What is a 'condenser' dryer ??
A condenser clothes dryer is a machine that looks just like a conventional tumble clothes dryer, but which does not require an external vent. For the user/owner, operation of both types of dryers is essentially the same - the difference is in the internal design.
In a vented clothes dryer, air is drawn from the surrounding area (i.e. the laundry room), then heated and blown through the clothes as the drum tumbles them about. This hot air evaporates some of the water in the damp fabrics, and the resultant moisture-laden air is then exhausted through a vent duct to the outside.
In a condenser dryer, there are two separate "loops". The inside "loop" of air is sealed from the outside environment - air from within the drum is heated, then blown through the tumbling clothes, then the moisture-laden air is passed through a heat exchanger, where the water recondenses. The same dry air is then reheated, where it is again blown through the drum and clothes, and the cycle begins again (this is a more-or-less continuous process).
The outside "loop" in a condenser dryer consists of either air or water. Some condenser dryer models are air-cooled, and use the ambient room air as a heat sink, by blowing it across the outside of the heat exchanger. These dryers will tend to heat the indoor air in one's laundry room significantly. Note however that ONLY heat is released - all MOISTURE is contained within the unit. The condensed water can be either pumped away to a drain line (e.g. into a standpipe shared with the clothes washer) or stored in a container within the dryer to be emptied later (not all models offer both options). All standalone Euro condenser dryers are of this type, i.e. units from Miele, AEG, Bosch, Asko, Malber, and Eurotech.
In "combo washer/dryers" (i.e. machines that can BOTH wash and dry the clothes), the ventless condenser system is also widely used, but in these cases the condensers are water-cooled. During a dry cycle, several gallons of cold water are used to condense the moisture evaporated from the clothes, which again is pumped away through the drain line. Most of the "combos" currently available in North America use this method - i.e. units from Equator, Splendide, Malber, Haier, Quietline, Thor, LG, and Eurotech. Note that unlike the air-cooled design, these models do NOT significantly heat the indoor air in one's laundry room - but on the other hand, the fact that they use extra water during the dry cycle must be taken into consideration, especially for anyone on a very limited (or expensive) water supply.
Since air-cooled condenser dryers heat the indoor air, they're most suited for use in cold to moderate climates (it would be counterproductive to run what amounts to a space heater at the same time you're trying to cool the house with air conditioning). However, condenser dryers do have one big advantage - they don't exhaust any air to the outside. And hence, on very hot days, a condenser dryer may surprisingly be as good...