Just finished up installing a Buderus G124X/32 with the Logamatic (dId it myself) and my old standalone hot water 50 gallons is 10 years old. I'm thinking of going indirect for the effiency but I'm not having any issues with regards to running out of hot water at the moment even though I have 5 adults in my house (3 of the will be moving out within the next few months). My question is if I purchase the Buderus St-150 40 gallons size will I still have enough hot water just for two people?
3 zone loops each zone covering 1000sf of living space for each floor; but we no longer use the third floor, so I'll be heating just the two floors.
A typical WH has maybe 30-40K BTU input (see what yours has). Your boiler probably has significantly more. Usually, when an indirect is installed, it becomes a priority zone so all of the heat from the boiler goes to it while it needs heat. So, a smaller tank will usually outperform a standalone WH. Look at the 1st hour delivery specs for what you have and that new indirect...the smaller indirect will probably have a greater quantity of water it can deliver in that first hour and will recover faster because it is more efficient and you have a greater source of heat to it.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013