The investor's and developer's goal at the beginning of every project is quite obvious: To build or renovate the building using modern technologies. Execution costs often play a huge role in the decision-making process. However, this innocent idea does not always lead to the expected savings. One such project is the redevelopment of the historic building in the center of Prague.
We are talking about a building full of greenery, glass walls, marble and elegance. Carefully planned, advanced lighting technology, individual heating and cooling controls, humidity control in rooms, automatic shading and many other solutions emphasize the exclusivity of the place. The building is environmentally friendly. The modern approach to ecology is proven by the LEED Gold certificate. Unfortunately, even such internationally recognized quality mark does not guarantee use of an ecological or efficient heating source.
Redevelopment of the building included renovation of office space on eight above-ground stories with a total area of 6830 m 2 as well as the addition of a new part of the building where the original inner courtyard used to be. Another element of redevelopment and one of the project's objectives was to also create an efficient heat source able to maximize the use of energy from fuel and to transfer it to the building as needed. These requirements were met by the design utilizing several gas condensing UltraGas® 800D boilers. The installation diagram is presented in the figure below:
Due to the budget reduction by the investor, at the execution stage, the Hoval condensing boilers were substituted for another type of devices which, according to technical documentation, seemingly displayed similar technical parameters and properties. Said change, however, entailed more modifications, including replacement of the hydraulic installation, and, above all, affected functionality of the entire heating system of the building.
If we take a closer look at the enclosed system diagram, we will find significant differences.
No Primary Pump
The UltraGas® boiler diagram operates using a large water capacity boiler without the minimum water flow requirement which means that, in the drawing, we will not find a primary pump with hydraulic switch or a differential pressure regulator (torus).
Separate high- and low-temperature returns
On the side of the device, we can see separate heating circuits. A circuit for higher temperatures e.g. for ventilation with a temperature gradient of 80/60 °C and a circuit with a low temperature gradient of 35/25 °C for floor heating, and other heating circuits with lower temperature requirement
We know that the dew point of flue gas is around 57 °C. It was shown that if there was a
movement of return water from all the circuits, the temperature would oscillate above this point, as a result there would be no flue gas condensation or no use of
its heat. Therefore, it is important to consider separation of hot and cold return water in both the boilers and the distributor. Thanks to this, using cold return
water, the condensation process may be initiated.
Another advantage of the UltraGas® is, that it doesn’t has a delta-T limitation (temperature difference between flow and return). The deciding factors for a system are the maximum flow-temperature needed and the return temperature (because of condensation).
Boilers with aluminium heat exchanger only operate with a ΔT up to 25K. UltraGas® doesn’t have this limitation. Compared to a system with aluminium-heatexchanger-Boiler, a system with UltraGas® doesn’t need additional components (hydraulic switch to increase the return-temperature, pumps e.g.)
Comparison with the used diagram of the executed installation suggests that, when doing the replacement, one did not take into account the heating system design but solely the price of the solution.
In view of the fact that, in the described case, the source consists of two condensing boilers whose structure and hydraulic resistance require an own primary circuit pump supposed to ensure the flow, a group of relatively large pumps was installed that operates practically non-stop.
In order to ensure proper operating conditions for individual pumps, the circuit of the source and the circuit of devices are separated with a differential pressure regulator. Due to their structure, the installed boilers do not allow the temperature difference at the inlet and at the return to be higher than 25K; the flow is therefore designed for this temperature gradient. At maximum combustion power in both boilers, the temperature gradient amounts to 25K. However, in a situation where the power is limited, also the boiler circuit's temperature gradient is lower (maintaining a proportion). The boilers must keep the temperature determined by requirements of distribution, and so the temperature at the return to the boiler is significantly increased which constitutes a further reason why, in the described situation, boilers operate beyond the limits of true condensation.
10% less efficiency without condensation (if condensation doesn’t work)
Condensation occurs if one manages to achieve cooling of flue gas down to a temperature below the dew point, when the heat of condensation is released. The heat of condensation constitutes about 11% of energy in burned gas, and efficiency increase depends on the achieved level of flue gas cooling. The true difference between condensation and non-condensation operations in the case of condensing boilers amounts to ca. 10% (interesting note: compared to low-temperature boilers with flue gas temperature of ca. 200 °C, there are differences of up to 25%).
In the given case, such substitute boilers – despite even very good conditions allowed by the device – are not and, actually, cannot be used in the condensation mode.
Figure 2 Diagram of the actual boiler plant including return temperature.
Although it may seem that replacement of a given type of condensing
boiler with another type is quite simple and effortless, the entire operation is much more complex, and replacement of one boiler with another will not
suffice. In our case, replacement of condensing boilers reduced the
cost of devices, however, significantly increasing operating costs.
Let us assume that a gas boiler plant with an output of 750kW will on average cost 63 000 euro per year. The savings that would be generated by the previous boiler variant with an efficiency of 109% operating in the condensation mode correspond to 10% of gas costs i.e. 56 700 euro per year. Consumption of electric energy in the case of the boiler's circuit pumps, assuming the scale and time of operation as well as the current electricity prices, corresponds to consumption of ca. 5 000 kWh per year, and thus around 1700 euro per year. Summing up, operation of the system in the described case costs 8000 more per year.
Savings in implementation of the project whose objective was to replace the boilers were ruled out by purchasing costs of circulation pumps and their controllers, the differential pressure regulator as well as, in this specific case, of the flue gas exhaust system (an UltraGas® double boiler would have needed only 1 flue gas conduit). Moreover, as far as the material and structure are concerned, in the case of the selected boilers one should not assume an excessively long lifetime that would be ensured by the original boilers being part of the design: up to 30 years.
Change in numbers
• Approximate costs of the boiler plant with an output of 750kW: ca. 63 000 euro/year
• 10% savings on gas costs ~ 6300 euro/year
• Increased electric energy consumption 5000 kWh/year ~ 1700 euro/year
Total increase in operating costs = 8 000 euro/year
• Circulation pumps, their control, differential pressure regulator, flue gas exhaust system
Total savings on the project compared to the original design: ca. 3700 euro
• Current solution – Al material – greater emphasis on water quality – expected lifetime: 10–15 years
• Hoval technology – combination of aluminum and stainless steel – expected lifetime: up to 30 years
Investment in modern technologies and intelligent control systems entails numerous decisions and requires in-depth knowledge of the subject matter. Unfortunately, certificates awarded to buildings do not guarantee optimum use of technical solutions. The planning and implementation process is influenced by several factors which, independently of one another, shape the final form of the building including its performance. Savings on investments are not necessarily savings in the proper sense of the word because they can have a much more profound, long-term effect on maintenance costs, lifetime and functioning of the system as a whole. Unfortunately, in the first case study on redevelopment of the historic building in the center of Prague, we are dealing with a process that shows the reality of the building industry where the consequences of increased operating costs are borne by the tenant or new owner.
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