Wednesday, May 4, 2016

Mini Split System HVAC

Do split systems make economic sense for general HVAC applications?  In most cases, no.  They are a specialized system that works well for specialized applications.

A reader writes, after reading my posting about hydronic heating, asking whether it would make sense to build a new house with resistive electric radiant heat in the floors (for emergency heat) and a number of split-system HVAC heat pump units for regular heating and cooling.

Such a system can work, but it is very expensive to purchase, install, maintain and run.  And the perceived benefits, in my mind, are fleeting.   And I have had a lot of experience with these systems, having owned them, worked on them in the lab, and having installed them as well.   They have particular specialized applications, but beyond that, a more convention system is far cheaper to install and own.

What is a split system or "mini split system" HVAC unit?   Well, most air conditioners are split into an inside and outside unit, so the term "split system" is a little broad.   A window air conditioner is an example of an un-split system, with both "sides" of the HVAC unit in the same box.   Our Japanese friends took this design and cut it in half, basically, making an inside portion that hangs on a wall and and outside portion that can be mounted on a slab or platform.

Here are two of the three Hitachi grey-market split systems (outdoor portion) that I installed in our New York home.  We had a total of four of these systems!  Note the refrigerant pipes going up the side of the house - I covered these with some used gutter downspouts to hide them.  The manufacturers make covers for these, but they are kind of ugly.  Putting the pipes in the wall is problematic from a service standpoint.

From a technical standpoint, mini split systems are great.  I tested one in the labs at the large air conditioning company in 1983 and we had an option to sell the units under license.  The suits at the company came in and looked it over, and said, "Americans like window units!" and took a pass.  True story.  (They changed their mind later on and  now carry them, of course).

They were much nicer than window units, as befits the Japanese sense of design aesthetic.  They were quiet and very well made - with a lot of attention to detail.  And at the time, they had a revolutionary infrared remote control - very cool (literally).

Mini-split systems are not, as my superiors had thought, a substitute for window units.  In Japan in the small houses they have there, they work well for cooling and heating in their relatively mild climate.

In the USA they are particularly useful for remodeling older historic properties or adding A/C to houses without central air systems.    Drill two holes in the wall, hang the inside unit like a picture, and then run the pipes outside (it is a little more complicated than that, of course).  But you can add A/C to a house without ductwork.

Our house in New York had no central A/C (!!!) and it was a hothouse.   So the split systems were really the only option and I was able to do them cheaply because of the deal I got.   The alternative would have been to tear out closets and walls and ceilings and run duct work or (ugh!) use window units, which would have been hard with casement windows.

So they have really specific applications where they work well because they are the only option.  If you already have ductwork, though, it is a lot easier to go with a central system heat pump or central A/C.  If you are looking at new construction, a central system is also cheaper to install by a large factor.

The house in New York came with one Mitubishi mini-split system unit in the main room (added after construction when the owners realized their mistake), but that was not enough to cool the other rooms of the house.  I added three grey-market Hitachis  (instructions in Japanese only!) and they worked well.  But since I got a "deal" on them, they were cheap - an astounding $600 apiece.   Most today are in the thousands, with installation.

(I also added two portable A/C units in the top floor.  They are not efficient, but the rooms were rarely used - so the trade-off of installed cost versus operating cost came into play.   Adding split systems to the top floor was problematic because of the long runs for the refrigerant lines and new electric lines.   The lower levels were much easier.  Since the top floor bedrooms were guest rooms, it made more sense to go with the portable A/C units for the few days a year we had guests AND needed air conditioning.)

The split systems are quiet and efficient, but as heat pumps, they only can heat the house when the outside temperature is above about 40 degF.  So you have to have another source of "emergency heat" to supply the house when the temperature goes below 40 degrees.

A traditional whole-house heat pump has a "strip heat" (resistive coil) that can kick in when the temperature is too low for the heat pump to work.  It works, but is horrifically inefficient.  In the South where the weather is rarely cold, this solution works OK.   In more northern climes, you may want to have a "dual fuel" system where you can use a gas furnace (as part of the heat pump installation) to supply the supplemental heat when it is too cold for the heat pump to operate.   This "dual fuel" system was what we had in our home near Mt. Vernon, and it worked very well.   It was cheap and easy to operate, and when the time came to replace the furnace portion (a vintage Chrysler AirTemp) it cost only a few thousand to replace.

The problem, from a cost perspective, is that mini-split systems (as sold today in the USA) are so expensive.  Even one system can cost as much as a whole-house central HVAC heat pump (which has built-in emergency resistive heat).  I am not sure why this is, but some contractors have told me stories about price-fixing.   The units do all tend to look the same, and indeed, regardless of "brand" you may be buying the same unit as another brand.    I had contractors quote me $3000 to $5000 a unit to install, which is kind of ridiculous for what is a glorified window unit.

I thought about replacing my existing heat pump here in Georgia with split systems, as they are quieter, allow for zoning and whatnot (although it means having a big ugly box on the wall in every room I want to heat/cool).  But sadly, since I don't have access to those grey-market Hitachis (that was sort of a one-time under-the-table kind of deal, with no warranty, of course) I would have to pay over $10,000 (or more like $20,000) to set up the whole house with split systems.   A new Heat pump would be maybe half that.

And then there is the problem of emergency heat if I went split system.  I would have to keep my existing air handler and resistive coil system (and ductwork) to heat the house if it went below 40 degrees, which it does here in Georgia on occasion.

If money is no object, it is not a bad idea.   But the extra cash to go that route would pay my utility bills in their entirety for about eight years at least.   And the payback in increased efficiency would be negligible - over the life of the units I would not likely recover any savings due to reduced energy consumption, if indeed there were any at all.

And I am looking at retirement, so it is not an option for me.  I want to spend less, not more.

A contractor suggested to the reader that they install resistive heat in the floor.  Basically this is a lot of heat cable running through the floor of the house to provide radiant heat.  It is like hydronic radiant heat without the water, but it is horrifically inefficient, as all resistive heat is.

We had electric radiant heat in the basement of our office building at 917 Duke Street in Old Town Alexandria.  There were three problems with it.

First, electric heat is the most inefficient heating source available - you are taking a KW of electricity and converting it into a KW of heat (unlike a heat pump, which takes a KW of electricity and uses to pump 2 KW of heat into your home).

Second is the hysteresis effect.  It takes a long time to heat up the floor and by the middle of the day, the room is overheated.   In the cold North where it is cold all day, this might work out OK except during the spring and fall.  In Georgia where it is rarely cold for long periods of time, you might find yourself over-heating the house and wasting a lot of money on electricity.
Third, right before we bought the property a pipe burst and flooded the basement.  It cleaned up OK (they had rose slate flooring) but it shorted out the electric radiant heat and it never worked again.   Jackhammering up the concrete (and all that lovely rose slate) didn't seem like it would be worthwhile, particularly in Virginia where cooling, not heating, was the main issue.  We put some space heaters in the basement and it was fine for the few cold months we had there (The basement was ducted for HVAC, but since there were open stairwells, the heat tended to rise, making the basement feel cold in colder weather).

So resistive radiant heat has its own set of issues.   I am not sure that the combination of these two expensive systems would result in a cost-effective HVAC system.  When you add up the cost of all those individual split systems, plus the cost of electric radiant heat, you end up with an installation bill that is 2x to 4x higher than a basic heat pump forced air system.   In order to recover those costs, it would take years - if ever.   The ultimate operating costs might be a wash or even higher when you have multiple expensive systems to repair.

We didn't have many problems with the split systems, of course.  But since we had four systems, that was 4x the chance of failure.  The Mitsubishi system that came with the house ("Mr. Slim") worked well even though it was about 10 years old.  The cylindrical fan on the inside unit did shatter in places and I was lucky to be able to find the parts diagram online, google the part number, and find a replacement fan for fairly cheap and install it myself (once the blades broke, the fan would wobble at high speeds and make noise).  So I can't say these are maintenance-free units.  And since you didn't work as an A/C tech, maybe this level of maintenance is not your bailiwick.

One argument made in favor of split systems is zoning.   You can shut off the units to rooms not in use and thus save energy and money.   That sort of works, of course, but it is the same argument made about electric fireplaces - you only heat part of the house.  This does not mean the underlying method of heating is more efficient, of course.  And in fact, with electric fireplaces, just the opposite (resistive heat, again, literally burning electricity to make heat).

You can just close ducts (registers) in your home to achieve the same effect with a central HVAC system.  So I am not sure the zoning thing is really a selling point - or one to justify a 2x-4x price delta.

So why do people use them?   Well, if you have an historic house that you cannot cut up to put in ductwork, they make sense.  If you have a house with no central air (as I had) they might make sense (particularly if you get them grey-market for 1/4 retail and install them yourself, again not recommended for the unhandy).  If you have an addition or converted garage (as my neighbor has) with no ductwork installed, a split system can be an easy way to add A/C.

But absent these compelling reasons, the split-system makes less and less sense.   Today, many contractors are pushing these as a high-end HVAC system, much like the built-in refrigerators and restaurant-grade stoves (or their lookalikes).   The idea is selling status and luxury.   And many people are buying them, convinced that "more expensive is better" or at least is more impressive to show off to friends and neighbors.

It is like the folks who ride a bicycle once or twice a year and go down to the boutique bike shop and buy an expensive racing bike costing thousands of dollars.  It is a "better bike"?  Yes, for certain applications.  When Mom puts her flowered basket on the front of the handlebars, however, and uses it to ride to the corner store, it is kind of ridiculous.   Like delivering pizzas in a Ferrari.

There is such a thing as appropriate technology.

Would such a convoluted HVAC system "add value" to the house?   Maybe, maybe not.  Generally, the value of homes is determined by the location and number of bedrooms, period.  You can fancy up your house all you want to, but if it is in a bad neighborhood it will not be worth much.  Similarly, a basic house in a good neighborhood is going to be worth about what the fancy houses are, particularly if it has more bedrooms.   You can always add fancy.   But for every dollar you spend on fancy, you would be lucky to get back 50 cents in added retail value.

Expensive HVAC systems are like any other expensive home appliance - you don't necessarily get more for your money, other than bragging rights.   And it is kind of hard to brag about an HVAC system, unlike say, a fancy stove or whatever.

And quite frankly, having a big box hanging off the wall in every bedroom of the house is kind of ugly.  Oh, and be sure to change the air filters on all of those units on a monthly basis, right?

Sometimes less is more!

EDIT:  Prices for split systems have gone way up since I installed mine.  A basic unit these days is $1000 online, and that does not include installation.  This is a cooling only unit:

Larger units are $2000 and up:

Again, not including installation.   So to cool your basic house, with a unit in each room (some units have more than one "head" of course) is going to run at least $10,000 or more, particularly if you have a multi-story house with long refrigerant runs.

They are lovely HVAC units.  The Lexus of air conditioning.   I drive a KIA and it works just as well.