Heat pumps: system sizing

Let's see how to design a heating system, both as a sizing that from the economic point of view. Should add at once that there are two different "technical": the "well done" and the "-thumb". In the first part we see the design "well done", however, if the system is simple, there is nothing wrong to use the second one, because otherwise the costs of the design alone could become comparable with the entire system!

The most appropriate design that can be done is divided into the following phases:

Evaluation of the dispersion of the building / apartment This phase tends to simply establish, for each room, the power required to maintain the set temperature, when they occur outside the most severe climatic conditions.

Evaluation of how to use the heating The theoretical value of the first phase is adjusted with considerations on the performance, the system losses, intermittent use, on the contributions of "free" heat (eg, exposure to the sun). Here only begins to show the difference between the various heating systems.

Comparison and economic evaluation It compares the cost-effectiveness of each heating system considered (heat pump, natural gas boiler, ...). The assessment is based on annual consumption and cost. You choose the most economic system globally.

Sizing you finalize the design dimensioning the elements necessary to heat each room: size of the pipes, radiators or fan coil, the boiler or battery, etc.. etc..

The design of the heating system is neither fun nor an option: the law 10/91 in fact make it mandatory as part of the overall design of all new buildings, and in case of replacement of the system (also individual! ) for the existing ones. Obviously in these cases can redarlo only a licensed professional. The law also constrains the "Normalized Energy Requirements" (FEN) of the new buildings, so that we should not spend too much fuel to heat them, and sets minimum efficiency levels for appliances used. It is also right, if the buildings are large, it is tens or hundreds of millions saved, to the benefit of the purchaser and the environment (that is, all of us!). Keep in mind, however, that for now there is only an obligation to design for heating, not for air conditioning. In addition, methods of calculation for these two are slightly different, as we shall see.

First step: to evaluate the dispersion of the building / apartment

To be able to buy the right heating system, you must (of course) to evaluate how much heat will consume. There are several ways to do this, the method good (which incidentally is the one to be used in Italy in the case of obligation to the project) is described by the UNI (there are several on the subject). The calculation is not particularly complex, but it is long because it takes into account a long series of data, such as:

• Measures for floors, walls and ceilings bordering the outside, the ground or unheated;

• The materials with which they are made (ie, their ability to disperse or insulate heat, and the heat capacity), together with the nature and thickness of the various layers constituting (plaster, brick, cavities ...);

• The orientation of the external walls and their exposure to the wind;

• The size, shape and the material they are made of the fixtures (doors and windows), which are important sources of heat loss, or dimensions of the frame and the glass, as well as whether it is a thermal break fixtures, if the glass is single or double, if it is filled with air or inert gas, its air tightness, if there is a damper ...

• The size and conformation of the various junctions wall-wall corner (internal and external), wall-ceiling and wall-floor, or the so-called "thermal bridges";

• The amount of air exchange with the outside and non-heated;

• Exposure to sun and wind, including any obstacles and shielding object (balconies, buildings);

• Any "free energy contributions" people present, kitchen machines, refrigerators, light bulbs, etc..

With these parameters, and (a nice little ') calculations, I have not the slightest intention to describe here because it would take pages and pages of tables, we arrive at a measure of the dispersion of individual rooms and the whole house. Knowing the dispersion is very easy to know, with sufficient accuracy, how much energy (heat) must be entered in each room to maintain a certain temperature difference between inside and outside.

In fact, if we fix the internal temperature at 20 °, and the one outside the minimum we expect, reasonably, may occur during the winter (this is also indicated, town by town, in the UNI - the so-called "temperature value of project "), we can know with any precision the warmth that we must provide our plant. If, for example, our calculation provides us to our house a dispersion equal to 0.44 kW / ° C, which means that, when outside there are 5 °, to keep the inside temperature of 20 ° must be a power of 0 , 44 * (20-5) = 6.6 kW; we then install a boiler with a power output greater than this, or a heat pump which renders this energy.

The usual DPR 412/93 also sets a maximum limit value for the coefficient of dispersion, ie the dispersion per cubic meter to be heated, for newly constructed homes (Cd lim.).

What do you use to make these accounts? As already mentioned, you need to have in hand the UNI (can be bought on the Internet, at the site of the CEI, but buy all the required is not cheap, certainly not worth it for just one plant), a plan building, a spreadsheet and a lot of patience to measure every room, every corner of the wall, each dimension of the frames, each layer thickness and material of each component every wall, floor and ceiling ...

If you want to make your life easier you can adopt a program just for this. There are several products in the United States is free (but for DOS) that a fee, but obviously do not apply to drawing up a draft "official" because they do not follow the methodology UNI, or you can take a good paid program Italian, understandable and perfectly normal, but you must always have the UNI, otherwise I probably would not understand anything ... I suggest the site of Kadmos, which forms a simple and effective programs, but there are several Italian producers.

Second phase: evaluation of the methods of use of heating

The first stage gives us the housing needs if we keep the heating on constantly. But if, as always happens, the heater is in operation only during certain hours of the day, the capacity is slightly different: first of all we must have a reserve of power because the optimum temperature is reached in a reasonable time, and then the needs will change depending on whether the heating is switched on only at night (when it's cold) rather than during the day (when it is warmer and there is also the action of the sun). All this is slightly alters the required power to the system, both its cost management. To always follow the methodology UNI, we need to evaluate:

• The number of hours of operation of the plant and their distribution in the day;

• The type of adjustment (a thermostat, differential);

• The type of plant (boiler, electric heat pump) and components that distribute the heat (air terminals, radiators, fan coil units ...)

• Climate data statistics of the resort, or temperature, sunlight and wind. These data are provided, town by town of Italy, from UNI.

Everything seems difficult to assess, but in reality most of the values ​​to be adopted are provided beautiful and ready in the tables of standards, and however it comes to adjustments of a few percentage points.

With all these data we can calculate, with a reasonable accuracy, the need for heat to warm for an entire winter, knowing the cost of each kilowatt of electricity or in the form of methane (as we did in the beginning ...), we can get Finally, the most important parameter of all: what it costs us to keep us warm throughout the cold season.

To better assess the comfort we can draw a graph of the temperature reached in the home during the off-periods of the daily heating.

Again, for new buildings, the usual DPR 412/93 sets a limit value for the energy spent for heating the building: the so-called FEN (Normalized Energy Requirements), which is calculated in Joules / cm to day: in practice it is the energy spent on average each day to heat a cubic meter of building. We now know exactly how to transform this data into calories, and then in kW of electricity or cubic meters of gas, and then in pounds, knowing that the number of days in a winter heating is fixed by law (guess which one? Bravi: the 412th / 93!)

What can be used for this second phase? If you are using the spreadsheet continue to use it, otherwise usually the program that you used for the first phase will be able to make the necessary calculations for this.

Third phase: comparison and economic evaluation

If we repeat the calculation of phase two for the different heating systems that we adopt, we can finally assess what is what makes us save more.

But be careful, there are two costs to consider: an installation cost (the cost of buying the system) and an operating cost (what we spend every year to use it). The purchase cost we find it in the lists, and the operating cost includes not only the "fuel" to get the system (be it gas, oil or electricity), but also that the periodic maintenance, repairs, etc.. , the figure is therefore only be evaluated with a certain approximation. And, as always happens, the plants that have a lower cost of ownership also have a higher purchase cost.

The DPR 412/93 says that - with regard to the government - you have to choose the system that is most convenient after five years. It is a reasonable valuation. But be careful: do not confuse these five years with the "expected life" of the system, which is much longer, it is in fact a non-technical and financial evaluation. Interestingly, only in the case of heat pump system in buildings of higher capacity to 10,000 cubic meters, the limit is raised to eight years (ten in town centers, given the importance of the problem of environmental impact).

An individual may also be taken as the basis for calculating a longer period, and here it would open a complex financial discourse. If you want to be precise, in fact, we should consider that what we save on installation of the system, we could invest it in some way to produce money which lower the operating cost of the plant itself, but it is also true that the returns over the long period of an investment can be evaluated only with a certain approximation ... In short, in addition to being engineers we should be financial wizards, to do this heating! So, according to your tastes, evaluated for a period of five and eight years, and this is just fine.

How do you make this assessment? Even here, though perfectly suited the usual spreadsheet, things are simplified using a special program. The DOE (Department of Energy) U.S., the equivalent of the Ministry of Environment, distributes a free program that allows you to do if this calculation, drawing up a "ranking" of convenience of the various types of systems from the dispersion of house to be heated and the area where you live, this is because every American who saves on heating is also good for the environment ... (En passant, is there any "there where is power" which can take a cue?).

The DOE program is downloadable from the site www.eren.doe.gov, but if you do not have the climatic data of the place of installation, and you do not know how to translate them to shape the program, its use is almost impossible.

Fourth stage: the sizing

We calculated the capacity and the type of facility best suited to keep us warm and to save money, and now we just have to go and buy. To do the "shopping list" we still need to calculate a few things which depend on the type of system chosen. Let's see:

Plant boiler and radiators: if we had estimated that this is the most economical heating system, we need to calculate the number of radiators - one for each room and their size (ie the number of elements to be joined to compose them - each has a specific power yield), the length of the connecting pipes, and a small surplus of energy that must be dispersed in the pipes

Ducted heat pump: we have to calculate the size of vents depending on the heat to be supplied to each room. Furthermore, we must calculate the size of the pipes that carry air, always based on the amount of heat to be supplied and to the load losses; also exists a constraint of maximum air velocity which, if too high, it creates turbulence and consequently noise (many do not know, and that's that they feel works with hisses and boos ... very annoying). In addition to the air discharge is necessary to calculate the size of the piping of recovery, to the central unit for sucking the air to be treated. Important: Do not you ever install vents in the bathrooms, because the air drawn from these would blend and would be fed back into all the other rooms, with easily imaginable consequences ... However, in the case of large baths, it may be necessary to install a small electric heater, or even a heat pump separate (split), in the case of bathrooms derived from integers studios :-).

Fan coil system (heat pump or boiler), we must calculate the loss in the pipes and the power of the individual fan to install, given that we find the catalogs of manufacturers. Often to save money, you do not climatizzano small environments, and this might seem like a limitation of this type of system, however, air systems, as the name implies, the air circulation is much greater than in the case of radiators, and Usually there are no difficulties to heat small areas by adjacent ones, in contrast to systems with radiators, while in the bathrooms of the cheapest you can install radiators for heating only (at times even electrical). The dimensioning of the fan coils can also be very coarse seen that each has its own internal thermostat, and, consequently, stops the consumption of heat when the room is at the optimum temperature, making it available for the other.

Heat pump split: we have to decide which rooms to put internal drives and what capacity. The calculation is very simple, because the cuts use of the indoor units are very few (usually those used in the apartments are from 7000, 9000 or 12000 BTU, rarely - in huge rooms - 18000), even here, as each unit is thermostatically controlled , any errors are compensated automatically by the system, and normally you do not install units in bathrooms and small rooms.

Air conditioning

In the case of air conditioning (cooling) the calculation method is similar, but there are some changes, for example the influence of solar radiation, which obviously has a much high in summer, and the climatic data that are completely different , remember that the "free contributions" are those in winter, while in summer they are more heat from "suck out". Then there is a problem: when it cools the air, almost always creates condensation, or moisture in the air (which in the summer is always quite high), can not make you stay turns into steam and water to do so, inevitably loses heat to the air. So, in essence, the air conditioner is forced to absorb heat from the air is both moisture that wants to turn into water. This heat is practically lost, it is an additional price we must pay to freshen the air. This is called "latent heat", should be calculated according to the degree of humidity typical and considered in the capacity of the air conditioner (usually is indicated in the technical data of the device). To be honest, even during winter operation is often creates condensation; winter but this occurs only in the outside unit (those that cool the air), and among other things in this case it is a positive phenomenon because improves the performance.

 

 

 

READ THE SPECIFICATIONS

 

 

The units of measurement

A friend of mine wanted to indicate when a given so that you do not understand if it was a little or a lot, said it was measured in "Fulton month." Here, for measuring the performance of heat pumps have been created units that are very reminiscent of the "Fulton month", but who has the relevant information and a good calculator can always get out of the impasse. To begin with we specify better the differences between work, power and heat. The work is, intuitively, what is accomplished by something, regardless of how long it took to do it. The power is the capacity to do work per unit of time: an engine power double compared to another has the capacity to perform twice the work at the same time, or the same work in half the time. When we do an electricity contract, we determine the maximum power that we can draw from the power supply, regardless of the amount of energy they consume into reality. The electric bill we pay depends on both the maximum power - in fixed size - both by the amount of energy (measured at the counter) that we consumed. The heat, in fact, as already mentioned is no other than work; we can transform through the machines heat into work and vice versa (with certain limits). The heat, therefore, is another form of energy. The ability to provide heat per unit of time, again, is no more than a measure of power.

Here is an example of the unit of measurement used; forgive the obstinacy of certain distinctions, but it is because of my training.

• The Joule (J) is the basic unit of energy in our international metric system.

• The watt (W) is the unit of measurement of power in the international system, with its multiples and submultiples (eg kilowatts (kW) = 1000 W) is equal to one joule per second (J / s) should be the only unit of power used in Europe. Maybe.

• The kilowatt-hour (kWh) that contrary to what many think, is a measure of energy or work (not power), is equal to 1000 Watt * 3600 seconds (one hour) or, as can be imagined, to 3.6 million Joule.

• Cal (calibration), is a measure of heat. As mentioned above, the physicist Joule has shown with his famous experience that work and heat are the same thing, also establishing that a calorie is equivalent to 4.18 J. In general, the capacity of a boiler or a generic heating system is expressed in Kcal / h, ie with a power; sometimes is directly expressed in kilowatts, especially for electric radiant plates (a plate that consumes 1 kW of electricity provides for definition 1 kW of heat).

• The BTU (British Thermal Unit) is another unit of heat, such as calorie, one kWh is equivalent to 3413 BTU; convenient is the fact that 4 BTU equivalent to 1 kcaloria. The ability of almost all air conditioners and heat pumps is shown in BTU.

• The Ton is an ancient unit of Anglo-Saxon power, yet there is some conditioner with capacity in this unit. 1 ton is equal to 12,000 BTU / h, ie 3.52 kW.

At the level of curiosity also carry out how these units:

• The Joule is the work done to move an object of 1 meter under the force of 1 Newton;

• The calorie is the amount of work required to raise a grade A liter of water;

• the BTU is the amount of heat required to raise a degree Fahreneheit a pound of water

• The Ton is the power required to freeze (or thaw) a ton of water in 24 hours;

They can return the following convenient conversion tables:

Energy (work = power * time)

 

J (= W * s) kWh kcal BTU

1 0.2778 * 10-6 0.239 * 10-3 0.948 * 10-3

3.6 * 106 1860 3413

4186.8 1.163 * 10-3 1 3.9685

1055 0.293 * 10-3 0.25198 1

Power (capacity = work / time)

 

kW (= kJ / s) kcal / h HP BTU / h TON

1860 1.34 3412 0.284

1,163 1 * 10-3 1.56 * 10-3 3.97 * 10-6 330.7

0.7457 642 1 2550 0.2123

0.293 * 0.252 0.393 * 10-3 10-3 10-6 1 83.3 *

3.5168 3024 4.71 12000 1

 

Some sites where you can find more information on units of measurement and their thermodynamic conversion can be found in the section useful sites.

The technical

Without this necessary introduction on units of measurement, let's read the characteristics of a typical device.

Power consumption: is the amount of "fuel", be it electricity and gas, the pump uses to operate: it is, in essence, what we're going to pay the bill.

Power output: is the quantity of heat which, in an hour, our pump will be able to provide or absorb from the conditioned. The higher power output (equal that absorbed), unless we're going to pay the bill ...

Attention must be paid to the fact that the characteristics are measured under environmental conditions of installation and precise, the variation of which can occur even significant variations in yield. The most important is the temperature of the plant where he works, almost all by measuring conditions:

Heating mode Cooling operation

Indoor 20 ° Dry bulb (DB = Dry Bulb) /

12 ° wet bulb (WB = Wet Bulb) 27 ° Dry bulb (DB = Dry Bulb) /

19 ° wet bulb (WB = Wet Bulb)

Outdoor 7 ° Dry bulb (DB = Dry Bulb) /

6 ° Wet bulb (WB = Wet Bulb) 35 ° Dry bulb (DB = Dry Bulb) /

24 ° wet bulb (WB = Wet Bulb)

What do they mean dry bulb and wet bulb? Normally we, as you can imagine, we measure the temperature with the thermometer bulb dry, but if we put a wet cloth around the bulb, the water evaporates from this will lower the temperature, the more the air is dry, the more is fast evaporation, the greater this reduction. In reality, therefore, this lowering is a measure of moisture, it is also the latter, in addition to the temperature, to influence both the absorbed power that (mostly) the yield.

Another parameter to consider is the length of pipes: the greater the length and number of curves, the greater the losses and thus lower the yield, the manufacturer normally indicates the length with which the measurement was performed, assuming that it is straight pipes and horizontal. Another variable may be the supply voltage (eg 230 volts rather than 220).

In reality, however, the greatest influence is given by the outdoor temperature (since the inner one is fairly constant) and moisture from the colder side, ie outside in heating and in cooling inside.

The relationship between power output and power consumption efficiency is called (usually indicated by the COP, by "Coefficient of Performance", coefficient of performance). A COP of less than 3 is to be avoided, and one above 3.5 is great. However, as we have said, any variation from nominal conditions certainly affect these parameters, to a large extent unknown, as we will see, then, there are parameters that indicate more accurately the quality of the pump at all temperatures.

The COP real, therefore, varies according to the difference between indoor and outdoor temperature. If there are 15 degrees out, the COP is very high, plus the amount of heat required is lower, then the cost of heating collapses. Vice versa if the outdoor temperature falls several degrees below zero, the COP becomes much lower, so that the heat introduced in the house comes almost exclusively from electricity consumed, with a very high expenditure; there are heat pumps that, in these conditions , to help you make an electrical resistance for groped to maintain acceptable indoor temperature, obviously at the expense of the cost, if you live in areas where the temperature is often below zero, it is virtually essential to add a traditional boiler heat pump, limiting the 'use of the heat pump at the beginning and at the end of the cold season (there are studies that say that even in these situations, you can have a savings) ..

In substance, the efficiency of the heat pump is a function of the temperature difference between inside and outside. Depending on the climate for which it is designed, the curve that describes the COP can be very different. For those who have some knowledge of thermodynamics, the heat pump must obviously respect its fundamental principles, (the third in particular) from the experimental data shows that the pumps today have an efficiency of about one third of the theoretical maximum, and this means that technological development can still do a lot to improve them. Depending on the climate of the place where you are located, therefore, the same heat pump can have very different costs. Perhaps it is because of all these uncertainties that we have the heat pump is still little known. To know which one to install, you should collect statistical data in place for several years, then apply them to the different heat pump, choosing the most suitable.

Just to get an idea, at the side found the performance graph of a heat pump business.

As we see, the decrease of the temperature drops both the absorbed power that the yield, and therefore the output power (which is the product of the two) drops more rapidly. A zero degrees the power supplied is approximately 75% of nominal (ie of that - remember - to 7 ° C).

I said that the efficiency is referred to as COP, this is inaccurate, because it speaks only of COP during the heating operation, the cooling operation is spoken instead of EER (Energy Efficiency Ratio, Energy Efficiency Index), which is always a measure of efficiency but, just to complicate matters, it is not given by the Anglo-Saxons as a pure number (as the COP, in kW / kW or kcal / kcal, or BTU / BTU), but in BTU / Wh; since 1 Wh (= 1/1000 kWh) is equal to 3,413 BTU we have that, to get the EER "local" (the COP "summer") must divide the EER for 3,413.

Without knowing the performance at various temperatures is virtually impossible to estimate the average return of a certain heat pump, but let's face it, even knowing the difficulty remains ... In the U.S., like many other things, are far ahead of us on the diffusion of heat pumps as a heating source, while there is almost no home or office with no air conditioner. So, to make life easier for users and installers, have been invented a series of parameters that are used to evaluate the goodness of a product in a much simpler way. There is also a government agency, the EREN that works to promote the diffusion of heat pumps, and the independent association of producers, the ARI, which certifies features, and provides a list of all brands and models .

However, we said that it is the COP that the EER measures are limited, since they refer to a single operating conditions (temperature and humidity inside and outside), but do not say absolutely what the average return that we expect in real conditions , ie at different external temperatures. The Americans have now created two other parameters are very useful for the users: the HSPF and SEER.

Can you imagine to put the heat pump / air conditioner in an atmosphere typical, "sample", and make it work for an entire season, from October to May or June to September, and to measure the relationship between energy consumed and energy yield during throughout the period. It is clear that, in the course of the season, the temperature difference is minimal at the beginning, you will make up in the middle of winter or summer, before decreasing again. The HSPF (Heating Seasonal Performance Factor, for heating seasonal performance factor) is, as the name implies, the parameter for the winter season, while the SEER (Seasonal Energy Efficiency Ratio, seasonal energy efficiency ratio) is the summer, it is clear that these parameters do not depend on performance at a single temperature, but rather the entire range of temperatures that can occur in an entire season. They indicated and certified on all equipment sold in the U.S., are never present on the Italian specifications. This is partly justified by the fact that the SEER and HSPF are calculated on the average climate "statistical", a series of different temperatures, each with its own "weight" percentage, representing the "average climate" of the U.S., but not it would take much to make a European version or Italian, since the manufacturer has no difficulty in measuring them.

If you go to compare, for example, the SEER and EER of two air conditioners, you will have several surprises, for example, two devices with very similar EER SEER may have very different, and the units with the highest SEER EER have nothing at all of the best. This confirms that the reading given by the COP or just dall'EER is definitely not enough.

American regulations prohibit the marketing of heat pumps with SEER less than 10; currents are average values ​​of about 13/15, the maximum achieved so far is 17. Compared to a few years ago, the SEER and HSPF for heat pumps are constantly increasing, and therefore the savings are increasing. The fact that there is a constraint on the SEER and not sull'HSPF indicates that, while many Americans do not have a heat pump (but still more than by us), virtually all have an air conditioner, in fact, there are regions, such as the Florida but also the Washington area, which have their own population growth to the invention of air conditioning, without which the very few people would find attractive ... That's why there is a greater attention to these issues.

Knowing the HSPF of the heat pump of interest, if you live in an area with a climate not too dissimilar from the "sample", it would be easy to calculate the total operating cost of an electric heat pump than methane: just inferred from the bill the consumption of the latter in cubic meters, turn it into heat (as usual, every cubic meter is equal to 9200 kcal.), and divide by the HPFS, which yields the consumption of electricity, more over a season, and by the cost per kilowatt is calculated the cost of the electricity bill!

To know then how much you can save with a heat pump should know not so much his performance at 7 ° C, but for every outdoor temperature, making an average calculation based on the statistical distribution of the winter temperature in your area. Complicated, practically impossible? Not at all! The UNI 10349, available to every designer, statistics specific conditions (wind and temperature) for every town in Italy, to be used in the design of heating (!), While the performance of the heat pump are perfectly known to each manufacturer ... It would take only a willingness to provide it (is there anyone who can hear me out there?). In the usual United States, there are programs that, using the corresponding climatic data of those places, and the characteristic curves (which there most manufacturers provide, and certify, too!), They can calculate the dollar, and to compare, the cost of a winter with each model of heat pump, and even different types of heating (traditional, air-to-ground pump, etc ...)!

Here in Italy we still go of thumb, keep in mind that in Rome the design temperature (ie the minimum normally expected) is 0 °, and the average winter is just under 7 ° C already mentioned.

 

03/04/2005