Heating and cooling installation use water for the energy transfer because it’s the most cost
effective medium and most readily available, stable resource. Water expands when is heated, and contracts (volume reduces) when cooled. To accommodate expansion and contraction storage is required. In general there are two types of system for accommodating thermal expansion, Sealed systems (Closed) and Open vented systems.

•  In an open vented system(open loop) an open tank at the highest point of the installation is used to store the expanded fluid (figure 1.1).
•  In a closed system (pressurised/sealed) an expansion vessel is used to store the expanded fluid (figure 1.2).

It’s common that older heating systems will most likely home an open vent boiler with a feed and expansion tank. When refurbishing a plant room should you retain this or opt for a closed system?

We take a look at the key differences between open vented systems and closed systems to help you decide what is best for your heating refurbishment project.

What is an open vented system?
A heating system which is open to the atmosphere and has a feed and expansion tank.

How does an open vented heating system work?
To accommodate water losses from evaporation or leakage in the system the tank water level is maintained and topped up automatically using a float valve in the tank connected to the mains supply. Expanded water is also accommodated within the open expansion tank. The system pressure or head is achieved through the height of the tank location above the plant room.

Feed and expansion tanks are typically installed in roof voids and the height of the tank above the boilers must satisfy minimum operating pressure for the installed boiler and have sufficient height above the highest point of the circulating system.

This system uses the existing cold water lines to return the cool uncirculated water back to the cold water supply in the home before the heating source. Typically used in retro fit and minor remodeling situations, an open loop circulator pump will draw water directly from the hot or cold lines on which it is installed. Open loop pumps can be installed by adding a tee or adapter before the fixture shut off valve as well as inline along the supply or return pipe. In an open loop system the pumps are typically located in a cabinet, under a shelf or counter top very near to the fixture. The pump is controlled by a thermostat or timer, circulating water whenever it is needed to keep incoming water to the fixture at the desired temperature.

What are the downsides of an open vented system for modern boilers?
An open vented system is typically used in older heating systems and cannot get to high pressures. It can allow pollutants to enter the system water. Feed and expansion tanks will require periodic cleaning and enable oxygen to enter the system that can contribute to corrosion. The pipe work run from the tank location to the plant room can sometimes be arduous and will need insulating to protect against freezing.

What is a closed system?
A heating system that is closed to the atmosphere and does not have a feed and expansion tank.

How does a closed system work?
Closed systems will use a pressurisation unit to provide automatic replacement of water losses and ensure minimum head requirements are maintained. A dedicated expansion vessel will also need to be installed to deal with the effects of heating fluid expansion and contraction that will occur as the heating fluid temperature changes across the entire operating temperature range of the closed heating system. Both units need to be accurately sized for the system it is being used on.

Closed loop water circulation systems have a dedicated return line for carrying water back to the original heating or cooling source. Installing a dedicated return line prevents the possibility of warm or cold spots being mixed in the water supply as in an open loop system. Often installed in new construction or extensive remodeling applications, this also allows for the circulator pump to be installed near the water heater or other heating/cooling source.

What are the benefits of a closed system?
Closed systems take up less space, assist with system cleanliness, improve water quality and reduce oxygen ingress.

Closed systems have the benefit of all equipment – pressurisation units and expansion vessels – being located in the boiler room, making service and maintenance access simpler.

Pressurisation units offer the additional benefit of monitoring system pressure, so that appliances can be interlocked to prevent operation in the event of operating pressure conditions being too high or too low.

Some pressurisation units monitor water volumes introduced to the system which can help identify leaks as well as helping decide when corrosion inhibitors may need re-dosing.

More for solar water heating system
Open loop solar water heating system

In an open loop (direct) system, water heated in the collector panels goes back to the cylinder and then to taps and appliances for household use. A system such as a temperature controlled pump to allow hot water to be circulated through the panel on cold nights to prevent freezing must be integrated into the circuit.

Closed loop solar water heating system

In a closed loop (indirect) system, a heat transfer fluid such as glycol circulates through the collector panels, absorbing heat. It carries this heat to a heat exchanger in the hot water cylinder, where the heat is transferred to the water.

Closed loop systems are slightly less efficient than open loop systems as there is some heat loss through the heat exchanger. Their advantage is that they can use a freeze-resistant fluid so are more suitable for frost-prone areas.

For both open and closed loop systems, reduce heat loss between the solar panels and the storage cylinder by:

• keeping the distance between the two as short as possible
• insulating all pipes
• running pipes through warm areas of the house.

To optimize performance, a pump controlled by the water temperature can be used to circulate the water/heat exchange fluid. This can:

•  provide flexibility in the location of the panel and the cylinder
•  increase the form part of the frost protection system by activating a reverse flow through an open loop system when there is risk of frost.