Heat-pump hot water heaters Stiebel Eltron Accelera 300 most efficient for Passive House

Domestic Heat-pump hot water heaters (DHW):
Stiebel Eltron Accelera 300 most efficient for Passive House Design

Stiebel Eltron Accelera 300, Domestic Heat-pump Hot Water Heater

In a typically constructed home, domestic hot water heating accounts for approximately 15 to 25% of the overall annual energy costs.  On average an 80 gallon hot water heater will cost anywhere from $400-$700 annually to operate depending upon usage patterns, temperature setting, and cost of electric.

In a passive house, however, the percentage of energy consumed for water heating rises dramatically--as costs to heat and cool the building are substantially reduced.  A lot of attention and construction costs, in Passivhaus, are focused on reducing primary heating and cooling needs.  However for a home to be truly energy efficient, other components which consume electricity must also be addressed.

Typical energy consumption profiles of varying construction designs
Fortunately these choices, as a percentage of overall construction costs, are far less costly to make and yet can signficantly reduce overall annual energy consumption.

We have already touched on the use of efficient LED and CFL lighting and ceiling fans, but the 800lb gorilla in the room, remains: domestic hot water heating.

The Far East countries are way ahead of us on this front.  Having available heat-pump compressors that use high-pressure CO2 (carbon dioxide) as their refrigerant, a material that is far more efficient than the typical coolant used in heat pumps found in North America--try COPs of 8!  Systems to heat domestic water, based upon this ultra-efficient heat-pump technology, are called Eco-Cutes.  Unfortunately these wonderful devices can not be found in the States, try as I did to acquire one.  I hope this changes soon; 800% efficiencies are difficult to conceive when COP efficiencies here generally fall between 2-3.

Since Eco-Cutes aren't available here yet, alternatives must be considered.

We initially looked at solar hot-water systems, comprising of solar panels position on the roof.  In our climate, solar hot water was expected to provide about 70% of our total domestic hot water (DHW) needs, leaving 30% that would need to be supplemented in some fashion.  In the final analysis we simply couldn't justify their high-costs (even with tax credits) for what amounts to a partial solution, anyway.

We briefly considered in-line tankless hot water heaters, but we discarded that notion, because their operating savings over conventional electric hot water heating amounts to merely 15%, not enough, and they require some serious ampage

After some extended consideration, we opted to use a heat-pump hot water heating system. While not as energy efficient, to be sure, as an Eco-Cute, their COP efficiencies can range from 4-5 on the very best units available.

Heat-pump hot water heaters work by extracting heat from the surrounding air and transfering it to their water storage tank.  In warmer climates, heat-pump hot water heaters can (should)  be placed outside for maximized efficiency, but in colder more moderate climates, they really need to be located inside a conditioned space.

The by-product of heat-pump hot water heaters is cooled and dehumidified air.  This present some additional advantages during periods where air-conditioning is desired.  One gets hot-water while getting some air-conditioning for "free."

This works against you, though--during colder months when heat is needed inside--since the heat-pump hot water heater robs heat from the air to heat water (the classic rob Peter to pay Paul).

While there have been discussions as to whether or not it makes sense to do this, we opted to do so because we figured the relative costs to generate interior heat is small--between solar heat gains during the day; waste heat from electronics & appliances; heat given off from the home's occupants (both two legged and four-legged); and high-COP heat sources such as ductless mini-splits.

To be sure, heat pump hot water heaters are more expensive than their conventional counterparts (typically about $600-1000 more than a conventional electric hot water heater of similar capacity), but reduced operating costs are expected to pay back the difference within a two year time frame, sometimes even less.  From then on, it adds up to some real savings over time.

Models we considered were:
Always looking for the best technology, we selected the Stiebel Eltron Accelera 300.  Stiebel Eltron suggests their 80 water heaters can save as much as 80% of DHW heating costs.  In my mind, that had the potential of surpassing savings from using solar hot water systems, at a far far less cost.

Other aspects that attracted us to Stiebel was that they provided a full 80 gallon tank capacity, were German-made (hey we are living in a Passivhaus here) and had an exceptional warranty.

Yes it it true that these units do make a little bit more noise than some others, but we have it located in our basement's mechanical room and its noise levels are not objectionable at all especially when one considers that it essentially will operate for an hour or two per day.

The unit can be configure to operate fully on heat-pump mode (which is cheaper to operate but takes longer to heat) or a blend of resistive heating and heat-pump modes.  The 240 volt unit consumes 500 watts of electricity during its heat-pump cycle and an additional 1700 watts (for a total of 2200 watts) when combined with the resistive electric heating.  The resistive heating mode can be configured to only operate to heat water a certain level, initially, followed by the remainder of heating to be accomplished by the heat pump only.

Stiebel claims that the unit can expend 50 gallons before noticeable temperature drops require heat regeneration.  Furthermore, the interior tank itself is surrounded by the heating system for the purposes of maximizing timely heating effectiveness.  The unit is heavy, weighing in at 300lbs empty.

Exhaust temperatures are typically four degrees lower than their intake values.  Expect a 10ft by 10ft room to cool about 2-4 degrees during its operating period.  The during operation, the heat-pump circulates air at approximately 325CFM

With its heat-pump running, the Accelera 300 creates approximately 3500btu/hr of interior cooling.  So in a typical day, expect about 7000btu of total cooling of the space it occupies.  For that 3500btu/hr, you effectively get 7,000-8000btu/hr of water heating capacity at merely 500 watts input power or about 13,000-14000btu/hr when operating in both modes.

But, all was not rosy.  When we hooked the unit up we didn't realize that the top cover needed to be removed prior to operation--to remove some packing foam protecting the compressor components during shipping.  For a short amount of time, I was appalled at how loud it sounded.  Fortunately, I discovered the error in my ways and removed the packing material.  Oops.

Unfortunately, things didn't improve much subsequently as we found that the water wasn't being heated too well even after extended amounts of heat-pump operation.  We contacted Stiebel Eltron USA and found them quite helpful and supportive.  We ultimately had to have a refrigerant expert inspect the unit and recharge the unit with R134a refrigerant.  Apparently the schrader valve caps may have come loose during shipping which allowed the coolant to slowly leak out.

Things were looking pretty good for about a week, that is until the unit stopped effectively heating water again and ran continuously.

We had our local technician come out again and with guidance from Stiebel's tech support determined that a leak existed in the interior of the unit and that it was not fixable.  To their credit, Stiebel offered to reimburse me for all of the expenses associated with both repair attempts and provided me with a replacement unit at no cost.

Things happen, so we didn't get too upset about our experience, that was until we received the replacement unit.  This one showed damaged on its exterior (some good dings).  After some back-and-forth, Stiebel offered us a discount (in terms of a partial refund) for the cosmetically damaged unit while fully standing by their warranty.

Our friend Jason, who built his passive house in Ohio, had some technical faults with his GE GeoSpring, shortly after he installed it, that also needed repair.

Certainly, these units are more complicated than conventional electric hot water heaters, but we are hoping that their very high energy efficiencies and long-term operational savings will justify their use.

We are also considering some novel (home-grown) alternative ways to heat hot water passively.  I will cover this in a future article.  When it's all said and done, heating our hot water for pennies a day versus dollars per day sure has its appeal.

What we have done is systematically eliminate or significantly reduce electricity consumption with our heating and cooling, ventilation, domestic hot water and lighting.

What remained to be addressed were cost economizing selections of appliances and miscellaneous electronics. Taking this holistic approach we were able to absolutely minimize our consumption requirement (and its associated carbon-footprint).


  1. Hi Bob
    Thanks for all your recent posts. They have been very useful in helping us to decide on our mechanical systems. Particularly, I was leaning toward a heat pump water heater. After reading your experience I will look a little harder at the other options before us. Thanks again for your well detailed posts. I look forward to the next one.

  2. Glad you are finding them helpful!

    I am hoping these articles will assist you and others in their endeavors. Being technical, I am spending a lot of time modeling the actual energy consumption that is taking place in the home during what is turning out to be very cold winter season. We are having extended weeks of <23F temperatures during the day and teens during the overnights. Are minis' COPs drop quite a bit in those temperatures. I believe they are at 1.80-2.0.

    I have also been examining the home with NO heat on during these periods. In that manner, I can really observe the heat we are losing over time and determine the BTUs that we are losing. I believe the home is doing quite well. In future posts, I will be detailing some real technical results, something that I haven't really found on the net. I always prefer empirical analysis, over theoretical ones!

    Despite the headaches we have had with the Stieble unit, I am encouraged by its use. Its efficiency appears quite good. I suspect it is the best of breed at the moment.

    How is your project coming?

    1. Thanks for asking. We are just finishing up all the last details before hitting the local building department for permit. That should be interesting as this will be their first passive house. Hopefully, there'll be no delays there.

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