Rules of Thumb for Ductless Minisplits

musingsheader image

Rules of Thumb for Ductless Minisplits

How to design a house that can be heated with one or two ductless minisplits — and how to operate the units once the house is occupied

Posted on Jan 30 2015 by Martin Holladay
prime

Since 2008, when Carter Scott built a pioneering Massachusetts house that was heated and cooled by just two ductless minisplits, GBAGreenBuildingAdvisor.com has endeavored to publish reports from the field to guide people designing homes that are heated and cooled by ductless minisplits. We’ve learned a lot on this topic since 2008.

My article on Carter Scott’s approach to heating and cooling was called “Just Two Minisplits Heat and Cool the Whole House.” Since that article was published, builders, engineers, and researchers have shared their minisplit experience and data. Carter Scott has given technical presentations at several conferences (including the Westford Symposium on Building Science and NESEA’s BuildingEnergy conference); energy consultant Marc Rosenbaum has written several valuable articles on the topic for GBA (including “Minisplit Heat Pumps and Zero-Net-Energy Homes” and “Practical Design Advice for Zero-Net-Energy Homes”); and researchers Kohta Ueno and Honorata Loomis have published useful monitoring data (“Long-Term Monitoring of Mini-Split Ductless Heat Pumps in the Northeast”).

We now have enough information on the use of ductless minisplits to heat and cool cold-climate homes to set out some rules of thumb. The nine rules of thumb that I present below are based on the work of Scott, Rosenbaum, Ueno, and Loomis, to whom I am indebted.

1. Design your building to have an excellent thermal envelope

If you want to heat and cool your building with just one or two point-source heaters, you want an above-average thermal envelope. That means that the building needs a very low rate of air leakage; above-code levels of insulation; and high-performance windows.

2. Consider snow loads when placing outdoor units

If you live in snow country, your outdoor unit needs to be protected by a roof — but not a roof that inhibits air flow — or needs to be wall-mounted at least 4 or 5 feet above grade. It’s better to locate the outdoor unit on the gable end of a house than under the eaves.

3. Most two-story homes need at least two ductless minisplits

A single ductless minisplit unit located on the first floor of a two-story house is often capable of heating the whole house. However, a first-floor unit is incapable of cooling the upper floor. In a two-story house, if you want both heating and cooling, you will need at least two ductless minisplits: one downstairs and one upstairs.

The downstairs unit will do most of the heating, and the upstairs unit will do most of the cooling.

4. Big windows can make things tricky

If you are heating your house with one or two point-source heaters, it’s a good idea to avoid oversized windows if you want to minimize room-to-room temperature differences.

The temperature in a bedroom can be within 4 F° or 5 F° of an adjacent hallway or common room as long as the heat flow rate through the exterior walls and windows is no greater than the heat flow rate through the uninsulated partition walls. Corner bedrooms are more of a challenge than bedrooms near the center of a building, and bedrooms with big windows are more of a challenge than bedrooms with reasonably sized windows.

During the winter, large windows lose a lot of heat at night and on cloudy days. During the summer, large windows can cause heat-gain problems on sunny days, especially if the windows face east or west.

5. Bonus rooms need special consideration

A bonus room — for example, a bedroom over a garage — cannot be heated and cooled by a ductless minisplit head located outside of the room. Bonus rooms usually have 5 out of 6 surfaces facing outdoor temperatures, so this type of room needs its own thermostat.

Bonus rooms violate a basic tenet of good building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. design: namely, to create a compact shape. If you want your building to be energy-efficient — and if you expect to heat and cool the building with just one or two minisplits — you need a compact design with as few bump-outs and ells as possible.

6. In Climate Zone 5 and warmer locations, you don’t have to worry about cold-weather performance or lack of capacity

All reports are consistent on this point: cold-weather ductless minisplits from Mitsubishi and Fujitsu are performing well at outdoor temperatures that are lower than the units are rated for. (Mitsubishi provides performance data on some of its ductless minisplit models at outdoor temperatures as low as -13°F; many users report that these units are performing well at -20°F.) Marc Rosenbaum has reported, “In temperatures below design temperatures, the units have enough capacity to heat the houses even though we don’t size them with an intentional safety factor.”

According to researchers Kohta Ueno and Honorata Loomis, “The MSHPs [minisplit heat pumps] seldom hit maximum power draw, indicating substantial excess capacity even during worst-case winter conditions (much colder than local design temperatures). These results are consistent with the installed capacity of the equipment: the oversizing (compared to calculated loads) typically ranged from 150% to 200%. Although oversizing cooling equipment is commonly criticized, oversizing of heat pumps (for wintertime loads) can be beneficial. This is particularly true for MSHPs that modulate their capacity, as their highest efficiency is obtained when the unit is running at the lower end of their capacity range.”

Of course, it’s still important to perform a heating and cooling load calculation before specifying a minisplit, and it’s still important to choose equipment designed to perform at typical outdoor temperatures in your climate.

The point that I’m trying to emphasize is that occasional comfort complaints arise from heat distribution issues, not appliance capacity problems.

7. One ductless minisplit head can serve a maximum of about 1,100 square feet

This rule of thumb was proposed by Ueno and Loomis. To clarify, and at the risk of repeating myself: this rule of thumb is based on distribution issues and room-to-room temperature differences, not on limitations to the heating or cooling capacity of the equipment.

8. Leave bedroom doors open during the day

If you want to heat your house with a ductless minisplit located in a living room or hallway, you’ll need to leave your bedroom doors open during the day. When the bedroom doors are closed at night, bedroom temperatures may drop 5 F° between bedtime and morning.

If family members don’t want to abide by this approach, or don’t want to accept occasional low bedroom temperatures during the winter, then supplemental electric resistance heaters should be installed in the bedrooms.

9. Ductless minisplit units should run for 24 hours per day

Turning your ductless minisplit on and off, or controlling it with a setback timer, will result in higher rather than lower energy bills. These units are designed to modulate, and operate at higher efficiencies when they run continuously than when they are turned off and on.

Ueno and Loomis wrote, “One homeowner complained of temperature unevenness; when the data were examined, it was clear that they operated their MSHP [minisplit heat pump] in an ‘on-off’ manner, rather than using a fixed setpoint. This resulted in wide swings in interior temperature (between 60°F and 70°F+). The electricity use showed many hours with the MSHP running at maximum capacity, followed by periods with the unit shut off. When operated in this manner, the MSHP is heating at its least efficient (maximum output) state. Electricity consumption was a high consumption outlier; when compared with simulations, it was the worst-performing house (heating [energy] use 57% higher than simulation).”

Martin Holladay’s previous blog: “Simple Methods for Measuring Air Flow.”

Click here to follow Martin Holladay on Twitter.


Tags:

Image Credits:

  1. Alex Wilson

1.
Jan 30, 2015 3:10 PM ET

air transfer grill
by Scott Tenney

I've read about air transfer grills with a low wattage fan that kicks in when there are temperature differentials between rooms. Does anyone have experience with these? Or, is there any research re how effective these are?

I'm wondering if this might be an alternative to supplemental electric resistance heaters in the bedrooms - that might also be effective in the cooling season.


2.
Jan 30, 2015 3:31 PM ET

Response to Scott Tenney
by Martin Holladay

Scott,
There are two main problems with your proposal. The first is the specific heat of air, which is 0.0182 Btu/cf/°F. That means that you need a high volume of air flow to make much of a difference.

The second problem is a low delta-T. If your bedroom is 60°F, you may want to heat it up. But the adjacent living room is probably at 70°F, so you only have a 10 F° delta-T to work with. (That's a much smaller delta-T than you get when you move air from the warm air plenum of a furnace.)

When you do the calculations, you have to move large volumes of air to raise the temperature in the bedroom. Such a fan has its own drawbacks -- noise, for one, and the problem of drafts. It's simpler just to install a small electric-resistance heater.


3.
Jan 30, 2015 5:20 PM ET

Heat pump oversizing
by Antonio Oliver

Martin,
I'd heard about oversizing heat pumps before to increase efficiency. But I still don't quite understand why it works. Is it that compressors use less energy per cycle when they run at a slower speed? And if that is the case, does this apply only to a unit that is equipped with an inverter. Or should one expect a similar result for a conventional heat pump capable of running in a lower of two or three stages?


4.
Jan 30, 2015 8:43 PM ET

Martin, are you sure that the
by Eric Habegger

Martin, are you sure that the specific heat of air enters into the problem or at least in the way you explained it? My understanding, which is imperfect, is that a home with a given interior volume, outside air temperature, and existing level of insulation and air sealing whatever that is, will need a certain amount of BTUs. A heat pump will supply that heat, mostly in the form of warm air rather than radiant heat. If you have 2 bedrooms that aren't getting warm but your minisplit is putting out 10000 BTUS and heating the living/kitchen area to a comfortable 70 degrees then that specific heat of air is already included in that 10000 BTUs.

Obviously to heat the bedrooms you will need more BTUs coming from the minisplit to supply them and the main living area. So if you increase the output to 15000BTUs from the single minisplit and use the in- wall fans, assuming your whole house requires 15000BTUs to heat, then I don't see why .the specific heat of air would enter into it. In other words, the specific heat of air is already included in the total of 15000 BTUs required for the whole house.

It seems to me, and I could be wrong, that the in- wall fans would really help distribute heat. You can't ding them for poor energy efficiency when you would have to count the extra BTUs for heating the bedrooms that would always be required. It seems like its double counting.


5.
Jan 30, 2015 8:56 PM ET

Mini-Split Oversizing Causing Short-Cycling
by Jerome Lisuzzo

Hi Martin - Thought I'd offer up a personal experience pertaining to mini-split sizing and short-cycling. Bear with me on some background...Over the past year-and-a-half, I've been finishing a Passive House in southeast PA (certification is currently in-process). My wife and I moved in last October. The house is two floors @ 1,000 sqft/floor plus a 1,000 unheated basement. We've tested .3 ACH @ 50 Pascals, and our Passive House heat load calculation is about 8,500btu/hr. The house is heated with two single-source Mitsubishi 12,000btu mini-splits; one on the first floor and the other on the second. I had two units installed precisely for the reason stated in your post; concern about summer cooling.

In December I installed an energy monitor on all the main circuits, and one of the first things I noticed was that the heat pumps were short-cycling most of the time; often at three to five minute intervals. This occurred whether I was running both units or only the first floor unit (I've attached a screen shot showing one example).

Mitsubishi concluded that the problem was occurring because the heat pumps are oversized, but added even the smallest mini-splits (9,000 btus) would have resulted in the same issue (because the lower limit on both the 12,000 btu and 9,000 btu units is essentially the same). The proposed solution, which appears to have largely solved the problem, was the installation of remote thermostats on both units.

I offer all this up to make several points. First, apparently (at least from my limited experience) oversizing may be a potential issue with mini-splits in exceptionally efficient homes. Second, the problem may not be clearly evident to occupants who aren't monitoring the actual electrical usage. And third, maybe a ducted mini-split is the better option when it comes to extremely efficient building envelopes, even though they are somewhat less efficient (as was suggested by Mitsubishi).

I haven't been able to find any other reporting of this short-cycling issue, and didn't see anything about it in the Ueno/Loomis report. It leaves me wondering if anyone else out there has experienced the same problem.


6.
Jan 31, 2015 4:58 AM ET

Edited Jan 31, 2015 5:01 AM ET.

Response to Eric Habegger (Comment #4)
by Martin Holladay

Eric,
The amount of heat that a fan can move is calculated using this formula:
Heating BTUs = (cfm of the fan) x (delta-T) x 1.08
Where does the 1.08 come from? It's the heating BTU multiplier at sea level, determined by multiplying the number of pounds of air per cubic foot times the specific heat of air times the number of minutes in an hour (60). Another way of expressing this: the factor (1.08) is the volumetric heat capacity of air (in BTU * minute / hour * cubic feet * °F).

We learn from this formula that the smaller the delta-T, the fewer BTUs the fan can move. The specific heat of air matters because a cubic foot of air at 70°F holds less heat than a cubic foot of water at 70°F -- which is why hydronic heating pipes have a smaller diameter than ducts.


7.
Jan 31, 2015 5:05 AM ET

Response to Jerome Lisuzzo (Comment #5)
by Martin Holladay

Jerome,
Your observation has been made by others; the solution to the problem you describe is, indeed, to install a remote thermostat.

For more information on this issue, see Mitsubishi minisplit behaving very differently with external thermostat vs. without.


8.
Jan 31, 2015 5:19 AM ET

Response to Antonio Oliver (Comment #3)
by Martin Holladay

Antonio,
The high efficiency of ductless minisplit units under part-load conditions is a direct result of the fact that these units are inverter-driven. Single-speed air-source heat pumps without inverters don't share this feature.

For more information on this issue, see Ductless Heat Pump Impact & Process Evaluation: Lab-Testing Report.

Quoting from that report: "When looking at the full results from the steady-state tests, the fully variable speed nature of the [inverter-driven ductless minisplit] equipment becomes apparent. The variety of compressor and fan speeds combine to offer a huge range of input powers and output capacities to meet the space conditioning load. A clear trend from the data shows that the higher the output capacity, the lower the efficiency. Equipment performance is maximized when the loads are small. This is true for both heating and cooling. Additionally, the ability of the equipment to run at a very low speed will greatly reduce the amount of time the equipment must cycle on and off in low load situations. This on-off cycling is a performance penalty for single speed heat pump systems but, in comparison, is largely avoided with the DHP [ductless heat pump] variable speed equipment."


9.
Jan 31, 2015 1:15 PM ET

oversizing
by Keith Gustafson

I could not see any advantage to oversizing a mini split.

First, since they are selected for design heat load, they are already oversized over 90 percent of the time. This is added to the fact that simple heat load calculations usually over size equipment, and pretty much every assumption a HVAC contractor is going to make is going to err on the side of 'bigger is better'

Second, in many cases in a well insulated house, they smallest available unit is already oversized

Third, Larger units have a lower SEER rating for the same series, IOW, a 9000 btu unit may have a 27SEER, while the 12000 btu unit of the same series will have a SEER of 22. So while the unit might run at a higher efficiency percentage, it is of a lower number, thus you are not really saving actual electricity.

Lastly, while my personal experience is mostly using them for AC, almost all of my complaints with comfort or setpoint control relate to [in my opinion] the units being oversized.

I think the small ducted minisplits are going to excel in being sole heat source for efficient houses.


10.
Feb 2, 2015 3:37 PM ET

Load Calculations
by John Semmelhack

Martin - Perhaps, in a better world, this would go without saying....but folks ought to be reminded to get an accurate heating/cooling load calculation before considering their equipment selection. Anyone who peruses the GBA Q+A forum will notice that it's quite common to see questions about equipment sizing for a particular application in the absence of a load calc. The question from the "first responders" (typically Martin or Dana Dorsett) is always: "What are your heating/cooling loads?"


11.
Feb 2, 2015 3:44 PM ET

A caveat to #9...
by John Semmelhack

...as always, rules of thumb come with caveats...here's one for #9:

For homes with reasonably strong solar gains, and/or for any well insulated home on a mild winter day, it can be pretty easy to exceed the setpoint temperature (even with an external thermostat). In these cases, the mini-split will start to short cycle, adding small amounts of heat to the house at quite low efficiencies. For those willing to actively manage the device, there are some energy savings to be had (albeit small in most cases) by shutting it off until the sun goes down...


12.
Feb 2, 2015 4:03 PM ET

Response to John Semmelhack
by Martin Holladay

John,
You wrote, "Folks ought to be reminded to get an accurate heating/cooling load calculation before considering their equipment selection."

I agree. That's why I wrote (in Rule of Thumb #6): "Of course, it’s still important to perform a heating and cooling load calculation before specifying a minisplit."


13.
Feb 2, 2015 4:05 PM ET

Whoops...
by John Semmelhack

...I read too quickly, Martin.

Carry on with the fine work. ;-)


14.
Feb 11, 2015 2:16 AM ET

Ductless minisplit and ERV
by Alec Shalinsky

Hi Martin,
No mention was made to the use of a Ductless Minisplit in conjunction with a high efficiency ERV/HRV (like a Zehnder 350).
I presume there wouldn't be any conflict, but is there any evidence that the HRV would help evenly distribute the warm/cool air flows. You mentioned that ideally a heating unit should be used on the lower floor and a cooling unit on the upper. Wouldn't the HRV eliminate the need for 2 units?
Thanks
Alec


15.
Feb 11, 2015 9:27 AM ET

Edited Feb 11, 2015 9:28 AM ET.

Response to Alec Shalinsky
by Martin Holladay

Alec,
As you probably know, a ductless minisplit serves a different function from an HRV or an ERV. The ductless minisplit is used for heating and cooling, while an HRV or ERV is used for ventilation.

The air flow rates required for ventilation are low -- generally in the range of 50 cfm to 90 cfm, or perhaps 10 cfm to 20 cfm per room. These low rates of air flow aren't capable of transferring enough heat from one room to another to equalize indoor temperatures.

For more information on this issue, see Choosing HVAC Equipment for an Energy-Efficient Home. In that article, John Straube is quoted as saying, “Ventilation air doesn’t do much to move around heat. Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.”


16.
Feb 13, 2015 1:25 PM ET

Alec, I concur with
by Tom Bassett-Dilley

Alec, I concur with Martin--we thought your idea would work in a Passive House we did a few years ago (Chicago suburb, Climate Zone 5), but there just wasn't enough air flow to even out the temps. The other interesting thing we noticed in the summer is that, as the ERV runs, it dumps air just a little warmer and a little more humid into the bedrooms, so they get progressively stuffier; get a few kinds jumping around in that room, and they start complaining about the temperature. So yes, keep those bedroom doors open whenever possible, and watch the solar gain carefully for summer performance. Ultimately the owners chose to replace the Zehnder unit with a CERV to provide conditioning to the ventilation air-- it helped comfort considerably.


17.
Jan 10, 2016 1:38 PM ET

Edited Jan 10, 2016 1:39 PM ET.

Oversizing of MSHPs in Heating
by Kohta Ueno

Martin--great column as always; thanks for taking our research work and making sure that a wider audience gets to see it!

Unfortunately, we never got to study short cycling effects directly in our study--due to cost constraints, we set up our kWh meters to record 5-minute total power use, and record that. Also, we didn't do anything on recording output. James Williamson and Robb Aldrich at SWA did a nice study that I *think* you summarized earlier (http://apps1.eere.energy.gov/buildings/publications/pdfs/building_americ...). As a result, we couldn't directly measure short-cycling effects.

But I believe that Marc Rosenbaum and Robb Aldrich might have more useful information on this front.


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!