Question about Sizing HVAC split
Question about Sizing HVAC split
(OP)
Lets assume the following in a single room:
Sensible gain: 20662
latent gain: 1662
total: 22324
I am looking at a mr slim type rated at 24,000 btu unit with the following:
sensible capacity: 14800 btuh
latent capacity: 9400 btuh
I see the total for the unit is greater than the total heat gain in the room. But, the sensible capacity for the unit is far less.
1) In this case, what will happen to the room? (ie. is the unit undersized?)
2.) If the unit is undersized, that means i have find a unit that can handle my sensible gain in the room? wont this in turn cause my humidity to go crazy?
thanks in advance
Sensible gain: 20662
latent gain: 1662
total: 22324
I am looking at a mr slim type rated at 24,000 btu unit with the following:
sensible capacity: 14800 btuh
latent capacity: 9400 btuh
I see the total for the unit is greater than the total heat gain in the room. But, the sensible capacity for the unit is far less.
1) In this case, what will happen to the room? (ie. is the unit undersized?)
2.) If the unit is undersized, that means i have find a unit that can handle my sensible gain in the room? wont this in turn cause my humidity to go crazy?
thanks in advance





RE: Question about Sizing HVAC split
If the dry bulb, pick a unit that can handle the sensible load.
If the RH, pick a unit that handles the latent heat.
If close (within 10%) to latent heat, pick that one. The unit will run longer, drying out the air.
Double check your calculations too. Where is the latent heat coming from - people, cooking, ?
RE: Question about Sizing HVAC split
What matters is the latent load and the total load. Your selection meets the total load requirement. Though instead of being capable of removing more sensible, your unit is capable of removing more of the latent. But, if you dont have that much latent inside the room, offcourse it can handle that much (or more ) of sensible. Coil never knows which load it is going to take care. It is the enthalpy that matters.
RE: Question about Sizing HVAC split
Does that mean that the unit will actually be 'transfering' the rest of its capacity to sensible cooling? (ie. the unit could actually do 22000 btu sensible and 2000 btu latent?) even though it says the sensible for the unit is 14,800?
Thanks again!
RE: Question about Sizing HVAC split
- if air is humid, more vapor will condense on the cooling col, hence more latent removal but less sensible removal (i.e. air is not cooled enough)
- if air is dry, more sensible removal (because the less condensation process " steals less cool" )
The numbers they give you is for standard test conditions (likely ARI). All standard AC units for normal spaces will have about 25% latent, 75% sensible and should work for normal spaces. Since humidity varies, the system is very forgiving. In daily life you never will have the exact sensible/latent heat load that you calculate anyway.
If you have a specific application for dehumidification there are specific units, (they probably sub-cool the evaporator coil, and have lower air flow, or something like that). they might have a 50%/50% ratio under standard test conditions (but will vary as well). If you have sensible heat only (IT server room), there are units that have higher sensible ratio. I assume they cool the evaporator less and increase air flow, or something like that. As long as you pick the right type of unit, I wouldn't worry about more than the total load and capacity.
Even in non-canned systems (i.e. air handler where you regulate the flow) this works out pretty well.
RE: Question about Sizing HVAC split
RE: Question about Sizing HVAC split
The latent is more likely to get "transferred" to sensible. But it could not be the other way around. I mean sensible is not likely to get "transferred" to the latent.
the coil properties mentioned by willard (no. of rows and FPI (fins per inch)) are varied by the manufacturer to handle different sensible/latent ratios.
but latent load is the one which is more stringer and cost effective. and that determines no. of rows. Condensation (latent removal) needs more no. of rows. But if the latent is less in amount these no. of rows help to handle sensible load more effectively.
on the other hand if there is a coil which is designed to handle more of the sensible and less latent. (Lesser rows and more FPI), it cannot take more of latent, cuz latent requires more no. of rows.
RE: Question about Sizing HVAC split
but the start chilled water temperature (or refrigerant temperature/pressure) matter. The colder, the less humid it can get with the very same coil.
I know, the thread starter asked about split systems which are DX.
RE: Question about Sizing HVAC split
Coils are manufactured for differentr design concepts.
If a coil is mainly for sensible load, it will have high S/T ratio, less rows, higher air flow, high EER, higher approach (12 ~ 15oF typical), an higher leaving air temperature limit (such as 47 oF).
A coil mainly for latent load will have a low S/T ratio, more rows, lower air flow, high MRE (moisture recovery efficiency), and lower approach, and lower leaving air temperature.
Of course you can select a coil with high latent load for a application with high sensible load/low latent load situation, but it will be less economic; On the other side, a coil of high S/T ratio will not work for a high humid environment, because the demumidification is a water vapor transport (convection and diffusion) controlled process, it needs more rows and low air flow to achieve the goal.
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RE: Question about Sizing HVAC split
You have good arguments for different design conditions. But they may be good for Chilled Water Air Handler Coils. Where Manufacturers vary these conditions to achieve desired S/T Ratio.
For DX Split Units, i havent seen any different configurations to achieve different S/T Ratios. They are just made to take care of total load. (With some capability of latent removal within).
Living in a perfect world, your argument might be right, if illuzon is able get two or more selections which fulfill the requirement of total load, and latent is no more than required, and the no. of rows are lesser than the curramt selection and he saves the money out of this.
RE: Question about Sizing HVAC split
RE: Question about Sizing HVAC split
RE: Question about Sizing HVAC split
I assume that the Split Unit would be a DX Coil, and, that the Coil depth will not probably be more than 3 row deep. In your case since he total Enthalpy of the Room is lower than the Unit Capacity, it should do the job, albeit at a slightly elevated DB with lowered RH. You will still be within the comfort zone.
RE: Question about Sizing HVAC split
RE: Question about Sizing HVAC split
ok but you did not mention that airflow rate....
whats the airflow rate you used to arrive at 27C-35%
RE: Question about Sizing HVAC split