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Adding DHW capacity to Bosch Greenstar Combi

We had our Bosch Greenstar combi condensing boiler installed a little over 2 years ago. Our previous home had a 40 gal indirect DHW tank and we never lacked for hot water. Even though this Bosch is rated at 4GPM we are finding ourselves having to time washing and dishwasher use outside of shower and bath times. So we are thinking of either separate HW heater or an indirect tank.

Here are some additional details...the boiler is 150k BTU (input, 134k output) and feeds a century-old gravity loop with 9 cast iron radiators (house is 2 floors, 3 bed, 2.5 bath, 1600 SF). We are getting quotes now on replacing the gravity loop and having the radiators piped directly to a manifold system with O2 barrier Pex, and at the same time splitting them into two zones, upstairs and downstairs.

Since we are adding a zone, a heating contractor suggested just adding a 3rd zone and converting to indirect DHW with 40 gal storage tank, and abandoning the DHW coil in the Bosch boiler. He then went on to mention that another option that would be less expensive to install is an electric water heater unit in series after the Bosch. So the DHW output of the Bosch would become the input to an electric water heater, effectively making the heater a storage cylinder. It should use minimal electric since it is only maintaining water temp during periods of inactivity, such as overnight. Here on Long Island electric is very expensive where natural gas is relatively cheap, so energy use is a concern.

That got me thinking about why you wouldn't use a similar series approach in an indirect setup rather than abandoning the Bosch DHW altogether?

Occupancy is 3 adults, me, my wife and adult daughter, and the usual amount of showers, dishes and laundry. We will probably be here another 5 or so years, so I'm trying to balance capacity, efficiency, and upfront cost. One quote said it would be more expensive to add indirect DHW, but cheaper to put an electric unit in series after the combi. Heck I can buy a water heater at Home Depot for $400 and plumb it in myself, but if it's going to drive up the electric bill then I may as well just spring for the indirect and be done with is.

All comments and opinions welcome on this one...

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