A common misconception about the energy recovery ventilator (ERV) is that it's good for humid climates because it helps to dehumidify a home. It's usually the better choice for a humid climate when you're trying to decide between an ERV or an HRV (heat recovery ventilator) but not because it's a dehumidifier. It is not a dehumidifier. The photo above shows the inside of what could be either an ERV or an HRV. They both work on the same principle. The difference is what happens in the recovery core. Basically, this mechanical ventilation device is just an insulated box with two fans, four ducts, and a recovery core. It pulls in outdoor air (red arrows) and sends it into the house. It also exhausts indoor air to the outdoors (blue arrows). The two air streams both pass through the recovery core, shown above and below. Ideally, the two air streams do not mix at all but do exchange heat (both ERV and HRV) and moisture (ERV only). Exchanging heat is definitely a good thing.

It means that when you need to bring in outdoor air to ventilate a home in the winter, you can both preheat the incoming air and rescue some of the heat in the outgoing air. In the summer, you precool the incoming air. It's easy to get confused over humidity, of course, because the mixture of dry air and water vapor exhibits such interesting behavior. (See two of my favorite books, Understanding Psychrometrics by Don Gatley and Water in Buildings by Bill Rose, if you'd like to crack that nut yourself.) One point of confusion is the difference between relative humidity (RH) and absolute humidity. You might think that 100% RH is humid, but if the air is cold, there's really not much water vapor in it because cold air is dry air. There are different ways to talk about absolute humidity, but I like dew point. You can't tell which is the more humid air by their relative humidities without also knowing the temperatures. Dew point, however, is a single number describing a volume of air that does tell you what might happen when you exchange indoor air and outdoor air.

The confusion over ERVs and dehumidification stems from their ability to reduce the amount of water vapor introduced into the house when the outdoor air has more moisture than the indoor air. air handling unit space requirementsBut reducing the amount of moisture coming in is not the same at all as dehumidifying the air in the home.ac unit fan motor In fact, the ERV will usually raise the humidity in the home when the outdoor air is more humid than indoor air because it doesn't exchange water vapor between the two air streams perfectly. ac repairing guide pdfJust as only some of the heat is transferred between air streams, only some of the moisture is  exchanged. So the incoming air will be more humid than the outgoing air on a humid day.

That's not a bad thing because the ERV is still reducing the amount of humidity that would have been brought into the home. If you ventilate with an HRV, supply-only, or exhaust-only ventilation sytem, you're bringing all the humidity in. The ERV brings in less but doesn't dehumidify. If you want to ventilate and dehumidify at the same time, you need some type of supplemental dehumidification, such as a ventilating dehumidifier. That's one way to deal with the higher ventilation rates of ASHRAE 62.2-2013 in airtight, low-load homes in humid climates. 4 Ventilation Quotes That Will Rock Your IAQ World Why Is This Obsolete Supply-Only Ventilation Method Still Used? Are You Making This Mistake with Humidity?Ask This Old House TV demonstrates installation of a pool heater on a residential A/C unit. Skip to 14:15 for the good stuff! Window Trim, Pool Heater Learn MoreGoodman - A NameThe wrap around heat pipe is a heat pipe wrapped around a cooling coil.  It consists of two sections, the pre-cool section placed before the cooling coil and the reheat section placed after the reheat coil.  

The wrap around heat pipe eliminates the need for reheat and increases the dehumidifying capacity of an air conditioner by as much as 91%.  The technology uses about 50% less energy than electric reheat systems (hopefully you aren’t considering these) and about 25% less energy than other types of reheat. 5 Key Benefits of Using Wrap Around Heat Pipes Increase Moisture Removal of A/C Units.  The first section of the heat pipe module precools the entering air. This causes the approach temperature of the air to the cooling coil to be lower. The result is, that as the air leaves the cooling coil, it is colder with a lower dew point and with less moisture in it. Depending on the design of the heat pipes, the cooling coil can be made to extract over 100% more moisture than one without heat pipes.After leaving the cooling coil, the air is reheated by the second heat pipe section. This lowers the relative humidity of the supply air. In a typical system, the relative humidity is lowered from nearly 100% leaving the cooling coil to approximately 70% leaving the second heat pipe section.

This is in keeping with ASHRAE Standard 62-1989 which warns that if duct relative humidity exceeds 70%, fungal contamination can occur.Buildings frequently encounter serious humidity problems that need to be addressed. Buildings used for specific purposes like hospitals, certain food processing plants, and some manufacturing plants require humidity to be kept at a low level.   Wrap around heat pipes are usually the most efficient method of humidity control in these situations. By helping the A/C system remove more moisture from the air, the required humidity levels can be easily achieved. Energy Savings Through Passive Reheat.  Since heat pipes provide reheat by utilizing the heat from the entering air, there is no requirement for active reheat and there is no net heat added to the space. Using heat pipes to replace active reheat results in substantial savings. A payback of one year may be achieved when electric reheat is replaced with dehumidifier heat pipes. Equipment Savings Through Downsizing.