8       Integrating heating and cooling emission systems 

In cooperation with NKUA

8.1       General principles about heating and cooling emission systems

The fundamental principle of the HVAC system is to meet the requirements of various zones within a building in terms of:

 

 

Normally, either temperature control or IAQ determines the design criteria for the HVAC system’s equipment. In general, four types of HVAC system can be defined:

 

§  All air systems. These systems use the ventilation air to heat and cool. This can be achieved using one of the two methods described in ‘integrating mechanical ventilation systems’ (constant volume mechanical ventilation systems and variable air volume mechanical ventilation systems).

§  All water systems. These systems use water for the distribution of both cooling and heat throughout a building.  The ventilation system is independent of the heating/cooling or can be combined.  Such a combined system commonly consists of fan-coil units in the rooms, with outdoor ventilation air being drawn through the exterior wall into the unit. No central supply air is needed but, to extract air from the rooms, an exhaust air system is necessary.

§  Air-water systems. With these systems, part of the heating/cooling can be supplied by outdoor air through a central air system. The airflow rate can often be considerably less than with an all-air system. The rooms are also commonly supplied with induction units, fan-coil units or radiant panels. One advantage of using water for cooling distribution is that the all-air system ducts will normally require about 70 times more space than the water pipes of the air-water or all water systems [CAD92].

§  Unitary refrigerant-based systems. These systems consist of a number of air-conditioners / heat pumps distributed throughout the building. These systems have many possible design options. If both heating and cooling are likely to be required, the air conditioner can be changed for a heat pump which can be operated in either a cooling or a heating mode. Four commonly used types of unitary refrigerant-based system are briefly described below.

When supplementary heating and cooling is required, after all possible passive measures have been adopted three questions remain to be answered:

The answers to these questions are neither easy nor unique. There is often more than one acceptable solution for every situation in hand. The main issues that need to be taken into account are the following:

§  Level of control needed (number of zones);

§  Need for heating, cooling, humidity control, air filtration, etc.;

§  Occupancy type (density, schedule, intensity of internal gains, etc.)

§  Pattern of use (continuous or intermittent) and the level of building inertia, i.e., the type of response needed from the system;

§  Water-based or air-based distribution systems;

§  Radiative or convective systems, for aesthetic or energetic reasons (e.g., high floor to ceiling height) reasons;

§  Space requirements (interaction with architecture);

§  Investment costs, operational costs

 

8.2       Retrofitting of existing systems

In general the most effective retrofit measures are those which adjust and balance the existing HVAC system. This refers to balancing the air and water flows and adjusting the control so that the system is fully exploited.  Simple control additions can offer additional options such as efficient time-scheduling of the fans. A more comprehensive retrofit control measure is the installation of a computerized building management system, that allows checking that all equipments in the system is working properly. A BMS system also allows adjusting the controls in cases when a space in the building receives new tenants and thereby new time schedule is required.

8.2.1    Points of attention

Adjusting the water and airflow rates affects heating, cooling, and electricity use of the system. Also, insulating pipes and ducts leads to the reduction of heat and cooling losses in the distribution system. This affects the heating and cooling capacity of the system.

Often enough, existing cooling/heating features such as radiators are of historical significance (Figure 82) and thus they should be retained. When repairing these features, it should be paid attention not to damage adjacent floors and materials. 

 

Figure 82: Radiator of historic value [WBD08]

Retrofitting existing radiators in some cases may involve the use of thermal plates and/or radiator thermostats. Thermal plates are used in order to exploit the irradiation from the back of the radiator that is otherwise not beneficial for the room. It is a reflecting plate installed on the wall that redirects irradiation into the room. Materials used for these reflectors are aluminium foil, insulated aluminium foil or special insulation materials. Radiator reflectors can save up to 2-3 % of the total energy consumption of the building [ECO08]. Radiator thermostats can significantly improve the comfort and also saving energy. Additions should be incorporated in such a way that they do not disturb the aesthetics of the building.

 

8.3       New heating and cooling emission systems

During the designing of a system, it is of most significance to predict the way that it will be installed. More specifically, it is important to anticipate how potential damage to historic materials can be minimized, and how visible the new system will be. In some instances, it may be beneficial to put the systems in locations outside of the building. Alternative ways should be examined in order to reduce the heating and cooling loads, which in turn influence the size of the equipments. This may lead to a minor (and hopefully unimportant) reduction of the comfort levels in the indoor spaces, or to an increment of the number of climate control zones.

In the following various heating and cooling emission systems that can be integrated in historical buildings are described.

8.3.1    Fan coil units

Fan coil units require separate cabinets in each room, which is serviced by 2, or 4 pipes (approximately 11/2" each in diameter). Hot or cold water flows through the pipes. A fan blows air over the coils. Each fan coil cabinet can be individually controlled. Four-pipe fan coils can provide both heating and cooling all year long. Most piping exists of steel. Main disadvantages of the fan coil units: condensate pans may overflow if not properly maintained and also, fan coils can be noisy.

Since installation of fan coils is relatively simple (it only requires supply and return, condensate drain pipes, electrical power connections and control wiring) and ventilation air can be ducted to rooms that contains fan-coil units in order to minimize penetrations in the outside walls of the building, which can make this option attractive for historic buildings. If the fan coil terminal units are to be visible in historic spaces, consideration should be given to custom designing the cabinets or to using smaller units in other locations in order to diminish their aesthetic impact.

8.3.2    Radiant floors and ceilings

Floor and ceiling heating has a number of advantages compared to forced-air heating. No energy is lost through ducts and they provide uniform, even heat. They are also more quiet when comparing to other systems. There are three types of radiant floor heating: radiant air floors, electric radiant floors and hot water (hydronic) radiant floors. These types can be further subdivided to “wet installations” (which exploits of the large thermal mass of a concrete slab floor or lightweight concrete over a wooden sub-floor; and to "dry installations", in which the radiant floor tubing is put between" between two layers of plywood or the tubing is attached under the finished floor or sub-floor.

Radiant air floors are not cost-effective due to the fact that air cannot hold large amounts of heat and are seldom installed, and electric radiant floors (with electric cables built into the floor) are cost-effective only if they include a significant thermal mass (e.g. a thick concrete floor). Hydronic systems are the most popular and cost effective. In general piped systems are easier to be installed in historic buildings because the pipes are smaller than ductwork.

Care must be taken in order to ensure the heat transfer to be directed upwards (to the space that is planed to be conditioned), and not down, wasted to the space below. This can be achieved by insulating underneath the radiant floor and by not insulating on top of the floor.

 

Figure 83: American Thread building [WAR08]

If these systems are to be used for the cooling of the building, humidity sensors and floor slab temperature sensors should be employed to prevent condensation and ensure comfortable operation. These kinds of systems were occupied for the heating and cooling of selected rooms of the American Thread building [WAR08].

Utilization of radiant ceiling in some instances may be more beneficial than radiant floors – especially in the case of retrofitting a building. It is very inexpensive and easy to lower a ceiling to accommodate the radiant ceiling. On the other hand it is difficult to raise a floor. It should be mentioned though that if ceiling areas must be lowered, these should be in secondary areas away from decorative ceilings or tall windows.

Despite their advantages, the use of floor heating for the heating in historical buildings is not as common as other heating and cooling methods.

8.3.3    Mixing and displacement ventilation

Another kind of cooling / heating emission systems is the mixing and displacement ventilation systems. The principle of the mixing ventilation systems is to provide fresh air with high momentum. Thus the introduced air is sufficiently recirculated and adequate mixture of the polluting agents with the fresh air is established. Typically the air is introduced into the indoor building through the ceiling and is characterized by high momentum, so that a minimum of concentration or temperature gradients is achieved. The air movement inside the room is mainly affected by air jets.

In this configuration the flow is almost entirely created by the density differences because fresh air is provided at floor level with a low momentum and low velocities in the diffusers. The air is moved from the occupied zone to the upper levels of the room, where the air is extracted by means of extraction diffusers. The airflow is characterized by a stable thermal stratification with a vertical linear distribution of temperatures in the room created by the heat sources.

The mixing ventilation method uses high airflow. This in turn means that the diffuser has a high loss of pressure, high level of noise and pressure in fans. Thus, the electricity consuming by these systems is high. The most important advantage of the displacement ventilation when compared with the mixing ventilation is the use of small airflows. On the other hand displacement ventilation is significantly influenced by heat sources that may exist within the room. The provided air with a low velocity of diffusion and lower temperature than room’s air can produce some thermal discomfort if the temperature differences are too great throughout the vertical axis of heights.

 

8.4       Points of attention

8.4.1    Reduce the air-conditioning load

The size of the HVAC system should be reduced to the absolute possible minimum to limit cost and energy needs. This reduction should be made both in terms of peak needs (installed power) and integrated seasonal needs (energy consumption). The methods to reduce the exchanges through the envelope have been discussed in a previous section.

8.4.2    BEMS control strategies

Modern techniques based on microprocessors can, on the other hand, optimize systems operations and significantly improve overall efficiency. Following issues can be integrated into packages that may be very cost effective particularly in large buildings: 

§  Optimised ventilation control, proportional to real occupancy needs

§  Optimized economizer cycle to make the best possible use of free cooling whenever cool outside air is available.

§  Optimized use of the thermal inertia, switching the system on an off before occupancy starts and ends to dilute start-up peaks and to take advantage of stored energy at day- end.

§  Thermal storage "loading" along predicted needs for heating or cooling as a function of weather forecasts.

§  Optimized lighting control for taking full advantage of daylighting, load management, etc.

8.4.3    Cost and economics benefits

The cost-effectiveness of these measures increases with size (power), which reduces their potential use when conscientious architectural design is present, as it should. As the total loads decrease, there are less savings to compensate the required investments and payback periods increase. So, another important consequence of sound thermal architectural design is the simplification of the most appropriate HVAC system for a particular building.

HVAC operating costs can be further reduced if the fresh air intake is circulated through an energy recovery system. These systems are generally cost-effective in winter, but are usually not useful for summer because the indoor/outdoor temperature differences are quite small.

 

8.5       References

[CAD92]          Abel Erno, Aronson Stefan, Jagemar L, Nilsson Per-Erik, Learning from experiences with Energy Efficient Retrofitting in Office Buildings. Caddet Analyses Series No. 8.

[ECO08]          www.ecocouncil.dk/download/ji_brochure_english.pdf. Last accessed 16/09/08

[WBD08]         www.wdbg.org

[WAR08]         American Thread building, Manhattan NY, http://www.warmboard.com/, http://www.warmboard.com/public-relations/featured-projects/american-thread-building-new-york/