Heat Distribution

Forced Air and Hydronic: Ducts, Pipes, and Tubes Should Be Short and Leak-free

Forced Air

In a forced-air system, an air handler or a furnace houses a fan that pushes conditioned air through a network of ducts to individual rooms. Heat can be generated by burning a fossil fuel or wood, by electric resistance coils, or by a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump.. Careful duct design is a key factor in system performance. Forced-air systems are versatile — they readily accommodate both heating and cooling and generally make mechanical ventilation and filtration easier — but leaky ducts are a potential source of trouble.


In a hydronic system, hot water from a boiler or water heater is distributed through tubing to baseboard radiators or in-floor tubing. Hydronic heating systems are usually quieter than forced-air systems that blow air into rooms.

Space Heaters

Space heaters can be fueled by wood, electricity, propane, natural gas, or kerosene.

Increasingly, designers of superinsulated houses realize that building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. improvements allow for radical simplification, if not elimination, of heat distribution systems. In a house with an open plan, a single space heater may easily meet a home's heating needs.



Windows play a huge role in comfort, which is a key consideration in choosing a distribution system. Better windows can dramatically improve comfort and drastically change the requirements of heating and cooling distribution systems.


LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H Heating distribution affects 7 points in EA1 (Energy & Atmosphere), EA5 and/or 6; and 3 points in EQ6 (Environmental Quality).

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 7 — Energy Efficiency: 5 pts for ducts designed & installed per ACCA Manual D (704.4.1); 12 pts. for all ducts in conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. and no panned building cavity returns (704.4.4); 5 pts. for transfer grilles/jump ducts (704.4.5).


One form of heat (and its distribution system) is not necessarily any better or more efficient than another. Overall system performance depends on factors ranging from the price of fuel to the efficiency of the heating system to how it’s installed.

Heat can be generated by a variety of fuels and is usually distributed to individual rooms by either forced-air ductwork or hydronic (water-filled) pipes. However, some well insulated houses may not need any heat distribution system at all, relying instead on a centrally located woodstove or on individual space heaters.

European Standard for Low-Energy-Use Buildings
Most European homes built to the German PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard distribute heat through ventilation ducts rather than hydronic pipes or conventional ductwork. Such homes include a heat-recovery ventilator with ventilation ducts measuring 4 inches or less in diameter.

The homes are built to a high standard of air tightness and include thick layers of insulation and triple-glazed windows; as a result, very little space heating is required. Space heat needs are supplied through the HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. 's ductwork by an electric resistance heater, an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. that scavenges heat from the ventilation system's exhaust air stream, or a hydronic coil connected to a water heater.

For more information on the Passivhaus standard, see "Passivhaus for Beginners."

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Jan 5, 2011 2:21 PM ET

lifebreath clean air furnaces
by kimshanahan

Martin and Jerry,
Jerry's desire is not as complicated as you suggest. Airia, a Canadian company, has been manufacturing an hydronic forced air unit with built-in ERV or HRV and ECM motors for at least two decades. They typically are supplied with hot-water from standard tank-style hot water heaters, but can also work with boilers or on-demand heaters or even solar panels. We put 25 systems in new construction in 2008, but I have always believed their best application would be for exactly what Jerry wants to do. The old exhaust and supply air holes for a gas fired heater potentially can be used for the supply and exhaust of the ERV. I keep waiting for a GBA in-depth article on hydronic forced air heating, especially with a solar panel helping to supply the BTUs.

Sep 24, 2010 5:55 AM ET

Response to Jerry
by Martin Holladay

The short answer is, "Yes, but such systems are complicated and problematic, and the installer needs to know what he or she is doing."

If you have located an installer who is willing to do this:
1. Ask how many such systems have been installed, and ask for a reference so you can speak with the homeowners living with such a system; and
2. Contact a technical rep at the manufacturer of the tankless water heater you intend to use and ask whether the unit is suitable for space heating. Some are and some aren't.

Sep 23, 2010 10:14 PM ET

by jerry

I want to replace my 30 year old furnace and water heater. They are next to each other in a closet. Can I use a tankless water heater and a hydro system to blow the hot air through the existing air ducts? Thanks in advance--jerry

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