Mini-Split Heat Pumps are Quirky, Especially Capacity

The name "mini-splits" generally refers to heat pump systems with one outdoor unit and one or several smaller capacity air handlers. The efficiency of their operation comes via the inverter compressor and variable refrigerant flow technology, and they are available for single- and multi-family homes, as well as commercial buildings of any size. Here's a general overview of what the systems are and how they work, in case you're not familiar. Compared to conventional heating and cooling equipment, they're very quiet and very efficient. For most folks, ductless mini-splits are what they've heard of, but there is also a concealed ducted option that many of our clients choose to avoid the "wall wart".

Over the past several years, I have gotten to know mini-splits pretty well, and like the 2001 Land Rover Discovery II that we drove for 10 years, with it's backwards dashboard and odd noises, mini-splits can be rather quirky. Unlike the Discovery II, though, most of the quirkiness of the mini-splits is good, not just quirky! At this year's Building Science Summer Camp, Bruce Harley, Technical Director at Conservation Services Group, discussed his experience and the quirkiness of the combination ducted and ductless multi-split heat pump system installed in his Vermont home. Below are some of the key takeaways from his presentation with a little commentary from me:

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Key Takeaways from "The Quirkiness of Heat Pumps", a Presentation by Bruce Harley
(Click title to see entire presentation):

• MYTH BUSTER: "Air source heat pumps work in hot and cold climates. Most mini- and multi-split heat pumps are rated and have proven to deliver heating down to -20⁰ C / -5⁰ F, albeit at lower capacity." All of our homeowners from Maryland, Rhode Island, Toronto, Michigan, etc. have all reported back to us that they're 2-3 year old mini-split heat pumps are keeping them plenty warm in the winter. For the commissioning I did of a Providence, R.I. home we worked on (images below), I stayed in the home, itself. It was the dead of winter (17 degrees!). If it didn't work I would have suffered right along side him. No one suffered!

• ADVANTAGE VRF: "Efficiency of VRF heat pump is better than even the manufacturers tout or than they are rated." - B. Harley. Between the energy monitoring, homeowner testimonials, and review of our own energy bills, (we also have a ductless mini-split heat pump system in our condo) we find the systems are from 20-40% more efficient that promised. WooHoo!
• MYTH BUSTER: "Temperatures setbacks do not save" energy. - B. Harley. Think "set it and forget it" for most of the year, and "use only modest setbacks for when you're away." - B. Harley. We recently got our July 2014 electric bill, and compared to July 2013 when we experimented with setbacks as opposed to "setting it and forgetting it." We found, after factoring in a difference in weather, etc, a 25%-30% savings by not setting back temp.

July 2013 Electricity bill with set backs

July 2014 Electricity bill when we set it and forgot it

• ADVANTAGE VRF: "Efficiency in mild weather is highly dependent on user settings and direct solar gain, but neither impact annual cost or energy." - B. Harley
• QUIRKINESS: Most manufacturers have hand held remote controllers that are not necessarily intuitive. When Bruce's system is in "energy mode", temperature control is restricted to 2 degree increments. Some manufacturers have more user-friendly controllers (see image below) for these types of systems. Unfortunately, because of their unique way of communicating, NEST thermostats are not fully compatible with any mini-split equipment.

• ADVANTAGE VRF: VRF heat pumps, in the heating mode, consume less than half the energy of resistance heaters. Not a big surprise.

• QUIRKINESS: "Conventional concepts of "right-sizing" may not be relevant." - B. Harley. While conventional heat pump systems are selected based on the cooling load, VRF heat pumps can be selected the based on their heating capacity or cooling capcity, whichever is greater. In fact, it's highly recommended and practiced to avoid the need for auxiliary heat (e.g. resistance heat), which we see can be more than twice the cost. CAVEAT: I still recommend some back up heat. It may never be used, but a good safety net.

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Quirkiness of Mini-Split (VRF) Heat Pump Capacity

On the topic of the HSPF (Heating Seasonal Performance Factor, a.k.a. heating efficiency) of VRF heat pump systems, Bruce Harley said, "Be careful about what manufacturers specs you use" and "Adjust for climate using bin analysis for actual equipment and application". This is extremely important! Especially when it comes to equipment capacity. In fact, knowing the rated versus actual capacity of the outdoor unit (heat pump) and the individual air handlers is one of, if not the most critical design considerations with VRF heat pump system.

Capacities are considered "Rated" if their published via testing performed by a third-party and approved by the Air Conditioning, Heating and Refrigeration Institute, or AHRI. You can search capacity and efficiency of all "rated" equipment on their website: ahridirectory.org. AHRI regulates all 3rd-party testing for all types equipment based on standard rating conditions at full capacity.

As mentioned above, mini-split heat pump systems can have a single or multiple indoor units (as many as sixty four (64) ducted or ductless air handlers) connected to a single outdoor unit (heat pump), depending on building or home size, desired zoning, required heating and cooling, etc. Although equipment efficiency is better and costs can be slightly less to have one outdoor unit serving each indoor unit, it may be more practical to have a single outdoor unit with multiple indoor units connected. The more indoor units there are, the more it makes sense to use a single outdoor unit. Whatever the configuration, the actual capacity of each indoor and outdoor piece of equipment will vary. What factors change the actual capacity?

DESIGN CONDITIONS (Factors that effect system capacity)

1. Climate of building or home
2. Altitude of building or home
3. System configuration
4. Number of connected indoor fan coils
5. Length of line-sets (a.k.a. refrigerant lines) and number of 90 degree bends in each line set between the indoor and outdoor units.
6. Design and orientation of building or home
7. Location of outdoor unit (can be installed on the interior or exterior of building or home with proper ventilation)
8. Etc., etc. etc...

Below is a screen shot of the actual capacities based on design conditions (see "Factors" above), generated with a proprietary design software from Mitsubishi Electric Cooling and Heating, one of the leading manufacturers of VRF (mini-split) equipment. For simplicity, I'm only showing the first four indoor units, but there are 10 total. The project is a custom home on a small island near Great Exuma, Bahamas. Yes, it's hot and humid. All the time. Well, almost. Maybe one day of the year it drops below 80...to 79.9.

We decided to go with the CITY MULTI line from Mitsubishi, a commercial product that requires three-phase power, for this project, but everything I'm about to show applies to all VRF equipment. It doesn't matter if it's residential-type, commercial-type, any other manufacturer, or if the project is in Anchorage or Timbuktu.

In this configuration, I selected a 10-ton (120,000 btu/h) outdoor unit to serve all but the guest house. The model number and actual capacities at design conditions (see "Factors" above) for the outdoor unit are in the top left corner of this screen shot. (blue = total cooling, red = total heating). All of the indoor fan coils (air handlers) are concealed ducted, and their model numbers are directly above each image (to the right) of the air handler (starting with PEFY). The two numbers after the dash represent the nominal capacity (24 = 24,000 btu/h, or 2-ton), and to the right are actual capacities based on manufacturer testing at the design conditions.

If I move the house to Fargo, North Dakota, with design temperatures at 71 degrees F in Summer and -23 degrees F in Winter, here's what happens to the capacities.

The AHRI certificate below shows the rated capacities of the outdoor unit is 114,000 btu/h cooling and 129,000 heating at their standard rating conditions which is different that design conditions most of the time. If you look back to the first screen shot, the actual capacity is higher for heating and lower for cooling. That has a lot to do with the design temperatures of rated vs. actual.

It is normal to have actual capacities differ from rated capacities with all types of equipment, conventional or mini-split, but now let's see what happens when I change some of the other design conditions, like number of air handlers, line-set length and number of 90 degree bends in the lines.

In the screen shot above, I've added two more 8,000 btu/h air handlers to the Bahama configuration, still using a single outdoor unit. The outdoor unit (OU) capacity has increased by about 6,000 btu/h and the indoor unit (IU) capacities have decreased by as much as 1,500 btu/h. The reason the OU capacity goes up is that I did not have 100% connected capacity (nominal capacity of indoor units = nominal capacity of outdoor unit) in the original configuration. So, when there is less than 100% connected to VRF equipment, the capacity of the OU will match the total of the indoor units.

As for the indoor units, the more units that are connected to the OU, the more the capacity of the indoor unit decreases. How much it decreases also depends on the size of each added unit. This is fairly logical, since they all have to share the same OU, but it's critical to know so there's enough capacity to match the load of each zone.

The above screen shot shows what happens when I changed all the line-set lengths from the original configuration (10 air handlers) to fifty (50) feet from some of them being over 130 feet, and changing all of them to have only one bend. See how much the outdoor unit capacity goes up? From 99 kbtu/h to 110 kbtu/h. So, the closer the air handlers are and the fewer bends, the more capacity the system will have. You see, it's not just temperature that effects capacity.

You can also compare the actual capacities of the indoor units with their rated capacities in manufacturers published performance data. Here is an example of the PEFY-P24NMAU-E3, the first indoor unit (air handler) shown in the configurations above. It also states the rated conditions. See the difference, and see why it would be important to know the capacity?

Two Very Important Caveats

1. These actual capacities are at design conditions, and at full capacity, which means that it's the hottest or coldest day of the year, and all zones are calling for maximum heating or cooling. at lower (Summer) or higher (Winter) temperatures, which is most of the year, the system is NOT at full capacity. These are all the considerations that must be known when specifying equipment. The milder the weather, the less demand, the more capacity available. The above results show worst case. Screen shot below shows what happens when I lower the summer design temp by 20 degrees F. Also, don't forget if some zones aren't calling, other zones will have access to more capacity.

1. Look at the cooling capacity (blue) PEFY-P06NMAU-E3 air handler. It's the fourth air handler from the top. Number on the left is total cooling, and in parenthesis on the right is sensible (ability to lower air temperature). The difference is the latent capacity (ability to remove moisture load), which is very important in climates with humidity. The Bahamas are in one of those climates. A LOT of humidity. In the case of this particular air handler, it will has very little capacity to remove moisture. If you look at the screen shot before I lowered the outdoor temp (see below), it shows no latent capacity. This is because they have very little to begin with as low capacity units, so at extreme conditions, it's going to be even lower. Whole house de-humidification is necessary in cases like this, no matter how tight the home.

"If I Had More Time, I Would Have Written a Shorter Letter" - Mark Twain

I know this is a long post, but it's an important topic that needs to be known. I hope it's clear and useful to you and your life. If you have any comments or questions, please post them below. I'd like good discussion. In the meantime, if you want help with figuring all this out for your own home or building, please contact us.