Important New Solar Tank from Bradford White

Bradford White now has a natural gas backup solar tank available for single tank SDHW systems.

Here's the description:

http://www.bradfordwhite.com/images/shared/pdfs/specsheets/507-B.pdf

This means that the old solar rule "You must have a two tank system if your backup is a gas fired tank-style water heater" is now wrong.

Single tank systems ALWAYS win the efficiency and cost contests in every report I've ever seen.  There is less area for heat loss, less dollars spent on tanks, less real estate occupied.  

But here's the most important (but least obvious) advantage of a single tank system-- if there is solar heat available, it rises up to the portion of the tank that is normally heated with the backup source.   Now the heat loss of the backup tank is made up by solar, not gas or electric.  In a two tank system, a recirculation pump or thermosiphon would be required to accomplish the same thing.

In theory, I like this new tank.  It works simply by placing the temperature sensor higher up in the tank.  That means the lower half of the tank will be cool until solar is available.  What seems  a little magical is how does the heat reach the top of the tank without heating the bottom of the tank, since the burner is still at the bottom?

Well, in an 80 gallon tank, let's just say the burner has heated the top 40-60 gallons by 6am.  Now everyone in the household takes their morning shower and uses 40 gallons.  Since the thermostat is high up in the tank, it doesn't see the new cold water that is now in the bottom of the tank.   The burner doesn't come on, and solar will heat up the bottom of the tank, and if there is excess solar heat for the day, it rises up and helps keep the burner off all night as well.

This latter effect is significant for tank-style natural gas water heaters, because they typically lose 30% -43% of their heat per day to the room they are in.  You definitely want to replace that standby heatloss with solar if possible.  Single tank design is the easiest and best way.

There is one missing piece to the puzzle for winter and low solar days.   If the top of the tank finally drops below the thermostat setpoint, the burner will fire until the thermostat is satisfied.   Since the heat reaching the thermostat must travel through the lower part of the tank, the “solar” section of the tank will be hotter than necessary.   Hot water in a solar tank always hurts solar efficiency.   B-W doesn’t report by how much, and I think somebody needs to find out.   Otherwise, solar curmudgeons will never be convinced to use this tank.

Matt Carlson of Sunnovations is also recommending single tank systems:
http://www.sunnovations.com/sites/default/files/Sunnovations_Newsletter_August2010.pdf