Efficient Boiler for Small Heat Load and Multiple Zone Types
I have been reading this form for a number of months and have always been impressed by the depth of knowledge and wide perspective. I am a general contractor installing hot water heat in my own home. I have installed five hot water systems over the years. However, my current project has some particulars that I would appreciate your suggestions on how to best resolve.
The house is under construction, 2800 sf, highly insulated, with a max heat loss at Seattle design temperature (22*F) of only 25,000 BTU/hr. I was planning on using three different heat emitters. A 450 sf slab in the daylight basement with 1/2" pex at 12"c; 400 sf of pex under plywood floor 8" oc with light gauge stamped aluminum diffusers and tile above; and two zones of HW baseboard main floor 32 lineal feet and bedrooms 32 lineal feet.
My question is what boiler and DHW heat source would you recommend?
I have considered combi boilers with built in heat exchangers however, I need to have capacity operate two showers simultaneously. That would bump me up to a Rinnai E110CN, Bosch Greenstar Combi 151, Triangle Tube Challenger Combi 125, or Navien CH 180. All of these are way over capacity for my heating needs. Perhaps a smaller combi with an electric hot water heater as holding tank for reserve capacity?
The alternative would be to get an indirect DHW tank zoned off a smaller boiler. I would like to get the high efficientcy of a modulating and condensing boiler. However, I have been told my hot water baseboard will not work well at 140*F and I will need a higher temperatures to induce convection. That would leave me with a 150*F and higher boiler output based on an out door temperature reset and a mixing valve for stepped down supply to the in floor heat. However, I would loose some of the efficientcy of condensing boiler with the higher output. What might be ideal would be a boiler that could alter supply temperatures depending on the zone calling for heat. Weil McLain's Ultra 80 has two temperature output.