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  Backfilling With Waste Water

This is probably one of the most common complaints that we hear, the machine fills with water when it's off or fills with dirty water, we will explain what's going on.

To frame the perspective of this article I will explain a bit about the odds of a leaking valve causing the problem of slowly filling overnight.

I run a small service company with two engineers and we will attend about 800-1000 service calls a year and, of all those calls, the instance of a valve leaking is perhaps one or two in the year. However much of the work we do is under guarantee or extended warranty and, whilst under guarantee (less than 12 months old) we will receive at least two or three calls a week from customers telling us that their machine fills with water overnight or over a period of time. So you may see where I'm coming from here when I tell you that a faulty valve causes this (maybe) once or twice a year but on all other occasions it is what we refer to in the trade as "backfilling". In other words the machine, washing machine or dishwasher, is being filled from the drainage system due to a poor or faulty installation.

Make no mistake, this is almost certainly NOT a fault with the appliance and if it's been recently installed I'd stress that even further.

  Symptoms Of A Washing Machine Backfilling

Dead simple. The machine will partially fill with dirty water over an extended period of time.

Often this will start to smell, especially if it's left for a few days in summer, as there's all sorts of gunk in the water that lurks in your drain. More often than not these are directly connected to the sink trap by way of a spigot (they can leak as well) as it is a cheaper and faster fit for the installer. The problem is of course that there's nothing and, I do mean nothing at all, to stop the dirty water from your sink going straight down there and into the washing machine or dishwasher.

The short of it is that, if the appliance was installed correctly in accordance with the instructions (which installers rarely, if ever, seem to bother reading) then the problem should never occur.

Of course dirty, greasy water going back into the machine isn't exactly going to do it any favours. Pumps can get clogged up, seals eaten away, food particles in the sump' the list goes on and on. Some of these things you may not even notice until you go away for a few days or after you use the kitchen sink heavily for a time so it isn't a fault that always manifests itself quickly.

fat blockage found in a washing machine drain
This is an example of how fat from the sink can block a washing machine (Photo courtesy of Jim Banks, IADOM)

  Solution For A Machine Filling With Waste Water

I am afraid that the only real solution to this problem is to sort out the drainage issue, there really isn't much that the appliance repairer is going to be able to do unless he's prepared to get involved in some plumbing work.

Since this problem largely happens whilst the appliances are under guarantee it is very worthwhile pointing out that the appliance manufacturers will NOT pay for this sort of rectification work. So far as they are concerned (quite rightly) this is an installation issue and, whether you installed the machine yourself or a kitchen company, retailer or whoever did it for you, it's wrong. This is considered to be a poor installation and not a fault with the appliance.

If the manufacturer chooses to they are perfectly correct to charge you for an abortive call as, had the appliance been installed correctly, the service visit would not have been required.

Over the years I have had many arguments with kitchen fitters, plumbers and the likes that tell me I'm wrong on this, it doesn't matter. But it does, very much. I've been sworn at, called a liar and accused of all sorts just because I tell people what the actual problem is however, in this instance, I've seen enough washing machines and dishwashers installed incorrectly by anything from small local retailers, to national kitchen companies and even national home builders to know that they often all get it wrong.

If only they'd taken the time to read the installation instructions my life would be far more peaceful.

Sally Neilson
Washing machine backfilling from sink waste
My washing machine has been backfilling from the kitchen sink ever since I had a new combi-boiler fitted on the wall above several months ago. It had never done this prior to the boiler being installed and I can't help wondering if the new pipework fitted by the installer behind the washing machine was incorrectly connected at the time. The heating engineer says he's not responsible. Can you advise if my thoughts about incorrect connections are feasible, please?
Matt Rees said :
I thought washing machines & dishwashers had non-return valves on the waste pump? We've had this problem at home, but it didn't happen for the first year or so & nothing was changed in the installation during that time.
yes it this will work and the non return valve costs only £10.99. it's simple to install yourself. however some plumbers will hide this easy solution and insist to change the whole installation to charge you hundreds of pounds.

fitting of non return valve
Hi your info was very helpful re dirty water seeping into dishwasher. Could you please tell me where to fit the non return valve on our fully integrated Indesit model DIFP 28T9 A and where to buy it from. thanks
Matt Rees
I thought washing machines & dishwashers had non-return valves on the waste pump? We've had this problem at home, but it didn't happen for the first year or so & nothing was changed in the installation during that time.
Kenneth Watt
Not that we are aware of and that work, no.
So I realize that it's against most plumbing codes to install the drain hose connecting to a kitchen sink trap. I was wondering if any anti-siphon devices are usually used on drain hoses, or if those are only for the fill hoses.

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