cold water: pipe types & typical sizes in dwellings
Cold water: pipe types & typical sizes in dwellings
Learning objectives
- recognise common pipe types and entry sizes for domestic cold water
- understand how system layout (direct vs indirect) influences pipe sizing
- use practical starting sizes, then verify with a quick sizing check
1) What decides pipe size
- available pressure from the street main or private supply
- likely simultaneous demand (total flow)
- route length and fitting losses
- minimum performance needed at outlets such as showers and the kitchen sink
rules of thumb are a starting point — always confirm with a simple check
2) Typical entry into the building
- most UK dwellings use 25 mm MDPE as the service into the property
- usually terminates at an internal stop valve (add a PRV if required)
3) Direct (mains-fed) systems — typical choices
in a direct system, every cold outlet is fed from the main. this suits small to medium homes with a combi or unvented hot water cylinder.
common starting sizes
- internal rising or main: 15 mm in many small installs; 22 mm if there are many outlets or long runs
- branches to outlets: 15 mm is typical
- unvented cylinder cold feed: minimum 22 mm to satisfy higher flow through control sets
- larger homes or higher demand: consider 22 mm internally; occasionally 28 mm when justified
notes
- with a vented cylinder, the cold for hot comes from a cistern, but cold outlets can still be direct
- adding an unvented cylinder or mains-pressure heater increases simultaneous mains draw, often pushing the rising main to 22 mm or above
- expect pressure dips at peak times on mains-fed layouts
4) Indirect (cistern-fed) systems — typical choices
used where mains pressure is low or a storage break is preferred. only the kitchen cold (and sometimes a downstairs utility or cloakroom) is taken directly from the main. all other cold outlets are gravity-fed from a roof cistern.
common starting sizes
- rising main to cistern fill: 15 mm in many small homes
- distribution trunk from cistern: 22 mm minimum; 28 mm for longer runs or more outlets
- bath feed on low head: aim for 22 mm to keep fill times acceptable
head, protection and storage
- mount the cistern as high as practical to improve static head
- insulate sides and lid (not the base), fit screened overflow and warning pipes, and protect against frost and undue warming
- where the cistern also supplies an open-vented hot cylinder, size storage for both cold use and cylinder replenishment; as a quick check, ensure storage at least equals cylinder capacity, then adjust
5) Quick reference — starting sizes (verify afterwards)
| item / run | typical starting size |
|---|---|
| service to dwelling (MDPE) | 25 mm |
| direct system — internal rising/main | 15 mm (22 mm if many outlets or long runs) |
| unvented hot water cylinder cold feed | 22 mm minimum |
| indirect — distribution trunk from cistern | 22 mm (28 mm if needed) |
| branch to basin / WC / sink | 15 mm |
| branch to bath (cistern-fed) | 22 mm |
6) Pipe types and practical tips
- MDPE 25 mm for incoming service: use the correct inserts and fittings; keep out of UV
- copper: durable; be mindful of dissimilar-metal joints
- plastic or MLCP: fast and quiet; follow insert and clip spacing guidance; avoid UV exposure
- clip to manufacturer spacing; secure long runs to reduce rattle and hammer
7) Checks before sign-off
- confirm worst-case outlet flow at showers and kitchen
- verify cistern siting, insulation, screening and capacity (if used)
- ensure isolation valves are accessible
- record static and dynamic pressure for handover
Cold water: pipe types & typical sizes in dwellings
Learning objectives
- recognise common pipe types and entry sizes for domestic cold water
- understand how system layout (direct vs indirect) influences pipe sizing
- use practical starting sizes, then verify with a quick sizing check
1) What decides pipe size
- available pressure from the street main or private supply
- likely simultaneous demand (total flow)
- route length and fitting losses
- minimum performance needed at outlets such as showers and the kitchen sink
rules of thumb are a starting point — always confirm with a simple check
2) Typical entry into the building
- most UK dwellings use 25 mm MDPE as the service into the property
- usually terminates at an internal stop valve (add a PRV if required)
3) Direct (mains-fed) systems — typical choices
in a direct system, every cold outlet is fed from the main. this suits small to medium homes with a combi or unvented hot water cylinder.
common starting sizes
- internal rising or main: 15 mm in many small installs; 22 mm if there are many outlets or long runs
- branches to outlets: 15 mm is typical
- unvented cylinder cold feed: minimum 22 mm to satisfy higher flow through control sets
- larger homes or higher demand: consider 22 mm internally; occasionally 28 mm when justified
notes
- with a vented cylinder, the cold for hot comes from a cistern, but cold outlets can still be direct
- adding an unvented cylinder or mains-pressure heater increases simultaneous mains draw, often pushing the rising main to 22 mm or above
- expect pressure dips at peak times on mains-fed layouts
4) Indirect (cistern-fed) systems — typical choices
used where mains pressure is low or a storage break is preferred. only the kitchen cold (and sometimes a downstairs utility or cloakroom) is taken directly from the main. all other cold outlets are gravity-fed from a roof cistern.
common starting sizes
- rising main to cistern fill: 15 mm in many small homes
- distribution trunk from cistern: 22 mm minimum; 28 mm for longer runs or more outlets
- bath feed on low head: aim for 22 mm to keep fill times acceptable
head, protection and storage
- mount the cistern as high as practical to improve static head
- insulate sides and lid (not the base), fit screened overflow and warning pipes, and protect against frost and undue warming
- where the cistern also supplies an open-vented hot cylinder, size storage for both cold use and cylinder replenishment; as a quick check, ensure storage at least equals cylinder capacity, then adjust
5) Quick reference — starting sizes (verify afterwards)
| item / run | typical starting size |
|---|---|
| service to dwelling (MDPE) | 25 mm |
| direct system — internal rising/main | 15 mm (22 mm if many outlets or long runs) |
| unvented hot water cylinder cold feed | 22 mm minimum |
| indirect — distribution trunk from cistern | 22 mm (28 mm if needed) |
| branch to basin / WC / sink | 15 mm |
| branch to bath (cistern-fed) | 22 mm |
6) Pipe types and practical tips
- MDPE 25 mm for incoming service: use the correct inserts and fittings; keep out of UV
- copper: durable; be mindful of dissimilar-metal joints
- plastic or MLCP: fast and quiet; follow insert and clip spacing guidance; avoid UV exposure
- clip to manufacturer spacing; secure long runs to reduce rattle and hammer
7) Checks before sign-off
- confirm worst-case outlet flow at showers and kitchen
- verify cistern siting, insulation, screening and capacity (if used)
- ensure isolation valves are accessible
- record static and dynamic pressure for handover