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fleebell,

Interesting idea. I think I see a problem though.

In several of your systems, I believe the fluid in the return line to the heat exchangers will favor the cold side since it is at a lower pressure. It will continue until equilibrium is reached. It can't push against the hot side pressure. I also see nothing to prevent the fluid and/or gas from the hot side from flowing into the cold side.
If I am correct about the problem, it might be solved by adding a pressure regulator at the entry of the cooling coil to keep the pressure low in the cold side. Alternatively, you could place the booster pump at the entry of the heating coil with a check valve. In this way the booster motor could run intermittently.

I also think there is a problem with the temperature control mixing valve. The cold side pressure would be too low to go through the valve against the higher hot side pressure. If you are trying to regulate the heat flow into the system to prevent over-pressure, you could use a temperature operated valve to circulate the hot side fluid into the cold side. If your intent is to slow the system when power consumption is low, you could shunt the excess power into a dummy load.

You might also want to consider a safety pressure release valve to atmosphere for the propane in the event of a fire or other fault. All propane tanks have these. I wouldn't use this in any closed building without a floor vent at the lowest point in the room and gas detector. Any leak (and any system can leak) could be disasterous.

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