To buy or not to buy a mini-split HVAC system – that is the question?

 

Question: My heating and cooling system is getting old and needs replacement. Currently I have a standard fossil fuel forced air based heating and cooling system, but have heard that new mini-split heat pump systems are a replacement option; what are the advantages and disadvantages of such a system?

 

Ductless mini-split heat pumps are ideally suited for compact, highly energy-efficient homes and where outside ambient temperatures are moderate. They also make good retrofit add-ons to houses with "non-ducted" heating systems  and a good choice for room additions where extending or installing distribution ductwork is difficult and expensive; if at all feasible.

 

They are designed to provide zoned comfort and enhanced energy-efficiency over traditional heat pumps and other fossil fuel forced-air systems.  They are based on air-source heat pump technology powered by electricity and identical to the air conditioner in your home or car, the only difference is they can reverse direction and heat in winter and cool in summer. 

 

Mini-split systems comprise an outdoor compressor unit and single or multiple indoor evaporator/blower units that mount on interior walls, in ceilings or free stand on floors.  Cables and refrigerant pipes have to be plumbed between the units.  A typical installed cost of a single ductless mini-split system is $3,000 to $5,000 or about $1,500-$2,000 per ton (12,000 Btu per hour) of capacity; although many variables affect the overall cost.

<< New text box >>

The reason we use air based heat pumps is that they are uniquely efficient, using one unit of electrical energy to extract multiple units of solar energy from outdoor air.  They are also a natural partner to photovoltaic solar systems in achieving zero-net-energy buildings.

 

As we saw with the dual fuel heat pump system in a previous article air based heat pumps do not work well at low ambient temperatures because the heat energy in the air is low and needs more work to extract.  More work means more source energy, lower efficiency and higher operating cost.

 

Most heat pumps use a scroll spiral compressor.  There are fixed single stage units two stage units where a valve controls the compressor pressure levels and a digital unit that varies the pressure by vertically separating the compressor plates.  All three combinations are purely mechanical, leaving the electrical drive motor running at its optimized natural utility mains voltage (220V) and frequency (60Hz).  For more information on scroll compressors go to: http://www.emersonclimate.com/en-us/Products/Compressors/Scroll_Compressors/

 

Mini-split systems use scroll compressors, but in this case they use an electronic inverter driven compressor.  The inverter synthesizes a variable frequency drive voltage to the compressors motor varying its speed from 30Hz to 120Hz. As the ambient temperature drops it speeds up and as the temperature rises it slows down.  The advantage of variable speed technology is that the energy efficiency can reach a higher Seasonal Energy Efficiency Ratio of 26 (SEER) and a higher Heating Seasonal Performance Factor of 13 (HSPF). 

 

Purdue University analyzed this configuration and determined that as the speed increased, increasing the output energy generated, the input electrical motor source power went up, the inverter efficiency and compressor efficiency went down and the Coefficient of Performance (COP) dropped for a total loss of 49%.  What does this mean – as the speed increases and compensates for temperatures below 40°F, the unit consumes twice the energy, which in turn doubles operating cost. The promotion is that these units work at lower temperatures, but there is a price to pay. 

 

The diagram below shows how the concept works.  Most heat pumps work well down to an ambient temperature of between 40 and 45°F.  For single speed units efficiency starts to drop below 45°F as shown in the diagram; however, the variable speed mini-split starts to increase its speed to compensate for the normal loss and increases its speed until it reaches its maximum level shown at 120 rpm at 5°F.  Below this temperature the variable speed unit also starts to lose efficiency.  The limitation is that as the speed increases the unit needs more source power to run the compressor and as the temperature drops the compressor extracts less heat energy from the air.

<< New text box >>

As an example, specifications below from a major mini-split manufacturer shows that at a constant 47°F temperature the higher compressor rotational speeds produce more output energy. It is important to know what your heating and cooling loads are so that you can size the system correctly; otherwise you may be running at full speed just based on leakage levels alone.

 

Rotational Speed

Input Power (W)

Cost of Utility Power per Hour

Output Generated Energy

Watts to BTU Conversion

20rpm

1KWh

12c

3KWh

10.2Kbtu/hr

60rpm (nominal)

3.5KWh

42c

11KWh

37.5Kbtu/hr

120rpm

7.5KWh

90c

21KWh

71.6Kbtu/hr

 

Now comparing the energy generated against a natural gas based alternative, where gas costs $1.13 a Therm.  The heat pump at 47°F ambient and nominal speed of 60rpm generates 37.5Kbtu/hr and costs $1.12 for an equivalent Therm, but at lower temperatures it becomes a lot less cost efficient.

 

Ambient Temperature

KW Generated

Energy Generated (BTU)

Cost of an Equivalent Therm

47°F (60rpm)

11KWh

37.5Kbtu/hr

$1.12

17°F (120rpm)

13.4KWh

45.7Kbtu/hr

$1.96

5°F (120rpm)

10.5KWh

35.8Kbtu/hr

$2.51

 

As shown even running at full-speed (120rpm) the cost to generate an equivalent Therm of output energy is much higher than the gas alternative.  Therefore mini-splits suffer the same issue as regular heat pumps in cold climates, the electricity cost outweighs most of the advantages and a backup solution is required for low temperatures just like the dual fuel solution.

 

Because traditional forced-air heating and cooling systems are custom and assembled on site from many parts, there are a number of ways for installers to make mistakes.  Researchers have repeatedly shown that a high percentage of residential forced-air systems have major problems, primarily with system sizing and duct systems that are poorly designed, poorly installed, and leaky.

 

The classic solution to these problems is to insist on testing the installed equipment to be sure that it performs in accordance with the manufacturer’s specifications. Unfortunately few installers actually do this — or even have the specialized equipment needed to do it.  Another issue is that neither builders nor home owners request test results because they themselves have no idea what’s required.  However, Energy Star now makes this testing mandatory; and therefore another reason to request Energy Star certification.

 

Energy Star testing includes:

 

  • Duct systems tested at 25 Pascal’s to verify that the ducts leak no more than 6 cfm per 100 square feet of the home’s conditioned floor area, and that duct leakage to the outdoors is no more than 4 cfm per 100 square feet of the home’s conditioned floor area.

 

  • The air flow of the supply duct system will need to be measured to verify that it is within 15% of the design air flow.

 

  • The air flow at each supply register will need to be measured to verify that it is within 20% of the design air flow.

 

  • The pressure difference between each bedroom and the common areas of the house (measured with the forced-air system operating and bedroom doors closed) must be measured to verify that it is no more than 3 Pascal’s.

 

  • If the home has central air conditioning, then the installer will need to verify that the sub-cooling deviation is no greater than ±3°F and the superheat deviation is no greater than ±5°F.

 

On the other hand Mini-split systems overcome some of the custom systems limitations because they are packaged systems that are ductless, and have factory installed integrated components and controls, all of which result in fewer opportunities for installer error.

 

Another advantage is their small size and flexibility for zoning or heating and cooling individual rooms. There is no combustion and no need for a chimney or vent. Each one of the zones has its own thermostat, so you only need to condition occupied space. Living space can be heated or cooled to the occupant’s preference without affecting other occupants and areas in your home. Because they don’t require ducts they avoid the 30% plus energy losses associated with poorly installed and insulated ductwork, and most indoor units have profiles of about seven inches deep and usually come with sleek, high-tech looking packaging.

 

However, some people do not like the appearance of the indoor units and since the indoor unit contains a fan they are noisier than a well designed duct system. If the homeowners let the filters get dirty, output can drop dramatically as the blowers have a lower static pressure capability than most central HVAC systems. There must also be a place to drain condensate water from each indoor unit.

 

As with any heating and cooling system correct sizing is important. First the installer has to establish the heating and cooling load of the home and then each zone, matching the mini-split internal air handlers to those specifications.  Oversized or incorrectly located air-handlers often result in short-cycling, which wastes energy, puts excessive wear and tear on the equipment and does not provide proper temperature or humidity control. In cooling, the fact that these systems are variable-speed and have such a wide range of operating points means that over sizing appears to have little consequence.  Oversized systems are also more expensive to buy and operate.

 

A single zone unit in the main space of a tight energy efficient home may suffice if you have an open plan layout or the doors to other rooms remain open; a possibility with common rooms, but unlikely with bedrooms. In a larger home or one that isn’t as well insulated, several ductless mini-split units would be required, and in many cases professional installers would also recommend either an electric–resistance heating unit or a fossil fuel heater as backup for colder nights or rapid heating.

 

Unlike central systems implementing temperature setbacks during the night or while away during the day doesn’t appear to save a lot of energy because mini-splits run at full speed when the temperature is set back up, which is a less efficient operating point? And because they aren’t usually oversized for the house load, it can take a long time to get the house back up to temperature; also if the thermostat is close too or even in the indoor unit, distant parts of the conditioned space may have large temperature differences and cold spots.

 

Another issue is that mini-splits systems do not have a way to recirculate and balance the air as a duct system does.  Central air based duct systems are balanced so that the return air capacity matches the supply air capacity preventing indoor air turbulence.  Mini-split systems have no return path they only suck in outside air which has nowhere to go unless outside doors or windows are open, which defeats the object of heating and cooling the home. This causes inside air pressurization that in turn causes drafts, door slamming, difficulty maintaining indoor temperature, inner wall condensation, lost energy, higher cost, and envelop damage through air being pushed through crakes and cavities to the outside.

 

A mini-split passes the outdoor air only once through its input filter, where a centrally ducted based systems recirculate the air from supply to return many times continuously filtering the air. The central systems recirculation process also removes humidity more efficiently.

 

Based on the above, adding mini-splits as a way to add or extend existing heating and cooling capacity and where ducting is not readily available makes a lot of sense.  They can be installed with relative ease, require little to no construction changes, look a lot better than window or wall units and provide a lot of flexibility. 

 

However, for new construction or upgrading an existing central system the advantages are not so clear.  If a duct system is in place and just the main unit needs replacing there are a lot of less expensive and less complex choices.  These include gas and other fossil fuel units, electric heat pumps and duel fuel systems where minimal structural changes are required.  However, before installation it would be advisable to have the duct system tested for leaks, sealed and insulated; a small price to pay for the benefits of a fully integrated central system.

 

Some Mini-Split Manufacturers:

 

Mitsubishi mini split 

Sanyo mini split 

Toshiba mini split 

Lennox mini split 

Gree Industries mini split 

Fujitsu mini split 

Ambiance mini split 

Panasonic mini split

Goodman mini split

American Standard mini split

Trane mini split

York mini split

Heil mini split

Bryant mini split

Coleman mini split

Frigidaire mini split

Tempstar mini split