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The essential role of underfloor heating manifolds and flow meters in achieving a balanced system
What is the role of underfloor heating manifolds and flow meters?
The manifold is the heart of the underfloor heating system. It’s the central hub where the supply and return pipes from the different circuits converge. It connects the pipework to the heat source and sends the water from the boiler or heat pump into each circuit at the correct flow rate.
A manifold consists of a flow manifold and a return manifold. The flow manifold is fed by the warm water from the heat source and is nowadays typically equipped with flow meters that indicate the flow rate in each circuit. The calculated flow rate of each circuit is set on the flow meter or an adjustable check valve.
The return manifold is equipped with valves that open or close the individual circuits, depending on the settings on the room thermostat.
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Why is it so important to balance an underfloor heating system?
About 85% of old systems and 60% of new radiant heating systems are not hydronically balanced. Yet, balancing is an important prerequisite for the optimal functioning and energy-efficient operation of the heating system. Only if the system is balanced correctly, does each heating circuit receive exactly the amount of water it needs to cover the heat load. Neither too much nor too little. Moreover, a balanced system ensures fast and even heating as well as greater control accuracy, which in turn leads to better control quality and lower energy consumption.
Olaf Kloetzel, Product Manager Radiant Heating & Cooling at Purmo, explains: “Water always flow along the path of least resistance. This also applies to the heating water that flows through the parallel circuits of a radiant heating system. Due to different area sizes, the circuits have different lengths and therefore different pressure losses. If you don’t balance the system, the short heating circuits will get over-supplied, resulting in overheated rooms, while the long circuits won’t get enough water. You therefore have to adjust the flow rates to suit the length of the pipe in each loop. By balancing the system, you create additional pressure loss on the shorter circuits so that, in the end, every circuit has the same resistance and receives the water quantity it needs. This way you create an even and comfortable warmth across the entire floor surface.”
Hydronic balancing in underfloor heating design
Of course, before you can correctly balance the system, you need to know which values to set on the flow meters. These are calculated during the system design process and generally included on the calculation documents. The following steps are essential to reach a correct (subsequent) calculation:
- Determine the heating area of the circuit (m2).
- Determine the heat load of the circuit area. For this, you multiply the building-specific heat load in W per m2 by the heating circuit area. In existing buildings the heat demand is often a rough estimation based on the year of construction. For example, for older buildings (constructed circa 1995) the heat load is about 100 W/m2, while for modern buildings (constructed circa 2004) it’s closer to 50 W/m2.
- Determine the temperature difference between the flow and return. A commonly used reference value is 7 K.
- You can now calculate the water flow for the heating circuit (l/min) by dividing the heat load by the product of the temperature difference multiplied by 70.
For example, a heating circuit of 4,5 m2 in a building with a specific heat demand of 70 W/m2 has a heat load of 315 W. If we then have a temperature difference of 7 K between flow and return, this results in a required water volume of 0,65 litres per minute to heat up the room.
If you don’t have the resources or time to make this kind of calculations for your project(s), you can always call on our calculation service. Our trained experts are happy to help lighten the load and take charge of the underfloor heating design and all associated tasks.
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How to adjust an underfloor heating manifold and flow meter?
Once you know the water quantity required in each circuit, you can adjust the flow rates on the flow meters by turning the flow adjustment valve. You can easily read the water quantity in the glass of the flow meter to ensure that every circuit gets the quantity it needs to efficiently heat the room to the desired temperature.
“This is a great example of how product innovation and technology facilitate an everyday task for planners and installers”, says Olaf. “In the past, manifolds only had regulation valves and it was quite difficult to set the right flow rate. Based on the amount of litres needed in the circuit and the pressure loss you needed to read on a diagram how many turns you had to close the regulation valve. It is therefore not surprising that formerly the majority of radiant heating and cooling systems weren’t balanced. Now that most manifolds are equipped with flow meters, it’s much easier because you can simply read the water quantity directly on the flow meter.”
You don’t need to use any tools to adjust the flow rate on the flow meter. You can easily do it by hand. Once all circuits have been correctly calibrated, the flow meters can be fixed in place using the clip provided. This means that the heating circuits can be turned off if necessary without changing the preset setting. To go over the process of adjusting the underfloor heating manifold step by step, we recommend to watch our instructional video on how best to set, clean and replace a flow meter:
Should you have any more questions about underfloor heating design, balancing or components such as manifolds and flow meters, don’t hesitate to get in touch. We are happy to provide any information you need.