System Design for an Existing Home


Optimum Size


The heating and cooling capacity of the GXS installed in your home is the single most important factor that will ensure a comfortable home, long-lasting equipment and an efficient system.


The owner of an existing home, especially an older home, generally does not have the house plans showing the wall construction, ceiling insulation and other details needed to calculate heat loss accurately. You will therefore need to measure and estimate the insulation value of features such as the walls, the ceiling and the windows. This information will be helpful to the contractor preparing a quotation. Ideally, a drawing showing the direction the house faces, the wall dimensions, window sizes and types, insulation values and other features for each level provides enough information to calculate the heat loss. Since the wind affects heat loss and trees may affect the cooling loads if they shade the windows, information about wind patterns and trees on the property is helpful. Some contractors will also perform a blower door test. The contractor should provide a copy of the heat loss calculation to you.

To double-check the calculated heat loss of the home, some contractors will ask for the energy consumption in your home for an entire year. If the insulation has not been upgraded recently, or you have built additions, the annual energy consumption figures can be used to estimate the heat loss of the home.

In an existing home with a ductwork system, there is an additional reason to install a system that provides less heat than the calculated heat loss. Older fossil fuel furnaces or electric furnaces were designed to circulate less air than a GXS. It may be difficult or impossible to upgrade the ductwork to the larger volume capacity required by a GXS without creating unnecessary air noise. Remember – when you are designing a GXS for your home, bigger is not necessarily better.

Many of the principles that apply to the system design of a GXS for a new home, such as COPh, COPc, ratings for closed- and open-loop systems and heat load calculations, also apply to existing homes – see “System Design for a New Home”.


Alternatives for Homes Heated with Hot Water or Electric Baseboard Heaters


A GXS can be installed in an existing home with a hydronic (or hot-water) heating system, or a home with electric baseboard heaters. Here are some things you should consider if you want to install a hydronic heating system.

Hydronic Systems


There are several types of residential hydronic systems. They include the old, heavy castiron radiators; the more modern, compact baseboard radiators; and radiant floor heating. There are also systems that use hot water to transfer heat to a forced-air system by means of a fan coil unit.

Each of them can be used with a GXS, although there are presently no heat pumps that can produce water warmer than 50°C, so the heating capacity of the distribution system may be reduced. Many existing hot-water heating systems will not distribute enough heat to your home unless used with water at a temperature greater than 65–70°C.

If you have recently upgraded the insulation and airtightness of your home, however, its heat loss may have been reduced enough to allow you to use a water temperature low enough to install a GXS. Therefore, it is important that your contractor calculates your home’s heat loss again, once the insulation work is completed.

Cast-Iron Radiators


These decorative heavy radiators were designed for use without a protective cover. As they are often located where people could come into contact with them, the systems were usually designed to operate at about 50–55°C. A GXS is capable of generating 50°C and, with some upgrading of the windows and insulation in the home, should work satisfactorily with these systems. The piping to the radiators will almost certainly need upgrading, however. Contractors have successfully used 12, 19 or 25 mm flexible “PEX” tubing to run new lines to the radiators.

Baseboard Radiators


Most baseboard radiator systems were designed to be used with 60–70°C water. As a result, they are not compatible with a GXS. The heating capacity of a baseboard radiator drops by 30–50 percent when supplied with water at 50°C. In most situations, it will be difficult to make a GXS work with baseboard radiators without installing many additional units.

In-Floor Heat


In-floor heating systems are often designed for use with water temperatures lower than ones compatible with a GXS. However, if the system in your home uses pipe installed in the void between the floor joists rather than in concrete or with metal reflector plates, it probably will need water temperatures hotter than those produced by a GXS.

Fan Coil Units


The heating capacity of a fan coil unit is directly related to the temperature of the water circulated through it. You should have the capacity of the heating coil tested to ensure it is able to distribute enough heat to your home with a GXS.

Before deciding to use the existing hot-water distribution system, the contractor should determine that the distribution system will heat your home properly at the lower GXS water temperatures.

Electric Baseboards


Electric baseboards use electrical energy to heat the room in which they are located and do not use a heat distribution system. There are two options. The first is to build a distribution system into your home – either forced-air or hydronic – and use the appropriate GXS. The second is to use heat pumps designed to heat a small space without a distribution system. Several manufacturers build console-type heat pumps in various sizes. They are designed to be mounted against a wall and both heat and air-condition a single room without a distribution system. They are typically 120–130 cm in length, 60 cm in height, and about 25 cm deep.

Air Conditioning


Existing homes without a forced-air distribution system can be difficult to air-condition. Some types of heat pumps, like water-water models, for example, are able to provide chilled water that can be used in air-conditioning systems. However, most hydronic heating systems are not designed to provide cooling. When a castiron or baseboard radiator, or infloor heating system, is cooled with chilled water, condensation forms on the cold surface of the pipes through which the water is circulated. Some types of fan coil units can be used for air conditioning through the use of chilled water, but the condensation must be collected in a condensate pan under the water coil. Also, the pipes through which the chilled water circulates must be insulated. 

It might also be appropriate to use console-type heat pumps (see the previous section “Electric Baseboards”) to provide cooling in some areas of a home heated with a hydronic system.


Some manufacturers produce equipment that can heat water for use with a hydronic system and also heat or chill air for use in a forced-air system. With this equipment, it may be possible to add some ductwork to your home for air conditioning, while keeping your existing hydronic distribution system to provide heat.

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