What are the different sizes available?
This Post's Contents
- 1 What are the different sizes available?
- 2 How do I decide which size I need?
- 3 Are there exceptions to Sizing Rules
- 4 The danger of oversizing a Heat Pump
- 5 The issue of Frequent Turning off and On (Short Cycling)
- 6 The issue with Short Cycling and Defrost Mode
- 7 What Happens During Defrost Video
- 8 In Conclusion
All heat pumps are measured by the number of BTU’s (British thermal units) they produce when in air conditioning mode. Popular sizes purchased in Atlantic Canada by home owners are:
- 9,000 BTU
- 12,000 BTU
- 15,000 BTU
- 18,000 BTU
There are also 7,000 BTU and 24,000 BTU single zone heat pumps, but they are rarely installed on their own in residential applications. Usually they are part of a multi zone system.
For example, a 7,000 BTU indoor unit is most often used as part of a multi zone (multiple indoor units matched to one outdoor unit). A 24,000 BTU outdoor unit is most often part of a multi zone system with two or more indoor units connected to it.
The most popular sizes installed for HRM home owners are 12,000 BTU and 15,000 BTU.
Why are there different sizes?
Different sizes exist to accommodate varying layouts and square footages of homes. The higher the number of BTU’s the larger the area a machine can efficienctly heat or cool.
For example, a 9,000 BTU heat pump is suited for 250 to 450 square feet.
How do I decide which size I need?
There are two main factors when determining the size of heat pump you need.
- The square footage of the are you’re installing it in
- If there are any large doorways or hallways connected to that area or room
While inside your home we first consider the square footage of the room/area and using this guide determine which sized heat pump will suit that space:
- 9,000 BTU – ~250 sq.ft to ~450 sq.ft
- 12,000 BTU – ~550 sq.ft to ~800 sq.ft
- 15,000 BTU – ~750 sq.ft to ~1100 sq.ft
- 18,000 BTU – ~850 sq.ft to ~1250 sq.ft
If there are larger door ways or hallways connected we may consider upsizing the unit installed in your home.
Are there exceptions to Sizing Rules
In certain styles of homes, such as split entry or bungalows, we will suggest one size larger heat pump than the above table of sizing suggests.
Most layouts for split entries and bungalows are similar. One side of the home upstairs is open concept. Then there is a hallway that connects the open area with bedrooms, bathrooms etc. on that floor.
We suggest the heat pump be installed at the top of that hallway so the heat can fill the main living area, where the unit is installed, as well as flow down the hallway and provide spill over into other rooms (pending the door is left open). The drawing below is an example split entry upper level layout and how we often install heat pumps in these styles of home. In a full split entry, the heat pump size is either a 12,000 or 15,000 BTU.
This same concept can come into play if there is a large double door way or archway leading to another room or hall way.
The key to installing a larger unit in a room than the square footage suggests is there must be somewhere for that extra heat to go. Otherwise you risk having an oversized unit.
The danger of oversizing a Heat Pump
If you were to purchase a unit too large for the area it’s installed two things will happen.
The unit will turn on and turn off frequently.
The heat pump will often not run long enough to trigger what is known as a defrost cycle.
The issue of Frequent Turning off and On (Short Cycling)
Thanks to the inverter technology built into every heat pump a machine actually draws less electricity when it’s able to run on what we like to call “cruise control mode” versus if it is frequently shutting off and then starting back up.
When a machine turns on and off frequently it’s known as short cycling. This occurs when a heat pump gets a room to temperature very quickly and then shuts off. This is often a symptom of a heat pump being oversized for the spaces it is installed in.
While this concept of heating a space quickly and then shutting off once the space is to temperature makes sense for oil or electric resistance, it can cost you more on your power bill with a heat pump.
The issue with Short Cycling and Defrost Mode
Defrost cycles (see video below for explanation), are a normal part of heat pump operation. The purpose of a defrost cycle is to melt ice and frost off the outdoor units coil to ensure the system can operate at peak efficiency.
Most heat pumps need to operate for a certain amount of time before a defrost cycle will trigger. This time is often around the 15 minute mark.
If your heat pump is running for a couple of minutes, shutting off, then running for a couple of minutes and shutting off etc. Then the unit never runs long enough to trigger a defrost cycle in the outdoor unit.
The danger for you as the heat pump owner is the outdoor unit can become a block of ice as more and more frost builds up on the outdoor coil.
This can lead to your heat pump no longer operating, and worse, damage to the coil, fan and fan motor.
What Happens During Defrost Video
Bigger isn’t always better with heat pumps. Take into consideration not just the listed size (e.g. 12,000 BTU Fujitsu) but also the nominal heat the machine creates and how much heat it produces at lower temperatures.
Purchasing the right heat pump for your home is more complicated then just calling around looking for prices on a certain size. Be sure to have a professional visit your home and assess the best positioning, sizing and brand of a heat pump for you.