We all use domestic plumbing every day but generally give very little thought to what goes on behind the scenes. But if you’re a residential landlord you have ultimate responsibility for Legionella risk management in your properties, which means that you should have a working knowledge of hot and cold water systems. Even if you’re employing a professional firm like Compliance for Landlords to handle Legionella control on your behalf, we strongly recommend becoming acquainted with the key components of your system.
Your property will have one of three different types of hot water system available in this country: open vented, unvented or instantaneous.
- Open vented hot water systems
An open vented system uses various components to heat the water: a hot water cylinder, a cold water storage tank, an open vent pipe, and a heat source to heat the water, which may be direct (an immersion heater or a boiler) or indirect (via a heat exchanger coil linked to a boiler). When the heat source is indirect the central heating and hot water systems run separately to avoid any iron residues from the radiator contaminating the water.
The cold water tank supplies the water to the cylinder and is always positioned above the cylinder to create natural pressure to make the water flow (which is why water tanks are often found in the loft).
The open vent pipe, which ends in the cold water tank, helps the system accommodate the expansion of water in the system as it heats up (as it warms it expands by up to 4% in volume).
- Unvented hot water systems
Unvented systems are more complex as they have more components, but the advantage is that they operate at a much higher pressure and do away with the need for a water storage tank.
A pressure reducing valve is placed on the incoming cold water mains pipe, which controls and slightly reduces the water pressure, keeping it at a constant level. A line strainer is used on the pipe to filter out any debris or particles which could contaminate the water or cause the water system to malfunction.
There will also be an expansion vessel which stores the extra water volume which is created when the water heats up. Similarly, there’ll be a temperature and expansion relief valve which removes pressure from the system. Attached to this via a piece of pipe is the tundish, a device which tells you if there’s a fault in the system as you’ll be able to see the water flowing out of the safety valve.
- Instantaneous hot water heating systems
The most common method of heating water instantaneously is with a combination (or combi) boiler. There is no need to store water (so no water tank is required) and it provides instant, unlimited hot water. A combi boiler uses the circuit that powers the central heating and diverts it to a heat exchanger which ‘swaps out’ the heat from the central heating water to the hot water system of the property. This type of system is more often found in smaller properties or those with just one bathroom (if you’ve ever tried having a shower while someone is doing the washing up in a property with a combi boiler, you’ll know that the pressure and temperature are adversely affected!).
Making sure that you have a basic understanding of your water system will help you to grasp the main risk factors for Legionella control. As you can see from the comparison below, there are pros and cons to each system.
Stored hot water: advantages
- Deals with demand more easily
- Water can be heated by more sustainable sources of energy (e.g. solar, biomass)
- Allows the connection of different types of water heating methods
- Can have an emergency back-up system to heat water if the boiler fails
Stored hot water: disadvantages
- Can be more expensive to run as water is heated even if it’s not used
- Takes longer to heat the water from cold
- Can be more expensive to install
- Higher risk of bacteria growth due to water storage requirement
Instantaneous hot water: advantages
- Generally cheaper to run
- Cold water is heated up very quickly
- Often cheaper to install than a storage-based system
- Lower risk of Legionella and other bacteria growing
Instantaneous hot water: disadvantages
- Struggles to deal with high demand for hot water
- Can be hard to connect back-up or alternative water heating sources
- Can’t use sustainable energy to provide power supply
- If the boiler fails there’s no emergency back-up