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Discuss Basin waste flowrate in the Bathrooms, Showers and Wetrooms area at

  1. windyboy

    windyboy New Member

    I'm helping a client with a situation where a tap was left on and a bathroom basin overflowed. This wasn't via the overflow, the plug was out. I've been to site and confirmed that the waste/trap (which was completely clean) can't handle the flowrate from even one of the taps if it's fully open.

    The bathroom was recently installed. It seems like common sense that you'd check this, but is anyone aware of any regs that say that water should be able to run out of a basin faster than it runs in? I can't find any.
  2. scott_d

    scott_d Plumber GSR

    Are you trying to blame the installer because someone left on a tap and flooded the bathroom?
  3. windyboy

    windyboy New Member

    I'm not blaming anyone. I just think that if you leave the plug out and turn a tap on it's reasonable to expect that sink won't overflow.
  4. Ric2013

    Ric2013 Plumber

    I would imagine that if it were installed as per the building regulations approved document, then the waste would discharge the flow of a single tap. BUT it then depends on the tap.

    If there's a restrictive design in the basin or waste, then the waste pipework might be fine, but still not keep up. In fairness, I sometimes find I have to do things to work around the limitations of existing plumbing that I wouldn't do in a newbuild situation. To take up Scott D's point, it depends on what the customer is willing to pay for so not necessarily the installer's fault. I recently refitted an existing bath on an existing 1.25" shared waste run. The customer accepted the reduced drain speed as a compromise for not having to pay for a new waste run through the wall and modifications to existing hoppers etc etc. At least I fitted a waterless valve, so the basin no longer pushes water back into the bath!
  5. Ric2013

    Ric2013 Plumber

    Might be good to fit flow restricters to pipework to taps...

    Back to your blame issue, who supplied or chose the materials, and what is the waste pipework like? Is it an excessive run or is the basin waste itself restrictive?
  6. scott_d

    scott_d Plumber GSR

    The installer may have assumed the same
    • Funny Funny x 1
  7. oz-plumber

    oz-plumber Plumber

    I'm sure there is a Reg that would state that an appliance / fixture should be 'fit for purpose'

    If the waste can't handle the flowrate or the overflow can't do what it is designed to do, I would be questioning the people that approved the product and the suppliers of this product for the 'test certificates'.

    If the waste pipe isn't blocked, why can't the basin cope with the waterflow rate?

    I wouldn't be questioning the Plumber over the mess, I would be questioning the sellers of this product or the Certifiers of this product for an expalnation as to why it can't cope with the flowrate of the taps.

    Good luck - been there before
  8. rpm

    rpm Trusted Plumber

    Use the little search box for this topic, it`s been covered before regarding a bath.
  9. Chuck

    Chuck Active Member

    I can offer you something from a 2004 British Standard, which says:

    BS EN 274-1:2002 Waste fittings for sanitary appliances -- Part 1: Requirements §4.6 Hydraulic requirements:

    "When tested in accordance with clause 5 of EN 274-2:2002 the flowrates of waste fittings shall not be less than the values given in Table 3." And, roughly speaking, Table 3 says 0.6 litre/s minimum.

    The corresponding BS for taps also gives minimum (not maximum) flow rates:

    BS EN 200-2008 Sanitary tapware - Single taps and combination taps for water supply systems of type 1 and type 2. General technical specification

    says in §10.3 Table 10 that that a 1/2" basin tap should have a minimum flow rate of 0.2 litre/s (high pressure supply) or 0.125 litre/ s (low pressure supply)
  10. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    what size is the waste pipe and how long is the run?
  11. Stigster

    Stigster Plumber

    I have a tap in a basin giving 22 litres per minute (0.36 litre/s). This is a pretty decent flowrate for most practical uses. You'd need nearly twice that to overwhelm a waste if it complied to the regulation above of 0.6 litre/s waste discharge minimum.

    That said, at a little higher flow rate/pressure than that (I adjust the pressure reducing valve to give 3 bar), half the water leaves the basin over the edge because it's coming out with enough pressure to slosh out of the basin I have here. So even if the drain could cope you'd still get water damage by leaving the tap on full blast and walking away.

    Is the waste failing to cope, did the water come from getting backed up or did it leave the basin by sloshing over the edge because the tap was on full blast?

    If it's the latter I'd say overall it is the customer's fault. Ultimately, never walk away from a running tap. If you do any flood is on you in my humble opinion.
    Last edited: Jul 11, 2017
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