How to ensure trouble-free heating

Early 2020 will see the new government publish an update to part L1A of the Building Regulations and there is a strong possibility it will include mandatory low-temperature heating systems in new-build properties. It may also include measures, such as maximum permissible flow temperatures of 55°C. But we cannot forget that the vast majority of UK homes are existing retrofit properties, which will also need to be updated to low-temperature heating systems.

The CIPHE renewables and low-carbon technical working group agreed its position on this matter at the last meeting in November (read more about this on page 18) and will be asking for mandatory maximum flow temperatures to be applicable in all new retrofit installations by 2025 at the latest. This transition should not be taken lightly, however, as the majority of heating installers will need to upskill in various areas of heating design.

Warm-water underfloor heating (UFH) systems will become more popular as a means of delivering low-temperature heating into the buildings of the future. It is therefore important for installers to understand how to correctly size the primary pipework and pumps in order to ensure trouble-free, efficient heat for the consumer.

Basic pump theory

Typically, underfloor heating systems are designed with multiple pumps and manifolds to feed different floors. As such, anyone installing a low-carbon heating system in the future will need to understand how these multiple pumps can operate correctly, without reducing their lifespan.

The two diagrams (below) show two types of pump configuration – one running in series, the other in parallel – and how each configuration effects the available flow and pressure head within the respective system.

Figure 1a illustrates that when two pumps are fitted on to the same pipework (in series), the available pressure head increases, but the available flow remains the same. Conversely, when two pumps are fitted in parallel (Figure 1b), the available head of pressure stays the same, but the available flow increases.

Unfortunately, fitting pumps in series can cause issues in the system and may damage the pumps themselves, resulting in an unhappy customer. This type of scenario is quite common in heating systems that contain a system or combi boiler, which have their own integral primary pump.

Figure 2 shows a typical layout of a system boiler with hydraulic separation that prevents the pumps from operating in series with this increased head of pressure scenario. While this increased head of pressure will help distribute the heat more effectively, it will also accelerate the risk of pump failure in the future, as well as create unnecessary noise due to the high-water velocities.

How can you avoid pump failure and system noise?

There are two things you need to do to avoid pump failure and noise in an UFH system running two pumps in series: check the pumps are of the required size and ensure that the pipework is big enough to deliver sufficient flow.

The first thing to do is check that the size of the pump supplied by your underfloor heating designer and supplier meets the requirements of the index circuit (UFH loop) from your manifold. The index circuit is the loop with the greatest pressure loss, such as the longest UFH loop, and will need to be added to the pressure loss created through the UFH mixer valve and manifold at a given design flow rate.

When plotting your UFH system duty point onto a pump curve, you will need to add up the total flow volume of all of the UFH circuits, along with ascertaining the circuit with the greatest pressure loss (the index circuit).

The second thing to do is to ensure the primary flow and return pipework from the heat generator is sized correctly to deliver sufficient flow, with velocities no greater than 1.5m/s to the manifold to meet the peak heating load. The primary pipework from the heat generator will need to be sized correctly to ensure that the frictional pressure loss created through the pipe and fittings is less than the available head of pressure delivered by the integral boiler pump.

As the UFH pump supplied with the manifold will need to be hydraulically separated to avoid the primary and secondary pumps operating in series, there will need to be some further investigation into the system.

Create a hydraulic separation

One option for creating hydraulic separation between both pumps is by installing close-coupled tees. It is important to hydraulically separate the UFH pump and manifold from the primary pump circuit to avoid the aforementioned pump-in-series problem. A close-coupled tee configuration is a low-cost but effective solution.

Figures 2 and 3 illustrate a close-coupled tee installed before a UFH manifold with simple rule-of-thumb guidance. Although a close-coupled tee configuration is a low-cost option, there are other suitable solutions, albeit at greater costs – such as loss headers with built-in air and dirt separation.

What happens if you find a system with two pumps running in series and no hydraulic separation?

One of the issues with retrofitting hydraulic separation to solve the pump-in-series problem is that you may find that both the primary and secondary pipework is undersized. As described earlier, two pumps installed in series will have increased the available pressure head, so retrospectively adding some form of hydraulic separation may cause the original system to fail, as both the primary and secondary circuits will then be subsequently operating on a single pump. In addition to the hydraulic separation, you must ensure the water in the system is clean and free of air, then perform a thorough system balancing procedure. Do not forget that by making these retrospective incremental changes you will have changed the balancing requirements of your heating system.

What to expect in 2020

The CIPHE renewables and low-carbon technical working group is developing a training course that will help equip installers with the core skills required to deliver efficient and effective low-carbon heating to consumers. For more on the training and the switch to low-carbon heating, see page 18.

This article first appeared in the Jan/Feb 2020 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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