One-string systems: The uncontrollable rebels of heating
A beam of spring’s first sunlight falls through the basement window of the six-storey apartment building that Mads Kjær Birk manages in Aarhus, Denmark. He gesticulates enthusiastically. “This could really be the solution we have been looking for,” he says. He’s talking about the newest addition to the building’s heating system – a pump.
But why is he so happy? Well, our story begins with an inevitable and persistent challenge for building managers of buildings with so-called one-string heating systems.
One-string heating systems are common in many parts of the world. Although they fulfil their basic purpose of supplying heat to the residents of thousands of residential buildings, they are difficult to control.
In a one-string system, all the heating surfaces (typically radiators) in the building connect to one single string (see Figure 1). The hot water from the heating plant enters the system at the top of the building and rushes down through the string, heating up radiators on its way down. When the water returns to the heating plant, it is either much colder than it was at the top, or almost the same temperature as it was at the top – depending on how many radiators were open in the building when the water made its way through the system.
That means that a one-string heating system is a system with very little control. Often, the system is designed to be a constant flow system – which means that pressure control offers little help, since the pressure in the system is constant, even though the demand for heat varies from day to day.
Delta T is the difference in temperature between the flow and the return flow in a heating system. A low Delta T is a symptom of a system that is not using the heat energy in the water efficiently enough. In a one-string system, the poorly regulated return temperatures result in a delta T that is often too low, consumers that lack comfort, and heating plants that are dissatisfied with the high return temperature. The consequence can be substantial fines from the heating plant to the building owner.
So if pressure control is not the solution…
The delta T challenge in one-string systems has been known for years. But the solution to the problem did not present itself until a new pump technology was developed, enabling the pump to adjust its performance based on the difference in temperature instead of the difference in pressure. In other words, pressure control is not the solution to this problem, temperature control is.
One-string systems are the result of a time when there was a big focus on fast installation times and inexpensive systems – and they exist in large numbers all over the world. But they are very hard to control, and the temperature-controlled solution could be a breakthrough in that regard.
Anders Nielsen, Application Manager, Grundfos
A new kind of control
The Grundfos TPE3 in-line pump is so intelligent that it can control a heating system’s performance based on the differential temperature between flow and return flow temperature. So what does this mean for one-string heating systems?
In a typical, uncontrolled one-string system, a high return flow temperature can only mean one thing: The system’s load is low, meaning the hot water from the heating plant is not cooled sufficiently on its way through the system.
On the other hand, a low return flow temperature means that the load in the system is close to 100% and the system is thus delivering return flow in the temperature that it is meant to deliver.
The TPE3’s designers thus took up this one-string challenge. If a pump could measure the temperature of both the flow pipe and the return flow pipe and compare the two, it could automatically adjust its flow, because the differential temperature reveals the current demand in the system. And this is precisely what the TPE3 does (Figure 2).
Temperature control is the answer
So instead of relying on pressure control in a one-string system, you can now rely on temperature control from the very pump that controls the system’s flow. This way, you have complete control of the delta T, which will increase comfort for the residents of the building, increase the system’s energy efficiency and enable you to live up to your obligations with the heating plant.
“It’s really a completely new way of controlling one-string heating systems,” says Anders Nielsen, Application Manager in Grundfos. “One-string systems are the result of a time when there was a big focus on fast installation times and inexpensive systems – and they exist in large numbers all over the world. But they are very hard to control, and the temperature controlled solution could be a breakthrough in that regard.”
Taking back control with TPE3
Mads Kjær Birk manages a series of buildings in Aarhus, Denmark. A few of them have one-string heating systems, and the challenge is the same across the board: One-string systems are hard to control, and the high return temperatures have resulted in multiple fines from the heating plant.
“To be honest, I had never heard of a TPE3 pump when Grundfos called,” says Mads Kjær Birk. “But I thought it would be really cool to see if this solution could be the answer to our problems, so it didn’t take much consideration to accept their offer to do a pilot project.
Almost like an iPhone
Grundfos installed a TPE3 pump complete with temperature sensors in the building’s heating system in March 2015. The pilot project will run for a year, and the expectations are high.
“I do expect a lot from this project – in terms of system efficiency, customer satisfaction and a happy heating plant,” Mads Kjær Birk says. “What I am very excited about is how easy the pump is to use – it’s almost like an iPhone. The display tells you exactly what you need to know and the interaction with the pump is a pure joy. And you can perform some pretty complex things with the touch of a button.
Facts: The Langenæs building, Aarhus, Denmark
- Built in 1958-60
- 70 apartments
- Building Manager: Mads Kjær Birk
- TPE3 test conducted from March 2015
Facts on the TPE3: Grundfos’ new in-line pump
- Low energy consumption – better than IE4 and above the efficiency at benchmark MEI rating
- Integrated ΔT control, saving the costs of a differential temperature sensor
- Built-in heat energy meter for complete control
- FLOWADAPT and other intelligent control modes ensure energy savings, easy commissioning, elimination of pump throttling valve and more.
This link will take you directly to TPE3.