Understanding Short-Cycling with Ductless Mini-Split Heat Pump
I’ve been reading a lot on these forums but am still having trouble figuring out short-cycling. From what I understand, short-cycling can happen if either:
-You’re heat pump is oversized. For example, a quad-zone heat pump with a minimum BTU output of 12,600 attached to four 7,000 BTU air handlers will short-cycle if only 1/4 air handlers are turned on because the heat pump BTU > air handler BTU.
-You’re air handler is oversized. For example, a single-zone heat pump with a BTU output of 9,000 attached to a 9,000 BTU air handler will short-cycle if the air handler is too large for the space.
Can someone clarify the difference between these and if they would both create short-cycling?
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There is also the potential of a control feedback issue where the controlling sensor is exposed to the warming/cooling effect faster than the room.
The solution in this case would be to use a remote sensor that is placed properly.
Many units control off the entering air/return air temp sensor by default and are prone to short cycling. The return may draw some of the conditioned supply in and indicate it's satisfied prematurely.
The first issue you mention is after caused by selecting the outdoor unit based on how many zones it can serve, rather than load matching it to the heat loss. Better to stick to a unit that is probably no larger than 120-130% of your heat loss, or slightly under. If you have multiple heat sources (like a electric heated bathroom floor, or a bit of baseboard), it might be better to under size your core heat pump slightly so it load matches better for most of the year. Also, you want to ensure the BTU output is mapped to your floor plan appropriately. You don't want 12,000btu/hr of heat going to a bedroom with 2500btu/hr of heat loss. The min size of many wall mount heads is still larger than a lot of small rooms need. It would be more comfortable to have less zones, but a better distribution of size match that a ducted head will provide. If the outdoor unit's min capacity exceeds a single zone it will bleed the extra capacity thru the other zones which has been shown to cause overheating in some cases. There are sometimes work arounds like disabling the fans on some units when satisfied, but that can make cycling even worse too.
I think thermal mass doesn't get enough enough attention. or in other words a highly insulated but thermally massive space with a high air volume might be able to absorb more minutes of min capacity runtime without swinging the space temperature uncomfortably. But a highly insulated small space with the same heat loss, with low thermal mass, will swing quickly compounding the downsides of a oversized equipment selection.
Thanks for the info! I'm trying my best to understand this all.
For the first issue, if I can't load match (all I need is four 7k BTU air handlers, but the smallest quad-zone is 36k BTUs) does this create any problems?
I think I'm confused if the minimum BTU range is what needs to be connected to the heat pump, or if the minimum BTU range is what the heat pump needs to run at.
I'm also grappling with understanding this stuff.
It sounds like you are referring to minimum BTU/h (power unit) output of the outdoor unit? The minimum output is the smallest amount of power the unit can supply at a given temp before having to shut off (causing cycling).
It's not just about the individual indoor head size, but about the total heating load. I have a hard enough time understanding single zone systems, but my understanding is that multi zone systems struggle to modulate as low as most single zone systems. This becomes especially apparent if your total heat load on all indoor units combined is low due to a mild day. With a giant outdoor unit, it won't be able to modulate down to supply steady heat at low demand, but rather will turn on and off.
How much of a kick in the efficiency pants that really is, I don't know, but apparently it's not ideal. Why short cycling is such a bad thing in terms of efficiency still alludes me a bit.
It's worth mentioning that BTU/h outputs and efficiencies at those outputs vary across temperatures. Most 'minimum' outputs I believe are given for 47 F. There's also a difference between 'rated' output at a given temp and 'maximum' output, and each will have a different efficiency. Slight oversizing is beneficial because units are not as efficient at maximum outputs. On the other hand, oversizing leads to the short cycling issue.
This video featuring Marc Rosenbaum is interesting "The Ins and Outs of Selecting Cold Climate Minisplit Heat Pumps"
I am referring to the minimum BTU/h (power unit) output of the outdoor unit.
I was initially thinking that a multi zone system with a lower minimum BTU/h range would be able to modulate down better and supply at a lower demand.
Looking at 3 similar 24k multi-zone outdoor units, they all have different min-max capacities for cooling and heating on their submittal sheets. For example:
Fujitsu 24RLXFZ (3-zone; 24k BTU): BTU Cooling Range: 6,100—27,000
LG LMU240HV (3-zone; 24k BTU) : BTU Cooling Range: 8,400—23,600
Mitsubishi MXZ-3C24NA (3-zone; 24k BTU): BTU Cooling Range: 12,600—25,500
Based off the BTU Cooling Ranges, the Fujitsu 24RLXFZ would work better with smaller 7k BTU air handlers than the Mitsubishi MXZ-3C24NA because it it can modulate down lower. I'm not sure if that is correct though.
For the efficiencies at those outputs, only LG shows numbers on their spec sheet from what I see, but I don't understand that yet. I'll have to look into that as well. These are LGs numbers:
Cooling COP @95°F (Rated): 3.96
Heating COP @47°F (Rated: 4.20
Thanks for your help and the video. I'm going to watch that and see if I can learn more!
"Based off the BTU Cooling Ranges, the Fujitsu 24RLXFZ would work better... because it it can modulate down lower."
I think that is a reasonable take. That video I link mentions Fujitsu in the context of heating and how impressive some of their specs are.
The video also mentions that you can calculate the COP at a given temp by using the input power from the submittal sheet. Some sheets will give input power at a given temp for both rated and maximum or minimum outputs, so you would have to determine if your load at that temp would be closer to the rated output or maximum/minimum, then you could roughly calculate COP for any given temp.
What is your calculated heat loss?
You don't want to grossly exceed your overall heat loss with the unit's max capacity. 20-30% overshoot max. Otherwise you'll likely have very little turndown capability and defeat the purpose of variable capacity equipment.
Variable capacity equipment's purpose isn't to allow you to grossly oversize. It's to allow it to load match thru much of the year for better comfort and efficiency.
Do the areas served by the 7kbtu air handlers have a load close to 7kbtu/hr each?
You have some good responses here from Josh and Tyler. In a nutshell, the outdoor unit is sized to meet the combined max loads of all of the interior units. If you have 4 interior heads each rated at 7K BTU, the outdoor unit needs to be able to meet all of these if they were to all call at the same time, so 28K BTU. If your total heat load is say, 14K BTU you won't ever have all of the interior units calling for a maximum BTU at the same time. The exterior unit has to push all this refrigerant and have a fan and motor sized to meet this potential, so when a small load is called for by the interior heads the oversized fan and motor's energy usage even at its lowest setting (lowest turndown ratio) is using too much power for what is needed, thus leading to inefficiency and short cycling. This can account for 2-3x the amount of BTU's needed by the interior units.
I'm not sure of your climate zone, but one thing to keep in mind with oversizing and short cycling is that it can't remove humidity as well as a properly sized unit. You may want to consider using separate 1:1 units that are sized correctly that can also effectively deal with this latent load.
A ducted system is another option, but one needs to find the space for the ducts. Also, multiple 1:1 systems can be more expensive than the multi zone units depending on the circumstances.
Ultimately one needs to consider all of these factors to make the right decision. You can always have a company such as Positive Energy in Austin or Energy Vanguard in Georgia help you with this design, but it does cost $. That being said, the money spent on the design will likely be offset with energy savings and certainly with delivered comfort and durability.
This statement does not ring true to my ear.
“-You’re air handler is oversized. For example, a single-zone heat pump with a BTU output of 9,000 attached to a 9,000 BTU air handler will short-cycle if the air handler is too large for the space.”
General the single headed compressors can run as low as 10% of there max before they are forced to cycle 900 BTUs is a pretty small load that is about the same as 250 watt light bulb.
The trick seems to be sizing the head to the load of that room. All to often we see a mini head set in place to replace each and every register from the gas furnace it is replacing, putting 9k heads in all of the tiny bedrooms and one in the bathroom. Now they have a system that is 250% oversized and at its min speed is oversized for all but the coldest 5 day of the year.
I say if you can avoid the multi headed minis and not buy more than 125 % of a real manual J calculation you will not see the units cycle until the outdoor temps are in the upper 40s.