Desiccant Dehumidifiers DH2500 or DH3500?

When is it best to buy a 3 hole system or a 4 hole system?

The DH2500 is classed as a 3 hole unit and the DH3500 is classed as a 4 hole unit.  The DH2500 has one internal fan while the DH3500 has two internal fans.  Both dehumidifiers use a desiccant wheel and in both cases, this wheel rotates slowly inside.  Both take air from the room (Process Air-In) pass it through the desiccant wheel to the other side.  As the air passes through the wheel it is typically 30% drier as it is put back into the room (Process Air-Out) ready to be dried again as it passes back into the dehumidifier.

The desiccant wheel allows the Process Air-In to flow through it’s flat surface to the other side along small flouts.    The desiccant wheel therefore has a large surface area that is coated with a crystal called Zeolite.  Zeolite has the chemical property of absorbing moisture at normal temperatures but will give out that same moisture (Purged) when heat is applied usually as hot air.  This purged air that is hot and contains the moisture, can be  ducted away to the outside (Exhaust)   Rotating the wheel and drying a small section, while Process Air-In  is dried in the larger part of the wheel is a continuous process.

In a 3 Hole system 15% of the Process Air-Out is heated then passed back through the desiccant wheel to Purge it.  This means that with a 3 hole system, air from outside the room needs to come in to replace the 15% of Exhaust Air to equalise the pressure.  This air is termed Regen Air.  Three holes comes from having Process Air-In, Process Air-Out & Exhaust ducts.  Usual use is to simply place in the dehumidifier in a room that is to be dried and use an exhaust to put the moisture to the outside of that room.

With a 4 Hole system the two air flows are totally separate.  Process Air-In and Process Air-Out are one fan while Exhaust & Regen air are powered by the other fan.  The Regen air in this can case also be taken from outside the room.  Four hole is more effective at drying as firstly all the dry Process air is passed back into the room and secondly Regen air from outside of the room may be relatively wet.   A 4-Hole system can be used as a 3-Hole system simply by leaving the Regen duct off.

In 4-Hole mode the dehumidifier dries “Pressure Neutrally” .  Air in the room being dried, is not required to enter to leave the room because of the dehumidifier.  This is needed if drying a sealed container, room or drying tent (covered plastic area where focus drying is needed to save energy and quicken drying time), cold store, electrical cabinet, shipping container, test lab etc.


Note that the DH2511 is the 110V/50Hz version of the 220V DH2500.  Likewise the DH3511 is the 110V version of the 220V DH3500.  They both work in the same manner.   EPD150 & EPD200 sold in the USA market are 110V/60Hz versions of the DH2500 & DH3500 respectively. 

Both the DH8XX (EPD30) & DH12XX (EPD50) act as 3-hole dehumidifiers


What is a “Three Hole” System?

3 hole system ecor pro

DH2500 3 hole and 4 hole systems


The 3 hole system uses only one internal fan to create two air flows internally in the desiccant dehumidifier.  15% of the “process air” (or damp air in) that you need to dry, is turned back on itself after passing through the wheel.  This air is used to dry the wheel again to be vented out as saturated “exhausted wet air”.  This is the air that is vented away like a tumble dryer hose (only 80mm in diameter).


Application used for 3-hole systems in the majority of cases.  Dehumidifier is placed inside of the room to be drying.  Ideal for flood drying, loft spaces, crawl spaces, clothes dryers  and many more.  The expelled exhaust causes air to come in from outside,

3 hole inside negative pressure


Blowing dryer air into a room causes a positive pressure.  Very ineffective as the real benefit is compound drying where the process air will pass through the dehumidifiers over and over getting dryer and dryer.  Used only in some industrial applications.

3 hole outside positive pressure

Configuration Not Recommended for Most Applications , this method may have poor results if the outside air is damp.  Input air will only be around 30% dryer than ambient.  Applications for this mode of drying are usually very specific often in specific industrial processes.



Alternatively, the unit can be placed outside of the room and the dry air can be vented into the room.  This is a “positive pressure” system (the room has a slightly positive pressure compared to the outside of the room).

3 hole outside negative pressure



Effectively the same as system A.  The difference of A & C over B is that there is a compound drying effect.  i.e. air is circulated over and over and dried over and over.  The only time system B is considered is if there is a real issue with the ingress of outside air from outside entering the room.



dh2500 neutral pressure room not heated

The DH2500 can be made into a pressure neutral system.  Since more air is taken into the DH2500 than come out, the additional air can be introduced from a location that the installer dictates by T-piece rather than from openings in the room itself.  Air entering the room from openings may be more wet than the introduction of air from another location.  The system can be balanced with variation but in general the additional air inlet should be approximately 25-30mm in diameter.


What is a “Four Hole” System?

4 hole system ecor pro

DH3500 3 hole and 4 hole systems

The 4 hole system has two fans internally.  It can be used in the same way as a 3 hole desiccant dehumidifier.  However, it has the advantage of the two air flows being separate. When the wet air is exhausted the “Regen” or regeneration air can also be taken from outside of the room.


4 hole outside neutral pressure


4 hole inside neutral pressure


The 4 hole unit has the advantage it can operate in a “pressure neutral” way.  The room can effectively be sealed as with a metal container.  Air in the room can be continuously dried without any air from the room being vented away or ingress into the room due to a negative pressure.

Not only is it a more efficient as a drying system but it also conserves the heating energy. With a 4 hole system operating separate air flows means the heat energy is put into the room. This could be a disadvantage if heat is a concern.  However, to get around this a little the following configurations will make the 4 hole version act more like a 3 hole with less heat gain.

4 hole outside negative pressure


The 4-Hole model acting as a 3-Hole.  Simply remove the Regen duct.  This will mean the DH3500 is still better at drying in this mode than the DH2500.

4 hole inside negative pressure




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