Improve water heater heating capacity defaults?

[shorowit]

Just documenting a possible future improvement....

Currently we independently determine the default tank volume and default heating capacity (for conventional storage WHs) based on NumBedrooms per BAHSP Table 8. For example, if neither are provided, we might choose 40 gal and 4.5 kW for an electric WH. But if the HPXML has the tank volume provided (say 80 gal) and not the heating capacity, we'd still choose 4.5 kW.

Would it be better to default the heating capacity based on the tank volume instead? Then if the HPXML has the tank volume as 80 gal, we'd choose 5.5 kW instead.

That said, BAHSP Table 8 shows a pretty wide range (38-50 kBtuh) of burner capacities for 50-gal gas water heater (see below). Probably would want to look at AHRI and see what the relationship between tank volume and heating capacity looks like.

cc @jmaguire1

[jmaguire1]

There's a pretty surprising range of heater capacities for say a 50 gal gas WH:

This is from the AWHI product directory, and the 100 kBtu/h units are from major manufacturers so they're real products that have crazy high FHRs (more than twice the tank volume delivered hot in an hour!). I'm not sure if this to support combi or some other not just DHW application, space constrained homes with lots of occupants, or something else entirely.