Weather-dependent schedule of heating systems in houses and schedule of heat supply from thermal power plants

How is a constant room temperature maintained with radiator heating?

To maintain a constant temperature in our homes in winter, it is necessary to adjust the heating power in houses and apartments when the temperature outside changes.

This is achieved by using the so-called “weather-dependent heat supply schedule”.

It is known that heat loss through external walls and windows depends linearly on the temperature difference between the street and the room.

That is, the greater the temperature difference with the street, the more heat needs to be supplied to the room to compensate for these heat losses.

For water radiator heating systems, this “weather-dependent heat supply schedule” is expressed in a linear graph of the temperature of the water supplied to the radiators versus the outside temperature (see Fig. 1.)

Such a schedule is maintained in the water heating system with the help of special automatic control systems, which are located in the boiler room of a private house in an individual housing construction, in the ITP of a separate apartment building or in the central heating station of an urban microdistrict.

Rice. 1. Family of linear “weather-dependent graphs” of a radiator heating system. The choice of curve depends on the characteristics of the heating system. For radiators of a single-pipe system in panel houses with a supply temperature of up to +105C, this is a curve of 1.8. For systems with conventional cast iron radiators on the 90/70C curve, the curve is 1.6-1.4. Heating with heated floors in individual housing construction schedules 0.6-0.4. A bunch of curves can move entirely with their origin along an inclined scale when the desired temperature in the room changes.

Why can’t a weather-dependent schedule be established at city thermal power plants?

Heat supply from city thermal power plants and boiler houses is used both for heating houses and for heating domestic hot water (DHW).

At the same time, the heat supply schedule for heating devices in individual housing construction can, in theory, drop even to +20C (for example, in large and eternally empty private houses in individual housing construction, where heat inflows from people are insignificant in the overall heat balance).

In actual practice, heating of apartment buildings in cities begins at +8C outside, while the temperature in the radiators is about +40C at the start of the heating season (graph with a slope of 1.4-1.6 in Fig.)

To heat cold water from +5C in a cold water supply to +60C for DHW needs, it is necessary to supply heating water with a temperature not lower than +65C to the DHW heat exchanger.

Thus, water with a temperature not lower than +65C is supplied from the thermal power plant and boiler houses along the heating main to the central heating station, even in the summer.

It turns out that at the thermal power plant there is a certain section of the graph with “weather-dependent” regulation, but at relatively warm outside temperatures the graph becomes constant at the level Tgvs = +65C. (see Fig. 2-3)

Fig.2. Schedule 150/70-20 of weather-dependent heat supply to central heating stations from external heating networks with a “horizontal shelf” at a temperature of +65C for heating hot water. In the “DHW shelf” section, the return temperature also gets a break and pushes the graph upward, which means “overtopping” in the nodes with the elevator.

Rice. 3. Schedules for supply from the thermal power plant, as well as direct and return water in the heating system with dependent unregulated connection through the elevator unit. The heating schedule has an upward bend in the “DHW shelf” section of the supply schedule from the CHP plant.

Looking at the theoretical operating schedule of the thermal power plant, it becomes clear the reason for the strong “overflow” in the premises during the off-season at street temperatures Tul=+8..+1C, if the heating is connected according to a dependent circuit using an “elevator unit” with a constant admixture coefficient.

So, on an inclined section of weather-dependent heat supply from a thermal power plant, the elevator quite regularly maintains a weather-dependent schedule in the radiators of the heating system.

But when reaching the “DHW shelf”, some overheating of the supply from the elevator begins.

Moreover, the higher the temperature outside, the more pronounced the “overheating” is in the rooms. (see Fig. 4.)

Fig.4. Scheme of the occurrence of “overflow” in the off-season on a DHW shelf +70C and “underflow” in extreme cold with a horizontal “schedule cut” (in this case, cutoff of the graph at +110C with a weather-dependent schedule +130/70C)

To get rid of “overheating” in the off-season for heating systems from external networks, additional weather-dependent regulation is provided in the central heating point.

Unlike an elevator, with a constant mixing coefficient, direct weather-dependent control in the heating system from the central heating system requires changing the mixing coefficient at a constant flow of coolant through the heating system, which is only achievable by pumping the internal circuit of the radiator heating system. (see Fig. 5.)

Fig.5. The graphs show the zone where “overflow” occurs in the off-season at elevator units without additional regulation and without pump circulation.

Cutting off the graph from thermal power plants at high temperatures.

Even with extreme cold outside, the weather-dependent schedule from the thermal power plant is not fully observed. There is a so-called “schedule cut”, which for Moscow is expressed by a horizontal shelf at the level of +130C with a schedule of +150/70C.

This cut began to arise spontaneously back in the 1970s, and it is a non-design consequence of the complexity of centralized heat supply from thermal power plants to unregulated heating systems with elevator units.

Oversized heating systems of residential buildings with a large share of ventilation load in heating radiators create an interesting effect of self-regulation of heat loss in the premises by residents. So, during severe frosts, people begin to close the windows due to “drafts in the legs,” thereby reducing heat loss through ventilation, but thereby increasing the temperature in the rooms.

This covering of the vents leads to overheating in the premises and to an increase in the return temperature in the network to the thermal power plant.

Increased return flow is money down the drain for thermal power plants.

Therefore, at thermal power plants they begin to lower the supply temperature so that the return becomes colder.

Since the elevator units are not regulated, a decrease in the supply temperature causes cooling in the apartments, which causes an even greater decrease in ventilation.

As a result, the amount of heat needed for heating goes into the heating network, but the supply temperature does not increase, but the return temperature begins to fall.

Thus, it turns out that the CHP sends into the network everything that the heating system can receive, and the heat flow is regulated with a change in the return temperature at a constant supply temperature at the “cut of the schedule.” (see Fig. 6-7.)

Fig.6. An example of a weather-dependent schedule from a thermal power plant with schedule cuts. You can see an upward kink in the return graph under the DHW shelf, as well as a downward kink in the return graph under the “graph cut”.

Fig.7. Explicitly designated weather-dependent graph +120/70С with cutoff temperature supply at +95C and with a return bend downwards. Judging by the font and surnames, this document is from some former Baltic Soviet republic, and now a neighboring country.

Overflow in houses “with an elevator” is due to lack of traction and the low mixing coefficient of the elevators themselves.

Flooding in houses with elevator units has not only a spring “overflow” phase of the entire house, but also a form of constant year-round “overflow” of the upper floors and “underflow” of the lower floors.

That is, the local “overheating” of the upper floors is present throughout the heating season, especially intensifying in the off-season.

This year-round “overheating and underflowing” is no longer associated with the “DHW shelf” in the diagram of heating networks, but with theoretical problems in explaining the structure of the elevators themselves.

Thus, systematic “overheating” is associated with overheating of the supply above the design temperature due to reduced admixture of cold return at the elevator.

That is, less water is pumped through the heating system than calculated. But at the same time, the consumption of hot water from the heating network remains constant. As a result, the temperature in the supply pipe from the elevator to the first radiator turns out to be significantly higher than the calculated one.

Due to this lack of circulation, in houses with a vertical single-pipe system with top supply, a strong “overflow” of the upper floors occurs, which is compensated by excess ventilation from constantly open vents.

At the same time, the lower floors can be quite cool even without opening the windows for ventilation.

This phenomenon is not only not advertised, but rather “heavily hushed up” so as not to discredit the old “scientific schools” of elevator developers.

Replacement of “elevators” with ITP in individual houses

To combat “overheating” on the upper floors and “underheating” in the lower floors of the same building, modernized “individual heating points” (IHP) with pump circulation have been intensively introduced in recent years.

When installing a pump IHP instead of an elevator, it is possible to increase the circulation of water along the internal circuit of the heating system, thereby reducing the temperature difference between the first and last radiator in the system. This reduction in the temperature difference in the radiators also equalizes the temperature in the rooms on all floors.

On the upper floors the level of “hellish heat” decreases to a state of “normal warmth”, and on the lower floors, on the contrary, it becomes “pleasantly warm” instead of “chilling coolness”. Only residents of the middle floors of the house will not notice the difference from installing ITP.

That is, a fairly simple and relatively inexpensive procedure for installing a low-power circulation pump in an old heating system with an elevator dramatically increases the efficiency of the system and increases the overall level of comfort in the house.

In addition to the circulation pump of the internal circuit, the IHP must contain a valve-regulator for the flow of hot water from the city heating network. This valve changes the flow rate of hot water from the heating network to the internal network in such a way that a “linear weather-dependent curve” is maintained in the batteries. (See Fig. 8.)

Fig.8. ITP diagram of a house with independent connection of the heating system and weather-dependent regulation of the internal circuit by changing the flow of hot water from the heating network through the heat exchanger using a valve with a servo drive controlled from a weather-dependent automation cabinet.

When installing an ITP, residents have the opportunity to regulate heat consumption from the heating network directly by the temperature of the water in the batteries according to a “weather-dependent temperature schedule,” while ignoring the broken temperature schedule of the heating network itself imposed by the CHP.

If the heating system was designed with a large power reserve, taking into account excessively strong ventilation, then installing an IHP allows you to reduce the slope of the “weather-dependent schedule” curve, which will accordingly lower the temperature in the apartments and reduce the “overheating”.

Reducing the “overflow” will also reduce heat consumption for heating, which will allow residents to save a little on heating if they have a common house heat meter in the ITP.

The installation of heat meters or UTE (thermal energy metering device) has been increasingly promoted in heat supply systems in recent years in order to use heat resources more efficiently. It is at the moment of installing the almost mandatory UUTE that you can easily supplement the old elevator system with weather-compensated control and an internal circuit circulation pump.

Heat metering unit (UUTE) as part of the ITP

UUTE is essentially an ordinary heat meter.

UUTE consists of an instantaneous water flow meter (PPR-primary flow transducer), two direct and return water thermometers at the inlet and outlet of the heating network in the IHP, as well as an additional unit for calculating the thermal energy taken from the water. (see Fig.9)

Fig.9. Drawing of a full-fledged UUTE with two PPRs.

UUTE is a rather complex measuring and computing complex of devices, since it is physically impossible to directly measure the heat flow with any one sensor.

UUTE can be installed without any connection with the type of ITP, although for independent connection the ITP structure is a little more complicated. (see Fig. 10.)

The simplest version of UUTE with one PPR can even be installed in the system even with an old elevator unit without modernizing the elevator itself. (see Fig. 11.)

Fig. 10. View of a real UUTE. Judging by the thin outlet line with a paddle water meter and pressure regulator (brass device with a blue spring), the ITP itself has an independent connection with a separation heat exchanger, and the thin pipe is a constant pressure make-up line in the internal system.

Rice. 11. Installation of UUTE with one PPR in front of the elevator unit without modernizing the elevator itself and with minimal alterations in the pipes.

Dependent ITP circuit

ITPs are divided by type into “dependent” and “independent” connection schemes to the heating network.

“Dependent” connection scheme is an option with an elevator unit or an internal circuit circulation pump without a separation heat exchanger (see Fig. 12-16.)

Fig. 12. Schematic diagram of an elevator heating unit.

Fig. 13. A realistic cross-sectional image of an elevator and an elevator heating unit.

Fig. 14. Schematic diagram of a dependent heating unit with pump circulation of the internal circuit.

Fig. 15. View of the dependent connection unit to the heating network with a circulation pump (red on the left) and an automatic regulator of the constant flow of coolant from the heating network (black with brass nuts on the right). This unit was installed instead of the elevator unit. I note that the pump is installed incorrectly, since all wet rotor pumps must be installed in a horizontal rotor position. That is, this pump must be unscrewed and turned on its side at 90 degrees from its current position.

Fig. 16. The same node from a different angle. In the distance, two white PPR devices (primary flow transducer) from the heat meter at the inlet from the heating network are visible.

Independent connection scheme to ITP

The “independent” connection scheme is an option using a separation heat exchanger, which ensures that the heating network and heating system are separated by water pressure in the systems, and heat from the network is transferred to the heating circuit through the thin walls of the heat exchanger. (see Fig. 17-18.)

Fig. 17. Two IHP schemes with independent connection through a separation heat exchanger (the difference is only in the degree of detail of the drawing).

Fig. 18. Realistic Three-D model of the heating heat exchanger piping unit in the ITP. It is clearly seen that this is a rather complex and expensive-looking unit, but functionally it plays the role of a simple Elevator with an additional circulation pump. Control valve with a servo drive to change the supply from the heating network to the heat exchanger is shown in blue (behind the yellow water meter for recharging the internal circuit).

The “independent circuit” is much more complex and expensive than the “dependent circuit” both in installation and operation.

Plate heat exchangers themselves are not only more expensive than elevators, but they also have an unpleasant habit of clogging, which leads to a decrease in their efficiency.

To restore the original characteristics of the system, the heat exchangers are washed every summer using special aggressive detergents, which is quite labor-intensive and requires additional annual costs.

Therefore, in old houses with elevator units, it is more profitable to maintain a dependent connection scheme when upgrading ITP with the installation of UUTE.

When upgrading the ITP with the installation of UUTE, it is beneficial to replace the unregulated elevator with an adjustable valve with a servo drive, and also to ensure constant pumping circulation of the internal circuit.

The presence of an adjustable valve will provide direct control of the supply temperature according to a weather-dependent schedule, regardless of the schedule of the heating network.

Savings from upgrading ITP and installing UUTE

Initially, the mandatory installation of UTE was done not for the sake of saving money for residents, but precisely for the sake of collecting additional money from them for unaccounted for heat.

Based on the results of installing the heating system, heating payments may either increase or decrease.

If, instead of a dependent connection with an elevator, a separation heat exchanger is installed, then the house will definitely become cooler. This is due to the need to maintain a small temperature difference dT = 3-5C on the heat exchanger plates, so that heat is transferred through the heat exchanger plates from the hot water from the heating network to the colder water of the heating system.

As a result, with a constant return temperature to the heating network at the level of T2 = 70C, the graph in the radiators will have to fall from 90/70C to 85/65C, which will lead to a decrease in temperature throughout the house.

With such a lower schedule, the house will not freeze, but people will open the windows less often, which will additionally lead to heat savings and a reduction in heating bills.

That is, it is not the UUTE or IHP that saves energy, but the people themselves reduce heat loss for ventilation by reducing the heat supply to the heating system through the new IHP.

When the schedule drops by dT=5C from the initial 90/70C to 85/65C, savings of about

=((90+70)/2-22)/ ((85+65)/2-22)=1.094 or 9.4% in nominal value.

This is precisely the 10% savings that can be expected in the payment when upgrading ITP and installing UUTE.

The actual payback of modernization can exceed 10 years, which makes such modernization unprofitable for residents.

But for a heat supply organization, the installation of an ITP allows for more complete use of the available boiler capacity in the off-season to supply heat to new point buildings in old established neighborhoods.

That is, the cutting of the schedule from the CHP plant is a consequence of the excess rated power of consumers exceeding the capacity of the CHP boilers.

So, to balance the heat consumption of enterprises and heat production at thermal power plants, there is a practice of planned shutdowns of supply ventilation systems at enterprises when the outside temperature drops below minus-15C.

Enterprises and offices can easily do without heating the supply ventilation for a couple of weeks a year, while radiator heating continues to operate at full capacity.

The result of such a shortening of the schedule for thermal power plants is increased profit due to the long-term, multi-month full load of peak boilers at thermal power plants, starting from street temperatures of minus -15C, and not only at nominal calculated minus -30C for a couple of weeks during the winter.