Heating

Heating (from Latin calefactio, -onis, derived from calofacere, to heat, compound of calere and facer hacer), from a generic point of view, is the method or system by which heat is provided to someone or something in order to maintain or raise its temperature.

Ancient cast iron radiator.

Applied to buildings, it refers to the set of devices and accessories that are installed to achieve and maintain thermal comfort conditions during cold seasons in one or many rooms. It is a component of the air conditioning.

History

The human being, whose body has no protection of hair or feathers, has needed to warm up during cold times. He has achieved it by making dresses and suits (coats) or taking advantage of the fire, through various heating systems. Since man mastered fire, he was able to live in latitudes where temperatures were low for a certain part of the year, heating living spaces with him.

Fire mastery seems to have been achieved by Homo erectus around 450,000 years ago. An Israeli team traces it back to 790,000 years at the Gesher Benot Ya'aqov site on the banks of the Jordan.

The fuel was firewood or other plant remains, and dried herbivorous animal droppings. In China it seems that coal was used since time immemorial.

But fire has several drawbacks: naked, in the middle of a room, it can be dangerous for humans, especially their puppies, it needs air for combustion, air it gets from outside, that is, cold air, and it has as result of combustion fumes, which must also be evacuated through ventilation. First it was the fire in the middle of the premises and then various systems were invented to evacuate the smoke better, to consume less fuel, etc.

Pompeii Brass

Perhaps the most important innovation consisted in removing the heating of the fireplace from the inhabited premises, avoiding smoke and the entry of cold air for combustion and this, according to the data we have, was achieved with the heating system called hypocaust, Greek invention, used in the baths of Olympia and Syracuse from 300 BC. C. The fire was in another room and the hot fumes were carried through a series of ducts, which ran through the floor and sometimes the walls of the rooms, leaving the heat before going out through the chimney to the outside. The most important installations were made in the baths. This system survived until very recently in Muslim and then Christian Hispania, in which a system directly related to the hypocaust, the glory, was used quite widely, using straw as fuel.

The spicy little girl, with a brazier, Romero de Torres, from the beginning of the centuryXX..

But although they knew about the fireplace, the Romans used it little as a heating system for homes. In the excavations of Pompeii and Herculaneum no fireplaces of this type appear, which indicates that they used mobile braziers. These artifacts have been used until quite recently in Mediterranean countries using carbonized vegetable fuels (oak charcoal, oak cisco, picón...)

Home for kitchen

Probably, hearth fireplaces appeared in more northerly latitudes, where heating is more necessary and, although there was something similar in the kitchens of countries further south (with a large hood to collect the fumes and a couple of seats at the sides), those dedicated exclusively to heating do not form part of architecture until the XI century. Although the chimney improved technically over time, it was never an effective heating system. In the 19th century, a domestic fireplace required between 800 and 1000 cubic meters of air per hour for the draft, air from the outside, cold, and therefore the cooling is all the greater the more the fire is stoked. And there was rarely a suction cup (external air intake duct directly into the home) to provide the air necessary for combustion and smoke drag, so the air had to enter through the windows and doors sweeping (cooling) the premises.

An important improvement was the invention of the stove: the hearth was closed, protecting people from burns and, most importantly, they had a system to regulate the intake of combustion air, which avoided having to introduce it into the premises in large quantities to carry the fumes that, confined to the home, had no other outlet than the flue, without the possibility of revoking towards the premises, and this limited the sweep of the environment to the air necessary for combustion, avoiding the entrance of the necessary to evacuate the fumes. And for the same reason they had other advantages: one of them is that, by pricing the air intake, the emitted power could be regulated because, with less air, less flame and slower combustion; the other is that by being confined, higher combustion temperatures were achieved, and the heat of the fuel was better used. For these reasons, the stove had a much higher performance than the hearth fireplace. The first complete work on stoves published by Franz Kessler appeared in 1619. This work describes the heating principles used in Germany at the time, which were little perfected until the 17th century XIX.

Heating by thermoosphere
1 Heat
2 Radiators
3 Open expansion vessel

Central heating

Since the Industrial Revolution, steam began to be handled in motor machines and the technique of driving fluids through pipes was developed, fluids heated in boilers from fuels, generally solids, mainly three: firewood, peat and coal.

At the beginning of the XX century, these techniques began to be applied to heating systems in buildings with coal-fired boilers, pipes and radiators, using steam as a heat transfer agent. The oldest of the facilities consisted of a coal boiler, a network of pipes and radiators. The boiler was a cast-iron stove, whose walls were double and between the two layers circulated water that was heated until vaporization, in the steam layers; later at temperatures below boiling point. The heat exchanger circulated through the pipes by thermosiphon or thermal draft, so it was convenient for the boiler to be located at a lower level than the emitters. The advantages of the system are: the fire is in a specific room (which can be as ventilated as necessary), not in the rooms to be heated; a single home is used to heat several premises or even an entire building (later entire neighborhoods: district heating).

As has been said, the initial heat transfer agent was steam, which was later replaced by water. When this happened, in the thermosiphon system, the pipes had to be quite thick to facilitate circulation. In addition, the outbound ones had to go close to the ceiling, above the emitters and the return ones on the floor. Later on, a recirculation pump was added, which allowed thinner pipes to be taken along any route and the boiler could be in any situation with respect to the radiators. On the other hand, it is not very convenient for the boiler to be of continuous combustion, that is, solid fuel (coal or wood pellets), because a cut in the electricity supply would stop the pump and the boiler could get too hot.

At the same time that steam was abandoned, moving to hot water heating, the fuel also changed, first city gas and fuel oil replaced coal, little by little diesel, and later, from the 1960s, natural gas.

Heating installations

Basically, a current heating installation has three parts:

• A heat production system, which can be a fuel boiler, an electrical resistance system, or use of natural or residual heat energy.
• A distribution system, through channels through which a heater circulates, usually water or air.
• A system of emission, by means of terminal elements (radiators, radiant paraments, air impulsion grids,...)

Heat production

Central solid fuel boiler

Depending on the extension of the installation, there are two types: local or unitary and centralized. In the first, a single device produces heat and emits it in a room. In the second, heat is produced in one place and distributed through pipes to the premises that need to be heated.

For fuels

In the centralized installation, the most logical and most economical heating is a solid, liquid or gaseous fuel boiler. The drawbacks of fire and air for combustion are limited because this boiler is located in a specific room.

Solid fuel boilers differ from the others because their combustion is continuous, that is, once they are turned on, they only turn off when the fuel runs out or the air intake is completely cut off. Power regulation is done by varying the air intake. Currently, the use of coal tends to be prohibited, as it is the fuel that produces the greatest amount of CO₂ per unit of heat, however boilers with wood pellets are used, which is also a solid fuel.

As a liquid fuel, the only one in normal use is diesel. Gases are of two main types: natural gas and liquefied petroleum gases (butane and propane). The boilers work with a coupled burner that, by means of a fan, makes the right mixture of air and fuel, and burns in the home. Both with liquids and with gases, the operation of the boilers is intermittent, that is, the regulation of the power is done by starting and stopping the burner. There are burners called modulating that vary the power of the flame depending on the demand, regulating the flow of fuel.

Candle series of a boiler with atmospheric gas burner

There are also gas boilers with atmospheric burners, which do not require a fan; the gas flows under its own pressure, comes out under pressure through fine injectors and mixes with air by the venturi effect before reaching the burner itself. In these boilers the burner consists of a certain number of candles arranged in rows on a plane.

There are unitary combustion devices, such as butane gas stoves, called catalytic stoves. They have an acceptable performance, but they require outside air for combustion and produce a large amount of water vapor as a result (approximately 1.6 liters of water for each kilogram of fuel), so they are not very advisable, since they add a lot of amount of moisture in the ambient air. Unitary kerosene stoves are also used, with the same drawbacks, although with slightly less steam production.

By electricity

Another heating system is electricity. And this in two ways: by electrical resistance, that is to say, taking advantage of the Joule effect, or by heat pump. It is rare to find centralized boilers with resistances, but the Joule effect is used in local or unit heaters, radiators or electric stoves. They have the drawback that, despite its high performance, the price of electrical energy is higher than that of other fuels, that is, the price of the unit of heat obtained is higher than that which can be obtained with another fuel..

In any case, it can be economical to use electricity through a heat pump, whose principle is that it takes heat from an external source (cold source) to introduce it into the premises. The performance of a heat pump is great, compensating for the higher unit price of electricity, but it depends on the temperature of the cold source; when this source is the outside air and it is very cold, the performance drops a lot.

Hybrid system

To alleviate the problem of low performance of the heat pump at low temperatures, a hybrid system can be made with a boiler and a heat pump. An electronic programmer determines when the pump is performing well (taking into account both the outdoor temperature and the price of fuel) and it is appropriate for it to work and stops it, and starts the boiler, when the performance of the pump is low.

Other heating systems

Although it is thought that solar energy can be used for heating, the disadvantages of the installation do not outweigh the advantages: solar collectors have lower performance the lower the outside temperature (and, therefore, the days when that heating is most needed) and also, the coldest days of the year are also the shortest, with fewer hours of sunshine. Technically it can be done, but the number of collectors needed is large and, when heating is not needed, they will produce significant amounts of heat that will have to be dissipated in the environment or for another use. An interesting possibility is to take advantage of that excess heat by making a seasonal accumulation, which requires large deposits, but is feasible.

More interesting is the use of waste heat from industrial processes, such as the production of electricity. It is also interesting, although only when it exists nearby, the use of heat from a hot aquifer (geothermal energy). Both systems require hot water distribution ducts (as in district heating) and that the heating of the buildings be centralized collective. Instead of a boiler, in the technical room of each building there will be a heat exchanger.

Heat distribution

The distribution of heat in centralized systems is currently done only in two ways: by water and by air. Steam is no longer used because it is a heat transfer agent that is difficult to regulate (it must be done in each radiator) and because the temperature that reaches the surface of the emitters is very high (around 100 °C) so that it can cause burns due to contact.

By water

The classic heating system (by hot water) uses water as a heat transfer agent, which reaches the terminal or emitting elements through pipes. The pipes can be made of black steel, copper and, currently, plastic materials. Galvanized steel should not be used, because the temperatures reached by the water destroy the galvanic protection. In a network of metal pipes, different metals should never be mixed (not only in the pipes, but also in the emitters), because the most electronegative ones can corrode the others. If there were no other remedy, to avoid it, they must be interposed between dissimilar metals, links or splices made of electrical insulating material (nylon, for example).

By air

Another system of bringing heat to inhabited premises is through air. In this case, the ducts are quite bulky and are made of different materials: galvanized sheet metal, fiberglass agglomerated panels, plaster and even copper. Construction spaces can also be used as holes over false ceilings or even corridors, plenum (in these cases it is usually only used for returns).

The use of air for heating is almost inexcusable when there is also a cooling installation, which must generally be by air. It does not seem logical to use two different installations for the same purpose: air conditioning spaces. In addition, a well-conceived air conditioning installation solves another absolutely necessary installation: ventilation.

The air can be heated directly in the heat-producing element or in an air treatment device (air conditioner), to which the heat is taken from the boiler, using water through suitable pipes, being in this case a system for water and air.

By water and air

Not only is water distribution used in classic heating, but it is also used in air conditioning installations, to carry the heat from the boilers to the air conditioners, where the air will be treated, which will be the heat transfer agent that it will reach the premises; that is, there is a primary transport by water and a secondary one by air.

The system that most appropriately deserves the name of water and air is the one that uses both heat carriers for air conditioning. Indeed, the air flow required for ventilation may be insufficient as a heat transfer agent, which is why in facilities it is common to mix outside air (ventilation) with return air. In this case, only the ventilation air is brought to the premises and to complete the required amount of heat, another part is brought by water to specific emitters (fan coils).

Terminal elements

Distribution of pipes of a radiant floor, showing the heel of hosting of collectors.

Devices that emit heat in environments are called terminals, sometimes emitters.

When it comes to a water system, the most classic are radiators, but radiant walls are also used. These are not proper devices, but consist of a circuit of pipes embedded under the coating, turning the wall into a heat emitter. The most common is that this facing is the floor, but sometimes the ceiling or walls are also used. The roof is not a good solution because human skin absorbs thermal radiation very well and alopecia suffer from headaches with this system.

Another terminal used in water systems is the fan coil.

When it comes to air systems, terminals are simply the various types of grilles or diffusers through which air is blown into the environment.

Regulation

The purpose of regulating heating systems is to provide at all times the power appropriate to the needs of the building or premises. The installed power in the system is the maximum power required at the coldest time of an average year. The rest of the heating season, outside temperatures are higher than the minimum and the need for heat is less. For this reason, the power must be regulated according to the needs at all times. The lower the outside temperature, the more power is required, so that the required power not only varies throughout the cold season, but also throughout the day. There are three ways to achieve this: by time, by temperature and by flow. In certain installations there are several types of regulation simultaneously.

Time regulation

The simplest regulation system consists of stopping or starting the heating system, so that, over a certain time, the power supplied by the heater is the necessary one (the less power needed, the less walking time). The simplest and most effective system is to use an ambient thermostat located in a representative room (the living room or main room) which, when the room is at the desired temperature, cuts off an electrical circuit that can directly control the boiler (in unitary or in individual centralized installations) or a solenoid valve that opens or closes the water inlet to the apartment bypass (in collective centralized installations).

It is not convenient to regulate using the boiler thermostat, which limits the temperature at which the water is prepared, because as has been said, the heat needs vary throughout the day, so the temperature of the boiler would have to be changed. consignment several times throughout the day.

With this type of regulation (room thermostat) in a home, in the other premises served by the same installation, emissions can be regulated by opening or closing the stopcocks or, for better regulation, placing thermostatic valves in each one of them. the emitters to regulate the temperature of the place they heat (which in both cases will regulate by flow).

Regulation by heat transfer temperature

The heat transfer temperature regulation system, or proportional regulation, is more sophisticated: it consists of an electronic system that receives a signal indicating the temperature of the outside environment of the building, sent by an external probe, and based on it, regulates the temperature of the water sent to the emitters. Indeed:

• The power emitted by the emitters depends on the surface of these and the temperature leap between that surface and the environment. Since the surface is always the same and, once fixed, the temperature of the environment can be considered constant, the only variable is the temperature of the emitter or, what is usually the same, the temperature of the heater.
• On the other hand, the need for heat depends on the heat losses of the building and these, in turn, depend on the isolation of its constructive elements that separate it with the exterior, the ventilation needs and the temperature difference between the exterior and the interior. As in the previous case, the constitution of the separators is fixed, the ventilation must be fixed and, as the same occurs with the inner temperature, then the only variable is the outer temperature.

The mission of the control unit is to relate this outside temperature to that of the heat transfer, so that the lower the outside temperature, the higher the temperature should be in the discharge heat transfer.

Illustration of a three-way valve by mixing one-way water (incarnated) with return water (blue).

The temperature of the heat transfer unit is regulated by means of a motorized multiway valve (three or four ways) that mixes the water from the boiler with the return water (already cooled) to achieve the right temperature. The control unit knows the outside temperature at all times through a probe located outside and the outlet temperature, through another probe in the flow (or outlet) duct; its programming indicates the temperature that it has to send to the emitters, and moves the valve motor accordingly, for which it follows the so-called heating curve that relates these two variables, the outside temperature with the emission temperature.

This system is called proportional regulation and it is compulsorily applied in collective centralized installations (it would not be logical to regulate with a thermostat located in the living room of the third-party neighbor). It is also the only one that works correctly with underfloor heating, where the supply temperature is almost always lower than that produced in the boiler.

In these systems, the regulation of the boiler is independent, and is done with a thermostat (by time), as in the previous case or, sometimes, by flame power (stage burners or modulating burners) in function of the heat transfer return temperature (which is also measured with a probe). The mission of this regulation is to maintain an always equal temperature in the boiler circuit (the temperature at which the boiler works best), so that the impulsion finds enough hot water at all times so that the mixture has the temperature demanded by the switchboard

In the case of air systems, temperature regulation is the most appropriate, especially when the installation also supplies the ventilation flow, because, although it could be done by flow, there is a minimum limit which is the mandatory ventilation flow, which would prevent it from working with low heat needs. In this case, the regulation is done by measuring the temperature of the return air (extraction) and based on it, the temperature of the supply air is established (the lower the temperature of the return air, the higher the temperature will have to be pumped). However, the mixing system described for water is not used, but rather the temperature of the heat exchanger or battery is changed by the flow system.

Regulation by flow rate

Regulation by flow consists of varying the flow of the heat transfer unit as the heat needs vary (the lower the flow, the lower the heat input). It is sometimes used in air delivery systems, but with the drawback noted in the previous paragraph. It is also the system used by the aforementioned thermostatic radiator valves: as they detect less need for heat, they close the heat transfer inlet to the emitter a little, directly regulating the flow that runs through it.

But where it is a very appropriate regulation is for air conditioners, in air systems. By means of a three-way valve, more or less heat transfer flow is allowed to pass through the heat coil (exchanger), returning the rest to the return through the other route. The lower the flow rate, the lower the heat input and the air that is blown into the premises heats up less. The control of the valve is done by means of a probe that measures the temperature of the return air, so that it is used for the area served by the air conditioner and only for it.

Accessories

For the proper functioning of a heating system, a series of accessories are necessary.

Circulators

For the heat transfer to move through the network, circulators are used, mechanical devices that are hydraulic pumps in the water circuits and fans in the air circuits.

Expansion vessel

Closed expansion vessel.

When the water contained in the circuits is heated, it increases in volume, which must be collected in a specific tank called an expansion vessel. When the water cools, it reduces its volume again and the expansion vessel returns the water contained in it to the network.

They can be open or closed; Normally, they are now used closed, having come to prohibit the open ones in some national regulations, a prohibition that has been lifted when continuous combustion boilers have begun to be used again (using biomass as solid fuel) since the open vessel is an excellent security system; When there is a power cut for the recirculation pump, the boiler continues to heat up and can produce steam, which would find its way out of the vessel.

Dilators

For the same reason that water dilates, the pipes of the installation also dilate, for which reason, in long straight sections, it is necessary to insert dilators. Expanders should also be used when the pipes pass through an expansion joint in the building.

Bleeders

In closed circuits run through water, it is especially important that there is no air at all. Due to its lower density, it accumulates in the upper parts and the pumps are not calculated to circulate water in these circumstances. To do this, traps must be located at these points to extract the air. They exist of manual type (in the majority of the radiators there is one) or automatic. They are not only placed in the radiators, but also in the upper part of the distribution columns, in this case most of the time automatic.

Others

In addition to the sayings, other minor ones are used, such as stop and cut valves, regulation valves (which must be seat valves), thermometers, thermostats, pressure gauges, etc.

Energy saving

Energy consumption in a heating installation is very important, which is why systems that help reduce it are of great interest. First of all, and although it is not part of an installation, it is essential that the building is properly insulated and prepared to lose the least amount of heat possible.

Regarding the saving measures to be taken, two types can be distinguished: those inherent to the installation and obtaining heat from alternative external, residual or natural sources.

Installation efficiency

Efficient boilers should be used, among which we must highlight condensing boilers whose performances are much higher than conventional ones.

The size of the boiler is quite important: the larger it is, the better performance it has, so the collective central heating works much more efficiently. And the larger the installation, the better it is, so a neighborhood heating is more efficient than one of building Another important factor in performance is ignition time: the fewer times the boiler is switched off and on again throughout the day, the higher its performance, so in large installations it is advisable to divide the power between two or more generators so that, at least some of them almost never go out. For this purpose, modulating burners can also be installed, whose flame power varies with the heat needs of the installation.

The maintenance (tuning) of the boiler also influences; It is much cheaper to tune up a large boiler than many small ones, so the big ones are usually much better maintained and tuned up than the small ones.

It is not exactly saving energy, but it is economical: fuel is often cheaper for large consumers.

Alternative sources of heat

Geotermia

There are natural sources of heat in certain places, in the form of hot aquifers. The use of this geothermal energy is done with two perforations: through one of them the water is extracted from the aquifer, it is passed through an exchanger (they are usually waters with many dissolved salts) in which a heat transfer medium is heated and injected again. in the aquifer by the other drilling. This is necessary because only a little of the heat it carries is obtained (its temperature is usually lowered by about 20 °C): reinjecting it will overheat again at depth and because it is a question of not emptying the aquifer. Obviously, once the heat has been obtained, it must be taken to the facilities, for which a distribution network of the heat transfer agent is needed through the neighborhood to the different building centers.

Aerotermia

Aerothermal energy follows geothermal energy in performance and efficiency.

Solar

Although used directly it is not advisable for heating, solar energy can be used through seasonal accumulation. During the sunny season, heat collected by flat solar collectors is stored in a large water storage tank, which is used in cold seasons. The deposit (or deposits) are made buried. It is about making a kind of artificial geothermal energy. Naturally, it requires a significant initial investment, which is offset by energy savings over time. This possibility will be all the more effective, the larger the system, so it is more advisable to apply it to neighborhood or urban heating installations.

Waste energy

But there are also sources of usable heat in many other places: all industries that require cooling do so by dissipating heat into the atmosphere or into a heat sink (such as a river). A typical example is power plants that produce electricity. Obviously, as in the previous case, this heat must be brought to the buildings, through an urban distribution network.

Other

There is another possibility linked to this: in certain large buildings (airport terminals, department stores,...) it may be profitable to produce electricity in situ and use the excess heat for heating; in the production of "surplus" at least 50 percent of the energy as heat at best. This system is called cogeneration.

Although it is beyond this article, in hot countries, such as Spain, this heat can also be used to produce cooling in summer, in what is called trigeneration.

Using the installation

It is not unimportant, from the point of view of savings, how the user manages the installation.

When it comes to an individual centralized installation, it is essential that it have a room thermostat located in the place of most common use: living room, for example. The kitchen may not be a good place, because there are often sources of heat running (stove, oven) that would falsify the measurements, although if you usually live in a place where the kitchen is, you can put yourself there, knowing that When these sources of heat are turned on, the installation will be unbalanced for a while and the other premises may remain slightly cold. It is not convenient, as has been said, to use the boiler thermostat to regulate the interior temperature, since performance is lost and it must also be changed every time the outside temperature changes. The room thermostat keeps the interior temperature stable whatever the outside temperature. If on very cold days the comfortable temperature is not reached, it is most likely that the boiler does not have the necessary power, or that its thermostat is set at a low temperature.

Programmer Termostato

Regarding the use of the system, the user must find, with the room thermostat, the temperature at which he is comfortable and leave it at that point during the heating season; a temperature of 20 to 23 °C is adequate (a couple of degrees less for underfloor heating). If this seems low to anyone, they must think that it is winter, that it is not about being in shirt sleeves, and that wearing something warm (vest, sweater) is not too much to ask. When the house is cold it does not make sense to raise the temperature of the thermostat a lot: it will not heat up faster, since that depends exclusively on the power of the boiler. If the heating was off, there is no other choice but to wait, without touching the thermostat, and if after a while you still feel cold, the correct thing to do is to raise it one or two degrees, at most (the air heats up faster than the walls and the equivalent temperature will be low due to the cold radiation from the walls), to lower it again later. On the contrary, if the user has heat in the place where he is, he should not open the window (literally, it is the same as throwing the money that is being paid for fuel out of the window), but close the stopcock a little. of the emitter (or in the case of a fan coil, lower the fan speed). Of course it is important to ventilate the premises, but it will only be done for a short time (10...20 minutes a day) and if possible when the programmable thermostat (see below) is marking the reduced temperature.

It is important to balance the installation at the beginning of the season: with all the emitter stopcocks fully open (they open and close like running water taps) the heating will be left running for a few hours. If any of the premises does not reach a sufficient temperature, all the stopcocks of the other premises will be closed a little (for example, a quarter of a turn) and we will wait a long time to see the result. After that time, the premises will be checked and the valves of those with the highest temperature (regulation by flow) will continue to be closed a little, until they are all more or less at the same temperature. If a room is used little, it is convenient to close the stopcock of its emitter a little more, but not too much, because it will cool the rooms next to it.

It is very convenient that the room thermostat is a programmable thermostat, so that at times when heating is least needed, the temperature remains low, although not so low that it is difficult to set the temperature again later regime environment; a temperature of 15 or 16 °C would be adequate. Those hours would be when the house is empty (the elderly at work, the children at school) and at night, when you can sleep with a good blanket. It can be done manually, but the advantage of the programmer is that it automatically raises the temperature for an hour or an hour and a half, before it is necessary and when the time comes, the house will be ready.