As has been noted, the dehu would add about 1/4 ton based on wattage, but an additional 1/4 ton or therabouts based upon water removal - 1000 Btu per pint removed, essentially trading latent for sensible.
The dehu should NOT be operated "whenever the cooling system is running"...that's the ONE time when you should NOT need to run a dehu. For a separate dehu to be valuable, it needs to be able to operate and have its air distributed throughout the home during times when cooling is NOT operating - damp cloudy / rainy days when temps are too low to provide sensible load to the cooling system.
Install a dehu if you must, but duct it so it can operate independently. Ensure air through it does not short circuit. Do everything possible to minimize latent load and manage latent load better...Our 15 point dehu avoidance strategy:
Bottom line - I regard a dehu, whether portable or central, as a noisy, crude pricey band-aid for crappy buildings and sloppy HVAC. We advocate a 15 point strategy to stave off the need for supplemental dehu under nearly all circumstances:
1) Minimize enclosure air infiltration - outdoor air is MORE humid, not less all summer in nearly all US climate zones. The home needn't be anywhere near as tight as a Passive House, 2-3 ACH50 will do fine.
2) Properly size HVAC system to minimize short cycling - endeavor to install the smallest feasible system. Use the Manual J safety factor to err on the side of smaller, not larger. Tonnage cools a home, but run time is crucial for drying.
3) Ensure ductwork is configured to provide individual room design air flows - stave off thermostat wars, zone where appropriate. Properly sized and configured ductwork supports smaller HVAC systems, as does zoning - provides ability for system to direct its capacity to where needed, whether occasioned by the sun's daily movement or intermittent concentrated loads such as intense cooking or gatherings in the public areas of a home. Zoning makes a system smarter and allows it to be smaller. Smaller dehumidifies better.
4) Ensure ductwork is entirely within conditioned space or at least minimize duct leaks to / from unconditioned space. Provide large filter cabinets that maintain good airflow over longer media change intervals.
5) Avoid single stage HVAC systems - they short cycle during part load conditions.
6) Include controls that modulate system airflow (reduced CFM per ton) in response to humidity excursions.
7) Manage point sources of humidity with ventilation - encourage use of bath vent fans by installing quiet models controlled by timers, spot humidity and / or motion. Ensure range hood is quiet, properly sized, selected, positioned, and ducted so that it both works well and is reasonably likely to be actually used. Position kitchen supply registers so that their discharge does NOT interfere with rising cooking fumes entering hood. (It is amazing how often this small but crucial mistake is made)
8) Discourage use of continuous fan in cooling mode - it re-evaporates water off coil while compressor is off.
9) Discourage use of natural gas or propane for cooking - a scant 20% of the fuel's heat goes into the cookware, and a byproduct of gas combustion is water vapor. Granted a properly configured hood will evacuate the water vapor, but why subject a southern kitchen to all that extra heat and humidity when viable alternatives exist - consider an induction range.
10) Evict panting dogs, sweaty children, and thirsty houseplants to the extent possible during summer months.
11) Endeavour to include a heat pump water heater, its thermodynamic / psychrometric effect within the conditioned envelope of a home. The free cooling and drying are significant, though the noise and appearance are concerns to be managed.
12) Resist with every fiber of one's being Pecksniffian demands to arbitrarily over-ventilate in response to an arbitrary and capricious(A&C) standard. Failing that, provide the "required" A&C ventilation system with an OFF switch!
13 Thermal mass - Flattens and delays sensible load, providing repeated opportunities to meet latent load after dark and during early morning hours
14) Decrease CPH for longer runtimes, better dehu
15) Select AHU profiles for improved dehu during short runtimes |