Frequently asked questions about installing heating controls

Frequently asked questions about installing heating controls

The technical team at heating controls manufacturer Danfoss Randall offers answers to some of the most frequently asked questions about choosing and installing domestic heating controls.

What do you need to consider to ensure that new heating controls will be compatible with a new or existing boiler?
This is a common enquiry and the main consideration is the type of boiler. In many three-port zone valve and two-port zone valve systems, for example, the motorised valve actually fires the boiler and the room thermostat switches the valve, so a standard mains voltage room thermostat should be used. With a combination boiler the room thermostats are normally wired directly which means that you need to find out the boiler’s switching voltage in order to select a compatible control; the boiler’s installation manual should tell you whether it is a mains voltage unit, often referred to as low voltage, or an extra low voltage model. Low voltage refers to any voltage between 50V and 1000V but is most commonly 230V (-6% +10%). Extra low voltage is under 50V and can be as low as 12V. You may find that some combination boiler manuals do not state what the voltages are across the thermostat contacts and will just state that the thermostat must have volt-free, or dry, contacts. This means they have no voltage on the contacts and, generally speaking, will switch any voltage across their contacts.

To sum up, if the boiler is mains voltage switching then a mains powered room thermostat should be used. If you are not sure, then a volt-free switching room thermostat is probably the best option as many of these are suitable for use with either 230V or below 50V boilers.

How do you decide whether to install wireless or hard wired heating controls?
Wireless room thermostats provide a particularly quick and convenient solution when upgrading an old heating system where room thermostats may not have been fitted when the system was originally installed, or the thermostat is no longer in a practical position. Installing wireless rather than hard wired controls is ideal for this type of job as it doesn’t disturb existing décor. However, wireless controls are not exclusively for retrofit installations. Although room thermostats in new build homes are mostly hard wired, wireless controls can still be an attractive solution for new heating systems. With no hard wiring involved, most qualified plumbers can complete an installation without the assistance of an electrician – saving time and cost on a job.

Is it possible for wireless thermostats to be affected by other wireless products in the home?
This is a question that installers are often asked when they suggest wireless controls to a customer, and the answer is no. There is no chance of interference from a home computer’s Wi-Fi system, for example, because wireless heating controls use unique identity coded radio signals which ensure secure communications.

How do you calculate the number of thermostats required?
Since it was introduced last October, Part L 2010 has required domestic heating installers to include at least two temperature-controlled heating zones and one hot water zone in all new systems in dwellings that are not open plan. Each zone must have at least one room thermostat and individual radiator control, such as TRVs. Of course, room sizes should also be considered when assessing the number of thermostats required. According to the Part L 2010 Guide produced by TACMA (The Association of Controls Manufacturers) rooms up to 150 sq m can have a common time control; any zones above this size should be timed independently. Installers should also be aware that when replacing a boiler in an existing system it is now considered good practice to install TRVs in all rooms with radiators except those with a reference room thermostat.

Having worked out the number of thermostats needed on a job, are there any guidelines about where these should be located?
Traditionally, thermostats were positioned at 1,500mm above floor level for optimum efficiency in terms of room temperature, and to be at a convenient reading level for most adults (but beyond the reach of a child’s inquisitive fingers!). With the introduction of Part M this has been reduced to 1,200mm to enable easier access for wheelchair users, without being too low, where the air will be cooler. Apart from mounting at the regulatory height, a thermostat should be installed in an area with good air circulation and where it will not be affected by draughts, dead spots behind doors, radiant heat from the sun, hot or cold air from ducts etc. Also, it is not recommended to have an outside wall behind the thermostat, and areas with concealed pipework are best avoided.

This information has been provided by the technical support team at Danfoss Randall. If you have further questions about heating controls, you can call the company’s technical helpline on 0845 121 7505.