A few reasons why performing meter readings and cost allocation of heat consumption in residential buildings using the degree day method that works automatically on-line 24 hours a day. The CEM System monitors the day degree meter reading, presenting graphs you can watch in real time!
The so-called smart apartment building, or intelligent apartment building is basically a building that is fully under online the operational control. Heat consumption costs are the key element of total costs of housing.  In order to prevent losses and leakages and to provide users of housing units with fair billing, the on-line monitoring of heat and water consumption, including the control of their supply, is therefore the basic feature of the system.
Methods of Meter Reading and Detection of the Heat Supply in Apartments
In terms of technology, there are several methods of heat detection and its meter reading. For heating cost allocation to end-consumers in residential buildings (users of apartments), four main methods of meter reading are commonly in use:

1. meter reading of the heat quantity ( “calorimeters”)
   2. calculating the amount of the supplied heat using radiator indicators (“RTN”)
   3. meter reading of the amount of maintained heat using temperature meter reading (“degree day method”)
   4. meter reading of the maintained heat by reading the temperature at the radiator outlet pipe (indirect degree day method according to VIPA patent)

All these methods are, in terms of the current legislation, equivalent and can be used for the purpose of heating cost allocation in residential  buildings with no fear of violating any applicable laws, regulations or standards.

The Principle of the Degree Day Method in an On-line System

Of all the above-mentioned  methods on the Czech market, the degree day method is the oldest (used already in the 80s of the last century), but least well-known to the general public. It is also known by the name of  “thermal comfort method” or a “gradden method”, which is of Scandinavian origin, as it was there, where this method was employed first.

The degree day method is mainly used in residential buildings, but is suitable for use not only at residential buildings, but also in some similarly designed types of commercial buildings (e.g., office buildings).
The method is based on the principle of continuous meter reading of the difference between the temperature maintained in a room or an apartment and the reference outdoor temperature. The method calculates the amount of maintained heat in the room or the apartment so that the difference between the inside and outside temperatures is being continuously measured (for each room the so-called “degree days” are calculated). The apartment´s proportion of the heating costs is then calculated by multiplying the measured degree days in individual rooms by the volumes of such individual rooms.
From the perspective of on-line monitoring in buildings, the degree day method is the most suitable one. In the past, the measured data had to be calculated based on outdoor temperature data gained at  meteorological stations, so that the process of meter reading was not actually continuous.

In an on-line system, however, such calculation takes place automatically, calculating differences between external and internal temperatures at very short intervals. Therefore the degree day method in an online system provides the owner (or a housing unit owner association committee) with vital information about the building performance, very much unlike other methods of temperature and heat meter reading. Continuous on-line meter reading of temperatures in individual rooms of a residential building provides you with complete information about the thermal performance of such building during the heating season. This information will enable you to optimize settings in order to find the right balance of the heating system, so as to supply heat to the individual parts of the heating system as needed.

In other words, we prevent overheating or insufficient heating of various parts of a building. In addition, we quickly detect apartments with disabled radiators. We will ensure fair cost allocation of heat consumption to all apartment users. Those who previously relied on the sunshine or the central location of their apartment to cover their heating expenses will not like it. On the other hand, those who are disadvantaged by the position of their apartments and must pay for the heat several times more than others, will get some justice, because they will only pay for the warmth of their own apartments.

Online Automatic Meter Reading in Real-time = The CEM System

We will describe the degree day method procedure of measuring and calculating an apartment cost allocation  on an algorithm example as used in the CEM system:

1. Temperature meter reading and calculation of degree days for each room

The reference external temperature is  read by a separate sensor installed at a suitably specified location outside the building. This temperature is used as the common value for the entire building.  The internal temperatures are read in every apartment living room. The temperature sensor should be installed at a suitable location so that it is not exposed to sunlight, and should be kept from both  heat and ventilation flow.

The differences between internal and external temperatures for each room are calculated  at relatively short intervals (about 20-60 minutes) and the resulting values are then multiplied by the lengths of time for which they were measured. The result is the number of degree days ( “DS”) for a given time interval. For example, if the difference between internal and external temperature equals 20 degrees and the length of the meter reading interval is one hour (which is 1/24 of the day), for the given interval  20 * 1 / 24 = 0.83 degree days is added.  If the temperature difference is negative (it is warmer outside than it is inside), degree days are not counted. Degree days are never counted during the days when the building is not heated (in the CEM system a special sensor on the radiator pipe is needed).

2. Standardization of degree days for the volume of the room and summarization of degree days for an apartment

The calculated  differential temperature of a room is standardized by multiplying the counted degree days at the room by the volume of the room. The result is the re-calculated number of degree days for the room. To calculate the volume of the room the heated floor area is multiplied by the height of the room. The sum of re-calculated degree days (RDS) of  individual rooms is then calculated.  For the remaining part of the apartment the weighted average method is used (see figure). The result is the summarized number of  heating degree days for the given apartment.

3. Calculation of cost allocation for an apartment

The resultant number of re-calculated heating degree days for the billing period in an apartment is used as the standard means for determining the consumption component share of the costs, very much in the same way as the annual total of  “re-calculated portions” when radiator meter reading is used or like the annual KJ total when calorimeters are used. The cost allocation for each apartment is then calculated as the ratio between the re-calculated degree days for that apartment and the sum of all re-calculated degree days in all apartments in the building. The calculation can, if necessary, be corrected by various coefficients, however, such corrections are only very rarely used, e.g., when some apartments have plastic windows and other have wooden windows.
Benefits of the Degree Day Method

What are the advantages of the degree day method in comparison with methods based on calculating heat quantities?
First of all, we should realize that the service we expect, as the apartment users, is not really heat supply, but the fact that the temperature in our apartments is being kept at the required level. Since heat is constantly leaking from any apartment, the heat provided is merely the means that the house owner uses to maintain the required temperature in the building.
First, how it works when using other methods:

Methods based on heat quantity reading or calculation  ( “calorimeters” and “RTN”) are actually based on the assumption that all apartments in a building show roughly the same heat loss (relative to the size of the apartment). If any apartment consumes more heat than the other, we assume that the “extra” heat is used to maintain its internal temperature at a higher level.
The fundamental problem of these methods is the premise upon which they are based – that equally large apartments lose roughly the same amount of heat. All of us, who have ever lived in an apartment blocks, know from our experience, that equally large apartments certainly don´t lose the same amount of heat at all.
Prefabricated homes are not completely isolated from their surroundings and their losses of heat are significantly influenced by their locations within the building, including what such apartment is adjacent to or how the neighbours behave (whether they heat more or less than we do). So, in order to compensate for losses, to keep all the apartments in a building at the same temperature, we have to deliver to each apartment a different amount of heat. This difference can be quite substantial. A ground floor apartment situated over unheated garages can consume up to several times more heat than the same apartment above the ground floor one,  just to maintain the same temperature. Therefore users of similar apartments enjoy the same comfort at radically different costs.

How it works using the day degree method in the online CEM system
This method reads the amount of heat maintained in an apartment (i.e. the amount of heat that has remained constant after all the losses). If the same temperature is kept in two identical apartments, the cost allocation for such two apartments will always be the same. Above standard payment is charged from such apartment, in which his/her user maintains above standard temperature.
The comparison of the principles themselves already implies that for the environment of Czech residential buildings the degree day method is far more suited, since, in most cases, the difference in heat losses among apartments has not usually been factored into the purchase price or rent. It is therefore not fair that a user of an apartment should suffer or gain advantage from the fact that his apartment has greater/lower heat loss than the apartment of his neighbours´.

It should be noted that suppliers of radiator meter reading systems are legally bound to compensate for  such injustice inbuilt in such types of systems by setting various coefficients (apartment location coefficient, apartment orientation coefficient …). If these coefficients are set fairly by real research  or calculation (as the law anticipates), the compensation may be sufficiently accurate.
In practice, however, due to high costs (that would make the use of RTN disproportionately more expensive), such coefficients are based on neither real research nor calculation, but are merely estimated. In addition, such estimation is often made by a person, who has never really been in the building in question.  In cases like these, the fairness of the RTN method is rather questionable.
So, returning to the question of what the benefit of the degree day method is, we must say in the first place that the method is fair.

Other advantages include its accuracy and simplicity of calculation without the use of various constants and corrective coefficients dependent on the “human factor”.
The degree day method is already by its nature very accurate (temperatures can be read with the accuracy of tenths of a degree).  The appropriate location of temperature sensor is a really important thing, making sure that the actual meter reading is not influenced by ventilation. If  the sensor is placed near the ceiling away from the window, then with heating on, no short window opening will have significant effect on the temperature within the room, as the incoming cold air from the window flows in at the bottom of the room, so that, due to thermal stratification, the temperature at the meter reading point shall remain relatively stable.
Another advantage lies in its independence of the type of heat delivery. The method can be used in rooms with radiators, under floor heating, air conditioning or convectors, or even in apartments and rooms with combinations of these technologies.
The big advantage of the day degree method lies in the fact that it is the only method that provides owners with a comprehensive set of data on temperature conditions in various parts of the given building that can be used for both building performance monitoring and compliance, but mainly as a solid basis for the correct adjustment of the its heating system.
Disadvantages of the Degree Day Method

Obviously, the degree day method has some disadvantages: it ignores internal heat sources of the user  (mainly heating from electric appliances) and does not adequately penalize apartment users for excessive ventilation.
According to published comparison studies on various methods of heat consumption meter reading, the impact of the above-mentioned circumstances on the overall accuracy and fairness of billing is minimal and creates inaccuracies of  several percent, many times less than the RTN method.
The most reliable day degree method meter reading is the online CEM System.
The degree day method of on-line monitoring of the heat and costs allocation has revealed in most cases that buildings are overheated by 2-3 degrees, most especially the insulated ones. The use of method resulted in the reduction of disparities in cost allocation between the outlying apartments and apartments located in central areas of buildings. Practical experience gained from implementation of the day degree method shows that the method is more fair, as it is taking into account different positions of apartments within a building, a fact that can be verified by users of two thousand apartments, where the CEM online monitoring system is already being used.

Authors: Ing. Ján Vlček, Ing. Jaromír Charvát, Mgr. Lucie Charvátová