I have a enviornmental test chamber in which engines are tested, so the system was fitted with a dry air makeup system to provide combustion air, however when it was sized the company appearantly did not account for post cooling coil and so we can only just barely use it or we cannot maintain temperature in the space. Since we cannot use the dry air makeup system as we wish we ultimately invite wet, hot air into the space causing the coils to freeze up. Now here is the good part, the company who installed said there would be no need for defrost as they had fitted the system with a dry air purge system, I insisted they install some amount of heat for "potential" defrost but it just cannot do an effective job of removing all of the moisture and in the end we have iced up coil. The question I am posing to all is where should we invest our time? Providing a post cool coil so that we can use the dry air system at will, which in theory should allow us to run with very limited if any defrost requirements or invest in providing the proper heat in the proper place to defrost the coil? by the way the second option requires a new style evaporator as we want to put the heat in it.