What is the role of underfloor heating manifolds and flow meters?

A 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, nowadays, is 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.

Why is it so important to balance an underfloor heating system?

Balancing is an important prerequisite for the optimal functioning and energy-efficient operation of the heating system, yet around 85% of old systems and 60% of new radiant heating systems are not hydronically balanced. Only if the system is balanced correctly does each heating circuit receive exactly the right amount of water it needs to cover the heat load. Additionally, 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.

Julian Clegg, Myson’s Product Manager for underfloor heating, explains: “Water always flows 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 sufficient 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.”

UFH_ Floortec graphic cropped

Hydronic balancing in underfloor heating design

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 in the calculation documents. The following steps are essential to reach a correct (subsequent) calculation:

1. Determine the heating area of the circuit (m2).

2. 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 100W/m2, while for modern buildings (constructed circa 2004) it’s closer to 50W/m2.

3. Determine the temperature difference between the flow and return. A commonly used reference value is 7K.

4. You can calculate the water flow for the heating circuit (l/min) by multiplying the temperature difference (7) by the heat demand (70) and then dividing the heat load (315) by this number – 315 divided by 490 = 0.64.

For example, a heating circuit of 4.5m2 in a building with a specific heat demand of 70W/m2 has a heat load of 315W. If we then have a temperature difference of 7K 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 calculation for your project, Myson has trained experts available who are happy to help lighten the load and take charge of your Myson underfloor heating design and associated calculations.

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 facilitates an everyday task for installers”, says Julian. “In the past, manifolds only had regulation valves and it was quite difficult to set the right flow rate. You had to consult a diagram to find out how many turns were required to close the regulation valve, based on the amount of litres needed in the circuit and the pressure loss. It’s therefore not surprising that most underfloor heating systems previously 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 do not 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.

Should you have any questions about underfloor heating design, balancing, or components, you can contact Myson on 0330 041 5474 or email underfloor.uk@purmogroup.com