Heat pumps: an introduction
Heat pumps there are of many types; applications, then, are the most diverse, being able to be used not only for heating the environment, but also for the production of hot water, or in industrial dryers, and in general in all situations which require heat at a temperature not very high.
But we have chosen to deal with the heat pump for domestic heating. In this part I have tried to give an overview targeted to belong to this category, and indicate strengths and weaknesses, as well as highlight, for each of the characteristics to be evaluated.
• Heat pumps and air conditioners. The first produce heat, while the latter cool. While heat pumps on the market are all reversible (ie can produce both hot and cold), air conditioners can only produce cold.
• source of primary energy. It is that we'll have to pay the bill. It may be interesting to know that, in addition to electrical appliances (the vast majority), are present also in Italy with natural gas appliances, which are beginning to make their timid appearance mall. Are devices with a high purchase cost, but of reduced exercise; must agree if air condition areas of greater size.
• Mode of operation. The heat pump, as we said, "suck" heat on one side and moves it to the other, however, is not at all certain that this heat must be removed from the air and used to heat more air, and even that needs to be exchanged with fluids of the same type. Thus there are also heat pumps which "aspire" heat from the water or soil, and can directly heat both the indoor air, is water which in turn is used by internal fan.
• Type of internal distribution. The heated air inside can be produced in a single point and then distributed, or be produced inside each individual room. In the first case we will have the large pipe to bring the hot air in the rooms, in the second of small pipes that carry air or fluid that feed of the heat exchangers (fan coils) which in turn heats or cools the air in the individual rooms.
Heat pumps gas
The heat pumps, to work, they need a source of mechanical energy for driving a compressor; this is normally supplied by electricity, but are already available some models that use the gas (methane). The York, for example, has already realized several years ago a heat pump driven by a Stirling engine to methane, which can heat and cool a large home; Triathlon is called. The cost is high, but can not change the electricity contract, and also the consumption of gas is negligible in practice, it is used as a boiler with a yield of 140% instead of the usual 80 to 90, which moreover can produce fresh, what obviously prohibited to a normal boiler. I've also seen other similar products from other manufacturers, but both are the same price that capacity (over 10 thousand Euro to 17 kW in cooling and heating in 28 - definitely needs a lot more than a single accommodation).
Pumps to 220 or 380 volts
If you opt for electric heating, consumption, not for tiny apartments, can easily exceed 3 kW, and in this case it is necessary to make a new electricity supply contract, giving the discounts offered for the users 'homey'. There is the option of paying a little more than an agreement facilitated by a contract so-called "heat pump", and there speak later seen that almost no installer can give you information about it.
However, in these cases should, if possible, jump over the classic supply 220 volt single-phase and three-phase a request directly to 380, if you compare the characteristics of all the devices, in fact, you will discover that the 380 volt versions, with the same all over the rest, consume 5-7% less, and the advantage grows as the power increases.
Heat pumps with auxiliary resistance
As already said, the power supplied from any heat pump drops as the outdoor temperature drops, ie just when it is needed most. Below a certain outdoor temperature, the pump "does not make it more"; some manufacturers (eg Daikin) do enter in function of the electrical resistances (placed in the indoor units) that provide the auxiliary heat, but of course they also make up fearfully bill. The power of these resistances is of the order of a few kilowatts; careful not to confuse them with the defrost heaters, which are used to heat a few external components, and that do not exceed 30/50 watts.
Type of compressor
The compressor is in fact the "heart" of the heat pump or air conditioner: its function is to transform the refrigerant from gas to liquid, thus allowing him to liberate heat that will be used where it is needed. Among the models "household" are normally heat pumps with compressor "scroll" or alternative. The name of the first (literally "roll") depends on the fact that it is realized in the form of a metallic sheet rolled up; normally scroll compressors are more efficient, but they have a lower power, and for this in greater power pumps are not normally employed.
External heat source
The majority (in Italy say almost all) of those in circulation are heat pumps that use the outside air as a heat source, for reasons of simplicity and cost. Unfortunately they are also the first to falter when it starts to get really cold outside, in the U.S., however, are not uncommon plants using as the source water or the earth. These in fact have a much more constant air temperature, in addition to the heat capacity much greater. For example, if you live in a mountainous area where winter is really cold, but next to you there is a pond or a stream of sufficient size, you can use the water to warm them: at the bottom will not drop ever below zero! Obviously you must not disturb too much the water temperature, adversely affect the life forms that live there, but with some quick calculation we can verify it.
If you have no water sources nearby, you can always pick up the heat from the ground ... is very simple: just dig a space large enough, at least half a meter deep (you need one square meter every 25 W), lie down in a special plastic tube, and cover it, then obviously you need a heat pump fits, which will subtract heat from the soil into the tube by circulating water and salt or antifreeze fluid. The land is a source of warmth and coolness perfect (do you remember a wine cellar?) During most of the winter the soil around the pipe will be ice cream, but this improves the thermal contact, while in summer the ground heated it loses moisture and becomes a poor conductor of heat, so this is a solution particularly suitable for areas with a cold climate.
In short, the solution exists for almost everything, and in Italy it is difficult to find a good supplier for a heat pump "normal", there is one specialized in geothermal installations.
The internal terminal
The "terminal" is nothing more than that object in the room that heats it. It can be a heater, an air vent, a fan, or anything else. Normally, independently from the external source, within the choice arises between two possibilities: a "battery", or a device that heats water, which in turn heats the circulating "radiators", or one or more terminals which directly heat the 'indoor air. If the external heat source is air, in the first case we speak of an "air-water", while in the second a system of "air-air" and similarly we can have implants "ground-water", "water- air "and so on.
Air-to-water
This type of system requires an external unit (in most cases a parallelepiped with a large internal fan, but it seems impossible that you have ever seen one), and a so-called "battery" internal which in practice has the same function of an individual boiler, with the only difference that it is able to produce even cold water.
Despite the apparent similarity, the water pipes for these plants are very different from those for heating only: in fact, the passage of the cold water through poorly insulated pipes causes the formation of moisture and condensation, with the result that within a short time, you would find a magnificent crop of mold on the walls at the pipes, while the floors begin to jump (it is said that all of this happen, but the risk is real). For this you need to replace all the pipes with other guaranteed to operate up to about 5 ° C without drawbacks and dispersions.
In addition, while the old radiators efficiently play the role of space heating (due to convection, ie the natural circulation of air caused by heat), they work very badly to cool them, and for this we need to replace them with the fan coil units (fan-coil ), ie the batteries equipped with a fan, normally controlled by a thermostat. To work, of course, the fan also needs a power supply, but more about that later.
The air-water systems are commonly considered the most refined that you can accomplish, of course the cost is the highest. But they are not entirely convinced they are worth what they cost. The Aermec has designed a system, called Idrosplit, which in practice consists in a battery for the air-water cooling only, which must necessarily be combined with a traditional boiler for heating and production of hot water. The idea, called in two words, is that in winter they warm up burning conventional fuel, and in summer there are refreshments pressing the battery to electrical operation, which consumes about 2 kW and thus can be used with the traditional electricity contract 3 kW of any home. With this little power you can cool those only two or three rooms where there are actually people. In this way it is possible to obtain the maximum power in the winter without increasing the power consumption, while in summer you can enjoy a discreet comfort without the need to increase the electricity supply contract.
But let me make a quick view on the economic aspect: in practice winter I have no savings, because I have always heated with gas, oil or whatever, in the summer I have the advantage of not only having to increase the contract electricity supply; few tens of dollars a month, compared with a cost, for a plant of this kind, of nearly 10 thousand euros for an apartment around 100 sqm. And if I have an existing plant can save just the boiler, having to throw radiators and pipes and redo everything. With much less than half of this figure can install a self-traditional (or even leave it, if it already exists), and in addition install air conditioners "split" in every room in which I need. I will have a more reliable system, which consumes less, and above all much cheaper. Would have been different if this system was reversible, that is able to function also as a heat pump, in this case I could, if the power had not been sufficient, preheat the water before heating with the conventional boiler, in which case I could go with the heat pump only in spring and autumn, or choose to warm up with the heat pump or gas, depending on the price of the two "fuel" ...
Certainly some advantage compared to traditional radiators there, thanks to the presence of the fan in the indoor unit, the room heating is done in a much more rapid, and additionally the presence of a thermostat improves the stability of the temperature, and therefore the comfort. But I do not think the game is worth the candle ...
The fan (or fan-coil) for air-to-water
"Accessories" indispensable in air-to-water, apart from the classic fan from the floor, ubiquitous in offices, which are installed in the same positions of the radiators, there are also models from the ceiling and recessed ceiling, right angle and so on, particularly suitable for housing, for some years models are available with remote control wall that look like a lot, for its size and price, the well-known "split". They have fans in 3, 4 or 5 speed, allowing for maximum speed heating / cooling, or the quietest, they are becoming increasingly popular models with infrared remote control.
The fan coils, as already said, for the connection pipes employ appropriate; addition to the two inlet pipes and outlet water will be one-third for the discharge of condensate, which invariably occurs where the air is cooled (in the summer so, inside, outside and in the winter). Finally, there should be a 220 volt power supply for the fan and the indoor electronic control, and an electrical connection between the fan coil unit and the outdoor unit (normally two wires).
The air-to-air: the channeled
In an alternative to water, it can, within the home, directly cool the air. If the cooling takes place at a single point, and then the cold air is distributed through penstocks in the various rooms, it creates a channeled system. The realization of this type of system is deceptively simple, especially in homes where there is a corridor onto which all rooms, if this can be false ceiling, just find a space for the internal exchanger and install a pipe to bring the ' air at air diffusion placed in each room. But things are not quite so simple.
The first problem that arises is that of the "recovery" of air: the system blows air into the rooms yes, but what of the one that was there before? If there is not an adequate intake system, the room goes in overpressure and begins to download hot air in those adjacent or even outside, with consequent difficulty of opening and closing of doors and windows, noises of drafts and / or increase immoderate consumption ...
In systems that you see in your office, then, the pipes of the air are two: the "flow" and the "recovery" of the air, the air conditioning is "blown" into the room, while the one already is sucked away , keeping the pressure balance; in large systems the intake air, which is hot but also flawed, is mixed with a regulated amount of external air (not less than 10%) before returning to the heater, and then again in the room, but normally in homes this mixing is not expected, since the parts are sufficient drafts and air leakage of the fixtures. Always in the home, where it is difficult to install a double pipe, normally shooting is done "in free air", ie directly through the environmental movement, because air can circulate from the central unit to the rooms and vice versa, it is therefore necessary for a journey the air. It is normally carried out through the doors between the rooms, which are drilled and fitted with a grill, or leaving a gap of a certain size below. If you install one of these systems, this is the first thing to keep in mind.
The second problem is the noise, if the air is pushed or sucked too hard, you create turbulence, which in practice are manifested as an annoying "whistling" in the background, to prevent this from happening, the pipes must always be large enough to bring the air flow provided at a sufficiently low speed. The air flow rate, in turn, depends not only on the power of the fan that moves the air, but also by the "loss of load", ie the size of the duct and its length, by the number of curves, from its shape , and the number of nozzles that feeds. In practice, to make this calculation is anything but simple, it would take a real designer, but normally does not fall in the "budget" typical for this type of installations. In most cases, therefore, it is aimed at a "praticone" that, with experience, is able to impinge on the system right, maybe a little 'oversized, without going crazy too calculations. But as it does, who is completely ignorant of the matter, to distinguish a true expert? Frankly I do not know, then you know that you are risking. But maybe, if you are reading these lines carefully, you will no longer matter so fast ...
A part of the increased complication of calculation, the system channeled presents undoubted advantages. For one thing, it is a plant with a good efficiency, since there is always balance between the heat outdoors and that absorbed from the indoor unit. We will see that this is instead a central problem for systems such as multi-split. In addition, if it is well designed, the channelized has a considerable noise. The fact of being able to put a large number of vents for release and suction of air allows a spare uniform at all points, providing excellent temperature uniformity, which is reached in a very short time. Also you can insert vents in smaller spaces (closets), so as to improve the exchange of air. On the other hand, you can not influence the bathrooms, and some problems can be caused even the kitchen, because all the smells spread quickly through the house (you must also think about this ...!).
If you do not want to switch on the system at the same time in the whole house, you can divide the system into two: from experience, agrees that a part acclimate the living area, the other the sleeping area, this saves turning on only the part that serves, differentiating maybe times, and sometimes also simplifies the installation of the pipes.
Usually the air vents are simple fixed grids, so that, by turning on the system, all rooms are heated equally. However, you can use the adjustable vents, which allow you to adjust the air flow, so the temperature, or even completely close the heating in a room, if not that their use always creates a problem in a house, as close a nozzle makes pouring more air on the other, and consequently increases the noise and imbalances of temperature. To avoid the latter there are also vents thermostated, which open and close in proportion to the temperature of the environment. They are a great invention, since they allow to compensate both the external temperature changes or opening a window, both the possible design errors, but unfortunately, at least when I did some checking on their cost (in 1996), this was prohibitive : 200/250 euro each!
The air-to-air: the "split" or "multi-split"
Large development has had in recent years especially this type of plant; the reason is easy to imagine: the installation is usually the less destructive of all, since it is sufficient to pierce the outer walls at the points of installation, without the need to lay long pipes or open traces on walls and floors. In fact, today the vast majority of air-conditioners is of this type, and begin to have a significant spread even heat pumps.
The splits are the direct descendants of the packaged conditioners, those that are mounted on a perch making a hole on the glass of the window, unlike these, the "split" are divided into two blocks, an internal and an external, connected other via two pipes (gas and liquid), hence the name. Needless to say, their efficiency is infinitely superior to that of their ancestors. The multi-split, however, with a single outdoor unit can feed more indoor units (up to five), also of different capacity; returns though very often the need to make tracks and install piping.
The multi-split systems have the advantage of being simple, robust, and also allow to air condition only the rooms actually used, you might think, therefore, that the split is the most efficient among those seen so far. This is only true if you do not use multi-split systems. If we examine the technical data of a multi-split "traditional", we will make a bizarre discovery: they consume more when you turn one indoor unit, and as you turn the other always consumes less! The problem, in fact, is that the external compressor, controlled by the thermostat inside, or is off or operating at maximum power. If there is only one indoor unit in operation, the heat transferred to the fluid is not entirely absorbed from the indoor unit, the compressor will then return to a warmer fluid which is further heated, and this is exactly as if we were heating the apartment 25 or 30 degrees and not just 20. In this situation, as we know, the efficiency of the heat pump decreases considerably.
Would then be concluded that should turn on the heating in all rooms, as well you save. However this is not true. In fact, although less efficiently, lighting a single unit of the heat produced by this is much greater, and therefore the time taken to heat is lower, the compressor is operated for shorter periods, and therefore the overall consumption will be lower. In other words, if two units of work we do with just one, the savings achieved will be 20 or 30% and not 50% that we would have expected. This becomes a big problem if the use of the environments is intermittent, especially if the multi-split must feed several indoor units.
To overcome this drawback, manufacturers have proposed different solutions; their level of quality, and consequently the improvement of performance, determines the price, so, once again, the facilities which enable the biggest savings are those that cost more .
The first solution is to employ two compressors and two separate fans in the same outdoor unit, it is, in effect, two separate plants in the same container, each of which manages one or two indoor units. In the case of operation in reduced mode, only the external compressor relative comes into operation; efficiency is therefore greater. An example of these devices are Henf models of Mitsubishi Heavy Industries. If you were to opt for a system of this kind, you connect to the same compressor indoor units that are most likely to work simultaneously, and in this way you will have the greatest savings. Just installed the system, then, immediately a try: Turn on only one indoor unit and check that, after a few minutes, you begin to heat or cool. Sometimes, in fact, in error the installer can connect the electrical controls of a compressor, and the pipes on another, and the result will be that, while a compressor will work like crazy to feed the indoor unit that is not in operation, that unit into operation will condition a damn, and this problem which is even likely to damage the compressor, you do not notice if, full of enthusiasm as always happens, the first test will come on all indoor units!
A solution technically more refined is to use a single compressor, instead adjusting the power according to the needs with an electronic system: the inverter.
A brief look at technical, dedicated exclusively to the curious like me (you can skip it clean over): The name "inverter" suggests a reversal of something, such as between hot and cold. Not so: and it took me a bit 'of time to understand how this name was born. Almost all compressors of heat pumps using asynchronous motors, the same as washing machines, so to speak, that are robust and silent. The problem with this type of engine is that it is easy to adjust the power as you can do with the brush motors, such as those of the drills, which for their part are adjustable yes, but also much more noisy, consume more and are less resistant. The only way, in fact, to set an induction motor is to vary the frequency of the supply, it is necessary to turn the AC constantly, and then convert it back to AC at the desired frequency. This second conversion is performed by an inverter, hence the name, an inverter is also what is used on the "UPS" often used for computers. Given the powers in play, this double transformation is far from simple and economical, and, however, involves an increase of fuel consumption. Today, fortunately, thanks to the power electronics sophisticated that we have, we can solve the problem by feeding directly from a DC motor using pulse modulation, the motor is simple and robust as an asynchronous, but it needs control electronics quite sophisticated, on the other hand, its performance is the non-plus-ultra. So, gone the asynchronous motor, a heat pump "inverter" is named after a part of that today does not even exist anymore.
The application of inverter technology is not limited to multi-split systems: on the contrary, is becoming more widespread, even among single split. In fact, it allows the heat pump to operate at optimum power, thus improving heat transfer and reducing consumption; performance is excellent in the intermediate seasons, when the required load is reduced, while consuming about 5% more when used at maximum power. The characteristic of the systems inverter is in fact never to be extinguished: after working at maximum power just turned on, when the temperature in the house is close to that ideal their power is automatically adjusted on the maintenance value; energy efficiency, therefore, is significantly better, and the savings in midseason may reach 30% (or so they say), even comfort is enhanced, because the air flow is no longer hot or cold. If this is a big advantage for the single split, it is even more for multi-split, the MXZ-32 model of Mitsubishi Electric, for example, one of the first multi-split inverter products widely distributed, has an automatic adjustment between 2 and 9 kW of power output, with a COP greater than 3.2, then regardless of whether there are one or four (maximum) operating indoor units.
It is important to note, however, that while a traditional model works best when it is maximized, an inverter works best when it is not, then, while the former should not be oversized beyond the 10/15%, the second should be at least 15% more powerful than necessary.
In conclusion, if you opt for a system of type "split", consistent with the other problems of installation and availability in the catalog, the preference, it should be, in order, to:
• Many split (each unit corresponds to an external drive) is the most economical and more efficient, but the jungle external drives certainly does not help the aesthetics of the building, and sometimes there's just enough space ...
• A multi-split inverter: is the optimal solution with regard to the space occupied on the outside, but also the most expensive, and ultimately the most technically refined.
• A multi-split in separate motors (max 2 internal for each motor), allowing the use of 4 indoor units to one outdoor, offers excellent firmness and a good efficiency, especially if the indoor units are bonded so "smart".
• More multi-split standard two internal drives.
• A multi-split to multiple indoor units (3/4/5), the single split more expensive and consumes more, unless they are not always used for all indoor units simultaneously.
In conclusion, as you can see, there is not the ideal setup, and this is true even in comparison to other types of heating, so I picked a table in a comparison between air systems; valued qualities which will enchant the most, and which negative sides have less importance to you ...
Characteristics of types of air-water system (fan coil) air-to-air (ducted) air-to-air (split)
Pros:
• You also can condition the bathrooms and kitchen;
• You can adjust the temperature room by room;
• Optimum temperature distribution, automatic compensation of errors in the project.
Cons:
• Plant expensive and complex;
• It is virtually indispensable to install pipes concealed in the rooms;
• The existing piping for heating are not suitable; Pros:
• The system is easier to achieve if there is a hallway controsoffittabile, do not need to install plumbing in the rooms;
• Low noise (only if properly designed);
• optimum aesthetics;
• It may also affect small environments, and using more vents you can better distribute the temperature in the big ones.
Cons:
• Inability to air-condition the bathrooms, kitchen smells waft throughout the house quickly;
• Difficult (or impossible) to adjust the temperature room by room, with the risk to unbalance the system;
• Project more critical for temperature uniformity;
• You can not switch on the system only in some rooms (for up to zone). Pros:
• Design simple, the possibility of errors is minimized;
• The kitchen smells remain confined;
• Possibility to adjust the temperature room by room and differentiate switching times;
• (for monosplit) maximum economy / installation flexibility and thermal efficiency;
• Can be cascaded to an existing system (eg, condominium)
Cons:
• Virtually impossible to use the air conditioning of the bathrooms;
• The existing pipes can not be used;
• Multi Split: You must install pipes concealed in the rooms, or plastic channel at sight;
• Split: high number (and size) of external drives;
• Multi Split Inverter not: low efficiency in the off seasons or with some rooms without air-conditioning;
03/04/2005
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Translated via software
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Source:
Italian version of CercaGeometra.it