Surprise! Your heat-pump water heater has a filter that needs to be cleaned. But you probably already knew that if you’re reading Green Building Advisor. It’s a thin little filter whose primary job is to keep larger particles from getting into the heat-pump guts of your water heater. It will do a sort-of OK job at keeping the heat-pump coil clean, but it’s not great. And worse, the filter doesn’t fit tightly in the slot, so some air will bypass the filter. So let’s talk about heat-pump water heater filtration today.

The standard heat-pump water heater filter

Gary Klein, one of the most knowledgeable people in the world on the topic of hot water, recently came to Atlanta and stayed at my house. I took the opportunity to shoot some video with him, and here’s part 1 of the two videos we made on the topic of this article.

Yeah, it’s embarrassing to ‘fess up to having such a dirty filter. But it’s been hard to keep up with it during our year-long basement renovation. Actually, that’s a lie. It’s not that hard to clean the filter, and I’ve been surprised how clean it has stayed during much of the work. The reason it was so dirty here is that we’d recently had drywall installed, and that’s quite a dusty process.

A serious filter upgrade

Now let’s get to that better way of filtering the air in a heat-pump water heater. My friend Ben Knopp in Virginia has installed the only external filter on a heat-pump water heater that I know of. The video below discusses the issues and shows what Ben did.

Voilà! There you have it. The photo below shows the water heater behind that opening in the back of this closet and the MERV-13 filter attached to it.

Yeah, the filter had collected quite a bit of dust at that point, but there was no significant effect on airflow. And now you’re wondering what the airflow rates were, right?

Before and after airflow rates

Ben and I gave a little presentation together at the ACEEE Hot Water Forum in 2022, and Ben provided the airflow rates he had measured. First, here’s what it was in the clean stock filter.

Then he threw away the stock filter and did his retrofit. Because of the little bit of ductwork and the more resistive filter, the airflow rate decreased, but only to 121 cfm, or 3% to 7%. In terms of pressure drop across the clean MERV-13 filter, he measured 0.049 in. of water column (iwc).

After more than three years of use, he took the photo below and again measured airflow and pressure drop. This time there was hardly any change in either. The airflow dropped from 121 cfm to 119 cfm. The pressure drop went from 0.049 iwc to 0.051 iwc.

His first MERV-13 filter was sized at 12 in. high by 24 in. wide by 2 in. deep. Then he went a step further and installed a MERV-13 filter with the same height and width but double the depth at 4 in. He calls his first one a “five-year filter.” He says the 4-in. filter is a “lifetime filter.”

How to retrofit for an external MERV-13 filter

If you want to upgrade the filtration of your heat-pump water heater, you need to be able to add ducts to it. The Rheem generation 4 that both Ben and I have came with takeoffs that make it easy to connect ducts. You can see the takeoff on the exhaust side of the heat pump in the lead photo at the top of this article.

Above you can see Ben’s intake duct connected to the takeoff. He used two 45° elbows and a piece of flex duct to get over to the filter box shown below.

Then he popped a 12-in. by 24-in. by 2-in. MERV-13 filter in there and let ‘er rip. Since it was in a closet, he didn’t bother with the grille, but you could put this in a more public area with the grille on it.

And then there’s one last, important step. Be sure to remove the stock filter and seal the slot. You can use a foil-faced duct to do that. Clean the metal surface first, though. If you don’t seal that slot, you’ll get air sneaking in behind your nice, new high-efficiency filter, defeating the purpose of making the change.

Easy peasey, right?

The benefits of better filtration

I mentioned above that going with better filtration keeps the coil clean, but let’s list the benefits here:

1. Keeps coil, blower, and other internal parts clean
2. Reduces maintenance from regular stock filter cleaning to changing the MERV-13 filter once in several years or longer
3. Improves your indoor air quality by removing more particulate matter

On that last benefit, Ben makes an excellent point. “I like to take advantage of any appliance that moves air to do better filtration and improve my indoor air quality too,” he says.

Do you have a heat-pump water heater? Have you checked the filter? Have you done a filter upgrade like this?

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Allison A. Bailes III, PhD, is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.