Has anybody found a decent explanation of how to correctly size a heat pump? I have seen various rule of thumb but no definitive information.

Regards, Paul

Posted By: PaulDHas anybody found a decent explanation of how to correctly size a heat pump? I have seen various rule of thumb but no definitive information.

The best rule-of-thumb I've heard of is to never use a rule-of-thumb. Anyone who uses a rule-of-thumb method is doing a dis-service to their customers.

In the US, the correct method is to perform a "Manual J" heatload calculation (this is for both cooling or heating). Without this, it's just guesswork. Now there did used to be some rules of thumb that could be applied after the Manual J was performed to determine the ratio of cooling load to heating load (and accounting for latent versus sensible cooling too) so as to not oversize the equipment. This is more of a problem in a heating dominated climate where some cooling is needed - if sized for 100% of the heat load the heatpump will be oversized for cooling (unless it's a multi-stage unit as is now becoming the norm).

Of course, the raw peak heatgain/loss figures are just the starting point. The habits of the occupants have to be factored in: do they program in temperature setbacks during the day? What performance of setback recovery is required (to ensure that auxiliary heating isn't inadvertently engaged) etc.

A decent Manual-J program will be able to factor in the HDD data ... and programs such as hot2000 also have climate data available - though hot2000 doesn't do room-based calculations in some of its versions so it can't necessarily be used to size the heat distribution equipment - which is also key to a successful installation.

Paul in Montreal.

p.s. see http://www.acca.org/design/ for more details about Manual-J