Dehumidification and Reheat Talk

Dehumidification and Reheat Talk

In today's live stream, Eric Mele and Bryan talk about dehumidification and reheat on the residential and commercial sides of the HVAC industry. Recently, Eric diagnosed a commercial RTU that wasn't reaching the set dew point. In the hydronic system that Eric troubleshot, the analog output was flipped; it ran the valve for reheat in reverse. These hydronic systems use waste heat from the refrigeration rack to heat water in the system; that water then circulates to the unit in lieu of refrigerant circulation. Dew point is a critical element of psychrometrics from a moisture/condensation standpoint, especially in commercial spaces.
By Bryan Orr

Air that reaches the dew point temperature will begin to condensate on a nearby surface. Higher airflow can reduce condensation, and condensation may be a slow process, but it will happen after you reach the dew point.

Reheat results in dehumidification, but it’s not because we warm the air. Instead, the A/C runs longer without overshooting the setpoint. Longer runtimes improve dehumidification. Overcooling causes condensation, so we don’t want to overshoot the setpoint. If you still need to drop the temperature of a space, then reheat probably won’t be necessary.

Some common reheat controls are of the modulating variety; these have a variable signal depending on DC voltage or milliamps. You can control fluid within the reheat coil or control the amount of air moving across the reheat coil, though these utilize different strategies; you use a damper to control the air or divert hot gas with a valve.

We have two main dehumidification strategies. We can use mechanical cooling, which is when we use our A/C units or some sort of cold coil to dehumidify the air; in this strategy, the equipment cools the air below the dew point, removes moisture from it, and reheats the air. Another strategy is desiccant dehumidification, which treats the air by removing moisture with a desiccant. A desiccant is a medium that adsorbs moisture from the air.

Your typical stand-alone dehumidifier will have an evaporator and condenser in a reheating cycle. The air leaving the dehumidifier is typically hotter than the air going in, so the dehumidifier increases the sensible heat load. That higher sensible heat load can have a negative impact on your existing HVAC equipment. However, oversized systems can actually benefit from a higher sensible load because the greater load will increase the equipment runtime and pull even more moisture out of the air. But overall, while proper equipment sizing is important for dehumidification, it can only do so much to help with longer runtimes and latent removal.

If you discharge that high sensible heat from the dehumidifier into the return, you will likely derate your equipment’s latent capacity. So, you’ll be better off discharging the high-sensible dehumidified air into the supply duct, but you have to take static pressure and backdraft possibilities into account.

The best ways to improve your air conditioner’s dehumidification efficacy are to keep moisture from entering a space in the first place, have a longer runtime, and use supplementary dehumidification.

Poor dehumidification can result in vents sweating and other moisture problems in the conditioned space. However, sweating may also occur on the ducts in the unconditioned space. If your ducts are sweating, you can either heat the ducts or decrease the dew point of the air in that unconditioned space. You can only control the air in those unconditioned spaces via encapsulation or ventilation.

Humidity in the conditioned space can come from ventilation; the home may pull humid air in from the outdoors. Pulling air into a space can worsen the humidity problem if you pull the air in from an undesirable source. Sealing your home will help, as will using a ventilating dehumidification strategy that filters the fresh air.

If you need to increase humidity, such as in a wine cellar, the best solution is to make sure you run a warmer evaporator coil. The cold coil is the key to dehumidification in compression-refrigeration systems. Evaporator pressure regulators (EPRs) and automatic expansion valves (AEVs) can help regulate the conditions of those evaporator coils.

Eric and Bryan also discuss:

Insulation/sealing
Condensation and defrost on grocery equipment
Building pressure in commercial settings
Balancing pressure in residential spaces
Reducing setpoint for over-dehumidification during favorable conditions
Energy recovery ventilators (ERVs)
Suction pressure changes, compression ratio, and system capacity
Low vs. auto fan usage and dehumidification
Duct leakage
Ductless mini-splits and poor latent load removal
Caveats of overcooling and airflow for dehumidification

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