Need clarification on technical words or phrases used on this site or elsewhere? Want to "talk tech" like a Pro? Here's a glossary of important Hydronics and HVAC terms.

Condensing Boiler

A boiler such as a Viessmann Vitodens that takes advantage of the condensing gases that accumulate in a boiler. This type of boiler makes use of the extra heat that is created when the gas turns to liquid. These are the most efficient boilers available. Under certain conditions these appliances can exceed 95% AFUE This liquid is very corrosive and therefore the boilers are created to withstand corrosion.

Indirect Water Heater Tank

An indirect water heater circulates water through a heat exchanger in the boiler. This heated water then flows to an insulated storage tank. Because the boiler does not need to operate frequently, this system is more efficient than the tankless coil. In fact, when an indirect water heater is used with a highly efficient boiler, the combination may provide one of the least expensive methods of water heating. Example of these are the HTP UltraStor or the Viessmann Vitocell. These devices can also have 2) heat exchangers to allow for Solar Thermal water heating with boiler back-up.

Manual J

The Air Conditioning Contractors of America (ACCA) guidelines for sizing HVAC equipment, ACCA Manual J Residential Load Calculation, enables contractors to estimate heating and air conditioning loads more accurately. Using Manual J, a contractor calculates heat loss from the building through walls and ceilings, leaky ductwork, and infiltration through windows, doors, and other penetrations as well as heat gain into the building from sunlight, people, lights and appliances, doors, walls, and windows, and infiltration though wall penetrations. Design conditions for the area are also used as inputs into load calculations.

Manual D

Residential duct systems have a direct and significant effect on equipment size, equipment efficiency, equipment malfunctions, envelope infiltration, operating cost, utility demand loads, vent performance, exhaust system performance, indoor air quality, ambient noise, occupant comfort and owner satisfaction. Therefore, the duct system must be carefully designed and properly installed or the potential benefits that are associated with building an efficient structure and using high efficiency equipment will not materialize.

ACCA manual D presents the methods and procedures that should be used to design residential duct systems. The subject material includes information about system selection (constant volume or variable volume), system performance characteristics, duct materials, blower performance, airside devices and duct sizing procedures. ACCA manual D also includes information about duct system efficiency and the synergistic interactions between the duct system, the envelope, the HVAC equipment, the vents and the household appliances. Indoor air quality, noise control, testing and balancing also are discussed. Other ACCA manuals that pertain to residential HVAC system design include Manuals J (loads), S (equipment selection), T (basic air distribution) and H (heat pumps).


Annual Fuel Utilization Efficiency (AFUE)

The AFUE is the most widely used measure of a furnace's heating efficiency. It measures the amount of heat actually delivered to your house compared to the amount of fuel that you must supply to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that you supply to heat -- the other 20% is lost out of the chimney.

Viessman Vitodens boilers we install can often attain a 95% AFUE or greater.

The US Department of Energy (DOE) determined that all furnaces sold in the US must have a minimum AFUE of 78%, beginning January 1, 1992. Mobile home furnaces are required to have a minimum AFUE of 75%.

The DOE's technical definition of AFUE is as follows:

The measure of seasonal or annual efficiency of a furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls


We have heard it, used it, and in some cases do not know what it is, but when talking HVAC, we need to understand what S.E.E.R. means to be able to provide energy saving systems to our customers.

S.E.E.R. is an efficiency rating, much like how miles per gallon (MPG) is used to rate automobile efficiency. We all understand that if you drive 300 miles and consume exactly 20 gallons of gas, then your vehicle's fuel efficiency (MPG) is 15 miles per gallon (300 miles divided by 20 gallons). A vehicle that gets 18 MPG would be more efficient (less costly) to operate, and a 12 MPG vehicle is less efficient (more costly) to operate. For HVAC, S.E.E.R. follows the same rationale.

In 1975, there was no universal standard of measurement for HVAC energy efficiency. The Air Conditioning & Refrigeration Institute (ARI) introduced the E.E.R. (Energy Efficiency Ratio) for the purpose of rating the cooling efficiency of HVAC units. E.E.R. equaled the rated cooling output of an HVAC unit in BTU's per hour divided by the rated input of energy in watts of electricity, at specific humidity, and temperature input/output conditions.

The formal definition of Energy Efficiency Ratio is a steady state efficiency measurement of BTUH cooling output versus power (watts) input or BTUH/WATT at a specific set of indoor and outdoor dry bulb and wet bulb temperature conditions. The ARI rating point has been at 80 db./67 wb. indoor and 95 db./75 wb. outdoor temperatures.

While this sounds logical, no seasonality was taken into consideration. The climate zones across the U.S. vary, as do seasonal conditions from one zone to another. For example, Florida and Arizona have different summer conditions, which affect the performance and resulting cooling energy savings for the user of the HVAC unit. This means the seasonal conditions affect the value and must be weighed when interpreting an HVAC unit's E.E.R. rating.

Thus, in 1978 the US Congress passed a law requiring labeling of certain "appliances" (HVAC units under 65,000 btuh cooling) with an efficiency rating that took into consideration how certain variables (seasonality) affect cooling BTUH output, watts input, and an average cost of operation for the cooling side of a residential HVAC unit.

The new rating, S.E.E.R. (Seasonal Energy Efficiency Ratio), was born as an alternative to the original E.E.R., and to better approximate the actual cooling cost of operation of an "appliance" based on the installed climate zone. S.E.E.R. is a different efficiency rating than E.E.R. and is based on residential air conditioner usage patterns. S.E.E.R. includes a rating at a different temperature than E.E.R. and also includes performances such as conditions of cycling (off/on) thus includes cycling losses.

Currently all HVAC unit specification sheets are required to show only S.E.E.R. ratings.

E.E.R. and S.E.E.R. are two ways to determine the cost of operation of an HVAC unit. A 6 E.E.R. was the typical unit efficiency in 1974, and approximately 8 S.E.E.R. was predominant in the mid 80's. 10 S.E.E.R. became the first federally mandated minimum efficiency in 1992 for residential split systems and 9.7 S.E.E.R. for single package systems in 1993. Prior to 1992, individual states were free to establish their own individual minimum efficiency standards for air conditioners as well as other types of appliances. The new federal efficiency standards preempt any state standards and therefore we now have uniform efficiency standards for the entire country as opposed to a various array of different individual state standards. The minimum efficiency will increase to 12 S.E.E.R. in the US effective on Jan 23, 2006.

The higher the S.E.E.R. or E.E.R. the greater btu/h cooling delivered for the watts of electricity consumed (better miles per gallon, so to speak). You may find the chart below useful when talking with your customers about the energy savings associated with upgrading to a more efficient HVAC unit.

Reprinted from
The Comfort Zone
October 2003
by Maury Tiernan


The quantity of heat required to raise the temperature of a pound of water 1 degree from 59 degrees to 60 degrees. BTUH stands for British Thermal Unit Per Hour- this establishes a time reference to BTU input or output rates. MBH is shorthand for Thousand/BTU/Hour. A 100 MBH boiler is thus a boiler capable of producing 100,000 BTUs an hour. Most boilers we install in Residential applications produce between 80 and 200 MBH. Boilers for commercial applications may have much higher capacities.

Non Plus Ultra

Okay, so this is not an HVAC term, but we though we'd put it in here anyway.

It's Latin for "It doesn't get better than this"

...and it typifies how we feel about the Solar and Comfort Systems we provide our customers!

DX Coil

DX (Direct Expansion) coil AKA Evaporator Coil

A coil found in an Air Handler which uses the properties of a refrigerant to produce a cooling effect. Can also be used with a heat pump to produce both cooling and heating.


PEX is an acronym for cross-linked polyethylene. PEX Tubing is now being widely used for Plumbing and Radiant Heating applications. PEX is desirable due to its flexibility, durability, and thermal memory. Synonyms for PEX include PEX Tubing, PEX Pipe, and PEX Tube.

Grid-Tie PV

A Photovoltaic system whereby the energy produced is fed directly back into your main electric panel where it used by the homeowner, or during periods when electrical production exceeds demand, is fed back into the electric grid, generating a net credit. This is the most common type of PV system and also the most cost-effective.

Battery Back-Up PV

A Photovoltaic system whereby the energy produced is used to charge a bank of batteries. Energy is stored in the batteries and made available to be drawn out when required. These type of systems are typical is remote locations where no grid power is available or in situations where the customer requires protection against grid "black-outs". They are not common in urban areas because of the extra expense and space required for batteries and other equipment.


LEED = "Leadership In Energy and Environmental Design".

LEED for Homes is a rating system that promotes the design and construction of high-performance green homes. A green home uses less energy, water and natural resources; creates less waste; and is healthier and more comfortable for the occupants. Benefits of a LEED home include lower energy and water bills; reduced greenhouse gas emissions; and less exposure to mold, mildew and other indoor toxins. The net cost of owning a LEED home is comparable to that of owning a conventional home.


MERV, or Minimum Efficiency Reporting Value, is a number from 1 to 16 that is relative to an air filter’s efficiency. The higher the MERV, the more efficient the air filter is at removing particles. At the lower end of the efficiency spectrum a fiberglass panel filter may have a MERV of 4 or 5. At the higher end, a MERV 14 filter is typically the filter of choice for critical areas of a hospital (to prevent transfer of bacteria and infectious diseases). Higher MERV filters are also capable of removing higher quantities of extremely small contaminant (particles as small as 1/300 the diameter of a human hair). A higher MERV creates more resistance to airflow because the filter media becomes denser as efficiency increases. For the cleanest air, a user should select the highest MERV filter that their unit is capable of forcing air through based on the limit of the unit’s fan power. MERV 11 is considered highly effective for residential use without creating undue strain on the Air Handler blower motor.