Separate or combined boiler and hot water heater

Question

I'm looking to replace my boiler and hot water heater.

For background, I live near Boston, my home is about 2000 square feet, we have gas for heat, stove, and dryer, and we are two people in the house.

It seems that there are three basic options:

1. Combination boiler and tankless hot water heater
2. Combination boiler and hot water tank
3. Separate boiler and hot water tank

For a novice, the first option seems to be the most efficient option, but one plumber strongly advocated for the third option.

His reason is that the boiler is expensive and it is only really needed for 5 months of the year. A combined unit requires the boiler to be used all year and thus puts unnecessary wear on it.

What else should I consider in selecting between these three options? It is an expensive job so I want to make sure I do it right.

Answer 1

I have always used your item #3 since I believe it would be a lot less expensive to operate and service this arrangement. Why run the large boiler in the summer just to get domestic hot water? my 2 cents.

Answer 2

I'll go for #1 because

1-I have all hot water I need
2-I do not have idle consumption
3-I do not risk hot home floood due to boiler failure

It's enough to choose the right delta-T, so, if you want 50° water and your lowest winter water temp is 5°, you need a 45° water rise boiler for the water flow you need: 1 bathroom 10L/m, 2 bathrooms 18L/min, if you have more than 2 bathrooms storage is better because can deliver bigger flow. So for 18L/min and 45° delta you'll need a 48kCal (55kW) furnace (190K BTU/h).

Otherweise I'll go for solution #2 as combi are, by far, more efficient than traditional WH (95%HHV vs 80%HHV) and also because combi are safer because have air inlet and smoke outlet coming from outside so there's almost no risk of CO poisoning while standard WH have room air inlet and backflow is quite possible.