For dehumidification purposes, does a ductless heat pump use more or less energy than a dehumidifier?

Question

We have recently installed a ductless heat pump (Mitsubishi) in some soon-to-be-finished basement space. It will be used primarily for heating during the winter (we live in Maine). During the summer, we have always had a few months where we run a dehumidifier in the basement to keep it dry-ish.

Ideally, I would like to use the heat pump in dry mode during the summer rather than running the dehumidifier, as it's one less thing to own and maintain. My question: assuming that either is used only for this purpose (not for cooling as well), which will use fewer kWh? I know that heat pumps are more efficient than resistance heaters for heating, but I am unclear about how the dehumidification scenario would work.

In practice, I have tried for the last week or so running the basement heat pump in dry mode, and it seems to be cooling as well and leaving the basement unnecessarily cold (it stays perfectly cool on its own all summer). I suspect this additional cooling is using more energy than a dehumidifier would. I wish I had a simple way to measure the kWh consumed by the heat pump...

Answer 1

All else being equal, the dehumidifier will be more efficient.

Both devices utilize the same refrigeration cycle (from Wikipedia):

Both devices remove moisture from the air at the evaporator (3), where the indoor air passes over a cool heat exchanger, lowering the dew point so that moisture in the air condenses out.

The difference is that the condenser (1) on your heat pump is outside where it's hot, but on your de-humidifier it's inside where it's cool. This means that the heat pump has to work harder to get the refrigerant hotter than the ambient air in order to successfully reject heat outside. The dehumidifier doesn't have to work as hard since it's rejecting the heat indoors where it's already relatively cool.

The main caveat is that a very old, dirty, or poorly maintained dehumidifier might be worse than your brand new heat pump, but I'm assuming your dehumidifier is in good condition.

Obviously a potential downside to this is that the dehumidifier can end up heating the space, but since you say it's already cool enough this isn't an issue for you.

There are at least two other advantages to using a dehumidifier:

1. You can put it in the dampest part of your basement. Whereas the heat pump will draw air from the entire basement, you can put the dehumidifier in the dampest part of the basement to focus on that.
2. You can collect the condenser water. Whereas the heat pump has a drain line, many dehumidifiers have a collection bucket, so you can use the water to irrigate plants, flush toilets, etc.