I have two pipes from the floor to connect the radiator into. Current one has bizarre bends added on one side so wider one fits in, but I want to change it to "designers" one that would nicely fit between these pipes with standard valves.

Question is then, how to calculate the width of the radiator to fit knowing the distance between pipes?


I want to get rid of that monstrosity. I found drain valve outside the house.

  • Please ask one question per post. That's standard across SE. Why would you need to calculate the width of a device?
    – isherwood
    Commented May 23 at 18:19
  • Pictures will help, but usually you change the pipes to fit the rad, not change the rad to fit the pipes.
    – crip659
    Commented May 23 at 18:40
  • I'd like to avoid chiseling my screed and these buried pipes are covered with kind of mastic and very hard to solder - it's very costly to move them. Commented May 24 at 9:44
  • I've added photo. Commented May 24 at 9:50

1 Answer 1


The first step is to measure the pipe centres. This is the distance between the centre lines of the pipes coming out of the floor on each side of the radiator. Measuring the centre accurately can be a little tricky, so usually it is easier to measure the same edge on both pipes (e.g. the right hand side of both pipes, or the left hand side of both) which will be the same distance.

One you know the pipe centres, you need to factor in the size of the valves. Assuming you are using right-angle TRVs like the onces shown, typically this is ~45-50mm per valve (i.e. one each side of the radiator). So if the pipe centre distance was 1300mm, you would typically want a 1200mm radiator (1200 + 50 + 50 = 1300).

Radiators come in standard sizes, in the UK these are nowadays in metric, usually in 100mm increments (e.g. 500, 600, 800, 900, 1200 are common sizes). Depending on how old your plumbing is, it is quite likely you have an imperial sized pipe spacing, which don't match up with the modern widths. This would explain why somebody has build the extra section of pipework - to adapt the pipe spacing to a modern metric radiator.

If you are at imperial spacing, to compensate for this difference, you can either stick with the "monstrosity", or use a narrower radiator with either telescopic or extended length radiator tails. The tails are the section of pipe that screws into the ends of the radiator to convert to pipe (e.g. 1/2" BSP to 15mm). These allow the pipes to stay at the larger distance without the need for the "zig-zag" section. Be careful though as using a smaller radiator will reduce the thermal output, though modern radiators may be more efficient so this might not be an issue.

  • Pipes are 15 mm, distance between centres are 1835 mm. Commented May 31 at 19:40
  • @MichałFita Most likely was originally a 69" radiator (old standard size). That would have been 1750mm plus 85mm for the valves would have given the 1835mm centres. When replaced they probably used an 1800mm rad, hence the zigzag. Using a 1700mm radiator with a pair of say 20mm radiator tail extensions would do the trick (1700 + 2 x (20 + 47.5) = 1835mm), or a 1700mm rad with some long telescopic valve tails. Commented May 31 at 19:56

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