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Hey Ed, Explain the Acceptable Over-Sizing on a Single Stage Two Speed and a VRV Heat Pump.

In this edition of "Hey Ed," Ed explains if it is acceptable to oversize a Single Stage Two Speed and a VRV Heat Pump

Hey Ed, explain the acceptable oversizing on a single-stage two-speed and a VRV heat pump. Is that a trick question? I think it is because there is no acceptable oversizing of two-stage or VRV equipment. There isn’t. And if you want to say, well, there’s condition B, and yeah, you have a point if you have a sensible heat ratio of the structure of greater than .95, there is more wiggle room to oversize those situations. But let’s face it: the majority of us work in climates where condition B doesn’t exist. So, this is another one of those banes of my existence. I will hear people say, I oversize a little bit; it’s okay; I put a two-stage in. No, it’s all about our goal. And our goal is to maintain 75 degrees and 50% relative humidity on a design day. When you oversize equipment, you lose the ability to do that. Now, people say, "oh, the two-stage system, look at the sensible heat ratio of a two-stage system when it’s running in low speed." If you look this stuff up, you won’t be making these statements that it’s okay to oversize. Same thing with the VRV equipment. Some manufacturers, or let me say a few, are starting to publish partial speed capacities, but for the most part, they don’t. And it brings us back to this thing in our industry that we’ve been doing for decades. We spec equipment that we have no idea what it does. Did you hear what I just said? We specify equipment. We have no idea what it does. What is wrong with us? But we do it. I’ve done it. Maybe I’m a little bit ashamed. Maybe I’m not. But it’s something that as an industry we don’t have to do. Do the proper calcs, select the proper equipment, and enjoy the predictable results you’re going to get from the fruits of your labor. And that’s the way I see it.

Ed Janowiak is the Manager of HVAC Design Education at ACCA.

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