3 or 4 Hole Desiccant Dehumidifiers

When is it best to buy a 3 hole system or a 4 hole system? It’s not about price of course it’s about application. Basically a 3 hole system will use one internal fan motor while a 4 hole system will use 2 internal fans and have 2 very separate air flows.

A desiccant dehumidifier uses heat to remove moisture.  This moisture is exhausted in an air flow much the same as a tumble dryer exhausts moisture.  In a home desiccant dehumidifier this water is condensed into a water container.  The more industrial products that are designed to work 247 exhaust the moisture.

Industrial dehumidifiers donate the fact they are designed to work 247 all day every day unattended.  If they condensed water the water container would often be larger than the working part of the unit.  Many uses of which can even be applied to the home.  In addition, collecting water means the desiccant dehumidifier is limited to working to 0°C as the water freeze point.  Frozen water could damage the container or dehumidifier itself.

Not having the water container means the range that are venting that all tend to be made from metal, can work down to -20°C which is often important in unheated areas such as lofts, garages, crawl spaces, boats & out buildings.

Other applications include meat drying,

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 airflows 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).

3 hole inside negative pressure



If we imagine this unit placed in a room and the “wet air” vented out, then air from outside must come into the room.  Generally, this is not a problem if dealing with flood drying, storage areas etc.  The air coming into the room is generally low in humidity and the 15% of air will not cause an issue with overall drying.

If we place the unit in the room being dried and vent out the damp air this would be a “negative pressure” system (the room has a slightly negative pressure compared to the outside of the room).

3 hole outside positive pressure



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 airflows 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 ingression 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



4 hole inside negative pressure


More on the DH3500 showing the key features:

See our New INOX DH2500 3 Hole  HERE

INOX DH3500 4 Hole 

 INOX 100% stainless steel – Unique

INOX Protection Bars for Flanges & Carrying Handle  – Unique

INOX Strong Enough for even for the Hire Industry – Unique  

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