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Old 10-21-2014, 11:34 AM   #11
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Brian, your discussion of where the electricity comes from, efficiencies of converting other energy sources into electricity, etc were all part of my reading and discussions with the people I talked with and the reading I did before buying the new furnace/heat pump system.

First off, our main heat source is passive solar. Any sunny day, in even below zero weather, heats our home with little or no extra energy other than some fans to move the heat around at bit. So, for us, the heat pump or furnace just needs to keep the house warm until the next sunshine hits us.

Secondly, in our area, our electricity is primarily coal generated. In my discussions with our electric coop people, they told me that coal plants are essentially on or off. They have to make sure that they have the ability to meet maximum demand at all times. If they have demand over their capacity to supply it, they have to go onto the open market and buy it from whoever has excess and at the price that that seller sets. If they have to do that too often, then they need to build more generating facilities. (They are working to bring in more wind power, and I am pushing them toward coop member owned solar, but that is another story.)

So, their point was that the more consumers they can get on the demand control system like ours, the less likely they are to need to build more generation facilities. Whenever demand nears capacity they send a radio signal out to my control box that switches off electric flow to my heat pump and we use the LP furnace. When they are allowing power to my heat pump, it is essentially excess energy that is being produced anyway whether their customers need it or not - due to the nature of coal generators and being ready to meet max demand.

Thirdly, I would love to be able to use natural gas, but the chance of someone bringing a natural gas line out to our rural home is about zero.
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Old 10-21-2014, 02:47 PM   #12
grc
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thoer,

Up until yesterday I knew heat pumps did not work at or below freezing, however I was not aware of a new technique. So now I would have to say traditional heat pumps have that freezing limitation. Checking on this yesterday found that Mitsubishi has developed their flash-injection technology that allows their heat pumps to operate below freezing. It does not appear that Trane or Carrier heat pumps use this as they are still talking about using an auxiliary heat source. For Mitsubishi, after the refrigerant takes the primary pressure drop the flow is split feeding some back to the compressor and some to the outside coil. Each of these split flows also take another pressure drop. It looks to be operating on three different saturation lines/curves. But the primary point is that they get their heat from outside air even below freezing by having the refrigerant temperature even colder.

The link below has a schematic video explaining their flash injection:

You wouldn't believe the things we do. - Heat Pump Technology - Mitsubishi Electric
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Old 10-21-2014, 03:55 PM   #13
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Grc,

Thanks for that link to the new Mitusbishi models - it sounds very promising. The video on that page does a very good job of explaining the principals involved. I enjoy talking about newer heat pumps as I think most everyone has heard horror stories about the older ones and I think it is a technology that is now frequently overlooked because of that.

Even our 4-5 year old Amana, without the very latest systems has worked well for us below freezing. http://www.amana-hac.com/Portals/1/pdf/SS/SS-ASZC18.pdf

We were lucky enough to get an Amana rebate, an income tax energy rebate and a rebate from our electric coop that meant we eventually only paid 60% of the original cost. The other advantage has been a huge energy savings in summer over our old much less efficient central air conditioning unit.

I would say the main disadvantage with our Amana is the defrost cycle. When we bought ours, Amana still used a timed defrost. I was told that they had experimented with a system that only defrosted when needed, but that they were not satisfied with the reliability of it and went back to the timed cycle.
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