Sizing Combination Heaters (Delta Performance)

Shawn O'Connor

Hi...In sizing combination boilers, should one base estimate on space heating or combination of space heating and domestic hot water production?

For instance, my house has an estimated design heat loss of about 62,000 BTU and a base hot water volume of 133 Gal (2 bath/shower, 3 facets, DW, WM based on family of 4, using SlantFin Hydronic Explorer).

The Delta Performance (Triangle Tube) uses a tank within a tank design and offers a 100, 120, and 140 thousand BTU unit (~85% AFUE). The 100 BTU model has a first hour rating of 162 (120F outlet, 180F storage). It has priority switching for DHW. I've gotten a couple of quotes, with some suggesting the 100 BTU model and others suggesting the 140 BTU model. Any wisdom on sizing these (is this an issue of whether to add space and DHW heating or not?)

shawn