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# size high eff. h/p

Member Posts: 123
What is the best way to size up the new high eff. 2 speed h/p's? Use the low speed sen. load in a larger unit or high speed sen. load in a smaller unit.

• Member Posts: 1,380

It depends...

Are you referring to the maximum peak loads or the average load?

If you are referring to an average load, you want the system to operate on low speed. This way, when the system is to operate under design (maximum heat gain conditions), the system can throttle into high speed. Since more energy is used on system startup, there will be less energy used if the system runs for longer periods of time. In addition, it is easier for a system to maintain a constant temperature, which is what will take place with a smaller system running for longer periods of time. This also helps ensure a more evenly conditioned space.

So, the system should be designed to operate at high speed when the maximum heat gain (cooling) is encountered and at low speed during other times.
• Member Posts: 1,380

You want the system to operate at high speed during design conditions (maximum heat gain in the cooling mode and maximu heat loss in the heating mode) and at low speed during periods of average load.

The system should be sized according to the maximum loads that will be encountered, unless the house has AED (adequte exposure diversity), and then the equipment capacity can be lower than the sum of the maximum peak loads for each area.

From your original post, you indicated two options. Using low speed on a larger unit would be proper for average conditions and the system would switch over to high speed when the load conditions reach a maximum. The system should be sized to meet these load conditions at high speed. Oversized systems create operational problems. Using high speed on a smaller unit indicates system operation during maximum load conditions. I am hoping that when you refer to "a smaller unit", the system is still sized to meet the maximum load requirements of the space.

I am just a little confused about your use of the terms "larger" and "smaller" unit.
• Member Posts: 123
size high eff. h/p

Thank you, you gave us the answer we were looking for.
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