Choosing our Hot Water Heating System

If you're building a net-zero or green home, you need to pay attention to the energy details.  One of the largest sources of energy use in a home is the hot water heating system.  Thus, it is one of the key areas to consider  carefully.  There are a few likely candidates for hot water heating, and, if you're remodeling or building a net zero or green home these are the ones you'd most likely consider:

• Tankless on-demand electrical hot water heaters
• Solar thermal hot water heating system
• Hybrid heat pump hot water heater

Tankless on-demand electrical hot water heaters work by heating hot water as-needed, instantaneously using a small heating unit, when hot water is needed.  These can be gas or electric powered.  We investigated using an electrical unit since our home will be solar-powered and we wanted as much of our energy usage powered by electricity as possible.  One of the considerations is when doing Title-24 calculations you'll find that these systems are rated at a 0.9 energy factor for minimum efficiency, which is not that great.  That's because normal electrical resistance heating is not considered to be that efficient for Title 24 calculation purposes, even though there are potential real savings due to only heating water on demand.  Another consideration with tankless units is you may need several units in order to satisfy the load and also to reduce the time needed to run water before the warm water reaches the faucet (unless you're able to place one single hot water heater with sufficiently high heating capacity very close to all of the areas that need hot water).

Solar thermal hot water heating makes use of roof-mounted panels which heat waters held in pipes that are heated naturally by the sun to bring the temperature to a higher level so that the main hot water heater needs to do less heating work.  These systems require quite a bit of additional costs in terms of installing the panels and pipes and from my perspective, add more complexity.  Typically this may add a few thousand dollars to the cost of hot water heating systems (likely in the $1500 to $3500 range or higher [reference here]) so the cost-effectiveness of such a system may be hard to justify.  You may be able to get rebates and tax benefits from the system which can help offset some costs, depending on your area.

Finally, perhaps the most interesting system, and the choice we made, was a hybrid heat pump hot water heater.  These systems typically use electrical heating, however, they add a small, efficient air-source heat pump to the hot water tank which is able to make use of energy in the ambient air to generate heat more efficiently.  In fact, our Title 24 calculations show it has a 2.4 energy factor for minimum efficiency.  We chose the 50 gallon GE GeoSpring hot water heater which is one of the most efficient hot water heaters available.  Although it's more expensive than a standard electrical hot water heater (probably by about $600 to $1000 more), there are currently $300 Federal tax credits at this time,  and for instance in 2013 PG&E has a $500 rebate.  In other areas, other utility companies may also offer rebates which may offset the costs.  According to calculations from the GE web site, a standard electric hot water heater would use 4879 KWH per hear whereas the GeoSpring is estimated to use 1830 KWH per year [see calculator], which adds potentially several hundred dollars a year in electrical savings.  Also, the reduced electricity usage is an important consideration if you're sizing a solar panel system for generating your own electricity.  So far, it's working great and we have had no issues with it so far.