I'm of two minds about this. I see Joe's point about not adding running hours to an expensive heat pump. But what really kills compressors? I'm not convinced it is all about running hours. My Mom has a fridge in a cool New England basement whose age well exceeds my 44 years, and while it enjoys a mild basement it gets defrosted about as often as we change presidents.
I suspect short cycling is hard on compressors - an analogy would be to compare an urban taxicab to a long distance freight truck. The truck will go a million miles, but the cab is lucky to last a tenth of that.
Air source heat pumps seem to die from bad outdoor conditions - thick oil or refrigerant slugging in winter, excessive heat and pressures in summer, UV degrading paint, plastic and insulation year round, dampness corroding metal parts, fins clogging. Then there are errant landscapers and dogs lifting a leg on them...the list goes on.
My geo ran 2000 hours last year. It may have experienced 5000+ starts in that year. (I have come to wonder a bit about the effect of a 4 zone system on a unit - more starts and shorter cycles?) I expect it to run for 20-30 more years since it lives in a mild dry indoor area and runs at less than half of FLA.
Would adding several more starts and a couple more running hours per day dedicated to hot water substantially shorten its life? Could the effect of this be minimized by plumbing a sufficient large storage tank so as to guarantee fewer longer cycles for hot water?
In future I envision smarter, more integrated units that might respond to a need for hot water by precooling or slightly overcooling a zone that the controller has 'learned' will shortly need cooling anyway, pumping that heat into the water tank. I expect better humidity controls slowing blowers way down to just above the ragged edge of evaporator icing.
DOE figures the national average cost of heating household water with electricity at about $500, with wide regional variation. A properly deployed desuperheater may halve that figure, and dedicated hot water production may halve it yet again. I'm confident that can be done without substantially reducing compressor or system life, but it will require careful design and installation.
If the so-called smart grid cometh and saddles us all with demand and time-of-day charges, it'll rock our world, but the added complexity will be an opportunity for smarter systems specified and installed by smarter contractors adding much more value.
Just a few thoughts...
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