We hear this all the time:

“My heat pump is 4 tons”.

“I need a 5 ton unit for my house”.

This is based on the 12,000 btu’s = 1 ton of heat pump capacity.

Also many manufacturers (almost all) label their units in a manner that suggests the unit capacity of their heat pump1:

  • Watefurnace ND049 = 49,000 btu’s = 4 tons
  • Climatemaster TT049 = 49,000 btu’s = 4 tons

 Heat Pump Peformance Data

Two similar comparing units until you look closer at the specifications.  In order to achieve 49,000 btu’s, the Waterfurnace requires 15.5C entering water temperature (EWT) while the Climatemaster only requires 10C EWT2.   Big difference, but in either case, not really one that is achievable in my climate locally in winter.

In the thick of winter, the EWT may be 0C (depending on design).  The Watefurnace now has a 37,500 btu capacity and the Climatemaster has a 38,300 capacity.  Both now approaching 3 ton capacity.

I have shown two of the industries more efficient units and respected and proven companies.  There are a multitude of other options out there and, in some cases, the manufacturer labelling is not a good representation of the heat pump output3.  I can find units with labelling that only gets 45,000 btus (less than 4 tons) of heating out of 15.5C EWT.

If you are buying directly from a manufacturer as a building owner, you need to be very careful about this purchase.  You can not size a unit, if the manufacturer only gives you the btu output based on one temperature.  You need a broad range table that allows you to interpolate heat pump output correctly.

In summary, talking about the size of a heat pump in tons is good for generalities.  But that is where the usefulness ends.


1 – I have roughly interpolated the heating side only, but used similar variables for both units.

2 – To be fair, the Watefurnace 7 NV048 series reaches 48,000 btu’s at 4C EWT.

3 – I should name some companies, but I’ll try and keep it civil.  Chances are if  you can purchase directly from the manufacturer without being an installer, you need to pay attention.