How the heck does condensation drying work?

If you've looked at dishwashers, you've probably noticed something unique about Bosch. Their dishwashers are listed as “no heat”. In the event you did some further investigation, you probably found that Bosch dishwashers do in fact dry your dishes, but they don't use a typical heater.

So how do they dry?

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A real-life, relatable example to get you started…

In a regular dishwasher, if you've ever opened the door during the “dry” cycle, you were probably hit with a burst of steam. All the water on your dishes was evaporating.

Assuming you opened the dishwasher for a reason (beyond “i wonder what will happen”), you may have grabbed a dish you needed. And that dish was probably still hot from the rinse water, even if the drying element hadn't started up yet. And even when you took the dish out, it probably stayed hot for a while, and the water kept evaporating.

Chances are, with that 1 example, you already understand half of the evaporation/condensation system. Bosch takes the principle above, and expands it into their dishwashing system.

If you follow through with the above notion, where most dishwashers rely on an element to heat the dishes, Bosch uses a more economic (and arguably more sensible method).

As long as the rinse water is hot enough, the dishes will be hot when the rinse cycle is complete. The water will evaporate.

Of course, the inside of the dishwasher won't stay hot forever. Eventually it will cool off, and that steam will become water again.

Q: So how do you make sure that when the steam becomes water again, it ends up in the drain (instead of back on your dishes!)?

A: The answer is… a stainless steel tub.

…And that's exactly what Bosch uses. It works, because the stainless steel tub cools off quickly – much more quickly than the dishes do. So as the water evaporates from your dishes, the steam touches the stainless steel tub, cools off to become water, and runs down the tub into the drain.

And that's the principle behind condensation drying. The water evaporates from your hot dishes, condenses on the stainless steel tub, and runs down the drain.
Nifty, eh?

An imperfect system.

It sounds great (and in many ways, it is), but it's not perfect.

• A reliance on hot-rinse-water – Hot rinse water is usually ideal for a wash. And the hotter it is, the better that condensation drying works. However, not every cycle can get away with hot water. For example, “delicate” cycles tend to use warmer water (instead of hot), because thin delicate glass has a tendancy to crack if it's suddenly heated or cooled quickly. In addition, dishwashers continually try to use lower-temperature water (with extended wash times to compensate) to maximize energy efficiency.
• A lengthy dry – condensation drying takes a while to happen. Unlike a drying element which relies on brute-force-heating, these dishwashers have to wait for the natural process of evaporation & condensation to take place.
• Imperfect materials – while glass and metal items retain heat very well (which is ideal), some materials like plastic don't. The result is that plastic items (like tupperware) often don't stay hot enough, long enough, for the water to evaporate from them. Thus, it's pretty common to open your dishwasher, and find that your plastic items are still wet.
• Requires a stainless steel tub – this adds cost to the system. Now that may not matter if you wanted a stainless steel tub anyway for it's other benefits, but if you were hoping to get condensation-drying “on the cheap” in a plastic tub, too bad. A plastic tub won't work with condensation drying.
• Requires “rinse aid” – for condensation drying to be most effective, 2 things need to happen. First, as much water as possible must run off the dishes (and into the drain) – the less water on your dishes before the evaporation/condensation process, the better. Second, when the remaining water evaporates from your dishes and condenses on the stainless steel tub, it needs to stream down into the drain, rather than remaining on the sides of the tub as water droplets. Rinse aid (like JetDry) takes care of both these issues very well. However, it is virtually required, and an added cost. Compare this to a “standard” dishwasher – while rinse aid is ideal in those too, the heater doesn't really care – it'll eventually evaporate all the water anyway.

The advantages (now that I've scared you off…):

Despite the issues that condensation drying faces, there are quite a few advantages which happen to be quite strong:

• Energy Efficiency – despite the need for hot rinse water, condensation drying is still much more efficient than having a heating element to do the drying. After all, the rinse water is heated in both cases – it's just being utilized better in a condensation system (even if the water has to be heated slightly more).
• No melting – elements inside a typical dishwasher have a tendancy to melt/deform plastic items placed in the lower rack (near the heater). Since Bosch condensation drying machines don't have this heating element (they only heat the water), the risk of anything melting is extremely low.
• No “steam” damage to your nearby counter – in a “regular” dishwasher with a heating element, you have a vent (usually located on the front panel), where the steam can escape. Often, this steam ends up affecting your wooden counter/shelving (sometimes ruining it over time if your wood cabinet is delicate). In a condensation system, there's no front vent. That steam is being turned to water and going down the drain. As an added bonus, if your home has high humidity levels, condensation dryers won't add to the problem.

Is condensation drying for you?

Since Bosch uses this in virtually all their washers and for some people it's a deal maker (or deal breaker), it might be just as accurate to ask “is a Bosch for you?”

Let's be clear. There *are* certainly advantages. A number of key things in fact.

However, the one major disadvantage that nearly everyone will face is that their dishes may not come out perfectly dry all the time. In fact, given the right (wrong) conditions, it's possible that you'll open the door and all your dishes will be sopping wet.

If you're willing to use rinse-aid, hand-dry the occassional item(s) if necessary, accept that you may have water spotting periodically (particularly if you have hard water and/or refuse to use rinse aid), and deal with the long “dry” time (where literally nothing is happening except the water evaporating/condensing naturally inside), then hey, the dishwasher's advantages are probably going to please you.

On the other hand, if those things would drive you absolutely batty, you probably want to avoid these machines altogether. After all, a “standard” machine with a heating element may melt/warp some plastics, vent steam onto nearby surfaces, and suck down more electricity, but hey, they may not. And they'll get your dishes perfectly dry every time, gosh-darn-it!