HVAC Training

Training Manual: HVAC Systems Fundamentals

Refrigeration and System Principles

The Refrigeration Process

• Refrigeration process/cycle: AC or heat pump systems operate by transferring heat energy from indoor environments to outdoor environments through a phase change chemical compound called a refrigerant, commonly referred to as “Freon”. (Note: Freon was a registered trademark of Chemours company and has since become a generic descriptor of all refrigerant products.)

• The process begins with the refrigerant as a hot, high-pressure liquid exiting the unit’s compressor.

• This hot liquid releases its heat to the outdoors via a fan-cooled heat exchanger, becoming a liquid at approximately ambient outdoor air temperature.

• The liquid then passes through a small orifice, causing it to atomize. This atomization results in rapid cooling.

• This cooling effect absorbs heat from the indoor air passing across the indoor heat exchanger, causing the refrigerant to change phase from a liquid to a gas.

• The hot gas returns to the compressor, where it is compressed back into a liquid, restarting the cycle.

Dehumidification and Cooling

• HVAC systems initially function as powerful dehumidifiers, removing more water than dedicated dehumidifiers during the first 12-24 hours of operation.

• After this initial period, the system should primarily focus on cooling, as its moisture removal capacity diminishes.

• Effective dehumidification for structural drying requires dedicated units, as HVAC systems generally cease significant water removal below 55 grains per pound (gpp), classifying them as standard (non-LGR Low Grain Refrigerant) dehumidifiers.

• Sensible cooling: This occurs when air is cooled without removing moisture. The air temperature does not reach the dew point temperature.

• Latent cooling: This occurs when cooling meets or exceeds the dew point temperature, resulting in moisture condensation and collection.

Key HVAC Concepts

• What is a BTU? – BTU stands for British Thermal Unit, a measure of the heat content of fuels or energy sources. One BTU is the quantity of heat required to raise the temperature of one pound of liquid water by 1°, & it’s generally based on a rate of BTU’s per hour.

• How does BTU apply to an HVAC system’s cooling ability? The BTU value also applies to an AC’s ability to reject heat from a structure. All refrigeration systems cool by absorbing & moving heat out of one area, and discharging it into another.

  • A refrigerator pulls heat from inside its cabinet and rejects it outside.

  • A car’s AC pulls heat from the cabin and rejects it through a radiator.

  • A dehumidifier keeps all heat inside, removing it from incoming air with an evaporator and adding it back, along with electrical load heat, via a condenser coil.

• What is tonnage, and how does it relate to cooling btu/hr? The value “tonnage” comes from the cooling performance of a 1 ton block of ice melting at 32 degrees F. 1 ton = 12,000 btu/hr of cooling.

• How much CFM, or cubic feet per minute, is associated with each ton of cooling? 1 ton of cooling, will have a system airflow of about 400 CFM. As an example, a 2.5 ton AC system will have a total system air flow of 1000 CFM through the evaporator (inside coil).

• What is SEER? Seasonal Energy Efficiency Rating - is a rating scale that measures an HVAC system’s energy efficiency. According to Energy.gov, a SEER rating is a system’s cooling output divided by the energy it uses during the warmer part of the year. A higher SEER rating equals a more energy-efficient system.

• What are the different R #’s mean with the refrigerant type? R-32, R-410A, R-22 are all different types of refrigerants, and they all have different chemical signatures. They all operate at different system pressures, they have different boiling points, and they may also have different environmental risks associated with accidental discharge. As an example you can’t mix an R-32 indoor air handler unit, with an R-410A outdoor condensing unit.

• What happens if the AC system is oversized for the home? A 1500 square foot average sized single level home will generally have a 2.5 ton AC system, or 30,000 BTU/HR AC. Would it be better to use a 36,000 BTU or 48,000 BTU/Hr AC system? The common misconception is that the house would cool down faster, and require less operating time to reach that desired temperature setting. And while that is technically accurate, the short cycle time creates a situation where the unit doesn’t have the cycle time needed to handle latent cooling, which is again where water is removed.

  • What happens is the unit runs for a short amount of time, produces incredibly cold air, and the unit’s thermostat sees the desired set point very quickly. This leads to condensation forming around the registers and on surfaces the air blows on. This moisture can create an environment conducive to microbial growth.

• How does the size of the HVAC system determine how much equipment you can place? Every piece of equipment you place on a property adds heat to the environment. Not just the dehumidifiers, the fans because of #s used produce more heat than the dehumidifiers. Wattage x 3.4 = BTU/Hr. All of this added heat must be removed, to keep the refrigerant dehumidifiers in their ideal operating range of 70-90 degrees F. If you add up all the equipment on the property’s wattage, and multiply by 3.4. You’ll have a BTU load the HVAC has to reject. Say you have 25,000 btu per hour being produced by the equipment you placed, and the HVAC is a 30,000 btu per hour unit, the temperature will likely not spiral out of control.

• How do I know what size HVAC system the house has? The unit outside will generally have a model # on it, which will tell you what the size is. The numbers will generally be a multiple of 12. ie: 24 will mean 24000 btu/hr, or 2 tons, 30 will mean 30,000 btu/hr or 2.5 tons, 36 will mean 36,000 btu/hr or 3 tons. All the way up to 60 which means 60,000 btu/hr or 5 tons. Or, more reliably, you can google search the model # on the unit.

System Types & Components

Different System Types

• Ducted systems: These systems comprise of a set of duct work that carries air into & out of these rooms for processing. Ducted systems may have a single large return (intake air) and multiple supply (exhaust) registers. Or they may have a small return for each room so that the system is more functional with the door shut. Duct systems have been shown to leak ~15% of their airflow on average. Some systems will leak more, some very little or not at all. These leaks tend to be on the return side (intake), which means they’ll draw unconditioned air into the system, potentially overworking an already overtaxed system during a dry-out.

• Package systems: unitary system incorporating the evaporator (cooling) & condenser (heat rejection) in a single outdoor system. These are sometimes mounted to roofs on commercial buildings, and usually on the back or side of a home in residential settings.

• Roof Top Units: Also known as RTUs these are commercial HVAC systems that are mounted on rooftops, on a fabricated curb to keep the unit out of standing water. The ductwork is connected vertically into the building beneath it.

• Window Unit AC's: These units are typically only found in situations where the owner doesn't have central air, or in situations where the duct system is non-functional. They are incredibly inefficient due to their design. 

• Split systems: outdoor condenser unit with a separate evaporator unit indoors. The condenser may be mounted on the roof in commercial buildings and is generally on the back or side in a residential setting. They’ll have refrigerant lines connecting the 2 units together, with separate electrical feeds.

• Ductless systems: Some HVAC systems don’t require any sort of connected ductwork.

  • Fan Coil Unit: typically found in commercial/industrial applications – Although the categorization suggests there is no ductwork associated, there is often a short length of duct attached to the return & supply side of the unit.

  • Mini-split systems: Very much like the split system, but higher efficiency relative to the normal system, and requires no ductwork. These systems are generally mounted to the wall in a high position, or at the floor level against the wall, or as a ceiling cartridge.

Terminology & Components

• Condenser: Coil & blower assembly that rejects heat to the outdoors. Another term for an outside unit, in a split system.

• Evaporator: Coil & blower assembly that absorbs heat from the indoors.

• Compressor: Electric motor driven compressor that pressurizes vapor into a liquid.

• Supply: Air & the duct coming out of the air handler with conditioned air.

• Return: Air & the duct returning to the air handler. This side of the system will have the air filter assembly in it.

• Return Air Grill: This metal louvered door often houses the filter assembly for the system. 

  • Frequently systems will only have a filter in the air handler unit which only protects the air handler.

• Branch Lines: Duct that connects the plenums to the register boots. These may be metal duct or flex duct.

• Register boots: transition from typically round duct to a rectangular opening in the floor, ceiling or walls.

• Take off: metal connection to the plenum to allow branch lines to connect. These generally require mastic to be applied to seal the connection.

• Main plenum: Can be either the main line coming off the air handler’s supply or return side.

• Riser duct/plenum: Duct that runs vertical between floors, connecting plenums.

• Curb: A roof curb is a raised frame that is attached to the roof of a building and designed to support roof-mounted equipment, such as an HVAC unit, but may also support skylights, hatches, and more.

• VAV – Variable Air Volume: Using electronically controlled valves on the supply side of the system, these systems allow active control of the airflow entering an area of a building. This HVAC system will run at a constant speed through the air handler, with excess airflow being bypassed back to the return side of the system by means of a carefully balanced valve. When the VAV’s are partially closed, the weighted valve opens, allowing air to be drawn directly into the return air plenum. These systems can be commonly found in commercial systems & may also be found in large HVAC systems in residential settings.

• Turning Vanes: Turning vanes are installed in ductwork to allow a smooth movement of air around turns in ductwork. This organizes the airflow & cuts down on turbulence.

• Plenum Air Handling Space: Many commercial spaces will use the area over the dropped ceiling to act as an HVAC return air plenum. There will be return air grills in the ceiling in various locations with a centralized location for the air feeding back into the Roof Top Unit. This is generally done as a way to save the client money during construction. However, commonly the return air boxes will either not be outfitted with a filter assembly or building maintenance will not replace them as it’s a time-consuming endeavor. This makes this area problematic relating to fire & mold losses.

• Flex duct: a flexible connection between the main plenum and the register boot. This duct is made up of an insulated lining on the outside, and a smooth wire reinforced mylar duct on the interior. They often will get water entering the duct & cause damage to the duct &/or the insulation surrounding it.

• Metal ductwork: rigid metal duct that is typically used for risers & plenums. Some older construction will use rigid metal duct between the plenums to the register boots.

  • Internally lined: metal ductwork with insulation lining the interior of the duct. This material can be dense fiberglass, or it can be as loosely filled as hair. The method of attachment may vary, it can either be adhesive bonded or use of self stick or spot welded insulation pins.

  • Externally lined: metal ductwork with insulation wrapping around the exterior of the duct.

• Fiberboard duct board: Generally used for plenums, however it can be used with register boots directly connected to the fiberboard. Fiberglass material that’s generally about 1" with a foil outer coating/lining.

• Service Ports/Doors: The options available are endless. There are options available for all types and shapes of duct. These will allow time savings for future visits for cleaning by allowing no cutting, or tools required to remove existing patches, and repatch upon completion. These run from less than $50 to well over $1,500 per patch. There are also paintable access doors designed to install an access door through drywall.

• Sheet metal patches: should be cut to allow at least 1” overhang around the opening cut. Most of the time, sheet metal will be pre-cut into 12” x 12” sheets and fastened to the metal duct with self-tapping screws, spaced 1 screw every 3” and foil tape to ensure an airtight seal. Some municipalities require mastic coating around the perimeter.

• Patching: To facilitate effective duct cleaning, multiple holes of various sizes will require cutting into the duct system. Holes as small as 1” to allow entry for small air tools, to holes as large as 12” may be cut in the metal duct. These holes must be patched in a manner that will not leak air, and do not pose a risk of falling out.

• Cutting & patching fiberboard plenums: the intake or supply side should have a square, or oval but, not a round port. Reason being, perfect circles will not allow the patch that was removed, to be reinstalled, or maybe removed without causing additional damage. Return side, due to the systems negative pressure, will require the cut in the fiberboard to be angled to the center. On the supply side, due to the pressure associated, the cut should be made from the inside out so that the pressure of the system will hold the patch in place. Both patches should have a coating of mastic on the fiberboard cut to allow them to bond together when cured. Both patches will require some creativity with tape &/or zip ties to hold the patch in place as the system cycles. Otherwise they’ll fall out during operation, prior to the mastic curing.

• Foil tape: aluminum foil tape with a solvent type of adhesive. Used as a sealer between branch runs and may also be used in many situations where a wide gap needs to be bridged prior to application of mastic.

• Duct tape: There are 101 uses for duct tape and not one of them is the use in actual HVAC applications. Unless it’s UL printed on the tape so that an inspector can immediately see the information it shouldn’t be used. If you find unprinted tape, assume that it’s not applicable to use on duct systems. The reason is that the adhesives don’t hold up to the rapid temperature changes associated with HVAC use.

• Nylon Zip Ties: these are generally used to connect the internal flex duct to take offs & register boots, as well as the outer insulated tube. There is a tool used to sinch these nylon zip ties tighter than you are by hand. This sinching can be done with a pair of pliers.

• Mastic: Fire resistant putty type paste that’s applied as a coating on the exterior of duct connections to ensure an airtight seal. May be applied with a putty knife in most situations.

• Filtration: Most HVAC systems will have filtration handled either at the air handler itself, or at the actual return air registers will have filter housings built into it. Some systems may also have a separate overlooked filter at the air handler unit. Their level of filtration is typically limited to the filter housing opening. Most ceiling and room air filters will only accept 1” thick filters, so their MERV rating is typically 8 – 13. Whereas the filter assembly at the air handler may accommodate up to a 4” filter, which could provide up to HEPA level filtration.

HVAC as it Pertains to Restoration Projects

• HVAC as related to drying with barriers: If you’re building a barrier for the purpose of isolating the wet area from the unaffected area, include the return inside the barrier. Shutting a door or isolating the return outside of the contained area will reduce airflow into that area, reducing cooling. Sometimes incorporating the return into the drying chamber isn’t plausible, financially, or operationally so just understand that the room isn’t going to have adequate cooling.

• What should we do during demo/ dust generating activities? Either turn the system off while you’re doing dust generating activities or be prepared to remove & replace that system’s filter. Clogged filters will slow down the system’s airflow, reducing performance, and potentially cause the unit to ice up. Some units will allow the addition of a spun poly filter to protect the main filter.

• Duct system cleaning:

  • Cleaning commercial HVAC systems associated with mold remediation & fire losses with a Plenum Air Handling Space – This is complex because on a fire loss it’s performed early on in the project, with the intention of getting the system back up and running. However, the HVAC will have pulled smoke into this plenum space & now everything in that space is covered in soot & will require cleaning. The plenum side of the ceiling tiles & the grid will also likely have a large amount of dust & soot contaminating it. If you clean just the return air duct assembly where it feeds into either the RTU or the riser feeding the RTU and the supply side of the ductwork, the freshly cleaned ductwork will just be inadvertently recontaminated when the ceiling assembly is cleaned by the structure cleaners.

• What if the HVAC system is undersized for the load you intend to place on it? If the project requires you to install equipment creating 32,000 BTU/hr, and the HVAC system is a 2.5 ton system, you’ll have 2,000 BTU/hr of excessive heat. You have a few choices on how to handle this:

  • You can do nothing & the project will overheat as if there was a 600 watt heater running. This excessive heating may push your dehumidifiers out of their operating range.

  • You can reduce the equipment load – which will slow down the drying progress.

  • Add supplemental cooling, which is complex & may very well be hard to collect payment from the insurer.

  • Use a thermostatically controlled blower, to exhaust heat from the structure -

  • Cheat by using routing cooled air from the registers to the intake side of a dehumidifier with lay flat ducting. You must be careful not to seal off the intake of the dehumidifier with the lay flay. This precooled, and potentially pre-dried air will push the dehumidifier back into its operating range. However, the drying chamber will continue to overheat. This also creates incredibly dry air at the outlet of the dehumidifier, as well as inside the drying chamber.

• What if the inside unit is damaged by fire or water, do we have to replace the outside unit also? That’s a complicated question because although the outside unit wasn’t physically damaged by the loss, the unit may still require replacement.

  • SEER #s must match. An outside unit that’s a 13 SEER system will not work with a 15 SEER indoor unit. The newest current SEER Rating minimum rating is 15 currently. In this situation, the outside unit would require removal & replacement to match the inside damaged unit.

  • Refrigerants must also match. You can’t use a R-32 air handler, with a R-410A outside condenser.