Tel: 00 353 (0)46 9029444

Radiators FAQS

Radiators FAQS


What size radiator do I need to heat up my room?
The size will depend on the heat required for the room. This can be established by your plumber/installer.

However, as a guide:
• Calculate the volume of the room by multiplying the height, width and length to get the volume in cubic metres.
• For bedrooms, hallways and kitchens allow 40 Watts per cubic metre, and multiply the total by 40; for bathrooms, living and dining rooms, multiply the total by 50. This will give the output required in watts. Example: A bedroom 3.6m long x 3m wide x 2.4m high has a volume of 25.92cubic metres. Multiply this by 40 to get 1036 Watts.
• If the room has large windows or exposed walls, increase the total by 10%.
• Always choose the next radiator size up if the exact output is not available and allow more length and less height for radiators under a window. Please note that these are only guidelines. There are many other factors which need to be considered like : “tightness” of house (infiltration), walls construction, ceiling and floor construction, heat gain from electric appliances etc. We do not advise an estimated calculation. We recommend to contact our office for assistance.

How do I convert BTU’s into watts?
Divide the BTU’s (British Thermal Units) by 3.414. (for example 5870 BTU’s ÷ 3.414 = 1919 watts). Or to convert watts to BTU’s multiply by 3.414 (for example 1259 watts x 3.414 = 4298 BTU’s).

How can I vent my radiator?
To vent a radiator yourself, some technical skill is required. Either have this done by a technician or get detailed information in advance.

My radiator is making knocking sounds. What does that mean?
One frequent cause of ‘knocking sounds’ at the thermostat valve is interchanging of the flow and return. As a result, the flow in the valve is in the wrong direction. Please contact your technical specialist.

My radiator makes flowing sounds. Why?
There are sounds that could be heard as rushing sounds constantly over an extended period of time. These are generally flowing noises. Generally, these are due to excessively high flow rates (because of excessively high differential pressures) in certain sections of the heat distribution, for example in valves. Flow balancing by a specialist can help.

What does flow balancing mean?
Flow balancing is the limiting of the water volume flows to values that correspond to the heat requirement of the plant. By means of a ‘hydraulic regulation’, which generates the same resistances for all the radiators in a heat distribution network, flow noises, overheated rooms and rooms with a shortfall of heat supply are avoided. In addition, the energy consumption for heat generation and heat distribution are optimized.

My radiator does not reach the required temperature. Why?
There could be several reasons for this: Is the setting of the thermostat sensor correct? Is the radiator correctly deaerated? Is the flow temperature sufficient? Is the circulation pump working correctly? Has the heating plant been flow balanced?

What is the difference between a direct and an indirect system?
A direct system is where the domestic hot water (hot tap water) is connected to a radiator, whereas an indirect system is totally central heating.

What is the hydraulic pressure drop across a radiator?
The pressure drop across a radiator is dependent on the style, size & type of radiator as well as the flow & return temperature. Please contact our office for details.

What is the difference between outputs listed at ΔT60°C and ΔT50°C?
These are a calculation between Flow, Return and Room temperatures. For radiators tested to EN442 a Flow temperature is 75°C, a Return temperature is 65°C and a Room temperature is 20°C. Our outputs are published to ΔT50°C and also at ΔT60°C for those who may have older boilers that may be running at temperatures above 75°C. We can also make calculations for other Flow and Return temperatures if necessary, please contact our office.

What will happen if radiator selections are made from the ΔT50°C tables when the boiler in question has flow and return temperatures of 82°/71°C?
Unless the installer/heating engineer has taken it into consideration when calculating the heat loss requirements of the room, the radiator will be oversized by up to 25%.

The temperature can be moderated by the use of a thermostatic valve, and in the future, when the boiler is replaced with a condensing type the radiators will be of the correct size.

What will happen if radiator selections are made from the ΔT60°C tables when the boiler in question has flow and return temperatures of 75°/65°C?
Unless the installer/heating engineer has taken it into consideration when calculating the heat loss requirements of the room, the radiators will be undersized by approximately 25%, and the room will not be sufficiently heated.

How can one avoid making the wrong selection?
The installer must know the flow and return temperatures of the boiler in question, and select the appropriate ΔT output data for the room.

Are Zehnder radiators suitable for my existing central heating system?
Yes. Our radiators work with all central heating systems. All our connections are BSP and will accept manual or thermostatic valves.

Can I change the radiator(s) in just one room or must I re-do the whole central heating system?
There is no problem with integrating individual radiators into an existing system. When new or additional radiators are being installed, the system should be drained, cleaned and flushed in accordance with BS7593 and a good quality inhibitor, suitable for mixed metal systems, used to protect the system.

I want to change an existing horizontal panel radiator for a tall vertical radiator. Is this OK?
Yes, but if you have a system that has a central heating system expansion tank (usually in the loft), then there should be a 1metre clearance between the top of the radiator and the bottom of the header tank.

Why is it important with some of the models that the flow is in a specific connection position?
Some of the design radiators have baffles inside the water-carrying tubes to divert the water flow around the whole radiator to maximize efficiency.

Can steel and aluminium radiators be put on the same system?
Yes, as long as an inhibitor for mixed metal systems is used. In reality, most systems are mixed metal, as many already have aluminium heat exchangers in the boiler, plus copper pipes and steel radiators.