Replies

1. 
   Clewless1 | Dec 14, 2009 10:14pm | #1

   I think I've heard similar
   I think I've heard similar for the heating side ... 2-3% savings ... but that is a generalization. Set it for your comfort range and your budget. Saving energy shouldn't be a lot about sacrifice. If you are looking to save, think about improvements to the house that will do you well (e.g. air sealing should be high on the list).

2. 
   DanH | Dec 15, 2009 07:34am | #2

   The savings on the heating side would be about the same as on the cooling side -- maybe a bit more, depending on temperature differential.

   But keep in mind that you're likely not saving anything if you turn the temp up when you're cold and trigger the backup electric heat. So be sure set it warm enough to "live with".

   I'd personally find it difficult to live with colder than 66 or so, but I'm one of those old farts who needs things warmer.

1. 
   gj13 | Dec 15, 2009 01:38pm | #3

   Thanks, DanH and Clewless. We did some figuring at home and estimated it would be about an extra $20 per month to keep it more comfortable, in other words, we weren't saving much at all.

2. 
   Nuke | Dec 16, 2009 06:35am | #4

   At what point does the air-sourced heat pump start to become less effective at warming? Most heatpumps come with a dedicated electric booster (heating element), no? I know mine does (it is in the basement for the basement).

1. 
   gj13 | Dec 16, 2009 09:49am | #5

   I've seen figures online that
   I've seen figures online that say outside air temps from 40° to 47° require the auxiliary heat to kick in.

1. 
   mrsludge | Dec 17, 2009 09:59pm | #6

   We have a dual fuel setup. Makes you much more cognizant of when it's switching over to the gas furnace. The thermostat is set to switch over below 30F. It was down around there the other night and I could tell that barely any heat was coming out before it locked out the HP. Newer models perform better at low temperatures (ours is 5 years old).

   Remember that a HP moving heat around is generally going to be more efficient than making heat from scratch (via electricity or gas). Even at the low end of efficiency, a coefficient of performance of 2.0 makes 2x as much heat as the power it uses. Whatever you can to to optimize when the strips get used will save you $$.

2. 
   DanH | Dec 18, 2009 07:25am | #7

   It depends on the design of the unit, and there are three factors at work:

   1) Though the HP may still be a "win" at some temp it's effectively undersized (due to both increased load and decreased output) and needs "help" to maintain temp.

   2) Efficiency does drop as outside temp drops, but there's no real "cliff", just a gradual decrease until some point when it's more efficient to use alternative heat sources.

   3) However, there does come a point when the unit becomes too cold to do a "cold start".

   #1 is usually the first factor to kick in, but it obviously depends on the size of the unit vs the heating load.

   #2 depends somewhat on the design of the unit, and I'd guess that, for electric resistance backup, the crossover point is 25-35F. Higher, obviously, for stuff like natural gas.

   #3 also depends on the design of the unit (and the size of the sump heater) but likely ranges from 0 to 25F.

1. 
   HuskiesHowls | Dec 19, 2009 11:45am | #8

   We just put in a hp with natural gas aux 2 years ago, and was told the hp would go down to about 20°. Since then, I've been watching the temps when its cold, and some nights at zero, it still works great.

   The hp was put on the west side of the house, and when its sunny out, its been working great at -10°F. However, when its bringing the temp up, from a setback, the aux heat almost always comes on, since the hp can't produce enough heat quickly enough.

   I guess it all depends on they design, and install, along with environmental factors. (Maybe it works great at cold temps 'cause the dogs are laying around it???)

   Dick