This Brilliant Valve is in Every Home, And It’s Pure Engineering

Simple, and effective, it’s in millions of washing machines around the world.

Sometimes, the most brilliant engineering manifests itself in simple mechanisms. The dual, single acting solenoid valves that control hot and cold water flow in consumer washing machines as an example. Small and lightweight, the unit uses commodity grade polypropylene, handle water flow of widely varying pressure and temperature, and cycle repeatedly to regulate wash temperature. It’s one of the simplest solenoid valves in service, yet it functions for years in a difficult environment, and costs a little to manufacture. Unchanged in decades, it’s a design that has evolved to near perfection.

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Episode Transcript:

A few of you out there will know at a glance what this nifty little assembly is. This is a solenoid controlled water inlet valve. This one is from my long-suffering 25-year-old Whirlpool direct drive top loading washing machine. So why is it in my hand instead of in my laundry room? 

Well, because this one has failed, requiring a replacement, and like almost every machine in my life, if it breaks, I can’t resist the urge to take it apart and see what makes it work. I come from the automotive industry, and I think I have a pretty good idea about good engineering design when I see it, and this particular part is outstanding. 
This one is made by Invensys, and it uses two 120 V solenoids to control hot and cold water flow into the tub of the washer. All washing machines have these, including the modern front loading machines, and if you’ve ever installed a machine, these threaded nipples protrude through the back of the machine, they’re what you’re attaching the hoses to. 
There are two inlets, but only one outlet which is the key to this device. When you select the temperature control on the machine, the solenoids are programmed to cycle individually or together as necessary to blend hot and cold water to achieve the desired wash temperature. And obviously, some form of feedback control is necessary and is provided by this thermistor that senses temperature at the outlet, before the water flows into the tub. 
This one gave me 25 years of service, and finally suffered a failure of the thermistor. So what makes this part an example of great engineering? It’s not only durability, as this part uses an absolute minimum of injection moulded polypropylene, contains maybe two dozen parts at most, and is elegantly simple in design and operation. 
These are the kinds of things that engineers sweat bullets to design, under brutal cost constraints, and when they produce things like this, I tip my hat, because although these things are in service literally by the millions around the world, only mechanical engineers or maybe the Maytag repairman can appreciate the genius of this assembly. 
And by the way, one of the things that makes this assembly so brilliantly simple is that the solenoid valves are single acting. For the valves to close securely, that has to be sufficient inlet pressure from your house water system to seat the valves. If your water pressure is too low, or even if the hoses become kinked, it’s possible for these valves to bypass a small amount of water, which you will see as water dribbling into the tub.
So before you call the appliance repair man or tear into the back of your machine, look at the hoses and check your water pressure. But if you do need one of these, I ordered a replacement online and it will arrive at my home tomorrow for under 100 bucks. Compared to the cost of a lot of replacement parts these days, this is a bargain. 

Written by

James Anderton

Jim Anderton is the Director of Content for ENGINEERING.com. Mr. Anderton was formerly editor of Canadian Metalworking Magazine and has contributed to a wide range of print and on-line publications, including Design Engineering, Canadian Plastics, Service Station and Garage Management, Autovision, and the National Post. He also brings prior industry experience in quality and part design for a Tier One automotive supplier.