A DH2500 or DH3500?


When is it best to buy a 3 hole system or a 4 hole system? Its not about price of course its about application. Basically a 3 hole system will use one internal fan motor while a four 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 as vapour.  This overcomes issues with frost, flood risk and indeed reliability.

More importantly an insulated vapour duct-out can vent water upwards above the unit so you dont have to rely on gravity draining of water away.  They can also be mounted even upside down on ceilings.  Of course without collecting water the unit can run automatically without needing to be emptied of water either.  We recommed uninsultated ducting to run away from the dehumidifier on a decline to stop ccondensate running back into the unit. 


What is a “Three Hole” Ducted Dehumidifier?

3 hole system ecor pro

DH2500 3 hole and 4 hole systems


The three 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)

(B)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 an 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” Ducted Dehumidifier?

4 hole system ecor pro

DH3500 3 hole and 4 hole systems

The four hole system has two fans internally.  It can be used in the same way as a three hole desiccant dehumidifier.  However, it has the advantage of the 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 four 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 four 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 four hole version act more like a three hole with less heat gain.


4 hole outside negative pressure


4 hole inside negative pressure