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Fundamentals Of Hot Water Radiators

Hydronic heat is one of the most effective ways to warm a building. It is highly controllable, silent and maintains a much steadier ambient temperature than central air systems.



Hot water (hydronic) radiator components

Components of a hydronic radiator. Control Valve: This connects to the flow, or inlet side of the radiator, allowing hot water to enter the radiator. It might be manual or thermostatic. We usually recommend thermostatic valves because they add comfort and control, and can dramatically reduce energy use. Bleed Valve: Used to purge air, ensuring the radiator is completely full of water. Our hydronic radiators come with bleed valves included as standard. Lockshield: Lockshield The return valve is used to control the flow through the radiator. It’s set by the plumber during installation and shouldn’t be adjusted during normal operation. Hot water enters the radiator through a control valve and exits through a lockshield. On initial fill, air is vented through the bleed valve to ensure the radiator is completely full of water. Bleeding the radiator shouldn't be a frequent necessity (the need to frequently bleed a water radiator is a sign of a problem in the system). The control valve allows water into the radiator. It can be manual or thermostatic. A thermostatic radiator valve adds comfort and control. The modern energy efficiency of TRVs can give a dramatic saving on fuel bills. The lockshield is used to balance the system by controlling the water that exits the radiator, and therefore the resistance to flow. Balancing radiators ensures that the radiator furthest from the boiler reaches the same temperature as the one closest to it. All our hydronic radiators are supplied with a bleed valve included as standard. We also stock bleed valves for use in restoration projects. Read about one-pipe steam and two-pipe steam radiators.
Hot water radiator showing thermostatic control valve (bottom right), lockshield valve (bottom left) and  bleed valve (top right). The central wall stay secures the radiator against the wall.
Hot water radiator showing thermostatic control valve (bottom right), lockshield valve (bottom left) and bleed valve (top right). The central wall stay secures the radiator against the wall.

Delta T, or the impact of water temperature on heat output of a radiator 

Delta T, or ∆T, refers to the difference in temperature between the water circulating in the central heating system and that of the ambient temperature. If the ambient temperature is 20ºC and the mean water temperature inside the radiators is 70ºC, the ∆T value is 70 - 20 = 50º.
The heat output of a radiator is proportional to the temperature of the water inside it.
The hotter the water inside the radiator, the greater the heat output of the radiator. So, with a ∆T of 50º, the radiator might give off 1000 Watts (3400 BTUs), but reduce the temperature of the water inside so that the ∆T is 30º and the same radiator gives just 510 Watts (1700 BTUs).

Finding the correct radiator size for your delta T value

Our heat outputs are independently verified by BSRIA. We display heat outputs at ∆T50 as standard for the European market and 170ºF for the US market. We can use correction factors to find the actual output of any radiator at a range of delta T values; just multiply the output at ∆T50 by the correction factor listed below. 

Delta T Correction factors

Delta T Correction factor
75° 1.69
70° 1.55
65° 1.41
60° 1.27
55° 1.13
50° 1
45° 0.87
40° 0.75
35° 0.63
30° 0.51
25° 0.41
20° 0.3
15° 0.21
10° 0.12


Cast iron radiators and heat pumps

Ground- and air-source heat pumps are an immensely important part of the decarbonisation of home heating and we expect their usage to increase. The water in a heat pump-powered heating system is not as hot as from a traditional gas-fired boiler, and that reduces the output of the radiator.  Typically, the mean water temperature from a heat pump is 50ºC (122ºF). That's a ∆T of 30º and abn output that's half what it is at ∆T50. 
Do you need twice as many radiators with a heat pump? Almost certainly not. 
If you're fitting a heat pump, it stands to reason that you'll also have insulated your home and upgraded the draught-proofing. It's important to perform an accurate heat loss survey to ensure that the figures are correct. Increased insulation means a lower amount of heat is required. Our pricing is tailored to ensure that the per-section price falls the larger the radiator, making the larger radiators required for heat pumps more competitive than you might first think. Further, with the lower running costs of a heat pump, the payback time for the additional radiators could be as little as a year or two.  

A case study in rural Wales

Our How to Heat An Eco Home guide focuses on a property in the Welsh Brecon Beacons that uses an air-source heat pump to heat a rural retreat and includes tips for right-sizing radiators with heat pumps.