Do You Need a Bigger Air Conditioner If You Install a Dehumidifier?
Those of us in humid climates know the importance of having good dehumidification along with good cooling for comfort. Dehumidifiers, however, blow hot air into the living space. So what happens when you add a dehumidifier? A lot of people think you also need to add more air conditioning capacity. Is it true? Let’s look at the critical factors that will reveal the answer for us.
Dehumidifier number 1: Your air conditioner
First, the air conditioner should be doing the bulk of your dehumidification. But that depends on runtime, and runtime is determined by the capacity of the AC. And where does the capacity (size) come from? Well, it should be based on the 1 percent design conditions for your location.
If you actually have a system that’s sized according to the HVAC design protocols from the Air Conditioning Contractors of America (ACCA), you shouldn’t have much need for a dehumidifier…when you’re at the design conditions for your location. That’s even true for inverter-driven equipment (i.e., mini-splits). As I wrote recently, my ducted mini-split heat pump keeps my house in the 45 to 55 percent range of relative humidity when we’re having design condition weather.
To get that kind of performance from your air conditioner, though, it really needs to be sized as close to the actual load as possible. Mine is sized at about 90 percent of the total load, which meets the Manual S requirement. That helps a lot with dehumidification. But again, we’re talking about design load conditions here, which by definition we’re at only about 1 percent of the time. The remainder of the time, we’re mostly at part-load conditions.
Two kinds of heat
And speaking of cooling loads, remember (or know for the first time) that we calculate two components of the total load: the sensible load (think temperature reduction) and the latent load (think dehumidification).
When the dehumidifier runs, yes, it blows hot air into the living space. But is that an additional load? No, not much anyway. It’s mostly cooling load that has already been accounted for in the load calculation.
Another way of thinking about a dehumidifier is that it’s a device that converts latent load into sensible load. It’s the same heat, just transformed from being hidden (latent) in humidity to in-your-face higher temperature air. But yeah, there’s a little extra heat thrown in because the dehumidifier isn’t 100 percent efficient.
So, no, you don’t need to increase the size of the air conditioner just because you’ve added a dehumidifier. Now, let’s look at the issue of timing.
When does the dehumidifier run?
When your house is at design conditions, the AC should be running pretty much continuously at or close to maximum capacity if it’s sized properly. That should keep the indoor relative humidity where you want it without the dehumidifier coming on.
But design conditions don’t last 24/7. It’s there in the afternoon and maybe into the evening. But at night, the outdoor air cools down. And that’s when you might need that dehumidifier.
In technical terms, you have a lower sensible load at night. As a result, the air conditioner runs less. That may cause the indoor humidity to rise. And that’s when the dehumidifier swings into action.
So, your dehumidifier is mostly blowing hot air into the house at a time when the AC isn’t operating at full capacity. If the dehumidified air starts making the house too warm, the AC can run more.
Dehumidifier takeaways
Here’s the skinny:
- Dehumidifiers convert latent cooling load to sensible cooling load.
- Dehumidifiers add only a little extra cooling load.
- Dehumidifiers mostly run at night or on cooler days, when the air conditioner has extra cooling capacity.
- Air conditioners should be able to handle all or most of the dehumidification on hot days.
None of this means you won’t have comfort problems with a dehumidifier in the house. Everything I’ve said above depends on proper HVAC design. If your AC is twice the size it should be, you may well have some comfort problems in your house. And if you do increase the size of the AC because of the dehumidifier, you may well end up there.
Also, how you deal with the output of the dehumidifier—the hot dry air—can affect your comfort. If it blows directly into the living space, it needs to be well-mixed with room air before hitting the occupied zone. And if the dehumidifier output goes into the air conditioner ducts, you have to understand the intricacies of that process.
So, no, you do NOT need a bigger air conditioner just because you’ve also installed a dehumidifier. If this is something you need help with, contact us through our HVAC design pages.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
Related Articles
Do Mini-Splits Really Have a Dehumidification Problem?
The 3 Types of Heating and Cooling Loads
Comments are welcome and moderated. Your comment will appear below after approval. To control spam, we close comments after one year.
I have a whole home dehumidifier that we installed a few years ago after reading one of your articles. We ran into one of the con’s that turned into a problem. Where to put the warm outlet air. The output was always close to 90 degrees, we tried different locations but were never able to find a suitable location to discharge the air.
We recently installed a variable speed heat pump that has both a variable speed compressor and blower.
It keeps the humidity well under control except under certain conditions when is relatively cool outside but with 90% humidity. Running in the dehumidification mode it still can make the house slightly cooler than desired.
The dehumidifier is just sitting in the attic unused. Is there any suitable way to set this up or should we just let the heat pump do its job and forget about it and write it off as a lesson learned?
David: Did you try sending the dehu air into the supply side of the heat pump ducts? That should mix the two air streams well enough to prevent the problems you’ve had. See the last of my related articles above, 4 Ways to Duct a Dehumidifier.
Nice! One of the conditions that system design fails is air movement – aka ventilation. You touch on two things that impact comfort (aka heat and mugginess!): ” If your AC is twice the size it should be, you may well have some comfort problems in your house”, and “If the dehumidifier blows directly into the living space, it needs to be well-mixed with room air before hitting the occupied zone.”
I would suggest that many if not most complaints of “comfort” are due to poor layout and balance of the supply and returns, the pressures between spaces and rooms, and obstructions (aka contents).
I just left a project where 12 of 14 supplies were covered by large furniture. I have no idea what it took to cool down this house to the level of comfort with so little treated air mixing quickly and completely into occupied spaces.
I would rate this the #1 problem above sizing. Building tightness and insulation are not included in this assessment – they are what they are and difficult for most owners to correct.
Regards, S.
scott: I’ve seen a lot of blocked vents, too, but never close to 86% of them as you did in that house. Wow!
Using a dehumidistat output from the thermostat on higher end forced air systems can be super beneficial. Allowing the variable speed blower motor to slow down will allow more latent heat removal with the tradeoff of less sensible cooling. In non design conditions this is a win win.
Scott: You’ve just hit on one of the most neglected issues in HVAC installations. Even when they have a good design and do a good job installing the ducts, many people forget to go into the settings and adjust the air flow.
FWIW, we have a central ducted dehumidifier that we use pretty much only during the non-cooling season (In North Florida I hesitate to claim that we have a heating season) I agree that the warm air discharged by a dehu can be an unwelcome nuisance.
Similarly, our HVAC is a 4 zone variable capacity system that controls humidity reasonably well whenever there is sufficient sensible load for it to run at least 6 hours per day at or above its minimum capacity
The scheme I came up with for the dehu’s warm air is to direct it to full bathrooms – The one place and time folks like a bit more warmth is during and right after showering, and the warm dry air helps control bathroom moisture.
Curt: That’s a clever idea. I like it. But what about when you go to the bathroom for reasons other than showering? It seems like it could be uncomfortable.
Actually Manual S Section 4 Air-source Heat Pump, page 4-1, 4-4 Air-source Heat Pump Sizing Limits states: “When heating and cooling is required, the heat pump equipment should be sized so that the sensible cooling capacity is greater than the calculated sensible load and the latent cooling capacity is greater than the latent load. Ideally, the total cooling capacity should not exceed the total cooling load by more than 15 percent.” So the sizing of Allison’s heat pump does not meet Manual S requirements. It may be working for his home but technically it does not meet Manual S requirements.
Judy: The part you’re quoting there just references the sensible and latent loads individually. Yes, Manual S requires you to have at least enough capacity to cover them. But the total cooling load can be as low as 90% and still meet Manual S. How might you come in at 90% of total capacity relative to the total load and still meet both sensible and latent? When you have excess latent capacity, half of which can be counted toward the meeting the sensible load.
But you’re right, though. My house does not meet Manual S because that heat pump doesn’t meet the sensible and latent loads. But it does (almost) meet the total cooling requirement of 90% minimum. I say almost because the ratio is really at 88%.
I think something about the way you are stating that your equipment is sized at 90% is confusing. If you are stating that the equipment you installed has an additional 10% of capacity over your Manual J cooling load than you are in compliance of Manual S. Even if there is actually 12% additional capacity you are in compliance with Manual S, but if your equipment’s total cooling capacity at your design conditions is only 90% of your Manual J cooling load then you are not in compliance with Manual S. It does not matter that you can add half of the equipment’s latent capacity to the sensible load. The equipment is not gaining total capacity at your design conditions it is just reapportioning the latent and sensible capacities.
What you are referring to is in Section 3, 3-6 Excess Latent Capacity and is explaining how excess latent capacity will convert to sensible capacity when the wet-bulb temperature of the indoor air is a bit lower than the relative humidity value used in the Manual J calculations. “Since the entering wet-bulb temperature is lowered, about half of the excess latent capacity will be converted to sensible capacity. This converted capacity can be added to the sensible capacity value that was extracted from the manufacturer’s performance data.”
In Section 3, Cooling Equipment Selection Procedures, 3-4 Sizing Limitations the 2nd bullet point states: “If heat pump equipment (air-source or water-source) is installed in a warm or moderate climate, the total cooling capacity should not exceed the total cooling load by more than 15 percent.”
There is nothing about it being okay to size below the Manual J total cooling load. So, if you are stating that you can size equipment capacity lower than the Manual J calculated load I would love to know where you are reading that.
Judy: Well, you’ve had me doubting myself since you wrote that so when I got back to the office today, I looked it up. I can’t easily post an image here, but you have Manual S so you can find it. I installed the heat pump I was discussing back in 2019 so the 2014 version of Manual S is the relevant standard. Turns out it doesn’t matter because the new Manual S has the same lower limit.
Now, you have to look in the normative section of the standard at the beginning of the book. Table N2-2 on page N13 shows the size limits for heat pumps. They divide it into Condition A (warm climate) and Condition B (cold climate) homes and then further into Single Speed, Two Speed, and Variable Speed systems. For Condition A air-to-air heat pumps, the maximum total cooling capacity varies from 1.15 to 1.20 to 1.30 for single, double, and variable capacity.
Those numbers are the ratio of the total cooling capacity to the total cooling load. And the minimum ratio for all three equipment types is 0.90. So yes, Manual S does allow you to size the total cooling capacity at 90% of the total cooling load.
My 1960s ranch home has an encapsulated attic. I tracked the humidity in the attic and discovered it was spiking to about 60%- 80% during the afternoon (ping-pong water). I decided to install a dehumidifier. I read a lot of articles about how to properly duct the dehu but the idiosyncrasies of my home and HVAC made all ducted strategies difficult. I decided to simply install the dehu in the attic with no ducting while I figured out which ducting strategy I would use. The dehu dramatically dropped the humidity level in the attic as anticipated (45% – 55%), but I was surprised to find that the humidity in the rest of the home also dropped substantially. Previously, with only my properly sized heat pump running, I measured a consistent 50% – 60% humidity in the summer. With the unducted dehu in the attic, the humidity in the living space has dropped to a consistent 35% – 45%. That’s actually a little lower than optimal, but it makes the home extremely comfortable in CZ2. My point is that depending on your specific home and installation location, it may not be necessary to duct a dehu.
Ken, “encapsulated” attic? Can you add some info on what you mean – is it treated/conditioned? or just “sealed” to eliminate air penetrations from one space to another? (e.g., in to out; attic to living spaces below; attic to basement via chimney chase; etc)>
I’m wondering also the impact of solar gain, pushing vapor from the hot attic downward thru materials into the cooler living spaces (just like damp masonry exterior sheeting will do from hot sun).
Ken – that’s an interesting and happy outcome. In theory, water vapor rises since water molecules are lighter (18 vs 29-ish) than air. What little I’ve read on that is contradictory as to the intensity of that buoyancy.
Allison – comfort during non-bathing bathroom visits hasn’t been a significant issue for several reasons – we don’t need to run the dehu during warm weather, the air is distributed to three bathrooms, and I aimed it at the bath tub / shower in each as best I could.
I might try to replicate Ken’s situation by temporarily ‘unducting’ (or would it be ‘deducting’?) the central dehu and let it work just the attic.
I’m not sure if there are downsides to the strategy of a ductless dehu in the attic. Maybe Allison can impart some wisdom. But another upside of this strategy is the warm air in the attic raises the temperature of the attic rather than the living space. I don’t mind if my attic is warm because I’m not living up there (but note that your ducts do). Also, my understanding is that dehus run more efficiently in a warmer environment.
Curt, I disagree with your “physics” on air vs. water vapor. Both are gases under these conditions and both fill the space and mix uniformly. They do not separate due to differences in molecular mass. I suspect that the attic does not have a vapor barrier and that outdoor water vapor is seeping into the attic.
I am not a physicist, but Allison is, so he can correct me if I am wrong.
I would like Allison/physicist input also – my understanding is that like clouds, vapor is relatively lighter than ground level air – hence while it is molecular, i.e., should behave like gas, it has other properties that drive it’s movement differently. Hence, vapor not gas.
As an aside – heavier solvent vapors (e.g., from gasoline or diesel fuel) will fill lower air spaces; and like CO will neither rise nor fall and is essentially evenly distributed throughout a space – hence the recommendations to put detectors low or high are incorrect. Either put them in neutral position or better, they should be in the location where the air mass flows; since CO generated from fire will be in the hot air, it is probably wise to put the detector where that hot(er) air will be moving past as it goes from the fire to the non-fire zones.
A vapor is a gas that is at saturation (in thermodynamic equilibrium with a liquid or solid), but it still behaves like a gas. If you release a gas that is denser than air (e.g., propane), it will initially sink to to the floor. If you release a gas that is less dense than air (e.g., methane or natural gas), it will initially rise to the ceiling. If the air is still, either type of gas will eventually diffuse throughout the room until the concentration is uniform. If there is air movement due to a fan or other mechanism, it will mix faster but still end up with a uniform mixture.
Clouds are not water vapor. They are composed of tiny liquid or ice droplets that are attached to tiny particles (dust). They float due to rising air currents. When enough of these droplets coalesce, it rains or snows.