We’re seeing more and more folks making the smart switch to heat pump water heaters (HPWHs) to slash those energy bills – and rightly so, especially if you’re a hot water fiend. But a recent Insight from the legendary Dr. Joseph Lstiburek serves as a crucial reminder: these aren’t your grandma’s resistance element heaters.
Think about it: heat pumps pump heat. They don’t magically generate it; they grab it from their surroundings and concentrate it into your hot water tank. As Dr. Lstiburek puts it, they’re essentially air conditioners that dump the “cold” side into heating your water.
This simple fact has some pretty significant implications for where you put these clever devices. Because if a HPWH is sucking heat out of the air, it’s also exhausting cold air. And that cold air needs to go somewhere.
One common mistake Dr. Lstiburek highlights is sticking HPWHs in closets. Sounds logical, right? Keep it tucked away. But a confined space getting constantly cooled can lead to problems. If that cold exhaust air hits a surface, especially in a humid environment, it can cool it down below the dew point. And guess what loves cold surfaces with moisture? You guessed it: mould. Dr. Lstiburek even points out that the United States Department of Energy recommends installing them in a room with minimum of 12 square meters of floor area (that’s a middling size bedroom) to avoid this.
As we enthusiastically embrace HPWHs for their efficiency (and they do pay off, especially with high hot water usage!), we need to be conscious designers and homeowners. We don’t want to trade lower energy bills for unexpected mould issues.
So, where should you put your HPWH?
- Outside (with proper protection from the elements and ensuring you’re not blocking the unit’s airflow) is another possibility – this is my favorite location.
- Garages and basements can be good options, provided there’s enough airflow and the cold exhaust isn’t aimed directly at sensitive materials.
- If the heat pump compressor (the fan-equipped bit) is indoors, you absolutely need to design to manage that cold air. Think about where it’s being expelled and ensure it can mix with warmer air to avoid creating cold spots on surfaces.
- This article doesn’t mention this but one option if the compressor is indoors is to duct air into and out of the HPWH. This seems crazy but it has been done in New Zealand and as far as I can tell it has worked out well.
Finally, a crucial point to remember, though not explicitly in Dr. Lstiburek’s piece, is the impact on your home’s heating if the HPWH compressor is inside the home. In winter, as the HPWH works to heat your water, it’s simultaneously cooling the air inside your house. Your space heating system will need to work a little harder to compensate for this additional cooling demand.
Heat pump water heaters are a fantastic technology for reducing energy consumption. But they are different and have tradeoffs.