At what outdoor temp is a heat pump

TonyS

rated to give its stated BTU's?

Is there a scale of the BTU output as it gets below that temp?

Jack

It is my understanding

That HP's are rated by ahri at 47F. For the heating output curve you will probably have to go to the manuf web site. I think before buying any hp you should compare output performance against your needs/desires and go from there.

Tom Blackwell_2

ARI

The ARI rating system for heat pumps stipulate two rating points. One at 47 degrees and one at 17 degrees. If this isn't bogus enough, the manufacturers are allowed to apply a factor to this data based on the "zone" that the unit will be located in. No additional testing, only a derating factor. Obviously, as outdoor temps go down, the heating capacity also falls. Standard practice is to size the unit for either the cooling load or worse still by gross square footage. This leaves the unit grossly undersized for heating, and supplementary heat is required to make up the difference.  A good way to analyze this, if you have time, is to get the weather bin data for your location and the full rating of the unit of your choice.  Then calculate the number of hours at each temperature, and list the unit capacity and supplemental heat required for each condition. That way you can come up with an operating cost per hour for each condition and play with various unit sizes and supplementary heat sources.  There is a push underway to re-vamp the ARI rating system for heat pumps to be more honest, but don't know if it will happen.

Tim_75

The manufacturer

that I work for, publishes Gross Heating Capacities in tabular form for commercial products (over 5 tons) from -18 degF to 72 degF outside air temperatures (OAT) and for 60, 70, 75 and 80 degF indoor db temperatures (IAT). For residential producs, from 2 to 72 OAT.



Nominal ratings are at 47 OAT and 70 IAT.

Tim_75

Bogus

or just confusing for you?

I have never seen anything other than simple tables showing capacities at various conditions.



The standard practices you mention are not utilized by professionals. There are many examples or poor equipment selection practices and estimations, but that should not reflect poorly on the manufacturers. If the data presented is utilized by a compentent individual, nothing is grossly undersized, or oversized.

Tom Blackwell_2

Rant

Tim; sorry if my earlier post seemed like a rant-just pent up frustration.  I lived in the Atlanta area for years and the aforementioned practices are rampant in the Southeast.  Georgia power rules for "Good Cents" participation in the early eighties was to size the heat pump for 1,000 square feet/ton; in an effort to sell more supplemental heat in the winter. Made for really unhappy homeowners. The rating system also does not take into consideration high humidity levels in the colder months; and as such don't derate for frequent defrost. Obviously a heat pump in a coastal area should be rated differently than one applied in a dry climate.  2 years ago I did a lot of number crunching for a client in the Knoxville area and the best choice for operating cost was 2-speed heat pumps with 90% gas furnaces for back-up. The best would of course have been WSHP, but the site would have made this option prohibitively expensive. Also, by having 2 fuels the owner can choose what to use if either fuel price gets out of range.

Tim_75

Experience

To be honest with you, because of the climate where I work, my experience with heat pumps is limited. Its too cold most of the winter for air heat pumps to be useful and electric resistence heating is too expensive. Water source heat pumps are gaining popularity, though. Since I engineer systems for a living, I generally dismiss the poor guessing practices that are far too common in the market.

meplumber

Enthalpy is the overlooked quotient.

I was an installer in North GA in the late 80's when the heat pump craze started.



Installed and serviced many since, from Ga to Maine.  The number that never gets mentioned is enthalpy.  Enthalpy is the amount of heat that the surrounding air is able to hold.  It is a correlation of temperature and moisture.



It is my understanding (and I could be wrong) that heat pumps cannot work below 7 btu's/lb enthalpy.  The physics of heat transfer don't work below that number.  Hence, when the air outside is cold and dry, most of the winter, the condenser cannot absorb heat from the outside air to change the phase of the refrigerant.  That number is more important than the outside air temp.



If I have it wrong than maybe the good professor will enlighten us. 

meplumber

Jack, any input on Enthalpy.

You are the man Jack.  Could you pose this question to the manufacturers that you rep?

Jack

Just saw this post...

I'll see if i an get some info.