I have a geothermal system with Water Furnace heat pump setup with the Desuperheater, where the heat pump provides warm water to a non-powered storage or buffer tank, which then feeds another electric water heater that is connected to the power. The setup is connected per Bergy's diagram that has been posted many times in these forums. The system was setup about 8 years ago and I guess at the time there was no requirement for a thermal expansion tank, so I don't have one. I'm looking to replace the water heater that is connected to a power source with the Rheem hybrid/heat-pump water heater, and I would like to add at the same time the thermal expansion tank. My question is, should the expansion tank be connected to the cold water line feeding the storage/buffer tank, or the cold water line feeding the Rheem that is connected to the power source?
expansion tank should be on the cold water line feeding the unpowered buffer tank - ie before the overall buffer-DSH-finishing tank system. You shouldn't have a "cold water line" feeding the powered unit. The "out" line on the buffer tank should be piped into the "in" line on the powered tank (per the Bergy diagram).
Thank you gsmith22, that's what I thought, and my system is piped per Bergy's diagram and I intend to keep it that way, but would like to add the thermal expansion tank for peace of mind. I was thrown off by another post I came across in this forum (https://www.geoexchange.org/forum/threads/dsh-piping.1160/#post-11245), here it was suggested that the expansion tank should be installed to the hot water pipe coming out of the buffer tank and feeding the cold/in line to the powered tank. And then somebody has posted in a different forum that he had added two expansion tanks, one to the cold water line feeding the buffer tank and the other to the in-line of the powered tank. I suppose there is not only one way to skin the cat, but I am curious about the opinions of people in the know.
Can't remember the theory/why aspect but I have always been of the impression that expansion tanks go before the hot water heating. Hot water tends to go backwards out of the hot water tank into the cold water line as it heats and if your expansion tank is located there, expands into the expansion tank. With an unpowered buffer feeding a hot water heater, the buffer tank can and does get to 130 deg F sometimes (geo cutoff) when the desuperheater is working hard (mid winter/mid summer). If you put the expansion tank in betweeen the buffer and hot water heater, it is essentially in front of the hot water heating aspect when the buffer is above city water/well water input temp. Having it in front of the buffer preserves the general advised arrangement. Would it work if it was in between? Maybe but I can attest to no problems with an expansion tank placed prior to the unpowereed buffer tank on a buffer/hot water heater/desuperheater setup plumbed via the Bergy diagram (waterfuranace has the same diagram in their literature too)
Expansion tank is required by code (at least around here) in the hot water lines after the tank. In any case, cold water lines in, DSH lines, hot water lines out, are all one circuit from a pressure point of view, unless you have a check valve in there somehow.
