also the higher electric load, no? The graph must be showing a greater time period than you're talking about...
The electrical consumption is lower with the fan on high.
Unit was set to max capacity auto fan at 1640 on the graph. Unit was consuming around 2200-2300 kwh. I then switched to high fan at 1700 and it dropped to a little over 2 kwhs.
See Pauls comment above about lowering the compression ratio with more air across the evaporator.
20 minutes is way too short to gather any tangible results. I'd say at a minimum an hour for each setting, ideally early morning before sun rises so you aren't dealing with solar gain, and outdoor temps have bottomed out.
In heating mode, I think high fan speed will improve efficiency. I plan on testing this out on my ducted system when I get around to putting it in. This will include some amount of changing the fan pressure set point to get higher fan speed. The ideal setting may be in auto, with a higher pressure setting to up the CFM.
The Fujitsu Slim Duct service manual includes information on the fan and compressor controls. With the indoor fan speed set to Auto, high fan speed isn't reached until the room temp falls behind by 5F. 5F off set point is pretty far. It doesn't seem like the fan speed would up until the compressor is running for some time at peak output, which seems sub optimal. A well designed ducted system should be more tolerant to running the fan on high all the time than a cassette or wall unit. The manual does not give much detail on the compressor modulation strategy in heating.
Under low load conditions, it may make sense to drop the indoor fan speed to extend the modulation range. This will depend on the low modulation COP vs the startup efficiency hit. There are some systems that appear to push the modulation too far, but I have not heard anyone suggest this of the Fujitsu slim ducts.
Great find. 5F below the set point is ridiculous. I'm sensitive to +/-1F. No wonder people complain about comfort levels with heat pumps. My wall split will behave the same way even with the hardwired thermostat. Is this to hit some sort of fake efficiency number? Depending on how the units are sized, they may not be able to recover as the energy required to offset the home's heatloss + the delta in temperature on top of that may be too great, generally at low outdoor temperatures with defrost cycles. That's really a shame, and should be user adjustable.
I also connected a temperature probe on the return refrigerant line to the outdoor condenser. The fan was set to high overnight and the return line was around 48-58. You can see when the fan was set to auto at around 630 the return line temp went up to 67-72.
The takeaway is then whenever the fan is on high the efficiency is significantly increased as more heat is extracted from the evaporator.
What was the difference in kwh usage from this chart?
johns3km -
The compressor still modulates to maintain the set point. On the slim ducts, a low indoor fan speed will limit output to about 85% of the capacity available at the high fan speed setting.
I am surprised the indoor fan speed in heating is not tied to the compressor speed. I think I want the fan to run on high except when at the lowest compressor speeds (low end of modulation).
On the cooling side, the compressor speed is limited by a combination of the outdoor temperature and the indoor fan speed. This must be related to preventing freeze up. This is worth noting for those of us that live in humid, low cooling load places and are considering dropping the fan speeds to improve latent removal. I am surprised they are not using a supply air temp sensor to control the compressor speed in cooling.
Thanks Matt. I've certainly noticed in cooler temps that if it starts to slip maintaining the set point, in auto mode, it feels like it isn't trying to attempt full blast to keep up. If I switch from auto to high fan, it immediately blows faster so the capacity is there. I tend to run high fan overnight and auto fan through out the day as needed.
When set to high fan both my mini splits will still throttle the fan back, even at min modulation to always maintain a minimum 86 degree supply air temperature.
Here is a similar chart from the Gree service manual on fan speeds. It looks like Gree limits the fan speed on outdoor temperature if I am reading it correctly?
Great question... I have a Mitsubishi fe18na or some similar model #. Would be great to see comparisons for this brand as well.
I think that we avoid fluctuations in the compressor power consumption with a remote thermostat... But it would be nice to have more info on what would happen if we used the fan at higher speeds. If you turn up the speeds i guess that the filter also needs to be cleaned more often.
If I'm not home/ not in the room I often set the fan on high. definately better evenness and lower compressor speeds.
If I'm in the room, medium or lower. High air movement leads to the "chills."
I’ve switched to turning mine on high overnight and then auto/low for most of the day unless it can’t keep up.