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I recently had my oil burner serviced by my local HVAC company for the winter. During visit I advised tech that I was losing hot water to the house with the water being luke warm to cool. Tech checked out system and stated due to winter heat demands on the oil burner I was losing out on hot water and suggested a indirect hot water heater be installed. I have had this probelm before which was a mixing / temp. valve issue. I recontacted my HVAC company and they sent out the tech who replaced or repaired the mix / tempering valve. This did not correct the problem. I am getting extremely hot water out of the coil to the mixing valve. This was checked while the hot water was running while a family member was taking a cool shower. There was no loss of temp. coming out of the coil. After the mixing valve my domestic hot water pipes are warm but not hot. I don't think mix / temp valve was replaced, I think a repair kit was used by the HVAC company.