Increasing our Solar Thermal System vs. Gas Boiler Backup

The high efficiency Munchkin gas boiler is more expensive to install (>$3,500) than budgeted as both a backup to the Schuco Slimline II-80 solar thermal system spec’ed out for domestic hot water and a sole source of heat to our hydronic radiant heating system (more on these mechanicals here). Installing gas service to the house will also cost an additional $2,000 in permitting fees and additional plumbing costs to run the line inside the house. The high efficiency (97%) of the gas boiler is needed to accomplish a HERS rating of 52 that enables the 100K House to achieve a LEED Platinum rating. Electric alternatives to heating water are not nearly as efficient overall and will dramatically increase our HERS score.

Increasing our Solar Thermal System vs. Gas Boiler Backup


If we increase our solar thermal system parameters by two more 25 square foot solar panels and an additional 80 gallon storage tank with integrated electric backup heater, we will be able to eliminate the gas boiler while reaching the same efficiency as the gas boiler backup design without increasing overall cost. This design has the potential to be lower in overall cost with the eliminating of any gas service run to the homes at all.

Ross Mechanical Group Hired to Design Mechanicals for 100K House

Key Variables

The big hurdle in switching to all electric water heating while still achieving the same HERS rating is the difference in the source to site multiplier between gas and electric. What the heck does this mean? Basically, since the US is so lousy at efficiently generating and transporting electric power to our homes, the HERS rating system multiplies the power used by electric appliances by 3.16 to compensate for the roughly 30% efficiency level we are generating electric at currently. For gas, this multiplier is only 1.02. So, if we are going to replace our gas boiler with an electric backup, we must be three times as efficient in terms of total power used.

Over-Conditioned: The Super-Sized HVAC Problem

We have calculated that we will need to compensate for roughly 67% of our radiant heating demand in the winter with increased capacity in our solar thermal system. This leaves us with just over 2,000 kWh’s of supplementary electric backup heat for the entire year.

Yesterday a three man team from local solar experts, Solaris Energy, sat down with us for a marathon meeting to try and determine if our goals above are possible. We have a proposed design and will be sending it off shortly to the fine engineers at Schuco to see if we are in the ballpark or totally off base. Stay tuned for the update and lots more math.