I bumped this back up to top of the list since it’s one of the most popular posts I’ve ever done. I also just had the misfortune of losing ALL THREE indoor units during a recent storm and power surge that killed appliances all around my development. Argh! However, they’re up and running again, good as new.
I love these heat pumps! There’s one in the bedroom, one in the basement “party room” and one in the large, living room that’s full of windows – a space that has been uncomfortable for years.
After installing these systems, I don’t know how we dealt without them before. The summer comfort is waaay better than ever. And in our basement room, I turned off the main heating system and used the Fujitsu exclusively all winter.
Keep in mind that this is one special little unit. There are many mini-split systems on the market that look like this, but most of them are barely half as efficient as this one. They’re just not in the same league.
Comments on performance
In the video, I make some bold comments comparing the efficiency of these units with those of a geothermal heat pump. Is this just idle speculation based upon paper specs? NO! I’ve made more measurements on actual HVAC systems than pretty much any HVAC technician out there. I have years of operational data for my own geothermal system and these Fujitsu heat pumps and I can definitively say that, on the whole, the Fujitsu units are as efficient as the geothermal. Why? a couple of reasons…
- A mini-split has no ducts and hence no duct losses
- Variable speed compressor and blower operation
- Smaller, more efficient blower
- Much less fluid to move around
Let’s look at each of these points in more detail.
A mini-split has no ducts and hence no duct losses
It takes energy to move air through ducts, much more than most people think. Depending upon the type of blower used, the blower in a typical, centrally ducted system uses 10%-20% as much as the compressor itself because it’s pushing air through hundreds of feet of ducts. The blower in the mini-split also uses energy, but much less because it isn’t having to send air all over the house – the air goes straight from the blower into your room.
This isn’t factored into the efficiency rating of a geothermal system, so right from the get-go, the geothermal loses about 10%-20% efficiency compared with the mini-split.
In addition, there are real duct losses. Every duct system I’ve seen has leaks. I spent a lot of time sealing my own system and there are still areas I can’t reach where I know there are leaks. And I’ve seen hundreds of duct systems and they all leak, some horribly. Studies have shown typical duct leaks may reach 30% on average. But let’s be generous and say that geothermal systems are much better than average and only lose 15% of their efficiency. That’s still 15% loss compared to the mini-split that has 0% duct losses.
So right off the bat, we’ve lost something like 25%-35% of the rated efficiency of the geothermal system compared with the Fujitsu mini-split.
Variable speed operation
Most modern HVAC systems have variable speed blowers, but almost none, except for mini-splits, have variable speed compressors, and that’s where you really need it. High end heat pumps/geothermals have two speed compressors, and that helps quite a bit to improve the system efficiency and evenness of heat/cooling delivery.
The inverter-drive mini-split heat pump has variable speed compressors and blowers, allowing it to carefully modulate the output. It’s not infinitely variable (based upon my measurements), but has a handful of speeds. What makes it really efficient is that when the weather is mild, it can really slow things down. So slow that you can’t even hear the system operating. Most of the time it operates in a middle zone. And only under extreme conditions does it have to ramp up to max.
Even geothermals with two stage compressors do not run very slowly/low power. The “low” stage is still about 2/3 of full output. This means in climates like mine where air conditioning is much less demanding than heating, even on the low speed, the air conditioning from a geothermal is less than optimal because the output is so cold that the system only runs for short periods and does a poor job at dehumidification. Some systems with humidity sensing thermostats and controllers do a much better job, but in general, in heating dominated climates, geothermals do a poor job at air conditioning. Trust me, I’ve lived with this for years.
The mini-split does an amazing job at air conditioning because it can turn down to such a low flow that it very effectively sucks the humidity out of the air during the summer. Even when you have those muggy, 75 degree days, the mini-split is still useful, while a regular central air conditioner is next to worthless.
The geothermal people never talk about this when discussing efficiency, but it is a big deal. When you have thousands of feet of pipe in the ground, you need pumps to move fluid through those pipes. That takes a circulator pump that uses energy. So, even though the geothermal system is intrinsically more efficiently operating, the circulator is adding another 250-500 watts to the system’s power needs. So start with a compressor requiring 4,000 watts, add in 350 watts of circulator pump and 600 watts of blower and now the system is drawing about 5,000 watts. That’s 25% more energy required. To be fair, the blower in the mini split takes energy too, and you need several mini-splits to cover the area that you do with a single central air handler. But based on my measurements, the mini splits are using about half the energy to move the same air as the central system. So let’s say that overall, the geothermal is losing about 10% of it’s efficiency here compared with the mini-split.
There are other types of geothermal systems. Some use water wells and pumps instead of lots of tubing. But those well pumps are even more energy hungry. It’s very typical for those to draw anywhere from 500 watts to 1,500 watts. So, on average, those are no better.
Final thoughts on geothermals
I’m a realist. I am not condemning geothermal systems. In fact, in spite of the real issues I’ve raised here, they are still greatly preferred over furnaces and conventional heat pumps in some applications. There’s no other central heating system that works in so many climates and does it so efficiently. If I were building a large new home, I would most likely install a geothermal system.
But if I was building an addition, or supplementing the heating/air conditioning for an existing home, it’s a no brainer, I’d get the Fujitsu 12RLS. Not only in theory, but in practice, since I installed three in my own home. I definitely put my money where my mouth is.
The realities of air-source heat pumps
No matter how you slice it, the Fujitsu mini-split is an air-source heat pump. What that means is that, like other non-ground-source heat pumps, it extracts the heat from the air. So if it’s 10F outside, it’s trying to extract energy from that air. The basic physics of this is that the colder it is, the less energy there is to extract. There is also the reality of “defrost cycles” – times where the heat pump must run in reverse, actually putting out cold air in the house while it heats up the outside coils to melt ice. These two limitations are what prevent air-source heat pumps from being used in cold climates and what make ground source (geothermal) heat pumps so much more useful.
As noted, I live in eastern Pennsylvania. The climate here is moderate. Not too hot, not too cold. We get our stretches in the teens during January and February, and those times push the limits of air-source heat pumps. But for the most part, the temperatures are in the 20’s and 30’s during the winter – temperatures at which the air-source heat pumps work quite well. In fact, something like 80% of our winter temperature is above 30F.
But if you live in colder climates, you may find that conventional heat pumps aren’t for you. You might still use it for air conditioning and heating in mild weather, but when you have stretches of zero degree F temperatures, you’ll be hating heat pumps, so you absolutely need a ground source or fossil fuel system.
So choose wisely. I don’t want to be receiving hate mail from Minnesota or Upstate New York when you’re freezing in the dead of winter!
The U.S. Department of Energy, through their Building America Program, published this NREL Fujitsu12RLS report on the Fujitsu 12RLS and Mitsubishi FE12NA mini-split heat pumps. (Original source: http://www.nrel.gov/docs/fy11osti/52175.pdf)
Seeing the actual laboratory figures, I’m even more impressed than I was before. I encourage you to check out the report. In particular, the measurements show that the Fujitsu 12RLS is capable of putting out its rated 12,000 BTU/h all the way down to zero degrees F! From there, it gets better. At 30F, it puts out about 18,000 BTU/h.
You might be wondering, “what about the efficiency?” When the system is cranking away at full blast at zero degrees F, it has a COP of 2.0. This is pretty respectable because that’s still twice as much heat for your dollar as a conventional space heater. This increases to a COP of around 3 for most temperatures from around 15F to 30F. But, get this, when the system is purring away at mild temperatures from 35F up to 55F, the COP ranges from 5 to 6 (when running at low fan speed). This is absolutely phenomenal!
After reading this report, I’m much more optimistic about my recommendations for the unit in colder climates. If you can count on this type of performance down to zero degrees F, there’s little reason not to use the Fujitsu in most moderately cold climates we encounter in the U.S.