Magical Reheat Dehumidification: How It Works

Nerd Level 5/10 - I'll go slow but this is a bit technical. It's one of the most important concepts I teach though, it's often the key to a healthy home. Part 2 is about what it costs, read that once you've read this post. Link at the bottom.

A Dry Home Is A Healthy and Comfortable Home 

Reheat dehumidification is a fairly obscure topic, it's common in commercial HVAC but not residential. In all my years of house whispering, humidity control has been by far the most challenging part, especially in older homes that don't have a plastic vapor barrier under the basement floor, or homes with vented crawlspaces. 

Keeping a house dry during humid seasons is critical to indoor comfort, reducing thermostat wars between spouses,  reducing the growth of nasty stuff like mold, mildew, and rot, and keeping the house and indoor air quality healthy. I target 45-55% relative humidity (RH) which is 45-55F dew point in cooling season. 

In humid climates where the grass stays green without having to water it* there are typically several months per year where it's humid outside, but not warm enough to turn on the air conditioner or have it run very long. Sadly, the days you'd like to open the windows when it's 70-75F/20-24C out are often these times. I don't make the rules, physics does, I don't like this either!

If you have noticed your house feeling sticky in spring or fall, or noticing where the dog peed last year, humidity is probably getting too high in your home. If you break 60% RH in your home, the odds are high something bad is growing somewhere in your home, often inside walls, crawlspaces, or attics where you can't see it (but you still breathe it.)

As air conditioners have gotten more and more efficient, they are actually getting significantly worse at dehumidification. 

Myself and fellow building science nerds have noticed a significant bump in mold and humidity complaints in the last five years, in fact I did a presentation in 2018 called The Coming Mold Explosion. 

So what's the solution? HVAC systems that have excellent dehumidification capabilities, in particular those with "reheat dehumidification". I view it as real world magic like air conditioning and flight: even though I understand how it works, it still seems like magic!

Let's take a look at what reheat dehumidification looks like. First, we need to look inside your air conditioner. 

The Air Handler Unit (AHU) from a heat pump, the indoor coil that heats or cools the air is on the top shaped like a letter A. Below that is the fan or air handler. We set this unit up for downflow, the air is blowing down into the ducts in the crawlspace.

​The First Nerdy Part: The Indoor Coil

This is a picture of the air handler unit (AHU) now installed in my house, AHU is the name for the indoor part of a heat pump (a heat pump is an AC that can heat and cool where an AC can only cool.) If you have a furnace and AC, the coil sits on top of your furnace and the furnace unit has the fan in it. 

On the bottom is the fan, the air handler itself. Above that is the indoor coil. This unit is set up for downflow operation, where the air blows from the top to the bottom and into duct work in the crawlspace. If you have a basement your system is almost certainly setup as an upflow unit, and in an attic or crawlspace as a horizontal flow unit. 

​In this case it's an "A coil" because it's shaped like a letter A, some are flat (aka a slab coil), some are shaped like a letter N (you guessed it, an N coil.) 

When the indoor coil gets cold, it removes heat from the air inside your home so it can pump it outside. 

​When the coil gets cold and it's humid inside, the coil will get wet with condensation. I don't have a good picture of that, but here's condensation on the bottom of another air handler that is in a very humid crawlspace (since fixed.)

Hitting Set Point Too Fast to Dehumidify

When your air conditioner is running, it's removing both heat and humidity from the air inside your house. 

An AC needs to run for a while, 5-15 minutes, before the coil gets cold enough to dehumidify. Until then it is just removing heat from the house, which you think of as cooling.

This is problematic on mild days because the air conditioner will remove heat too quickly so the thermostat is "satisfied" and the AC shuts off before the coil gets cold enough to start dehumidifying. This leaves your house cold and wet which is a recipe for high humidity, mold, and discomfort. 

A Partial Solution: Variable Speed AC

If an air conditioner is too big to run long enough to dehumidify, one option is to make it smaller. You can choose a smaller size in a typical single stage on/off model, but the real key is to buy a variable speed AC or heat pump that can turn down to a very low output.

The best variable speed ACs or heat pumps can turn down to about 25% of their full capacity. A 2 ton/24,000 btu AC or heat pump can turn down to 6,000 btus. For reference the smallest window air conditioners are 5,000 btus and this is for an entire house. As small as this is, it still typically leaves several months where it still shuts off because it's not warm enough to run the AC for long before the thermostat satisfies.

The other key is that variable speed units need to turn down low while also running a cold coil so that condensation forms and they dehumidify. Only a few systems do this well. That's for another article. 

The problem is that on mild days in the 70-80F range, even very low outputs will often cool the house before dehumidifying it enough. 

The Actual Solution: Cool and Dry the Air, Then Reheat It

Reheat dehumidification is where you put humid room temperature air through an air conditioner coil which removes the heat and humidity, then you heat the air back up so instead of cold dry air coming out you get room temperature dry air.

Here's why I had to tell you about the coil inside your air conditioner. It's the first step in reheat dehumidification. It removes heat and humidity from air going through it, making the air cold and dry. 

But we've already established that this is bad, it overcools a house without dehumidifying it, and bad stuff can start happening. 

To fix this, we need to "reheat" that cold dry air so that it is room temperature dry air. 

We can do this two ways, by running it through a second coil with the now warm refrigerant in it often called "hot gas reheat", or by using electric resistance that looks a lot like the coils inside a toaster but larger that's called "electric reheat". 

In commercial systems, the second coil/hot gas is quite common, but in residential systems this adds a lot of cost, complexity, and risk of early equipment failure. Lennox has a product called Humiditrol that does this, but the two times I mentioned it to contractors they practically ran away screaming afraid of the potential failure risk. 

​By the way, traditional dehumidifiers work with "hot gas" reheat. The first coil runs cold and cools and dehumidifies the air, then the warm refrigerant goes through a second coil which adds the heat back. The air comes out above room temperature because of the heat the compressor and fan create. 

​Back to home HVAC systems, that leaves us the second option, "electric reheat", which technically most heat pumps are capable of. 

Electric reheat dehumidification is when you run the air conditioner (which takes out heat and humidity) at the same time as the resistance backup heat strips (which adds the heat back in.) To repeat, you put in room temperature humid air and you get room temperature dry air out the other side of the system so that you dry the house without cooling it.

I know it sounds crazy to run the air conditioner and the backup heat strips at the same time, but it uses less energy than you might think when done correctly.

In the next article I'll show data from multiple client homes and one of our AirBnbs, as well as comparing that use to three other homes we own that do not have reheat where we can look at the usage of the dehumidifiers.

Most Heat Pump Only Systems Have the Capacity to Do Reheat Dehumidification

Most heat pump air handlers have not only the air handler fan and the coil like we discussed earlier, but also electric resistance heat that is used for backup on very cold days or for when the compressor eventually fails.

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This is what backup electric resistance heat strips look like when they are on. This is a large setup, 15,000 watts aka 15 kw. It's better to run far smaller backup strips, preferably with 2 or 3 stages, not just on/off.

You can set up a standard heat pump system for reheat by running a Honeywell Prestige thermostat in commercial mode, but note that it will likely use a painfully high amount of electricity because the heat strips will be running for 15 minutes or so before the coil gets cold enough to dehumidify. And you'll want to be sure you have the airflow settings right so the coil can get cold. It takes a good technician with holistic understanding of how homes work and how to set up the equipment to get this right. I'd love to say that's easy to find, but it seems to be under 1% of HVAC techs in my experience. 

My equipment preference, shockingly, is variable speed equipment with 2 or 3 stages of backup heat so that they can run low and slow while dehumidifying which is both more efficient and more effective. More in a minute.

Let's look at a few other factors about reheat and alternative methods to see why I've settled on using it as the first line of defense in my projects. ​

Superior Longevity

I've been called crazy for my near obsession with reheat dehumidification, but there's a really important reason why: it basically never breaks. 

I've done energy audits on homes with ancient 40-50 year old heat pumps and while the heat pump may not work, the resistance backup strips worked every time. As long as the fan turns on to flow air over them and prevent them from melting themselves, they almost never break. 

Another Solution: Whole Home Ventilating Dehumidifiers

A Santa Fe Ultra 155 whole home ventilating dehumidifier. I had one of these in our last house, it's one of the nicest and most efficient models on the market.

I skipped one potential solution to high humidity on purpose because I wanted to teach you what reheat dehumidification was first. 

There's a lovely product called a whole home ventilating dehumidifier. They pretty much self define, it's a large dehumidifier that can serve the whole home and also brings in outdoor air aka mechanical ventilation. I've used them on a number of my House Whispering projects including one on our last personal home which was right on the Cuyahoga River outside Cleveland Ohio.

My friend Ken Gehring aka Teddy Bear invented these and also coined the phrase "green grass climate" I used earlier. 

While I quite like ventilating dehumidifiers and have used them in a number of client homes, they have two main drawbacks: they are expensive (typically $4-10K installed) and they have shorter lives than a well installed home HVAC system which should last 15-20 years. 

Santa Fe has the best built units and a generous 6 year warranty, the longest in the business. From my own experience and from talking to fellow HVAC pros who install them, they typically have 5-10 year lives. 

This means that if you install one with a new HVAC system, you will almost certainly have to buy two of them in the life of your new HVAC system, that's $8,000-20,000 total. 

Instead, I'd rather you spend that money upgrading from a basic single stage HVAC system to a higher end fully communicating variable speed heat pump with reheat dehumidification. (Say that 10 times fast...) Then one product can do all the work for you, and like I said earlier the resistance heat strips tend to outlast the units. 

My goal is to make it so the HVAC system with reheat dehumidification handles the majority of dehumidification work and all you need to pick up the slack is a basic $250 portable dehumidifier from a big box store. 

Where I use ventilating dehumidifiers is in client homes that have a newer AC that the client does not want to replace again, or occasionally when a house needs an unusually large amount of dehumidification. They are a useful tool in those situations. 

You Can't Do Reheat Dehumidification with a Furnace

I've skipped one other important piece about reheat dehumidification: you can't do it with a typical home furnace. The reason is that you have to add the heat (reheat) to the cold dry air after the air conditioner coil cools and dehumidifies it.  

In the typical home furnace and air conditioner system, the furnace comes before the AC coil, so you can't reheat the air**, you can only feed very hot air into the AC coil which is very likely to break the system.

Therefore home HVAC systems with a furnace can't do reheat dehumidification whether it's paired with a one way AC or a two way AC heat pump (aka hybrid or dual fuel system). 

What's the Best Way to Do Reheat Dehumidification?

Remember how I talked about the best partial solution to good dehumidification is a variable speed AC or heat pump that can run a cold coil at low 25% capacity to provide the best comfort without reheat?

The best reheat dehumidification is done with that nice system running a cold coil at low capacity matched with as small a resistance backup heat strip as possible. 

This means the system will run for a while at low capacity, sucking as much humidity out of the air as possible. 

What's the Worst Way to Do Reheat Dehumidification?

Basic single stage ACs or heat pumps are bad for reheat dehumidification - they use a lot of energy to do the job and usually have poor dehumidification performance.

While you can do reheat with a basic single stage AC or heat pump with electric resistance backup heat strips, it tends to burn far more energy. 

Remember how I said that an AC needs to run for 5-15 minutes to get the coil cold enough to dehumidify well? If you don't want to overcool the home, you need to run the resistance backup the whole time the AC is on. 

The resistance backup is running while the system is not dehumidifying, so you are wasting energy. Plus the AC is very oversized when it's 70-80F outdoors, so it will cool the house faster than you want, using more energy still. 

Ideally you also want to get the AC coil cold fast and run as little resistance backup as possible. More on that in a second. 

What's the Worst System for Reheat Dehumidification?

Many (frankly almost all) AC and heat pump systems are really bad at dehumidification. They focus on cooling over dehumidification. I'll write about this in depth another article, but it's called the Sensible Heat Ratio. An AC can do 65% dehumidification and 35% cooling which is a 0.35 Sensible Heat Ratio (SHR), or it can do 1% dehumidification and 99% cooling, a 0.99 SHR. 

The 0.99 SHR system does almost no dehumidification, even when it's running correctly. 

Most ACs used to be in the 0.7-0.8 SHR range, meaning they did 20-30% dehumidification which is about right to keep a house dry if the AC is sized as small as possible to the house. 

The trouble is that efficiency standards are driving basic single stage minimum efficiency systems into the 0.85-0.95 SHR range. Even if they are running perfectly these systems don't dehumidify well. 

Sadly, efficiency standards have made it where if you want good dehumidification you either have to buy a separate dehumidifier or a "communicating" HVAC system. 

Communicating Systems or Bust

In a communicating system the air handler (indoor unit), the outdoor unit, and the thermostat are constantly talking to each other and making small adjustments in how the system is running to deliver efficiency, comfort, and dehumidification. 

A "communicating" HVAC system is fully variable speed. It can vary the speed of the air handler fan and the speed of the compressor that actually cools (or heats) the house. 

Most importantly, communicating systems can run the indoor coil cold for the best dehumidification and at 25% capacity so it runs a long time and also maximizes dehumidification.  

The challenge is that only two systems are both communicating and do reheat dehumidification: Carrier Infinity and Trane XV heat pump only systems. Each one has multiple brands that are the same products with different badges. 

Carrier Infinity Heat Pumps

While I have two close HVAC contractor friends that swear by Trane XV heat pump systems with reheat, Tim Portman and Stephen Rardon, I have only used Carrier Infinity VNA8 and Carrier Infinity GreenSpeed VNA4 heat pumps. I absolutely love them and finally bought one personally last year. 

Not only do the regular dehumidification and reheat dehumidification functions work very well, but Carrier has the only 3 stage resistance backup heat strips. There is a 3/6/9 kw model (3,000, 6,000, or 9,000 watts) and a 5/10/15 kw model (the glowing photo earlier is the 5/10/15 model running at 15 kw.)

The 3/6/9 kw model of heat strips is an amazing match with a two ton heat pump because it will run at 3 kw and the system at 25% capacity. 25% capacity is 6,000 btus, remember that a small window AC is 5,000 btus. That makes it low and slow, exactly what we want for maximum dehumidification and minimum energy use, something I'll show data on in the next article. 

I should say that Carrier is not a sponsor in any way, in fact I've had zero technical support from them in 10 years of using their products, I've either figured things out myself or learned from other HVAC pros. I can consult on setting your Carrier Infinity system up properly if desired. 

Some Real World Results

This is the relative humidity data from the Haven Indoor Air Quality Monitor in Mothmanor, our AirBnb with a Carrier Infinity VNA8 heat pump system with reheat. It also has a dehumidifier in the basement, I'll show energy use next time. I've noticed that the relative humidity runs a bit lower than what the Haven says, 5-10%. 

It's been a dry year here in the New River Gorge National Park, but the temperatures have stayed higher along with humidity through October. 
​
Here's a chart from Game House AirBnb that has a Daikin Fit heat pump system without reheat, it has a small dehumidifier as well. 

Note how the humidity levels are much less steady. The energy use is comparable to Mothmanor which I'll show next time. 

These homes are quite comparable in size, Mothmanor is 620 square feet with a full basement, the Game House is 800 square feet with a full basement. 
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Wrapping Up and What's Coming Up

I warned you this post was kinda nerdy, but hopefully it makes sense now how your AC coil needs to be cold to dehumidify well, run as long as possible at as low a capacity as possible, and for the best dehumidification your system needs to be able to reheat the cold dry air that comes off the AC coil. 

To do this, it's best to run as small an AC as possible that is also communicating, which means it can run a cold coil at very low capacity. Only the Carrier Infinity and Trane XV heat pump only systems are capable of this, as well as their other brands like Bryant and American Standard respectively. 

​Done right, reheat dehumidification doesn't use a ton of energy, in fact I'll show how it's the same or less than running a dehumidifier. Like I said at the beginning, I view it like real world magic. 

Here's a comment on this article from cleantech entrepreneur KC Boyce in Atlanta about his Carrier Infinity system and my help setting up/commissioning it:

​Next Up: How Much Does It Cost?

​In the next article we'll talk about operating costs.  I'll show you actual energy use from multiple client systems with reheat dehumidification, friends' houses in warmer climates, and 4 of our own houses in West Virginia - 1 with reheat and 3 without. 

See you next time, and good luck choosing the best HVAC system for your home!


Footnotes
* Green grass climate definition borrowed from Ken "Teddy Bear" Gehring, inventor of the ventilating dehumidifier and a particularly good guy. He's semi retired from Thermastor/Santa Fe, but still active at hvactalk.com
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** Someone is going to say that you could install a furnace after an AC coil and use it for reheat dehumidification. While technically correct and actually how many commercial rooftop HVAC systems work, it tends to rot out furnace heat exchangers in just a few years because you are running very cold air conditioned air over a heat exchanger that has room temperature humid air inside of it, creating condensation inside the heat exchanger and rotting it out. Those commercial rooftop units are famous for rotting out within 5 years. It's just not a good idea. ​