Geothermal heat pumps are able to extract heat from the ground, even in Seattle when outdoor temperatures are well below freezing. They use a system of pipes installed in the ground below or around your home to collect this heat and then return it to your home where it can then be used to heat the air. While these types of heating systems are certainly more efficient the warmer the ground is, they can be effective even in very cold climates.
This is true even in areas where the ground freezes from time to time or for parts of the year because the frozen layer does not typically extend more than three or four feet below the surface. As long as the pipes for your geothermal heating system are below this level, they will still be able to gather plenty of heat from the warmer ground below the frozen layer.
In fact, there are two different ways that the pipe loop for a geothermal heating system can be installed. Most geothermal systems have a horizontal pipe system which sits about four feet below the surface and extends out from the house. This type of installation is typically cheaper than the alternative, but it also usually needs to be larger. Plus, you need to have the space for it to stretch across.
On the other hand, a vertical installation goes straight down into the ground below your home. With a vertical installation, you can usually get away with less pipe overall, but you will probably pay more for the installation because it is harder to drill straight down than it is to dig out a relatively shallow trench to lay the pipe in.
However, if you live in an area that has particularly harsh winters when the ground can be frozen for significant periods of time, it may be worth it to opt for the vertical installation. That is because the further below the surface the pipes go, the farther away they will be from the frozen layer of ground.
With a vertical pipe installation, a geothermal heating system can work quite well in a climate in which the ground usually freezes in the winter. While you will always want to have a backup heating system in place in case of emergencies, this type of heat pump should be all you need under normal conditions.