And how are they accurately sized?
Heating systems that use water to transfer heat from a heat source (such as Boilers, Ground Source Heat Pumps, Solar Heating) are subject to the various gas laws by Boyle, Lussac, and Charles. When one parameter changes (either pressure, volume, or temperature) the effect on the other two is inversely proportional.
In simple terms, as the system heats up, the volume and pressure of water will increase. Water is the only fluid that increases in volume during both heating and cooling.
Heating systems are either vented or non-vented. A vented system is open to atmosphere and therefore has minimal changes in pressure. The increase in volume overcome by a feed and expansion tank, mounted as high as possible above the system. A pipe from the system takes any excess hot water and dumps it into the tank. Once cooled, the water from the tank will feed back into the system.
There are many benefits of a non-vented (sealed) system, but for the purpose of this video we will focus on how to deal with the increase in volume and pressure as the system heats up.
Water will expand by 9% when heated from room temperature to boiling point. When water turns into steam, one litre of water increases in volume to one thousand litres of steam – extremely useful in steam trains or ships, but not ideal in a heating system!
In a sealed system the result of this is that the pressure can increase considerably and cause fittings to fail, potentially resulting in material damage to the fabric of the building. The incorporation of a correctly sized Expansion Vessel will accept the increase in volume and restrict such a dramatic rise in pressure.
An Expansion Vessel is open to water one end and connected to our system normally on the return side. It has a compartment filled with air separated from the system by a flexible membrane (air unlike a fluid will compress). As the water heats up it will enter the vessel and begin to compress the air.
When we size an Expansion Vessel, we need to consider certain parameters.
Firstly, the System Volume – if this is unknown a good rule of thumb is 10 – 12 litres of water per kilowatt of boiler output.
Secondly, we need the Static Head above the Vessel. The Static Head is the measurement from the Expansion Vessel to the highest point in the heating system (top floor radiators + 2 metres). For a two storey building, 10 metres will be adequate.
Thirdly, we need the Flow and Return Temperatures. In a LTHW (Low Temperature Hot Water) system with radiators, this would typically be 82⁰C Flow and 71⁰C Return, but can vary. For instance, underfloor heating will run at lower temperatures. Make sure to always calculate for the highest temperature that the system will reach.
Fourthly, the High Cut-Out. This is typically 2.9 bar for a system with a static head of 20 metres, but must always be at least 0.2 bar below the safety relief valve setting.
The results of the calculation using the four parameters will give us the exact expansion of the system. Erring on the side of caution, we would select the next size up Expansion Vessel. The results will also give us the Cold Fill pressure Switch On, and Cold Fill Switch Off. This is important as we will set the air pressure in the Vessel to this figure.
In a stationary system at ambient temperature, there should be no water in the Vessel. As the heating comes on, and the water temperature increases, the expansion in the water will cause an increase in pressure – this will overcome the air pressure in the Vessel and allow water to enter.
Our system will still see a change in pressure but it will be manageable, at most an increase of 1 bar.
It’s very important that Expansion Vessels are maintained and their air pressure matches the set parameter. Vessels should be clearly labelled with their required pressure setting – we recommend this is checked every six months. When the air pressure is checked, the system must not be under pressure. Special Ball Valves allow the Vessels to be isolated from the system and drained to allow for these checks.
We have compiled a table of standard values against our range of Expansion Vessels to allow for the easy selection of the correctly sized Vessel. For more detailed sizing, we are more than happy to enter the data from your heating system into our Vessel Sizing Spreadsheet.
Chilled systems also require Expansion Vessels. However, when we add the data for a chilled system to our Sizing Calculator, our Flow and Return Temperatures can be as low as 3⁰C and 6⁰C respectively – resulting in an expansion of next to nothing. Using the ambient temperature as the Return Temperature for the calculation will overcome this issue and give the correct expansion results (46⁰C should allow for the hottest day).
Systems that incorporate high amounts of Glycol, or Inhibitor, may have different coefficients of expansion and this must be factored into the calculations.
It is also worth noting that new systems that are using Ground Source Heat Pumps and Air Source Heat Pumps may need larger heat exchangers as temperatures are lower – this therefore rules out the old 10 – 12 litres per kw calculation.
Expansion Vessels play a vital role in sealed heating systems. They are essential in controlling expansion and changes in pressure as the temperature of water increases. They need to be correctly sized and maintained. There are four key parameters that determine the size; System Volume, Static Head, Flow and Return Temperatures, and High Cut-Out. Other factors can also result in deviations from standard charts. Please contact us at Arrow Valves for the accurate sizing of Vessels.
