I have actually been thinking a lot about humidity. I live in a very humid location, Washington, DC. It’s really common for individuals here to need a dehumidifier in addition to their air conditioning. I’ve been thinking about the concern of how one goes about sizing a dehumidifier.
Designing a home’s heating and cooling loads through the Handbook J procedure has actually ended up being regular. It would be natural to wonder if an analogous procedure could be utilized for sizing a dehumidifier. It ends up that the response is most likely no, and it relates to the nature of modeling. In economics, they like to state, “all designs are wrong, some work.” One of the things that makes a design helpful is if it is fairly invariant to the inputs, which is an expensive method of stating that little variations in the numbers you plug into the design don’t change the output that much. This will indicate that mistakes in measurement won’t alter the last conclusion. The issue with modeling humidity removal in the very same way that cooling and heating are modeled is that you wind up with outputs that are excessively sensitive to your presumptions.
To size an air conditioner you use a process called Manual J, where you approximate the cooling load based upon the construction of a structure and the regional climate. You take a look at weather condition records (available at https://ashrae-meteo.info/v3.0/index.php) and compute the 99th percentile of hourly temperature records, and then you approximate the cooling load at that temperature. When computing cooling loads, you look at whatever that would be putting heat into the structure: solar gain, conduction of heat through the walls, resident activity, air seepage, and any home appliances and equipment being run. These are called practical loads. You also take a look at sources of humidity, which will have to be removed, which requires cooling too. The sources of humidity are resident behavior like bathing, cooking, breathing and sweating, and seepage of humid outside air.
Just to give an idea of the magnitude of these elements, here is the summary of a Manual J for a 4-bedroom home in Washington, DC:
(in BTU/hr)
Sensible gains
Solar gets 13,939
Conductive gains 10,448
Resident activity (5 individuals) 1,150
Infiltration, 20 CFM 367
Internal 2,400
Latent gains:
Resident activity 1,000
Seepage 531
overall Practical 28,304
Overall Hidden 1,531
Grand overall 29,835
Practical heat ratio: 95%
The “reasonable heat ratio” or SHR is just how much of the overall cooling load originates from reasonable loads instead of dehumidification. The lower that number, the more dehumidification you require, a SHR of 100% means you don’t require any dehumidification at all.
Now to look at a comparable computation for dehumidification. The ASHRAE database also has 99th percentile details for humidity. In DC the cooling design day is 92F with 45% relative humidity, however the humidity design day is 82F with 77% relative humidity, a dew point of 74F.
In basic, the more practical gain you have the much better task the air conditioning unit does at removing humidity, so let’s presume that the humidity design day is going to be a cloudy day with no solar gain.
Running those numbers for the humidity design day with the very same presumptions, and the conductive gains changed for the lower outside temperature level:
Reasonable loads:
Solar gains — Conductive gains 4,302 Occupant activity 1,150
seepage 151 Internal gains
2,400 Hidden loads: resident 1,000 infiltration 855 overall reasonable 8,003 Total Hidden 1,855 Grand total 9,859 SHR 81
%At first look it would
seem
that modeling humidifier would be a basic matter of figuring
out what SHR your air conditioning unit is capable
of
, and then adding a dehumidifier that can comprise any difference. Ac system have a SHR ranking, right? However this is where the instability in the model begins being a problem. The very first source of instability in this design is the quantity of infiltration, which is at finest a quote notified by blower door screening and at worst just a straight-out guess. In the cooling load design, changing the seepage from 20 CFM to, state, 50 CFM, doesn’t truly change the photo. The overall cooling load goes from 29,835 to 31,182 and the SHR goes from 95%to 91 %. But in the dehumidification design you see much larger proportional modifications: the overall load goes from 9,859 to 11,369 and the SHR goes from 81% to 72%. To put those numbers into context, at 20CFM the latent load is 1,855 BTU/hr. Dehumidifiers are frequently measured in pints per day, a pint of dehumidification is about 1000 BTU
of cooling. So that’s 44,500 BTU/day, or 44.5 pints daily, or about 5.6 gallons each day. At 50CFM it’s 9.4 gallons daily. (Did I state it’s truly humid here?)But the point is a rather small change in assumptions– or resident behavior– brings in gallons a day of wetness that needs to be eliminated. The other input that is really conscious assumptions is the quantity of dehumidification provided by the a/c. Yes, it holds true that air conditioners have
SHR scores, but they are just helpful for comparing devices, they have no predictive power. What does have predictive power is the outlet temperature of the a/c. If air is cooled below its humidity, wetness will speed up out. If you know the outlet temperature of an a/c unit, you can forecast how much dehumidification it can do. Indoor air at 75F and 50%RH is at a dew point of simply under 55F. If the output of the AC is at 55F, no dehumidification happens, the SHR is 100 %. Reducing the temperature by one degree, at 54F the SHR is 90 %, and decreasing by another degree, at 53F it’s 86 %. So small modifications in the assumptions have a huge impact on the output. The distinction in between 55F and 53F coming out of an air conditioning system is literally the difference between a clean filter and an unclean filter. So what’s the answer? I don’t understand. I wonder to hear everybody’s ideas.