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View Full Version : Is my thinking correct about HSPF rating

djp0623
01-26-2011, 05:48 PM
My reading has left me with this conclusion.

HSPF is an a rating that says you get this much heat for this much electricity. Does this mean that you could realistically take the ratio of the (1- (old HSPF / new HSPF)) and multiply this factor by your avg winter HVAC usage to get a reasonable cost savings?

junkhound
01-27-2011, 03:10 PM
No, for the simple reason that if your new and old are the same (1-1/1) = 0, your heating bill would magically be reduced to zero...

Becomes more complicated as HSPF numbers change with temperature differentials. I'm old school, and only deal in COP vs. temp differential curves and mollier diagrams, but that is too straightforward for the consumer, eh?

BTW, 1kW-hr = 3412 BTUs per hour. I have no idea what percentage of time at 47F outdoor vs. 17F outdoor ARI uses to calculate HSPF, somebody else may? Then, with COP vs. temp data and your specific climate graphs you could come up with a seasonal savings that would be pretty accurate.

IMO, both SEER and HSPF are both techniques to 'dumb down' thermodynamics.

01-27-2011, 03:52 PM
The long answer is that (unlike COP, which is unitless) HSPF has units of BTU/Watt-Hr. So you can write the equation HSPF = BTU/Watt-Hr (meaning your old system HSPF = the number of BTUs needed to add to your house last winter divided by the number of Watt-hrs of electricity consumed to heat your home last winter). Rearranging the equation, we can write HSPF * Watt-Hr = BTU. Since the BTUs produced by the new system will be the same as the BTUs provided by the old system, we see that HSPF(New)*WattHr(New) = HSPF(Old)*WattHr(old). Rearranging, WattHr(New) = Watt Hr (Old) * HSPF (Old)/HSPF(New). In other words, you heating bill this year will be equal to your heating bill last year times the ratio of the old HSPF/new HSPF, or, as you stated, your savings will be equal to (1 - (old HSPF / new HSPF))* your average winter HVAC Usage.
This assumes that the weather next winter will be about the same as the weather this winter, that you didn't do any weatherization or insulation, and that the cost of electricity is about the same.
Junkhound took a very logical approach to anwering the question, it's just that he didn't see that the formula you proposed was for the savings, not for the amount of the heating bill. If your new and old are the same, then your savings would be reduced to 0, which is not magic.

djp0623
01-27-2011, 04:52 PM
Thanks. I knew it left out a lor of variables, but I wanted to wrap my brain around the rating.

I was trying to decide purchasing a new inverter driven infinity system ovar an infinity 19 was worth it. I think in a colder climate it will be, but in northern Alabam we have so few months of cold weather, that it would be overkill.