# Heat Pump Delta T

| Posted in General Questions on

I have the Manual J calculation for my new house and was trying to trying to figure the CFMs required for each room.  CFM = BTU / ( delta T x 1.08) … just need help with delta T … there is a bunch about delta T for heat pumps in AC mode (approximately 18-23 degrees) but not much for heating mode.  I have seen some information that mentions that delta T should be about 30 degrees in heating mode.  Planning on a air source heat pump in zone 6, possibly a Mr Cool system.

Thanks for any info,
Steve

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### Replies

1. | | #1

30F is a good number to use. For a specific system and location, you can take the heat output at design temp divided by (air handler CFM * 1.08). For example. my Mitsubishi SVZ-KP18NA puts out 21,600 Btu at 5F and is rated for 675 CFM. When I do the math, this comes to 29.6F.

Also worth considering, your ductwork needs to be sized to handle the full flow of the air handler, which may mean that the ductwork is slightly larger than the calculation based on required Btu would indicate.

1. | | #2

Thanks, for my initial calculations I used 27 to get a slightly higher CFM required (like you suggested) to base ductwork sizing off of.

2. | | #3

Note that if your unit will have a variable speed compressor you will only get a 30° delta on the very rare occasions when the unit is operating at its highest speed 11° is much more commonly seen number.

Use 30° for your calculations but understand you will only see that number a few hours per year.

Bigger ducts always work better than smaller ducts but they cost more and take up more space.

Walta

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