Quote:
Originally Posted by RobertSmalls
...I've heard several reports (one of them carefully controlled and documented) of heat pumps using more energy when allowing the house to cool at night and when noone's home. I know that some heat pumps will engage the auxiliary electric heater (30-50% as efficient) if they see more than one degree difference between the temperature and the set point...
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Mini-Splits are an Asian development. They come from the land where energy is very expensive, and the pool of engineering talent is absolutely huge. So there has been so much technical focus on improving mini-split efficiency, that it might not be a good idea to lump them in with the category of all air-source heat pumps any more. It would be like making estimates of personal automobile fuel costs based on national fleet mileage averages, when in fact you are driving an Insight hybrid car.
For instance, most minisplits being sold now, use variable speed fans and a variable speed compressor (AKA: Inverter Technology) to adjust their output to a particular load situation, rather than cycling on and off around a temperature set point.
I think that in time the Inverter Technology will appear in whole-house heat pumps, but under a different name. I understand that some new European construction uses continuously variable fans for ventillation systems.
Regarding the aux resistance heater, my mini-split doesn't even have one and I suspect Xringer's unit is the same.
There may be another reason that the resistance wires, in the unit you mentioned, switch on when a greater than two degree situation is sensed, that is that the air that ASHP's put out is not nearly as hot as fossil fuel (parched air) furnaces put out. So the resistance wire may be functioning to accelerate warm-up, even though efficiency is lowered. Cold people can be so impatient.
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But going back to the topic of this thread, the set back timer on my minisplit only goes down to 60 F, otherwise, I'd set it down to 50 F at night.
Regards,
-AC_Hacker
P.S. As another indication of the amazing efficiency increases of mini-splits, if you were the kind of person whose world-view was broad enough to consider that operating cost is an important consideration of heating alternatives, but that CO2 output is even more important, consider this:
All heat pumps use electricity. In almost all cases that electricity comes from coal, the most used fossil fuel in the world. So, when a heat pump is used it may be well over twice as efficient as resistance heat, but the CO2 produced is at the rate of burning coal. This situation is made worse because not all of the energy from burning coal is converted into electricity (conversion losses), and not all of the electricity sent from the power plants arrives at our homes (line losses). So when we flip the switch, we're burning coal.
Now, using
this very handy fuel cost calculator from Build It Solar, which takes into account typical conversion losses, and line losses, I entered current local Natural Gas, Propane, Fuel Oil, and Electricity costs and came up with this:
For the mini-split efficiency numbers, I used the Fujitsu Halcyon which has a Heating Season Performance Factor (AKA: HSPF) of 12. This converts to COP like this:
COP = HSPF/3.412 = 12/3.412 = 3.5169988276670574443141852286049
COP = 3.52
Efficiency = 352%
But look at not only the cost numbers, but also the CO2 produced in operating that mini-split. Even considering conversion losses, and line losses, a really good mini split only uses about 1/3 as much coal as if you burned it right in your own home. It is pretty impressive.
However, the Halcyon is only good down to 5 F, so it's not for everybody.
Regards,
-AC_Hacker