I own a 3-story home. Basement, ground floor and upper floor. Each floor is approximately 80m2 (860 sq feet large), combined volume of the house is cca 600m3 (21200 ft3). All stories are heated with floor heating and all have the same inner temperature. Basement is not cooler (max 1-2F) or more humid. Given the PHPP (passive house standard) are house falls in the passiv standard albeit we do not live by those standards, so we need a bit of cooling in the summer. Summers in our country are quite hot at 30-35C (86-95F)during the day, occasionaly even hotter. We have an ERV installed and we heat (winter) and cool (summer) our house with a heat pump. Since we do need a bit of cooling and I am highly sensitive to all kinds of A/C systems (blowing cool air), we opted for floor cooling. Not the best option but adequate and free, since we didn't need to do any additional installation. The problem is that floor cooling works in terms of cooling the house to requiered temperature, but it does not take care of condensed water. I have two portable dehumidifiers (delonghi dem10) - I put one in ground floor and the other one in upper floor and they manage to keep the RH below 65% in the worst conditions. This solution works but I am looking for more automatic solution and there is a lot of talk on american sites about wholehouse an basement dehumidifiers. The room where I have the heat pump and ERV has installation for direct drainage so I wouldn't have to manualy empty the tank. The amount of condensate is not very big, about 6-8 pints per day. So how can I "automate" dehumidification of our home? 1) Can I use both of my existant dehumidifiers as an whole house dehumidifier in the basement? I have been told yesterday that moisture will quickly even in my house since it is very air thight and I do not have closed stairways between stories. Additionaly there is an open ceiling (220 sq ft) between ground and upper floor. 2) If those two dehumidifiers won't work is there hope for any dehumidifer installed in basement and not being connected in any way to the air ducts. I am looking at a 40L (85 pints) dehumidifier but am not willing to buy if this will not work. I am borrowing a 20L dehumidifier tomorrow and will start all 3 (2x10L + 1x20L) dehumidifiers in the basement room where I would like to put a single machine. Will these 3 dehumidifiers perform the same as one 40L dehumidifier and can I assume the same result - that is either failure or success.

As long as there is at least some open convective or mechanically driven air flow between floors a small dehumidifier in the basement of a tight house that size will keep up with the humidity in the rest of the house, as long as you don't have excessive ventilation rates during periods when the outdoor dew points are high (say 16C or higher.) In fact, just one would do it for most homes. Three would be overkill. The key is to minimize the blower speed or duty cycle of the ERV whenever outdoor dew points are high. Only if you need/want very high ventiltation rates during hot-humid weather would you need more.

Most "whole house dehumidifier" models would be EXTREME OVERKILL in a very tight house, akin to using a cricket bat to swat mosquitoes.

Dehumidifiers convert latent heat (moisture) in to sensible heat (hot air), raising the cooling load on the house. Modifying the cooling system to allow for a chilled water air coil for dehumidification may be the best solution, but it would require a bit of design.