COP for Low-Load Operation
yargk
| Posted in Energy Efficiency and Durability on
Is the heat pump COP really that high for low load operation?
The Daikin 4MXL36TVJU has great turndown as listed here:
https://ashp.neep.org/#!/product/26570/7/25000///0
However, I was looking at the COP at 47 F outside temp and saw that
at 54KBTU (max) COP = 2.93
at 36KBTU COP = 4.26
at 6.6 KBTU COP =7.44
Is the last number real? That seems amazing for light loads.
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Their engineering manual is pretty good, you can find most of the information you want in there:
https://daikinac.com/content/assets/DOC/EngineeringManuals/EM-4MXL-T.pdf
Generally speaking, at lower output the COP of most(not all) heat pumps increases. This comes simply from the fact that you have a very larger outdoor condenser that needs to provide only minimal heat.
Daikin does tend to have better minimum range than some of the multi splits out there but keep in mind as you connect more indoor units the minimum also increases.
It is also not clear how this minimum capacity is split between indoor units so it is still best to size the indoor units for the actual load they serve (ie no single wall mount per bedroom).
The COP at minimum also warries a great deal with the actual installed capacity and combination. In most cases the COP is around 3 to 4.
Akos, I'm looking at page 29 of 712 of the Engineer Manual that you linked, which is titled "3.3 Combination Capacity." If you scroll down to page 30 of 712, several rows down you get the "07+07+07" three wall unit combination. It says that the capacity range for that combination is "5.50 - 26.80."
Am I correct in reading that as suggesting the minimum modulating capacity per unit would be 5.50 / 3 = 1833 btu?
If so, isn't an 1833 btu minimum modulating capacity pretty amazing for a multi-split? When I've researched this in the past I feel like I've come to the conclusion that most multisplits can't modulate anywhere near that low, but maybe it's been too long since I've researched it and this is the norm now?
In any event, at 1833 btu per head minimum, that seems like it's almost getting to a level that you could have one per bedroom and it wouldn't be THAT crazy.
Since they don't say how the min capacity is distributed, I would not make that assumption.
My experience with my multi split with a low-ish (9k) min capacity is that the calls don't overlap between heads all that much, chances are each head will run at max capacity until the setpoint is satisfied at which point it cycles off, repeat...
Even if modulation is better, just from the noise point of view, you don't want a head in bedroom if it will be used for heat. Refrigerant flow reversing during a defrost cycle will generally wake you up in the middle of the night.
First off page 29 is on 208V, but looking at the 230V the numbers are all almost the same, when there should be roughly a 10% drop in max capacity. Also the numbers for combined indoor units basically match a single indoor unit - see 9+9 vs 18k. The numbers are also not associated with a specific outdoor unit model. All of this implies the numbers are synthetically generated based on theoretical specifications of the indoor units. Since this data is not validated by external testing, or even implied they did empirical testing themselves, I would be highly skeptical. I would only assume ~50% turndown on multi-splits.
Don’t get to hung up on the numbers for 47° a pretty god house will not be needing many BTUs at 47° and the few it does are likely to come from the occupants and the waste heat of the electrical devices they likely are to be operating.
My guess is the COP differential will be much smaller when the outdoor temp is -.5 ° Closer to 100% as opposed to the 250% at 47°
Walta
We purchased and installed 3 separate 4 zone 4mxl (7 7 9 9) units and can confirm that the units do not operate according to the engineering manual. According to the engineering manual , 4 units should be able to modulate down to 6600 btu running at aprox 300 w. How ever the lowest we see the units running is aprox 800 watts and 15000 btu. This is very disappointing as it results in short cycling in outside temperatures above 40 degrees. Daikin support has not been helpful. Their last advice was that it’s just how the units run and that most people don’t look at power draw. It seems like Daikin is grossly over estimating their performance. We have three seperate units and they all perform the same. Does any one have any other real world performance data to report?
At neep.org there is a great resource for these kinds of discussions. Select a heat pump*, put in your zip code and heating load, and it produces a great report showing how that heat pump will perform in your climate. I'm going to try and attach a screen shot.
What I don't know is how much their numbers come from manufacturer's specs, and how much from actual testing. It does seem that their "maximum output" comes from testing.
They show COP at min, rated and max output at 47F, 17F and 5F.
*(unfortunately you have to select the heat pump, what would really kick is if you could just put in your zip code and heating load and it would find heat pumps for you.)
Another example of an unhappy multi split owner.
We rarely see complaints about miny splits.
Since it is short cycling should we assume the unit is also oversized?
Walta
Hi, in a previous flat, I estimated the cop of a Mitsubishi MSZ FH35 VEHZ (single head, hyper heating) and I could obtain COP values around 7 above 7°C (45°F) at minimum compressor frequency and hi or max indoor fan speed, once steady state has been reached (20mn), which never happens in real life because the amount of produced heat is too high.
I have now a Fujitsu multisplit and I can observe the following: each indoor unit will request at least 38°C (100°F) condensing temperature, or more if the setpoint is several degrees below the current room temperature (I have seen >50°C). Moreover, the minimum fan speed dictates the minimum air flow. For a typical 12kbtu wall unit it might be 360m3/h at "quiet" fan speed. So, unless I did not compute correctly, if the intake temperature is 20°C, deltaT*air_flow leads to at least 2.2kW (7.5kbtu) for each running wall unit.
So the displayed minimum modulation for a multisplit unit is probably only valid if a single head is requesting heat... Moreover, COP will be degraded (compared to single head) because the condensing temperature is never allowed to drop below 38°C.
I am trying to understand how this machine works in order to get the best out of it. I can see many questions around multisplits and how they work, and I would be pleased to share data...
Best regards, Benoit