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Why Aren’t My Radiators Hot?

April 15, 2021

Radiators are the emitters that radiate the heat energy produced by the heating appliance, which for the sake of this article will be a gas boiler. The water passes through the heat exchanger of the gas boiler where it’s heated to the set temperature before being pumped around a network of pipes and radiators known as a heating system. There are lots of common faults and problems which can occur within a heating system, this article should help with three of the main ones:

If you’re experiencing uneven heating, very slow heating, cold patches on some or all the radiators or some are just not working at all it’s likely due to one or more of the following reason:

  1. You have air in the system.
  2. You have a blockage in the system.
  3. You need to get the system properly balanced.

As a very general rule of thumb if the radiators are cold at the top but hot at the bottom, they’ll likely need bleeding, if they’re hot at the top but cold at the bottom they’ll probably need cleaning and if some are hot while others are cold the system might need balancing. This is a very general rule and, any of the faults can display any of the symptoms, this is just where an engineer would start their diagnostics.

Air in the system

This is perhaps the most common issue we come across when being called out to a lack of sufficient heating. As above if the radiators are cold at the top but hot at the bottom it’s a good indication they need bleeding, towel warmers, designer radiators, vertical radiators and loft extensions are all prone to air locks. It should be noted that if you need to bleed your radiators frequently, more than once or twice a year, this will suggest there is an underlying issue such as an incorrectly sized pump, poor system design or a build-up of sludge in the system and further diagnosis would be advised.

How to bleed a radiator

Follow our step-by-step guide:

  1. With your central heating on, check the radiators to see which ones are hot at the top and which are cold, take a note if you have more than one.
  2. Next, you’ll need a cloth or some tissue paper and a radiator bleed key - these can be picked up for about 50p from any hardware store.
  3. Once the system has cooled down, starting with the lowest radiator which was cold at the top insert the radiator bleed key in to the bleed valve, do not remove the screw entirely when you hear air or feel water the valve is open - note some designer radiators or towel warmers will need a small spanner instead of a bleed key.
  4. Once located, slowly open the bleed valve while holding the tissue or cloth just below until you hear a whooshing sound, that's the air.
  5. You’ll likely get a few splutters of water from the bleed valve before all the air has been released.
  6. Once you have a steady trickle of water from the valve close it firmly and wipe any excess water and you’re done
  7. If you have a pressurised boiler you will need to check the pressure of the system after bleeding, if no air or water is released from the bleed valve you might need to top the pressure up in the system or you could have a blocked cold feed and further diagnosis will be required.

Blockage in the system

If you notice an uneven heat across the radiators with cold patches and a noticeable difference between the top and bottom of any radiator it’s likely the system needs cleaning. There are a few checks you can do to determine the health of your heating system:

  1. Try bleeding the radiators which are patchy and use a clean piece of tissue paper, if the paper has turned brown or black it’s a sure sign the system is severely sludged and a powerflush is needed.
  2. Drain some of the system water into an apparatus known as a Turbidity tube. The tube has rings at the bottom, fill it up and see if you can see the rings. If not pour some water out, any reading above 200ppm will indicate the system needs cleaning, even if the water looks clean.
  3. While you have the water in the Turbidity tube, or another clean container use pH strips to test the chemical compounds of the water. A general rule is a good reading is 8.2-10 although different manufacturers will specify different readings.

Once you’ve confirmed your heating system is dirty and needs a clean it’s best to call a competent engineer, the cleaning process involves very strong chemicals and correct PPE must be always used.

Balancing the system

If you notice an uneven heat distribution across the heating system with some radiators getting hotter or heating faster than others it’s a sure sign the system needs to be correctly balanced.

Balancing just means manipulating the flow of the water to encourage it to flow effectively and fully around the system, to do this the “easiest path” must be restricted.

As with most things in the heating industry there is a correct way and a fast way to do this, the fast way involves closing all the radiators and then opening the furthest away from the boiler fully and restricting the valves a ¼ of a turn as you get closer to the boiler.

The correct way is known as hydronic balancing, this is a lot more effective and complicated, it involves measuring flow rates, pump speeds and temperature differentials across each radiator individually and at the boiler to make sure the system is operating at its highest efficiency.

It would typically require assistance from a trained plumber or heating engineer as it can be quite complex!

 These are just three of the more common issues we see daily but there are lots more reasons why your heating may be faulty.

Get in touch with us today, we have a variety of contact options.

You can call us on 0207 0999 035 or email hello@heatsy.co.uk and we’ll be happy to help.

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Thank you for reading.

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