Unvented Cylinder Discharge Pipe Size Calculator

What Are D1 and D2 Discharge Pipes?

Every unvented hot water storage cylinder installed in the UK must be equipped with a temperature and pressure relief valve (TPRV) and, in most installations, a temperature relief valve (TRV) as independent safety devices. Under abnormal conditions — including thermostat failure, loss of cold feed, or immersion heater runaway — these valves open automatically to discharge dangerously hot water and relieve system pressure before a catastrophic failure can occur.

The pipework through which this discharged water is safely conveyed away from the cylinder is divided into two distinct sections, referred to in the Building Regulations as D1 and D2, with a tundish forming the critical visible air break between them.

D1 Pipework — The Short Discharge Connection

The D1 pipe is the section of discharge pipework that runs from the outlet of the temperature relief valve (or combined temperature and pressure relief valve) directly down to the inlet of the tundish. Building Regulations and the manufacturer's installation instructions impose strict constraints on D1 pipework:

The Tundish — Visible Air Break

The tundish is a funnel-like fitting that creates an air break between the D1 pipe and the D2 pipe. This visible air break serves two critical purposes. First, it provides an observable indication that a relief valve has operated — a building occupant or engineer can see water dripping or flowing through the tundish and understand that an abnormal condition has occurred. Second, the air break prevents any back-pressure from the D2 discharge pipe acting against the relief valve, which could prevent the valve from closing properly after the discharge event.

The tundish must be positioned in a visible location — typically in the airing cupboard, plant room, or cupboard housing the cylinder — and must not be concealed behind panels or in locations inaccessible without tools.

D2 Pipework — The Long Discharge Run

The D2 pipe runs from the outlet of the tundish to the final discharge termination point. It is the section of discharge pipework that presents the most significant design challenge, as it must typically traverse considerable distances through the building fabric before reaching a safe external or compliant internal termination point. The sizing and routing of D2 pipework is the primary purpose of this calculator.

G3 Building Regulations — Discharge Requirements

Key G3 Discharge Requirements

Approved Persons and Certification

Unvented hot water cylinder installations must be carried out by a G3-competent installer — a person who holds a current qualification (such as City & Guilds 6189, ACS G3 assessment, or BPEC equivalent) and is registered with an Approved Inspector or a Competent Persons Scheme such as APHC, NAPIT, or Gas Safe (for combination systems).

The installer must issue a Building Regulations compliance certificate (notification to the local authority) upon completion. Non-compliant discharge pipework — including undersized D2 pipe, blocked tundish, or inappropriate termination — constitutes a breach of Building Regulations and is a notifiable defect that must be rectified.

D1 and D2 Pipe Sizing Rules

The pipe sizing rules for D2 discharge pipework are derived from the Approved Document G guidance tables, which in turn are based on hydraulic calculations of the discharge flow rates expected from temperature and pressure relief valves at their rated conditions. The fundamental rule is that the D2 pipe must have adequate bore to convey the maximum rated discharge flow of the relief valve to atmosphere without causing back-pressure at the tundish.

The Step-Up Rule — Minimum D2 Size

The absolute minimum requirement is that D2 must be at least one pipe size larger than D1 (the tundish outlet). In standard domestic installations:

Equivalent Length Calculation

The maximum permitted D2 pipe length is not simply the measured physical length — it is the total equivalent length, which accounts for the hydraulic resistance of bends and fittings in addition to the straight pipe run. Each bend or elbow in a D2 pipe run adds a penalty equivalent length that reduces the remaining allowable straight-pipe distance.

Worked Equivalent Length Formula

D2 Discharge Pipe Sizing Tables — Full Reference

D2 Discharge Pipe Termination Requirements

The termination point of the D2 pipe is as important as its sizing. A correctly sized D2 pipe that terminates inappropriately remains non-compliant. Approved Document G sets out specific requirements for where and how D2 pipework must terminate.

Approved Termination Locations

Termination Visibility Requirement

The G3 visibility requirement means the discharge point — whether the tundish in the airing cupboard or the D2 termination point externally — must be observable without using tools or moving fixed objects. The purpose is to alert occupants that a safety device has operated and that the system requires inspection by a competent engineer. Where the D2 runs a long distance, a secondary visible indicator (such as a sight glass or second tundish) can be used.

Worked Calculation Examples

Example 1: Standard 210L Domestic Cylinder — Short Run

Scenario: 210 litre unvented cylinder, 15mm TPRV outlet, tundish in airing cupboard, D2 runs 4.2m through ceiling void and exits through external wall. Two 90° standard elbows (22mm) in D2 run.

Example 2: Long D2 Run — Multiple Bends

Scenario: 300L commercial unvented cylinder, 15mm D1, D2 must run 12m through service corridor with four 90° standard elbows (28mm).

Example 3: Plastic D2 Pipe — Apartment Installation

Scenario: 170L unvented cylinder in flat, 15mm D1, D2 must discharge internally into floor drain. 6.5m run, one 90° elbow, one 45° elbow (22mm fittings). Plastic pipe (manufacturer-approved, temperature rated).

Common D2 Discharge Pipe Installation Mistakes

These are the most frequently encountered non-compliant discharge pipe installations identified during G3 inspection and remedial surveys:

Frequently Asked Questions — D1, D2 & G3 Regulations

Related Calculators for Unvented Hot Water Systems

Use these engineering tools alongside discharge pipe sizing for complete unvented cylinder system design: