How to pass Part G with a big bath or a rainfall shower
Clients love big baths and powerful showers. Under Part G, one of those is far easier to accommodate than the other.
Not all litres are equal
Each fitting has its own use factor in the calculation. Once the normalisation factor of 0.91 is applied, here's what a change of one unit adds to the final figure:
| Change | Adds to the result (litres/person/day) |
|---|---|
| Shower: 1 l/min more (with a bath present) | About 4.0 |
| WC: 1 litre more average flush | About 4.0 |
| Basin taps: 1 l/min more | About 1.4 |
| Bath: 10 litres more capacity (with a shower present) | About 1.0 |
| Sink taps: 1 l/min more | About 0.4 |
The shower and the WC dominate. The bath barely matters, as long as there's also a shower.
The big bath is usually fine
Going from a 185 litre bath to a 230 litre one adds only about 4.5 litres to the result. With efficient WCs and taps, that's easy to absorb, even at 125.
The exception is a home with a bath and no shower. Without a shower, the bath's use factor jumps from 0.11 to 0.50, and a 180 litre bath alone contributes 90 litres before adjustment. That almost always fails, which is why nearly every compliant home has a shower.
The rainfall shower is the hard one
Here's a worked example at 125 litres, with a 230 litre bath, 6/4 dual flush WC, 6 l/min basin taps and 8 l/min sink taps:
- With a 12 l/min rainfall shower, the result is about 136.9 litres. That fails.
- Switch to a 4/2.6 dual flush WC, 5 l/min basin taps and 6 l/min sink taps, and it's about 128.2. Still fails.
- Bring the shower down to 10 l/min as well, and it's about 120.3. That passes, with the big bath kept.
With the most efficient standard WC and taps, a 12 l/min shower and a 230 litre bath still come out at about 126 litres. The shower has to give a little.
Under 110 litres
At 110, the same 230 litre bath works with efficient WCs and taps if the shower is 7 l/min. At 8 l/min it fails by about 2 litres.
Practical options
- Choose a shower with a flow regulator, or one that's rated at a lower flow at the expected pressure.
- Fit 4/2.6 dual flush WCs. It's the single biggest saving after the shower.
- Use aerated basin taps around 4 to 5 l/min. Users rarely notice.
- Run the numbers before ordering. The calculator shows how much room every fitting has once the spec passes.
Run the numbers
The free calculator checks a dwelling against 125 or 110 litres, shows where the water goes, and finds the smallest change that passes.