Last month I had the privilege of hearing two really smart guys (who don't know each other) arrive at exactly the same conclusion. John Siegenthaler, P.E., speaking at the Radiant Panel Association's Building Radiant conference and show in Reno, Nev., and Gary Klein, speaking at the American Council for an Energy Efficient Economy’s Hot Water Forum in Ontario, Calif., said the future of space heating is an efficient water heater.
Siegenthaler, in a seminar titled Radiant in Low Energy Houses, said we will inevitably see some type of combination systems. These space/water heating systems are common in Europe. Most of them, such as the ACV International HeatMaster 35TC (http://www.acv-uk.com/heatmaster35.htm) are drop-in plug-and-play appliances.
"How do you heat a house with a 20,000 Btuh heat load," Siegenthaler queried.
That's not a hypothetical question. According to the National Association of Home Builders, the average size house has dropped to 2,100-sq.ft. and building envelopes are tight. Houses can be built with structural insulated panels, or insulated concrete forms or have closed cell foam insulation sprayed into the stud wall.
Siggy has run the numbers, and heat loads are down to 10 Btuh/sq.ft. Associate Editor Candace Roulo has a story in this issue about a certified passive house in Salt Lake City that's designed for loads less than 4 Btuh/sq.ft. During the ACEEE Hot Water Forum, Charlie Adams, director –Thermal & Mechanical Technologies Group, A.O. Smith Corporate Technology Center, said the heat loss in his Milwaukee-area home is less than 30,000 Btuh. Adams said his house isn't green, just well constructed.
Siegenthaler urged hydronic contractors to pursue this part of the housing market because the people building tight homes, he notes, only know the building envelope. They don't know what modern hydronics can offer.
The calculations that Siegenthaler showed during his seminar indicated that the load in one of these houses could be handled with a floor temperature of 73°F, too cold for a comfortable radiant floor. Hydronic practitioners need to consider radiant walls and ceilings instead. New panel radiators are coming over from Europe that work well with 120°F. Some radiators that incorporate tiny 2W fans will supply enough heat at 95°F.
Even with 10:1 turndown ratios, boilers would need a 20-30-gal. buffer tank to handle microloads, Siggy says, with the buffer tanks taking care of hydraulic separation, air separation and dirt separation. A variable speed pump from the buffer tank would move the water through the radiant panel or to a fan coil.
Europeans have lots of heating and domestic hot water systems that come with solar integration options, he says, and some are already available here.
At the ACEEE Hot Water Forum, panelists were convinced that combination systems will have to work with air conditioning to be palatable to builders and homeowners; they advocated pairing water heaters with fan coils.
There will be numerous contentious issues along the way — no codes or standards, no performance ratings, no matched up water heaters and fan coil units, previous failures with this concept, lack of training, the perception that water heaters are inferior to condensing boilers.
Then there are the cultural divides. Plumbing contractors and HVAC contractors don't always talk to each other. There could be internal corporate battles. As Charlie Adams notes, A.O. Smith makes water heaters and fan motors for HVAC systems. Will the Rheem Water Heater Division in Montgomery, Ala., duke it out with the HVAC Division in Ft. Smith, Ark.? I doubt that either company is willing to cannibalize sales from one division to another.
The issue, however, is not going to go away. With the emergence of LEED for Homes and the NAHB Green Home certification program, certified passive houses and demonstration projects such as the Solar Decathlon houses, tight building envelopes are becoming part of the mainstream. You can’t heat a house with a furnace or boiler when the load drops to 10 Btuh/sq.ft. or lower and the total load — at design temperature, no less — is 30,000 or even 20,000 Btuh.
It's time for contractors, engineers and manufacturers to come up with creative and economical ways to handle low heat loads.