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Heating & Cooling Load

Heating & Cooling Load

With our expertise and software program at High Mark Heating & Cooling you can expect a comprehensive Heating & Cooling Load analysis. The heating load is the amount of heat energy that would need to be added to a space to maintain the temperature in an acceptable range. The cooling load is the amount of heat energy that would need to be removed from a space to maintain the temperature in an acceptable range. The heating and cooling loads, or “thermal loads”, take into account:

• the dwelling’s construction and insulation; including floors, walls, ceilings and roof; and

• the dwelling’s glazing and skylights; based on size, performance, shading and overshadowing.

Lower thermal loads indicate that, relatively, the dwelling will require less heating and cooling to maintain comfortable conditions. Lower thermal loads do not necessarily correspond to lower electricity usage.

In practice, the heating and cooling loads may be handled by heating or air-conditioning equipment. The efficiency of the equipment, fuel type, and climate zone are also contributing factors that need to be considered. Lower loads will improve your Energy score. High Mark Heating & Cooling sets maximum heating loads and cooling loads separately, meaning that good performance in heating or cooling alone will not be undermined by poor performance in the other.

The 3 Types of Heating & Cooling Loads

Before we get into the three types of heating and cooling loads, it’s important to make sure we’re all clear on the difference between two words that are easy to confuse: load and capacity. If you’re new to HVAC lingo, it’s easy to miss the distinction so here is a quick definition:

LOAD – the amount of heating or cooling a building needs

CAPACITY – the amount of heating or cooling a piece of HVAC equipment can provide

Whenever you see the word “load” in the context of heating and cooling, it refers to the building’s needs. When you see “capacity,” it’s what the heating or cooling equipment can provide.

Now we can get to those three types of loads, knowing that we’re talking about building needs.

Design Load

If you ask a competent HVAC designer what the cooling load on a house is and they tell you it’s two tons, they’re talking about the design load. That means they’ve calculated the amount of cooling needed for the house using plans and specifications for the house or actual field data for existing homes. Mostly the inputs in the calculation are constant (e.g., insulation R-value, house orientation…), but some things change throughout the day or year (e.g., outdoor temperatures) and some can be changed by the occupants (e.g., indoor temperatures). 

Calculation protocols specify the temperatures you should use in the calculations, and these are called the design conditions. So your design cooling load is how much air conditioning you need when the indoor and outdoor temperatures are at the summer design levels. The design heating load is how much heating you need when the indoor and outdoor temperatures are at the winter design levels.

Extreme Load

Unless you live in a place like Santa Monica, California — where the temperature is always perfect — you probably understand that design loads are simply a guide. A house in North Idaho will never spend a whole lot of time subjected to design conditions, so if you size your heating and cooling equipment to meet the design loads exactly, you’ll have the wrong size equipment most of the time.

Extreme loads happen when you get the hottest or coldest temperatures your location experiences. In Sandpoint, we got down to single and even below zero digits Fahrenheit last winter, nearly 20 F° below our design temperature. In the summer we have set a record high temperature of 104° F. Should we install HVAC equipment with the capacity to meet the loads from such extreme temperatures? The answer is no.

The loads we calculate at High Mark Heating & Cooling will probably be 15-20% higher than the actual load at design conditions, which gives you a buffer to help meet the extreme loads. By the time the heat from that 100°+ Fahrenheit day starts getting to the inside of the house, the outdoor temperature has already dropped. That’s one of the ways insulation and air sealing help you. Extreme temperatures occur for about 1% of the time on average. Yes, there will be years with heat waves and years with cold spells, but HVAC equipment professionally sized according to the design loads and manufacturers equipment protocol should cover you for most of the extreme loads you experience.

If you’ve got a home that’s adequately air-sealed and insulated, properly sized equipment should do fine. If your home isn’t air-sealed or insulated well enough and your HVAC equipment can’t keep up, attack the building enclosure and distribution system before installing bigger equipment.

Part Load

For nearly 99% of the time, the outdoor temperatures will be less extreme than the design temperatures for your location. Think about it. You get up on a summer morning, look at the thermometer, and see that it’s 73° F outdoors. Maybe it’ll climb up to your design temperature in the afternoon, but even then, it won’t stay there very long. So even on a day when you hit the design temperature, your heating or air conditioning system will be operating under part-load conditions most of the day. Another factor is the seasonal change in conditions during the cooling or heating seasons. Early and late in the season, every day will be a part-load day. As houses get more airtight and better insulated, the imbalance gets worse. You can put in some supplemental dehumidification or use two-stage heating and cooling equipment. Or you could go a step further and put in fully modulating equipment like mini-split heat pumps.

It’s important to use professionals that understand the nuances of heating and cooling loads in North Idaho and the ramifications of the type of equipment you choose. The HVAC design world isn’t as simple as it used to be back when you could use rules of thumb or word-of-mouth protocols.  Actually, it never was that simple.  Let High Mark Heating & Cooling build a thoughtful and comprehensive Heating & Cooling Load for your new installation.  We want you to have the right equipment for your project and your budget.  And remember in the HVAC industry, bigger isn’t always better!