I am looking to install a geothermal heating system to provide hot water and heating in a small home. I am aware that constant cycling of the geothermal heat pump is not desirable, so it is usual to use a buffer vessel to store residual heat for the heating loop. The problem is that we are very constrained on space. We also intend to use the heat pump to provide hot water.
I have found some hot water cylinders that have dual heat exchangers. These are often used when you have multiple heat sources, but I was thinking that there is no reason i could use one of the exchangers as a sink instead of a source, and use it to heat the underfloor heating loop. Then the hot water itself is used as the buffer to reduce cycling.
The hot water cylinder thermostat that would be used to control the geothermal heat pump, aiming to keep the hot water at a sufficiently high temperature. The room thermostat would control the circulation pump on the underfloor heating loop.
Please see the attached diagram (I have ignored expansion vessels, etc, for simplification):
Logically, this seems like a sound solution to me: it avoids the need for a dedicated buffer tank, reduces the cycling of the geothermal pump, and eliminates the need for complicated valve controllers to efficiently balance the distribution of heat between hot water and heating.
Am I missing anything obvious? What are the caveats to a solution like this? Any advice appreciated.