Heat Pump Size Calculator | Auckland Heat Pump Installation
Heat Pump Size Calculator | Auckland Heat Pump Installation

Heat Pump Size Calculator

Room Measurements

As a general rule of thumb the size or output capacity of your heat pump required in kW is:

Older houses use 0.065 (kW) x Length (m) x Width (m) x Height (m) of the room
Newer Houses use 0.05 (kW) x Length (m) x Width (m) x Height (m) of the room

Most homes built after 1978 should have had insulation installed where the information regarding R-Values should available on the building permit documentation at your local council. It is now also a legal requirement that any house that is not owner occupied and intended for the rental market must have ceiling and underfloor insulation.
In each of the following years 2000, 2004 and 2007 the minimum level of insulation required for newly built houses were increased and thus newer homes will have better insulation than older homes.
The power of an air conditioner is measured in BTU’s (British Thermal Units). BTU is the amount of energy required to raise the temperature of ONE pound of water by ONE degree Fahrenheit. When it comes to an air conditioner or heater the BTU tells you how powerful it is. As an example, if a heater claims to be a 5000 BTU heater it means it can product 5000 BTUs of energy per hour. In relation to an air conditioner where it cools the room, a BTU measurements tells you the amount of heat energy it removes from a room.
Heat pumps in NZ always states the amount of heating or cooling capacity in kilowatts where the BTU capacity can only be found in the technical data. If you are ever faced with only BTU figures you can easily convert BTU’s into kW
kW = BTU x 0.0002928
From this moment forth, I will only be referring to kW for sizing a heat pump to a room.
Factors that influence the size of a heat pump to a room:

1. Ceiling height - A Typically in New Zealand home will have a standard ceiling height of 2.4 meters, however, designer, luxury, or old villas are most often designed with ceiling heights greater than 2.4 meters. Houses with very tall ceilings will find it harder to keep warm in the winter as the warm air rises towards the ceiling leaving the colder, damper air closer to the floor.  During the summer where the heat pump may be turned to "cool mode", high ceilings do not have such a prominent, negative impact as cold air already sits closer to the floor level. This is the same reason why hot air balloons can float high in the sky, where the basic principle of hot air rises is evident. High ceilings are used to make a space look bigger and feel more luxurious but often come at the expense of needing a larger heat pump and costing more money to heat and cool when compared to rooms with a standard ceiling height.

When we look at trying to size a heat pump for a room we always look at the volume of air that is needed to be cooled or heated. The taller the ceiling the greater the volume of air and thus could require a larger sized heat pump. Sizing a heat pump is important for the longevity of the heat pump unit and the comfort of the user. Heat pumps that are too small will use up more power as the heat pump will work in overdrive to keep up with the heating or cooling requirements. This leads to higher electricity bills in comparison to a properly sized unit as the unit is no longer working efficiently. When the heat pump is overworked the compressor takes a real beating where it works too hard for too long without rest and can shorten the life of the heat pump.

A simple solution to overcome high ceilings is to install ceiling fans which help support a heat pump system by pulling and pushing the hot air trapped around the ceiling to give the room a more even distribution of heat. One obvious disadvantage to ceiling fans is the aesthetic consequences. Ceiling fans give a room a dated look and often don't match modern contemporary designs. They require dusting and cleaning that requires the user to set up a ladder and manually remove dust and/or grease accumulated on the fins.  Often, a better idea is to size up a heat pump to match the room it will be servicing which any experienced heat pump installer can and will do.

2.Unit location: If the room you wish to add a heat pump to is exposed to a significant amount of direct continuous sunlight or is on the second or third level of a house it is recommended to add an extra 10% to your kilowatt calculations as these rooms are generally get much hotter. As we know a little bit of sun is good for our wellbeing, however, when it's too hot outside it's great to be able to hide inside or find a nice quiet shady place to relax. Houses do not have the luxury of being able to hide from the sun. When the sun is out, the sun is out and we rely on the house's ability to resist temperature changes to keep us comfortable. The amount and quality of insulation our houses have, has a positive effect on the comfort of our family. Investing in quality window coverings such as thermal curtains or roller blinds can help heat from coming in or out of the house.

As heat enters your house, it travels up to the ceiling and up to the second and third floor if any. Fighting this basic fact that hot air travels up is similar to fighting gravity. The most cost-effective way to ensure your house remains comfortable is to get an experienced heat pump installer to size the heat pump to the location of your room.

One important factor that is often overlooked is the direction the heat pump is facing within the room. A heat pump works by changing the temperature in the air surrounding the heat pump and forcefully blowing it out. This can often create a draft like effect and if the heat pump is directed towards seating, resting or, sleeping areas it can cause the user to experience headaches, discomfort, or disturb sleep. It is best to position a heat pump so it is not directly facings where people regularly sit and relax and instead directly adjacent or somewhere close to allow air to flow freely in and out of the heat pump.

If the heat pump is positioned inside a kitchen and is in frequent heavy use and additional 1.2 kilowatts (1.2kW) could be added to the requirements to overcome the heat produced when cooking or when electrical appliances are turned on. A Rangehood above the cooktop is important to have as it sucks out a lot of the oil-contaminated air and helps keep the heat pump filters clean and running efficiently. It efficiently removes heat and moisture produced by the cooktop below and would be a worthwhile investment if the homeowner hasn't already put one directly above the cooktop or if the current one is broken or undersized.

3.Insulation: The ability for a room to keep all the cool air or heat from escaping is important for sizing the correct heat pump for the room. An uninsulated room could require 30% more power in kilowatts as opposed to a room that has wall, ceiling and underfloor insulation in combination with double glazed windows. 

Insulation is a barrier that reduces the migration of thermal energy (heat) in and out of a room. Some insulation has been specially designed to reduce acoustic transfer thereby reducing sound being heard from the roads, neighbors or from the other side of the wall. Houses built after the year 2000s will have insulation on the inside of exterior walls. However, most residential houses will not have insulation in the internal walls due to a higher cost in the building process. It is worth noting that it is possible to add insulation in interior and exterior walls without removing the plasterboard lining by drilling small holes and blowing in insulation fill.

If you are choosing insulation it is best to choose one with the highest R-value for the budget you have. the higher the R-value the greater it's ability to retain heat and prevent it from passively entering or leaving.

Without proper insulation, your heaters could be running continuously but your home could remain cold. This is because heat is escaping as fast or faster then it is produced. The better the insulation, the better your electric bill will look at the end of the month as the heat pump will only run when it is turned on and when there is a perceived temperature difference from what you set to and what the room actually is. This also helps the air from becoming too dry and cause discomfort in some people.

4.Windows: Logically a glass window would not be able to provide the same insulation factor as an insulated wall. Windows can be either single, double, or triple glazed and the number of layers of glass will you have between the outside of your home and the inside of your home will positively increase your insulative potential.

Windows can provide beautiful natural lighting that has positive psychological effects on mood, reduce lighting expenses, and increases heat gain during the summer months, and facilitates heat loss during the winter months. In effect, this would directly increase the amount spent on heating and cooling your home or office.

In New Zealand, all new houses must be built using double glazed windows. Triple glazed windows can be done, however, the cost of doing so is too high for an average developer and thus rarely seen in Auckland houses. If you are living in a home built prior to 2007 you would most likely have single glazed windows. These windows will most like stay single glazed as the cost to retrofit double glazed windows can be quite an expense. The average homeowner and especially landlord would not have the budget to pay for such an upgrade. The most cost-effective way to increase the thermal insulative properties of a window is simple by investing in thick thermal curtains. Thermal roller blinds do have better aesthetic properties, meaning they look cleaner and make a property feel more modern. They do not have the same thermal properties as curtains and if your main goal is to keep the heat from escaping them thermal curtains especially for a single glazed home will provide you with a better outcome.  

The size and quantity of windows make a huge difference to how a room can keep heat from entering or escaping. The more area that is covered by glass, the worse off your home will be at being able to keep heat where you want it. This also leads to higher heating and cooling costs but is generally well accepted when compared to the emotional and psychological benefits from having a view of the outdoors. 


5.Expected number of Occupants: The human body naturally gives out heat, and in a confined space such as a closet or elevator the body heat comes together to increase the temperature of that room. In larger spaces, this is less noticeable and takes longer for the heat to build up. Over a period of time, you will begin to feel the room warming up and getting stuffier. For rooms that have the sole purpose of hosting meetings, conferences or religious gatherings a heat pump sized appropriately to the room may have to work extra hard to maintain a comfortable temperature. An average person will require an additional 0.12kw-0.18kw depending on activity. For a bedroom, this may not be necessary to factor in as you would generally only have 1 to 2 people. However, if you know you often entertain large groups of 10 or more people during the summer days when it does get quite hot you may want to consider having a slightly larger heat pump to accommodate for the extra guests.


6.Electrical Equipment: Lights, computers, televisions, and other electrical equipment all produce heat. In the residential setting, electrical equipment  do not product enough heat to be a significant factor unless the sole purpose of such equipment is to produce heat. To calculate the extra cooling capacity needed for a room with electrical equipment the following formula can be used: Wattage x 0.00099552 = kW. You would only want to include electrical equipment into your calculations if it was in a commercial setting where perhaps there were 20+ computers running at the same time.  By observing the formula used to calculate waste heat from electrical equipment, you may have noticed that the common multiplying factor is so small that unless you have a serious about of equipment in a room the amount of waste heat would not add up to much.

7. Building constructions: A house made of bricks or cinder blocks would generally be cooler than timer or metal framed houses. Exposed bricks or blocks would exacerbate this by absorbing more heat making the room even harder to heat.

For the most basic heat pump sizing requirements without much thought into the 7 influencing factors mentioned above use this table

Room Area Heat pump size Requires independent power supply from fuse box
10-15m2 2.6kW NO
16-20m2 3.5kW NO
21-30m2 5.3kW NO
31-40m2 7.0kW YES
41-50m2 8.8kW YES
51-60m2 10.55kW YES