NZ Passive House Forum

I am currently working on a mechanical system for a PH project with no access to natural gas (electric only) in Northern California. The idea so far is to have a solar thermal system to provide much of the DHW and space heating (our estimated %'s so far are admirable, given the mild climate and insolation levels.) The space heating will be delivered through the ventilation air stream, using a hydronic fan coil fed from the solar water tank. If the solar tank is insufficient, an electric resistance duct heater downstream of the fan coil will make up the difference.

My questions are two:

1) At what temperature differential should one NOT pump water through the fan coil? Obviously, if the water is cooler than the air coming past the fan coil, it's a bad idea, but my instincts say that as long as the heat transferred to the air stream via the fan coil exceeds the energy to run the pump, it is beneficial, no? Does anyone have thoughts and/or quantifiable techniques to inform such a decision?

2) At what temperature should the electric duct heater be triggered? In other words, is it necessary to have the electric resistance element turn on whenever the air stream is below indoor design temp, or can the Passive House "tolerate" a lower incoming air temp at times, perhaps compensated with "later that day" passive solar, solar thermal and internal heat gains? Again, how would you evaluate this quantifiably?

Thanks,
Graham