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Zehnder - Frequently Asked Questions

Click on the drop down links below to read a frequently asked question:

Below is a list of the most frequently asked questions from Zehnder about radiators and heating systems.

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1. Are Zehnder radiators suitable for my existing central heating system?

Yes. Zehnder radiators work with all central heating systems. All our connections are BSP and will accept manual or thermostatic valves.

2. 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.

3. 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.

4. 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.

5. 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.

6. What happens if I have a water softener on the system that uses salt?

It is important that the salt does not get into the central heating system. In an open or unvented system, if there is a leak, the water in the top-up tank for the central heating system will automatically be topped up with softened water, which will get into the radiators. This must be avoided. If the central heating system or a radiator has been drained and is to be refilled, the controls to the water softener must be switched over so that unsoftened water is fed into the header tank.

7. What is an inhibitor and why should inhibitor be used in a central heating system?

Inhibitor is a chemical mix that should be added to the central heating system water to prevent corrosion and prolong the life of the system.

8. 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.
  • To convert watts to Btu’s, multiply by 3.412.

9. What is the difference between outputs listed at ΔT60°C and ΔT50°C?

These are a calculation between Flow, Return and Room temperatures. All Zehnder radiators are tested to EN442 which has a Flow temperature of 75°C, a Return temperature of 65°C and a Room temperature of 20°C. Our putputs 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.

10. 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.

11. 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.

12. 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.

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