Dana, I've got some updates and was hoping I could get your thoughts. Unfortunately, there are no contractors in the DC-area with HTP or Lochinvar expertise (at least that I've come across). The primary brands are NTI, Triangle Tube, and Viessmann. Given that none of the Triangle Tubes go down below 16k, I've eliminated them from the running. While the NTI water-tube boilers go low, they make me nervous given their very low mass and the potential for durability and maintenance issues. So that leaves the NTI TFT154, which goes down to 15k and has 3 gallons of water mass, the Viessmann Vitodens 200 B2HA19, and the Vitocrossal CU3A-26. One contractor said that the Vitodens and the Vitocrossal would be the same price to install, given that the CU3A could be piped direct. No one has been willing to pipe the NTI direct (and the company generally discourages it, as you saw). So, given the design day heat-loss of ~45k, the 40F heat-loss of ~15k, and a smallest zone of 9-10k output at 120F supply temps, I'm wondering what you would choose. Do you think the extra mass on the Vitocrossal and the ability to pipe direct outweighs the 12k minimum modulation on the Vitodens? The Vitodens also seems to have a temperature sensor that attaches to their LLH - wondering how much of an advantage that is and whether it really helps to low return water temps. I also wonder, given the size of the heat exchanger on the Vitocrossal, whether it will prove to be more durable than any of the lower mass boilers, though of course don't have any evidence to that effect. I spoke with the Viessmann rep and unfortunately it sounds like the model-26 Vitocrossal is actually the worst seller of the line - they don't have any plans to bring the smaller Vitocrossal units with up to 7:1 turndown to the US in the near future. I think that's a shame, since a high-mass boiler that goes down to 8500 btu/hr seems like it would be a real game-changer!