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Reverse Indirect (TurboMax or ErgoMax?) with condensing boiler
I'm helping a friend who's rebuilding a large house in the Boston suburbs after a fire.
It looks like the heat loss on the house rebuilt with modern insulation and windows will be in the low 200s. There's fairly high DHW demand -- my friend has 6 kids and 2 or 3 are often home, there will be fancy high output showers, multiple laundry machines, etc. The GC's preferred plumber is suggesting a pair of Buderus or Viessmann boilers for a total of 500KBTU of output, and a 119 gallon reverse indirect (ErgoMax I believe) for DHW.
I'm...skeptical. I manage large residental properties and we've done a lot of boiler and DHW upgrades. We had a proposal several years ago involving TurboMaxes and buffer tanks and the Mech E on the job finally intervened with a pretty convincing calculation showing that running our heating loops up to 180F to store energy in the buffers/TMs (which after all are just buffer tanks with "tankless" DHW coils in them, right?) would basically ensure we almost never condensed in cold weather -- eliminating the point of our fancy new condensing boilers (these were large commercial units -- Bryans -- not cheap). I have another property we "inherited" with condensing boilers and buffer tanks feeding DHW exchangers and it's a constant problem -- the buffer tanks can't store enough energy to avoid having to haul the HHW loops up to "storage temperature" all the time to charge them, and we get nasty overheating we can't lick and a lot of wasted fuel.
I suggested to my friend that he might want to look into a more conventional indirect setup, maybe a double-coil rig or just two single-coil tanks, if he really thought he needed that much DHW and he was going to buy a huge fancy condensing boiler for his house. Or just get a commercial condensing DHW heater (my "preferred plumber" is partial to the HTP Phoenix) and a more reasonably sized boiler for heating. But his plumber thinks the reverse indirect he wants to sell him is the best thing since sliced bread and swears even though the manual says you have to send it 180F water, the boilers will still condense.
What's the real deal here?