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Old 01-13-16, 06:15 PM   #21
jeff5may
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With the system you propose, it doesn't need two reversing valves. Simply running the compressor discharge pipe through the water heater before it reaches the reversing valve will yield you ample hot water for all intensive purposes. In nearly all home systems, the space heating and cooling load requirements are much larger than the water heating capacity. Unless you are using your hot water for some other form of heating, that is. Or, say you have a family of eight girls, who take hot baths everyday. Or some other abnormal demand.

For the system you have drawn, you would need to size the water heating coil large enough to digest all of the compressor capacity to ensure that the TXV's would see only liquid. As the water tank temperature rises, the system would yield lower and lower performance until the demand is satisfied. Also, the rig would run short cycles at a high temperature differential to satisfy the water heating needs during periods of low (or no) demand. More importantly, the system cannot heat water and air at the same time. Some control would have to be devised to prioritize one over the other, and switch between them. You would definitely want to run some sort of high-limit sensor on the discharge line to make sure the compressor would not burn itself up or rupture some plumbing.

I'm not saying you can't do it that way. If you size everything correctly and account for the unknowns when this or that happens, the rig could do a good job for a long time. I'm just saying that a short (10 - 15 minute or less) cycle time will eat away at your energy efficiency.

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Old 01-14-16, 06:45 AM   #22
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Jeff you are absolutely correct. That is over kill for a water heater. Just wanted to share it to maybe inspire someone else for some other project. I will end up doing the de superheater idea as it is much more proctical and much less complicated. I haven't pulled the trigger on any parts yet as I told myself I need to get the oil heater running first and then I can play with this project. But I might order the parts just because they are cheap enough and I would hate for them to not be available when I go to start the project.
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Old 01-21-16, 07:09 AM   #23
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So I've been thinking about running r134a instead of 290. Looking at the other chart it looks like r290 needs 240psi to make 124* and 134a needs 181 to get 124*. Any particular reason people don't run it instead of r290 other than part availability like txvs and the cost of buying the refrigerant? Just curious.
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Old 01-21-16, 07:01 PM   #24
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R134a can be used to do the same thing, no problem. There are lots of manufactured units that use it, especially in systems with high delta t between sides. GE makes the geospring hpwh, and automakers have been using it in millions of autos and mobile refrigerators since R12 was phased out in the 80's. If it didn't work well, the majors would have used something else that did.

The big main difference between R134a and propane or R22 is the oil. R134a uses synthetic oil. It doesn't like water either. Then again, it is low flammability and lower pressure gas. There are literally millions of spare parts in auto salvage yards everywhere, as well as in scrap yards, in all imaginable capacities and shapes. Almost as common as old R22 stuff.

R134a isn't very expensive as far as refrigerant goes. It can also be bought without an EPA license. I found the generic cans at big lots a year or two ago for like 6 bucks a can. The super frosty freeze stuff is just plain refrigerant spiked with a little butane or isobutane to lower the vapor pressure somewhat.
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Old 01-21-16, 07:36 PM   #25
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well i guess the main reasons why i was thinking about going this route.

1.flammability
2.could theoretically run a higher pressure to heat H2o to keep the C.O.P higher? or do i have that wrong?

say run it at 210psi thats in the 130F range. as the water heats up to 120 you could shot it off to keep the Delta T higher?

i think i had another reason but i cant remember what it was
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Old 01-21-16, 08:12 PM   #26
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There are 2 main things that affect your COP while the unit is running:

1. Temperature difference between evaporator and condenser

2. Volumetric efficiency of compressor

The pressure difference that the compressor has to work against varies directly with the temperatures exposed to the two heat exchangers. As this difference rises, COP drops. Volumetric efficiency is not so simple. Most compressors are built to operate in a certain envelope. As you go towards one edge of the envelope from the middle, some don't suffer much and others do. You can look at charts and graphs all day, but the only way to tell how a certain compressor will act is to make it do it and watch.

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