In the introduction, let us review, in terms of legislation and technologies, how to perform metering of the heat supply to residential buildings and provide users of apartments with costs allocation. As far as technology is concerned, there are several methods of how to perform metering of the heat-supply.
For heating cost allocation to end-consumers in residential buildings (users of apartments), four main methods of meter reading are commonly in use:

1. metering the heat quantity ( “calorimeters”)
   2. calculating the amount of the supplied heat using radiator indicators (“RTN”)
   3. metering the amount of maintained heat using temperature meter reading (“degree day method”)
   4. metering 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.

In this article, I am not going to theoretically analyse various metering methods of the heat- supply. On the contrary, I would like to demonstrate on concrete facts the outputs we get using the RTN (radiator indicator) method in comparison with the relatively new degree day method that is based upon external and internal temperature meter reading.

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 the “thermal comfort method” or the “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 apartment and the reference outdoor temperature. The method calculates the amount of maintained heat in room or apartment so that the difference between the inside and outside temperatures is 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 online 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 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 heat several times more than others, will get some justice, because they will only pay for the warmth of their own apartments.

RTN radiator indicator meter reading method of the heat-supply is widely used and currently, due to massive media support, often presented as the only method used in meter reading of the heat supply in residential buildings. Looking in more detail at how this method is actually applied in practice and what its results are, it can be said that its results are very ambiguous and non-transparent. The RTN technology is based upon radiator indicators that are installed directly on heat radiators. Here is where problem begins; there are dozens of radiator types, each one of different performance characteristics, so that a little mistake, such as incorrect indicator placement during assembly, can lead to significant errors in the final result.  Another source of error lies in the radiator indicator contact, which can be dramatically affected by the surface finish of the radiator. Due to aesthetic modifications during its use and maintenance, its properties can and indeed will change drastically, and, to make matters worse, in each apartment they change in a different way.
In addition, the position of the room is another important factor that makes the calculation difficult.    The calculations must include various coefficients, which take into account the individual position of the room in the building. Such coefficients are specified by the Distribution Authority, which relies on situational plans of the building. This means that they have to take into consideration whether a given room is situated in the corner, above the garage, under the roof and so forth. To make matters even worse, even these parameters can change during the life of the building. This is often caused by new insulations, new roofs or by landlords themselves carrying out internal reconstructions.

Interestingly, I would like to note that for each indicator the Distribution Authority has to set about 10 different coefficients, so that the correct calculation for heating costs allocation could be made.  These coefficients include, for example, the number of cooled walls, the location in the building, the existence of thermal insulated pipe (riser), the heat transfer coefficient between the indicator and the radiator, and six other different coefficients. It’s not really easy and in practice rarely perfect, whether it´s the unavailability of certain coefficients or a low level of knowledge and experience.
When the degree day method is used, we only perform meter reading of external and internal temperatures, knowing the dimensions of the room. For all apartment owners or cost allocators, the method is transparent and easy to read. If we compare these methods of the heat-supply meter reading, it is obvious that the degree day method is simpler and more transparent.

What are the experiences and results of heating costs allocation of in practice? We currently rely on the data obtained from the sample of two thousand apartments. The introduction of the degree day method into billing has brought a fundamental change. Heating costs now reflect thermal comfort in apartments and are calculated in accordance with the government decree on heat supply services. For illustration, see the figure that shows the variance of the heating costs in individual apartments. In the picture, the boundaries of plus and minus 40% specified by government decree are highlighted.  This limit is there just because of the use of the radiator indicator (RTN) method of cost allocation. The differences are due to the fact that radiators in many apartments are turned off, either because of being located in the central part the building or because the riser leads through them. The result is that the consumer component of the heating costs for users of such apartments is zero or negligibly low. This concerns up to 35% of apartments in the building. Then there are the other apartments, which have to have their radiators fully on just to support these above-mentioned “vacuum cleaners” of heat, even though temperatures in such apartments still will not rise above about 21 ° C. Such apartment users pay up to 30% of the heating costs of the entire building. It is obvious that the RTN method of meter reading is very far from being fair.
Let´s have a look at the results of the degree day method in detail. The introduction of this method of heat supply meter reading has brought clear and understandable outputs into heat-supply billing. From the graph shown, it is clear that this method needs no assistance by any government decree, because when this method is used, heating costs vary only in the range of plus-minus 20%. This variance reflects the actual temperatures in individual rooms ranging from 19 to 23 ° C. This can only be confirmed by the long-term heat-supply meter reading data obtained from of the sample of two thousand apartments, where this method had been applied. Our experience with apartments where heating is completely turned off, shows that temperatures in such apartments fall rarely below 19-17 ° C. This means that these apartments are, due to thermal transfer within the building, heated as well and their owners should pay for their thermal comfort. I hope I have been able to sufficiently explain why the Decree 372/2001 includes the boundaries of plus/minus forty percent.
Whether or not the allocation of  heating costs using the RTN method is more fair than when the degree day method is applied, make your own  conclusion.   The amendment to Decree 194/2007 Coll. has just become effective, specifying clearly how to perform meter reading of the heat supply to homes, making the day degree method and the RTN method equal.  I am specifically stressing this here because of certain concerns by housing unit owner association Committees that the day degree method might violate the current legislation.  It never did, but now it is even included in it.
Decree 194/2007 Coll paragraph. 7, Art. 2
” A device for heating costs allocation is an indicator for the allocation of costs installed directly on each radiator or an indicator installed on the outlet pipe from the radiator or a device with the air temperature sensor reading the air temperature of the heated area and the air temperature of the outside environment associated with a building with continuous meter reading of air temperature difference per certain time interval. “