Hello, I am considering replacing a standard water heater and boiler that are at the end of their life span with either a Navien 240B or Bosch Combi 151 combined DHW and Boiler. My house has 4 bathrooms with showers. House is a quad level totaling approx. 4400 ft^2. The lowest level is a concrete slab of approx. 1200 ft^2 that is heated by a boiler of 37,000 BTU/hr output. The remainder of the house is heated by 2 Forced Air Furnaces of standard efficiency. All levels of the quad are open to each other so heat generated in the slab disperses to the rest of the house. I have been happy with the performance of the existing boiler. Appreciate any thoughts regarding whether either of the combi boilers would be an effective replacement for both the hot water and slab heat requirements. Pricing of the Bosch unit is almost double the Navien and the Bosch output is lower, seems like a simple decision between the two but I'd like to confirm I'm not missing something important
Is Navien 240B or Bosch Combi 151 right for me?
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Neither has enough burner to support 4 simultaneous 1.5gpm showers, let alone 4 standard 2.5gpm showers:
4 x 1.5 gpm = 6 gpm, which is ~3000lbs/hr.
In winter when the incoming water is getting down to 35F and you need to get to 105F for a decent shower temp, that's a 70F rise.
70 x 3000lb/hr = 210,000 BTU/hr...
...which even more than the input BTU for either of those combi boilers.
If it was 1-2 bathrooms it might be an idea worth entertaining, but for 4 baths (+ laundry + kitchen) there isn't any good single tankless solutions. They have enough burner headroom for 2 shower flows & change, but not more. You would be peak-flow limited.
A combi with a decent sized burner AND a decent amount of local storage might cut it though (eg HTP Versa series), which won't be burner-limited on peak flow rates, and has enough burner to recover quickly.
4 x 1.5 gpm = 6 gpm, which is ~3000lbs/hr.
In winter when the incoming water is getting down to 35F and you need to get to 105F for a decent shower temp, that's a 70F rise.
70 x 3000lb/hr = 210,000 BTU/hr...
...which even more than the input BTU for either of those combi boilers.
If it was 1-2 bathrooms it might be an idea worth entertaining, but for 4 baths (+ laundry + kitchen) there isn't any good single tankless solutions. They have enough burner headroom for 2 shower flows & change, but not more. You would be peak-flow limited.
A combi with a decent sized burner AND a decent amount of local storage might cut it though (eg HTP Versa series), which won't be burner-limited on peak flow rates, and has enough burner to recover quickly.
