Domestic Hot Water w/o Gas for an 7 unit apartment building…

January 28, 2012


Most of our new projects have no connection to the City’s gas piping, so we need to find an efficient way to produce hot water. First we were thinking about a solar thermal system with electric resistance backup, but the more we learned about it the more we realized that they are quite complicated: They need a pump to circulate the water from the panels to the storage tank, plus need to drain at low temperatures so the pipes don’t freeze… and in summer they can overheat. Here is an excellent overview at GreenBuildingAdviser.

Next we looked into Electric Heat Pump Water Heaters. We previously used a 50 gal GE GeoSpring on our project at 174 Grand St, but we then found that the US branch from Stiebel-Eltron sells the 80 gal Accelera and A.O.Smith the 80 gal Voltex.

While reading up on the specs and differences of the 3 products we found a field test done by Steven Winter Associates. It turns out that the Stiebel-Eltron outperforms it’s competitors with an average Coefficient Of Performance of 2.40. To be fair, it has to be said that the Accelera retails for $2,500 and is the most expensive of all 3.


In an attempt to keep the system affordable  we decided to go with a communal solution where all 7 apartments get their hot water by three 80 gal heaters which have a parallel connection and are located in the unconditioned basement — one of the known advantages of heat pump water heaters is that they dehumidify damp basements. If it turns out that the total 240 gal capacity is not sufficient, it will be easy to add another heater… and if one breaks, the building will not be not without hot water.

We still have to decide if we will install BTU meters to measure the individual usage.

If you have any questions or suggestions, we’re all ears:


Here the complete report

Stiebel Eltron Accelera 300

AO Smith Voltex Hybrid

GE GeoSpring

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