As I noted in my Hydronic Heating post, adding central A/C to an existing house that does not have a forced-air furnace can be a nightmare. Running duct work after the fact is expensive and cumbersome and moreover cuts into closet space or other living space. In most cases, it simply is impossible.And now, the stealth air-conditioner. A new generation of machines is nearly or totally invisible from the exterior of buildings and is being aggressively marketed this summer, mostly to people who live in apartment buildings where no window is available or where air-conditioning is not allowed by the landlord. Mounted on casters and advertised as ''windowless,'' these air-conditioners may, in fact, be hidden easily from landlords. But they also have drawbacks, say engineers, who much prefer conventional window models. The machines are sold in appliance stores in cities around the country for $700 to $900, substantially more than window models of the same cooling capacity. The important point for consumers to remember, engineers say, is that windowless air-conditioners, like all air-conditioners, create cold only by moving the heat somewhere else.

They employ electricity to drive a compressor, a device that takes a gas and puts it under pressure to turn it into a liquid and simultaneously squeezes out the heat. The heat is then released from the rear of the air-conditioner, and the coolant, now a liquid, is released from pressure and boils back into a gas. In that step, the coolant absorbs heat, thus producing cold air that is blown into a room. how to fix my air conditioner compressorBut there is a catch.ac unit for walk in cooler ''There has to be somewhere for the heat to go,'' said Dean R. Keuch, a senior engineer at Consolidated Edison. ac unit cleaning sprayIn addition to moving heat around, he said, an air-conditioner makes heat, because its electric motor and other parts get warm.

A Chill in the Air ''If you're not putting it out somewhere, obviously you're not doing yourself good, and maybe you're doing some bad,'' Mr. Keuch said. Conventional air-conditioners, he pointed out, can be set to chill air from the room, making it colder and colder on each pass and sending the heat out of the building. But a windowless model often does not segregate the heat it is rejecting from the air it takes in for cooling. So the heat stays in the room. The process is a little like trying to bail out a boat with a leaky bucket: some of the water gets handled more than once. The windowless air-conditioners vary in how they get rid of heat. Some come with hoses, which can be routed out a window or into the space above the acoustic ceiling tiles in an office or even into a windowless master bathroom to get rid of heat from the bedroom. ''If the heat stays there, what's the difference?'' said Mel Tillman, the owner of S&S Sound City, an appliance and electronics store in Manhattan.

''You go into the bathroom once, but you're in the sack eight hours.'' One model, the Cold Front, has an optional window vent, 2.5 inches high, that is almost invisible from the exterior. But it will also work with no vent at all, according to the manufacturer, Bionaire of Montreal. The proper use, said Juergen Puetter, company chairman, is to sit in the ''cold zone'' in front of the machine. ''As long as you have a door or window cracked open, the heat can ultimately get out,'' he said. The Bionaire can also remove humidity, he said, making the room more comfortable. The Bionaire is like a dehumidifier that suburbanites use in musty basements. In this case, a tank that holds just under a gallon of water must be emptied - on humid days, as often as every few hours. ''It is not a full replacement for a regular air-conditioner,'' Mr. Puetter said. ''But in many areas you cannot use a regular air conditoner.'' Another model, the Pinguino, built by DeLonghi, can be filled with water, which it uses to help remove heat.

It has an exhaust pipe that can be put out a window or into another room. Another type of windowless air-conditioner is meant to be connected to a water faucet, sending the heat into running water. This is extremely efficient in terms of electricity use, because water conducts heat far better than air does. But these air-conditioners use large amounts of water. For that reason they are illegal in many areas, including New York City. They are nonetheless popular. Windowless air-conditioners have been known to industry for years as spot coolers, the opposite of portable heaters. In a huge, hot factory, they are sometimes used to blow cool air where a worker stands. But they are far newer to the consumer market. Windowless models that are not cooled by water tend to be far less efficient than window models. The window air-conditioners commonly available, for instance, deliver almost 20 percent more cooling power than the Bionaire does for a given amount of electricity. Con Edison, for example, recommends that consumers buy air-conditioners with an energy efficiency ratio of 9.5 or higher;

the windowless models run less than 9. They are not required to list an energy efficiency ratio, and some do not. This inefficiency has two consequences. The first is higher electric bills. The other is that a windowless air-conditioner will challenge a fuse box more than a conventional air-conditioner of equal size will. So, like the stealth bomber, the windowless air-conditioner may not show up on radar. But particularly in a building with old wiring, its presence will eventually become obvious. Photo: Mel Tillman, at left, showing a windowless air-conditioner to a customer at his store in Manhattan (Steve Berman for The New York Times); The Cold Front windowless air-conditioner by Bionaire, which can also remove humidity from a room. It has an optional window vent, but the manufacturer says it will work with no vent at all.Your air conditioner, heat pump, or furnace probably uses a lot of energy. Heating and cooling makes up about half of the total energy use in a typical house.

For air conditioners and heat pumps using electricity generated in fossil-fuel fired power plants, the amount you use at home may be only a third of the total. A question I get asked frequently is whether or not it's OK to close vents in unused rooms to save money. The answer may surprise you. The photo above shows a typical vent for an ducted HVAC system (air conditioner, heat pump, or furnace). On the return side, you'll typically see plain grilles, but on the supply side, where the conditioned air gets blown back into the house, most HVAC contractors install registers like the one above. It has a lever of some sort that allows you to adjust the louvers behind the grille. You'd think that since it's adjustable, it must be OK to open or close it to suit your needs, right? The blower in your HVAC system is the heart of the air distribution. It pulls air from the house through the return ducts and then pushes it back into the house through the supply ducts. In high-efficiency systems, the blower is powered by an electronically commutated motor (ECM), which can adjust its speed to varying conditions.

The majority of blowers, however, are of the permanent split capacitor (PSC) type, which is not a variable speed motor. In either case, the system is designed for the blower to push against some maximum pressure difference. That number is typically 0.5 inches of water column (iwc). If the filter gets too dirty or the supply ducts are too restrictive, the blower pushes against a higher pressure. In the case of the ECM, a high pressure will cause the motor will ramp up in an attempt to maintain proper air flow. An ECM is much more efficient than a PSC motor under ideal conditions, but as it ramps up to work against higher pressure, you lose that efficiency. You still get the air flow (maybe), but it costs you more. The PSC motor, on the other hand, will keep spinning but at lower speeds as the pressure goes up. Thus, higher pressure means less air flow, and, as we’ll see below, low air flow can cause some serious problems. The important thing to remember here is that no matter which type of blower motor your HVAC system has, it's not a good thing when it has to push against a higher pressure.

In a well-designed system, the blower moves the air against a pressure that's no greater than the maximum specified by the manufacturer (typically 0.5 iwc). The ideal system also has low duct leakage. The typical system, however, is far from ideal. Although most systems are rated for 0.5 iwc, the National Comfort Institute, which has measured static pressure and air flow in a lot of systems, finds the typical system to be pushing against a static pressure of about 0.8 iwc. Now we're ready to address the question of closing vents. When you start closing vents in unused rooms, you make the duct system more restrictive. The pressure increases, and that means an ECM blower will ramp up to keep air flow up whereas a PSC blower will move less air. Most homes don't have sealed ducts either, so the higher pressure in the duct system will mean more duct leakage, as shown below. The more vents you close, the higher the pressure in the duct system goes. The ECM blower will use more and more energy as you do so.

The PSC blower will work less but not move as much conditioned air. In both cases, the duct leakage will increase further. In addition to moving air, your air conditioner, heat pump, or furnace is also cooling or heating that air that flows through the system. The air passes over a coil or heat exchanger and either gives up heat or picks up heat. In a fixed-capacity system—and most are—the amount of heat the coil or heat exchanger is capable of absorbing or giving up is fixed. When the air flow goes down, less heat exchange happens with the air. As a result, the temperature of the coil or heat exchanger changes. If air flow is low, it'll dump less heat into the coil in summer, and the coil will get colder. If there's water vapor in the air, the condensation on the coil may start freezing. You might even end up with a block of ice, as shown in the photo below. And ice on the coil is really bad for air flow. It's also bad for the compressor as not all of the refrigerant evaporates and liquid refrigerant makes its way back to the compressor.

If you want to have to buy a new compressor, this is a good way to do it. Same thing if you have low air flow over a heat pump coil in winter. You could get a really hot coil, high refrigerant pressure, and a blown compressor or refrigerant leaks. Similarly, low air flow in a furnace can get the heat exchanger hot enough to cause cracks. Those cracks, then, allow exhaust gases to mix with your conditioned air. When that happens, your duct system can become a poison distribution system as it could be sending carbon monoxide into your home. Let me now summarize the problems I've described above that can result from closing vents in your home. The first thing that happens is the air pressure in the duct system increases, which may give rise to these negative consequences: You're not guaranteed to get all the problems that apply to your system, but why take the chance. I recently wrote about all the IT folks who are trying to follow in Nest's footsteps and profit from the home energy efficiency movement.

I used the Aros smart window air conditioner as the example of companies that think you can solve problems just by creating a product with a smartphone app. Well, meet a more malignant idea: the E-vent. (You can find it easily enough by searching on the term "Kickstarter E-Vent.") It's just a Kickstarter project right now, and maybe it won't get funded. If it does get funded, however, it will be subject to all the problems I described above. It doesn't matter whether you close the vents by getting up on a ladder in your home or from the beach in Cozumel. It's still a bad idea. The E-Vent page on Kickstarter says they monitor the air temperature and open vents if the temperature gets too cold while air conditioning or too hot while heating. Of course, that’s not going to work unless they monitor the temperature right at the coil or heat exchanger. And that still probably wouldn’t work because there’s a wide range of acceptable temperatures for different systems. This is an HVAC product developed by people who don't know some very important principles of heating and air conditioning.

Let's hope they don't kill anyone. The fundamental problem here is that closing supply vents in your HVAC system changes what comes out in particular locations. It doesn't change what the blower is trying to do. Nor does it change the amount of heat the air conditioner, heat pump, or furnace is trying to move or produce. It's possible you may be fine closing a vent or two in your home, but it will depend on how restrictive and leaky your duct system is. If it’s a typical duct system with 60% higher static pressure than the maximum specified, closing even one vent could send it over the edge. If it’s a well designed system with low static pressure and sealed ducts, you shouldn’t have a problem as long as you don’t try to close too many. The only way something like this could work is if closing a vent signaled the blower to move less air and the air conditioner, heat pump, or furnace to move or produce less heat. (Properly designed zoned duct systems do this by using variable speed ECM blowers with multi-stage systems.)