System Design

 

Heat Pump Selection

 

How much heat does your home lose? Calculating its heat loss is the foundation on which your GXS design is built. The care taken in the construction of your home determines how much heat escapes through the cracks around its windows and doors, and how well its insulation isinstalled. The direction your windows face determines how much solar energy they let into the house. The heat loss calculation, therefore, determines the size of GXS you need. 

The contractor will need a set of plans with the dimensions and construction of the walls, ceiling and floors, and the size and types of windows and doors as well as the direction they face. Winds and trees (which may shade the windows) also affect heat loss. To measure accurately how tightly the home is sealed, some contractors will perform a blower door test. The contractor should give you a copy of the heat loss calculation.

GXS should have the heating capacity to supply at least 90 percent of the total heat required in your home annually. Auxiliary heat (usually electric elements installed inside the heat pump or in the ductwork) can supply the rest of the heat. Factors that influence the heating capacity you need for your home include the number of occupants, the appliances and lighting, the solar gain through the windows, the quality of the construction and the climate.

Why some standards recommend a GXS capacity of 90 percent (not including auxiliary heat)? Because it takes all heat sources in your home into account. The lights in your home give off heat. So do your stove, fridge, television, computer and freezer. The sun shining through the windows helps heat your home. Finally, the people (and pets) in it create a significant amount of heat as well. A heat loss calculation does not take this so-called “internal heat gain” into account. That is why a GXS that produces 90 percent of the calculated heat loss of your home will normally provide all of the heat your family needs. And it will cost a bit less. 

An auxiliary heater provides additional heat on just the coldest days (usually, electric heating elements are installed in the ductwork or built into the heat pump). The few hours the electric heat is needed affect your energy bills only slightly, but can reduce the cost of installing a GXS significantly. And because heating is more important than cooling in most of Canada, the lower air-conditioning capacity of the system is acceptable for most homes, and will perform better than a larger system.

The performance of a heat pump is rated for both heating and cooling efficiency. This is usually expressed as the Coefficient of Performance, or COP. The COP in the heating mode is referred to as the COPh, and in the cooling mode as the COPc. You calculate it by dividing the heating or cooling capacity of the system by the energy used to run it. For example, if the heating capacity of a system is 10.4 kW, and the power needed to operate the compressor, pump and blower is 3.25 kW, the COPh is 10.4 ÷ 3.25= 3.2. Similarly, if the cooling capacity is 10.55 kW (36 000 Btu/h x 0.000293 = 10.55), and the power needed is 2.51 kW, the COPh is

10.55 ÷ 2.51 = 4.2. (Note: Some manufacturers define the air-conditioning efficiency of their GXS as its Energy Efficiency Ratio (EER). The EER, expressed in Btu/h per watt, can be converted to COPc by dividing the EER by 3.413.)

Air-conditioning efficiency can be expressed in the same terms. You calculate the COPc by dividing the cooling capacity of the system by the energy input. So if the cooling capacity of a system is 36 000 Btu/h (36 000 x 0.000293 = 10.55 kW), and the power needed to run the system is 2.29 kW, the COPc is 10.55 ÷ 2.29 = 4.6.

The efficiency of a GXS varies as the temperatures and flows of the liquid and air pumped through the heat pump change. Manufacturers publish the ratings of their GXS on the basis of a specific set of standard conditions called the ISO 13256-1 rating. The rating for a closed-loop system is called the Ground Loop Heat Pump (GLHP) rating; the rating for an open-loop or ground water system is called the Ground Water Heat Pump (GWHP) rating. When comparing quotations on equipment, make sure you are comparing the equipment on the basis of the same standard ratings. As with any system, however, your GXS will only meet the performance ratings if the whole system is designed and installed according to the manufacturer’s specifications.

Loop Size: Is Bigger Better?

 

You can think of an earth loop as a rechargeable battery permanently connected to a battery charger. Heat energy is drawn from the loop, or “battery,” as it is needed in your home. If the battery is large enough, it is easily recharged by the heat energy from the surrounding ground, sun, rain, heat expelled during the cooling of your home, and heat emanating from the earth’s hot core. But if your loop battery is continuously drawn down more quickly than it can be recharged, it will be unable to provide enough energy to run your system. And there is no easy way to recharge it quickly.

So the ground loop has to meet the requirements of your home. Some of the factors that will affect the size of the ground loop you need include:

  • the heating and cooling requirements of your home;
  • the moisture content and type of soil;
  • the depth at which the loop is buried;
  • the climate;
  • the amount of snow covering the loop in winter; and
  • the size of the buried pipes as well as the distance between them. 

The larger the heating and cooling loads of your home, the larger the loop must be. Moist, dense soil conducts heat more quickly than light, dry soil. Pipe that is buried deeper has more soil to draw heat from and will perform better. A climate with long cold spells will require a loop (“battery”) that can hold more heat. Heavy snow cover insulates the earth and helps retain the earth’s heat. If earth loop pipes are buried farther apart, they are recharged by a greater mass of soil. 

A competent contractor will know the soil conditions in your area, and will design the earth loop on the basis of all these factors. Some heat pump manufacturers provide contractors with computer software to do this. GeoAnalyser, the CGC developed design software is the best tool available on the market as it is the only one with options to consider the exact ground conditions.

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