One big or two small

[hvacbs1]

I am thinking about replacing a 300k BTU NG boiler with a new mod-con. Should I buy one big boiler or two small boilers? Or, should I buy one small boiler and configure the existing boiler to assist on unusually cold days? The existing boiler is 6 years old and works fine…just inefficient. The system currently has two zones (north and south).

[beenthere]

2 small boilers, and have the tenants pay for their own heat.

[hotwaterworld]

If a house, one mod/con boiler properly sized will suffice. Size it properly by proper heat loss. I would bet old boiler is way oversized unless unusually large structure and in cold climate. The only other influence may be if you are really concerned about a breakdown, and house is very large, then I might do twins for redundancy if you have the extra money to "burn" ha ha. Hope this helps.

[hvacbs1]

The building is a 5-plex. I also have a need to add hot water capacity. The building currently has a 60 gallon AO Smith. So, perhaps, a new mod-con boiler could handle both tasks. Domestic hot water and building heat seem like two very different applications (differences in volume and heat rise). So, I am wondering how well-equipped a boiler might be to supplement hot water domestic requirements. And, should it be configured to backfill the tank or heat water leaving the tank?

[hotwaterworld]

hot water settings built in w/ priority. Yes, dhw and heat are 2 different beasts but the mcba control in boiler is set up to handle that. Works very well. Takes 2 pumps, one for heat and one for domestic hot water. May take 3 if you have individual zones for each unit but that can easily be determined.