Central heating radiators

This article is about various types of radiators used in Central Heating systems. There is more information about designing central heating systems in

• Central Heating Design, whilst
• Underfloor Heating discusses a significant alternative to radiators.

There are separate articles about:

• Central Heating Operation for help maintaining a working CH system
• Central Heating Repair for diagnosing and fixing a faulty system.

Types

Radiators comes in a number of types, shapes and sizes. Most are finished with a heat resistant white paint.

Standard radiators

These are available in a range of heights typically between 300 and 750mm with the greatest range of lengths and configurations in heights around 450 to 600mm high. Lengths range from 200mm to 3m or more with the greatest range around 450mm to 2m long. Colour is generally white. Radiators generally have one or two panels, although 3-panel types are sometimes found. Modern single-panel radiators have a corrugated panel forming a series of fins (called "convectors") attached to the back (wall-facing) side of the panel, which increases the convection output of the radiator. These are usually known as "single convector" (SC). Two-panel radiators may have one set of convectors attached to the back of the front panel ("double panel": DP) or two sets of convectors ("double convector": DC), with the second attached to the front of the back panel (thus in both types the convectors are 'sandwiched' between the panels). Older radiators comprised one or two panels without any convection fins; in the case of double types the two panels are much closer together than on modern DP or DC radiators.

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  Seam-top radiator

• 

  close-up of seam-top

• 

  Compact radiators

• 

  Round (or roll) top

The traditional standard radiator has seams at the top, sides and bottom of each panel (where pressed sheets of steel are joined together). These are sometimes known as "seam-top".

Nowadays most seam-top radiators are sold with decorative panels fitted to the top and sides (the top ones having ventilation slots to allow air circulation), and are known as 'compact' radiators. These types are widely available, from outlets such as B&Q, Screwfix, Toolstation and SELCO as well as traditional plumbers' and heating merchants.

An alternative to the seam-top design uses one sheet of pressed steel folded to make a radiator with a rounded (or roll) top. Typical of this type are Barlo radiators, although Myson also produce them (as their "Premier HE" range) as are Wickes' own brand (in a limited range of sizes, but typically much cheaper than Barlo and Myson).

Myson also manufacture a compact version of their roll-top radiators, i.e. with a grille top and side plates as fitted to seam-top radiators to make "compact" types.

Low Surface Temperature

LST radiators are designed so that accessible surfaces have relatively low temperatures when operating with flows at normal CH system temperatures. These are used where there could be a hazard arising from the temperature of a normal radiator, especially in areas used by the very young, very old or others with reduced mobility or ability to detect and react to dangerously high temperatures.

Designer radiators

There is a huge range of designs available which may be more or less pleasing to the beholder, and more or less outrageously expensive! Some are available in tall, narrow configurations which may be suitable for rooms with e.g. narrow walls beside patio doors where conventional radiators could not provide sufficient output in the limited wall-space available.

Some designer radiators give out much less heat than similarly-sized regular types.

Column radiators

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  modern column radiator

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  old radiators awaiting refurbishment

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  ornate types

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  circular radiator at Bearwood College

Traditional "school" style column radiators may be modern reproductions or genuine old units; the latter may be reclaimed (more-or-less as-found) or reconditioned where the radiator is split into its constituent sections, cleaned inside and out and re-finished. Being made of cast iron they are very heavy and for this reason, and due to the manufacturing process, they are often supplied in sections with longer units made by joining shorter ones. They are floor standing with a stay to anchor them to a wall. Large old-style valves (including thermostatic types) in various antique/retro styles are available to match. These radiators are much larger than equivalent modern types and add significantly to the volume of primary water which may require more water treatment chemicals, bigger expansion vessels on sealed systems, and make for longer warm-up times.

Towel-warmers

These are specifically designed for hanging towels over for warning and drying in bath and shower rooms. Chrome plated or white painted ladder types are widely available at relatively modest prices. However their heat outputs when covered in towels is negligible, and even when not covered with towels are much smaller than similarly-sized conventional radiators and are not usually sufficient for anything but relatively small and well-insulated bathrooms: in other cases they need to be supplemented by a normal radiator. Some designs of towel warmer comprise a normal radiator with towel rails above and sometimes to the sides of the radiator. These have better heat outputs but again this will be reduced when covered by towels. They also tend to be more expensive than ladder-type towel radiators.

Heat Output

Delta-T

Manufacturers publish the nominal heat output for each size and type of their radiators. Obviously the hotter a radiator is the more heat it gives out, so for the specification of a radiator's heat output to be meaningful this factor must be incorporated. Since a water cools on its passage through the radiator the mean of the flow and return temperatures is taken, and from this is subtracted a notional room temperature of 20°C, and the difference, known as Delta-T, should be given in the radiator specification. For non-condensing boilers which might have maximum flow and return temperatures of 90°C and 70°C respectively this gives a 60°C difference, and radiator outputs specified for these conditions are known as "Delta-T 60". For more modern installation with high-efficiency condensing boilers flow and return temperatures are likely to be around 75°C and 65°C giving Delta-T 50.

To convert Delta-T 50 to Delta-T 60C multiply by 1.2675. (from Barlo's FAQ)

If unsure about the output of a given radiator it can be calculated using the formula below (for standard and compact radiators) or deduced by comparing with similar-sized models from other manufacturers. If specifications are available but it is not stated whether these are for DT50 or 60 it is safest to assume DT60.

Manufacturers' specifications

• Output specification of Kudox compact radiators (PDF). These are sold by B&Q and others, and are typical of compact radiators of similar sizes from other manufacturers.
• Myson
  • Myson standard rads technical data
  • Myson compact rads technical data
  • Myson roll-top rads technical data
  • Myson compact roll-top rads technical data
• Barlo radiator technical specs and price guide

As would be expected the output of Double Convectors (DC) is approximately twice that of Single Convectors (SC), whilst Double Panel (DP) types are in between.

Radiator output formula

From the specifications of Myson's standard (seam-top) and compact radiators, and Kudox compact rads, it is possible to derive a formula to calculate the heat output of any given sized-radiator of these types. This formula should be applicable to other manufacturers' radiators of similar types.

 Heat Output  =  (Height + 12) * Length * FACTOR
   (Watts)         (cm)           (cm)

or

 Heat Output  =  (Height + 120) * Length * FACTOR / 100
   (Watts)         (mm)           (mm)

where FACTOR is (for Delta-T 50°C)

• 0.13 for SC
• 0.19 for DP
• 0.24 for DC

In other words:

• add 12cm to height (in cm) of radiator
• multiply by length (width) of rad
• multiply by FACTOR above

The formula can be used in reverse to calculate the size of radiator necessary to provide a given heat output (as given by a heat-loss calculator) e.g.

Length  =  Heat Output / (FACTOR * (Height + 12))
 (cm)         (Watts)                (cm)

For example suppose an output of 1kW is required from a radiator whose maximum height is 500mm. The length of radiator necessary to provide the required output is:

             1000      /  (FACTOR * (50   + 12))
            (Watts)                 (cm)

         =   1000      /  (FACTOR * 62)

Substituting the factors for the different types of radiators we get lengths of

• SC: 1000 / (0.13 * 62) = 124cm (1.24 metres)
• DP: 1000 / (0.19 * 62) = 85cm (850mm)
• DC: 1000 / (0.24 * 62) = 67cm (670mm)

Thus in practice one might have a choice of 1300mm SC, 900mm DP or 700mm DC.

Radiator Cabinets

Putting a radiator in a cabinet (usually made of MDF) may hide its "ugliness" but will typically reduce its heat output by about 50%.

Oversizing

With modern systems using TRVs and condensing boilers there are no technical drawbacks to installing over-sized radiators, although they will incur extra cost, weight and size. It is usually wise to design a little on the high side. Also, oversized radiators will enable faster heating up of the room from cold (a long as the boiler output is sufficient to provide the extra heat required to do so).

Location

• Where in a room should a radiator be placed? In some rooms such as a bathroom the position of a towel warmer is decided by the layout of the room. In most other rooms there will be more choice. The decision mostly hinges on whether to place the radiator under a window or not.

The arguments for putting it under a window are:

• it's not in competition with larger items of furniture as the latter won't be placed so as to block the window.
• it will convect warm air up against the descending air cooled by the window so stopping convection draughts.
• The outside wall may well be a stronger block wall better for hanging a radiator and than an internal stud wall.

The arguments against are:

• Where there are excessively long curtains there is a temptation to draw them in front of the radiator, blocking heat to the room.
• The wall behind and the window above will be warmer than the rest of the room and will marginally increase heat losses to the outside.
• Installation costs will usually be a little higher than with radiators on an inside wall due to longer supply pipe runs.

Even when a radiator is not fitted directly under a window it should be located close by and preferably on an outside wall. If the radiator is located at the opposite end of the room from windows and outside walls it is likely to result in the room being uncomfortably warm near the radiator and/or cold near the window & outside wall(s). This will be particularly so with large areas of single-glazed windows and solid or uninsulated cavity walls (although for energy-efficiency, and economy in the long-term, it is also advisable to improve insulation).

Fitting reflective insulating sheet behind radiators fitted on outside walls can help reduce heat losses directly through the wall.