Mini Split Heat settings based on room usage
Hi,
I have a 3 story home in Boston, MA. Heat is provided to the 1st floor w/ a single zone high efficiency boiler.
On the second and third floor I have several smallish rooms that each have a new mitsu hyperheat mini-split. As of now, I only use my bedroom from 7pm-7am and my office from 7am-7pm. These two rooms are heated by mini’s that are on the same outdoor unit. As we enter winter, I wanted to know if its most efficient if the units are either OFF or ON – or, if I should maintain a minimum temp while a room is not in use.
I.e. is the energy used to heat up a COLD room > or < the energy to consistently heat a room at a certain temp when not in use?
Thanks,
Joe
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Replies
Joe,
This article might be helpful: https://www.energy.gov/energysaver/thermostats.
Members who live in your climate may be able to give you more insight into what makes sense from a comfort and dollars perspective.
In general, doing any setback on a heatpump is not ideal. You generally end up asking the heat pump to do the most work when it's coldest outside. Set them and forge them.
> smallish rooms that each have a new mitsu hyperheat mini-split
So probably significantly over sized - which reduces fixed-thermostat efficiency. The right answer is "it depends, measure it".
Most (maybe all) multi splits do not module the indoor units, the indroor head is either on at full capacity or off. This means that you don't get the energy penalty you would with one-to-one setup from thermostat setback recovery.
Most likely in your case, it is better to reduce the setpoint in the rooms that you are not using and run them only when needed.
This would also get longer runtime on your outdoor unit which generally improves the efficiency.
Akos, is it true that the indoor units don't modulate when used in a multi-split configuration? That would be quite an issue and undermine the advantages of VRF. Could I ask for more info on that? Can one tell, for example, from the spec documents (e.g., those on Mitsu mylinkdrive?) Does it depend on the particular configuration? Do any manufacturers provide guidance on modulation for particular combinations (ie beyond 1-to-1). I know that Mitsu's software apparently does not.
This is what I have seen with my multi (LG) and the Mitsubishi multi splits without a branch box. I can't comment on the rest.
My unit essentially runs the compressor at fixed power levels depending on how many and which size heads are on. I have played with all the remote control settings and they don't change the compressor power consumption at all until you go above room temperature at which point the compressor ramps down to off.
This is the biggest issue with multi splits and why you have to match them carefully to your actual load. Some cycling is not a problem, but excessive cycling significantly reduces efficiency.
>"On the second and third floor I have several smallish rooms that each have a new mitsu hyperheat mini-split."
I cringe at that description. The "head for every room" approach is pretty lousy, in several ways.
Smallish rooms in tight insulated houses have tiny design loads, and miniscule average loads, and a half-ton head per room is extreme overkill. Consolidating zones using mini-duct cassettes will usually be more efficient and more comfortable.
As long as the average load for the whole house is higher than the minimum output of the compressor it's generally more efficient to "set and forget" the individual room temps. The heads will continue to cycle on/off, but the amount compressor cycling will be less, taking a smaller efficiency hit. If one uses setbacks the compressor output will be distributed between fewer heads, all oversized and cycling frequently causing more compressor cycles. With more heads active the cycles will overlap more often. But it depends on just how ludicrously oversized the heads are, and how short the cycles on the heads really are. At some oversize factor the cycles become too short for calls for heat/cool from different heads to overlap no matter what.
Also, even when a head is "off" during a setback there will be refrigerant moving through the coils of the head whenever ANY other head on the multi-split is calling for heat/cool. In rooms with very tiny loads this can result in passive overheating /overcooling discomfort when rooms/zones even though the blower isn't running, if there are zones with heads sized more reasonably for their zone load (or undersized) running long cycles.
In summer this bypass refrigerant can also cause condensation to build up on cool surfaces inside the "off" or low duty-cycle head causing it to spit droplets into the room when it starts.