Sizing Humidifiers & Ancillary Dehumidifiers
Whole-house dehumidifiers are not sized by square footage. The Manual J block load procedure most technicians have never been shown.
I recently read a post on an industry Facebook Group in which a technician was asking, “Which is the best whole-house dehumidifier?” Not trying to be a smart a$$, I immediately replied: The one that is sized correctly! Based on the follow-up conversation, it was quite obvious most were not aware that there is a procedure for sizing dehumidifiers, and like their more popular central A/C cousins, sizing is not based on square feet of the home!
The first step in sizing any whole-home comfort system is to complete an ACCA Manual J v8 load calculation. For the purposes of a whole-house dehumidifier, a block load is sufficient.
When looking at your summary report, the item of interest for sizing your dehumidifier is the latent gains of the house (Summer). Your ancillary whole-home dehumidification system should meet at least 85% of this latent capacity.
Of course, we calculate this load in Btu/hr and most dehumidifiers are rated in ‘Pints Per Day’ (PPD) or ‘Liters Per Day’. There is a little bit of math to get you to this rating, but if this is ancillary dehumidification then 85% is the target. If you don’t have air-conditioning, then sizing for 100% of the latent gains would probably be your best bet!
Pints per hour = Latent Load (Btu/hr) / 1,054
Pints per day = 24 x Pints per hour
Liter/Day = (Pint/Day) / 2.113
Want this done right on your next job?
Chris Morin trains and coaches HVAC contractors on ACCA-compliant system design — and runs the load calculations for you when you would rather not.