“Buffer tanks” are used when it is necessary to “buffer” water that is heated by a heat source so the heat source does NOT short cycle. When low thermal mass heat sources are combined with zoned hydronic radiant distribution systems, it is possible for the heat source to short cycle when only a couple of these zones call for heat. This occurs because the rate of heat produced by the heat source is much greater than the rate of heat used/released by these zones. Low thermal mass heat sources in combination with low volume hydronic circuit and distribution systems cannot absorb this excess heat rate without experiencing a rapid rise in temperature. This causes the heat source to reach its high temperature limit very quickly resulting in very short on times and short cycling. While electric heating elements may tolerate short cycling, gas valves, oil burners, ignition systems, and compressors will have a significantly reduced life expectancy. You can find buffer tank installation schematics in John Siegenthaler’s Modern Hydronic Heating and we have free DIY buffer tank design software on our website:
Borst Buffer Tank Design Software “Direct water tank heaters” are heated by their own heat source, either by a burner or an electric emersion element. “Indirect water tank heaters” are heated indirectly by the building heating system heat source. “On-demand tankless water heaters” heat water only as actually required in real time. On-demand tankless water heaters are by far the most efficient, but may not perform adequately when large flow rates of hot water is required (e.g., several folks taking a shower, doing laundry, washing dishes all at the same time) so they are best suited for small families. However, this flow rate limitation can be somewhat mitigated by using multiple on-demand tankless water heaters and a load shed system. Direct water tank heaters are the next best in terms of efficiency and can handle larger flow rates until they run out of hot water. Indirect water tank heaters only make sense where the building heating system heat source operates nearly year-round. It doesn’t make much sense to use a building heat source to just indirectly heat domestic water.
It sounds like your system has or had a buffer tank and now you have an indirect water tank heater thanks to your plumber. Personally, I am a strong advocate of always keeping the domestic hot water heating solution totally separate from the building heating system solution. Personally, I am NOT an advocate of ground-based heat pumps systems (often mistakenly called geothermal) because the return-on-investment is most often non-existent. Real geothermal requires having a hot spring under your property like you will find in Yellowstone and that would be as good as it gets... These days, air-based heat pump systems (e.g., mini splits) are nearly as efficient as ground-based heat pumps systems and are more likely to have an achievable return-on-investment break even point.
I would recommend finding someone who is very knowledgeable about ground-based heat pumps systems in general and who has actual significant personal experience installing Climate Master systems. When buffer tanks are required, they are absolutely required and they should not be bypassed. When buffer tanks are NOT required, the money should be saved for better use.