Recommended installing an overpowered boiler?

[JessBBBBB]

A contractor recommended I install a Bosch Greenstar Combi 131 for the dual purpose of providing hot water to my radiators and for normal household use. He stated that the boiler would definitely be over-sized for the space-heating purposes, but going to the next size down would risk not having enough hot water if, for example I wanted to shower and run the washing machine. I've had trouble figuring out how severe the loss of energy efficiency will be. Anyone know where I can find specs around cycling times vs . energy use for Bosch boilers?

Thanks!

 

[Leon82]

That's the dilemma with the combination units.

Short cycling won't help longevity or efficiency

 

[Sponsor]

 

[Dana]

The Bosch combi units are almost always a terrible fit for reasonably insulated average sized houses. The minimum input of the 131 is 36,000 BTU/hr, which can be problematic even for domestic hot water temperature control at low flow. But it would also require about 170-180' of fin-tube baseboard to emit that much heat at condensing temperatures without cycling on/off. The Greenstar Combi 100 is better, but at ~25,000 BTU/hr in still needs 110-120' of baseboard (per zone) to operated at condensing efficiency without cycling.

If it HAS to be a combi boiler, Navien's combi boilers have much better turn down ratios, and minimum firing rates that can SOMETIMES work well in average sized homes.

It's fine if the capacity at maximum-fire is way oversized for the whole house heat load, but not so great if the minimum-fire output is also oversized for the heat load and TERRIBLE if the minimum fire output is oversized for the radiation. To avoid that, measure up all of your radiation (zone-by-zone, if multi-zoned) and report back (or run the napkin-math yourself.)

If you have a heating history on the place, run a fuel-use based load calc to ballpark the actual 99% design heat load using the old boiler as the measuring instrument.

 

[JessBBBBB]

Hi Dana,
Thanks for your response! I have a single zone unit with 95ft of baseboard. My unit is on the 3rd floor and the boiler is in the basement so I'm guessing there are an additional 30 ft of piping leading to the baseboard. Should I be factoring that in somehow?

Regardless both, 200 BTU/hr * 95ft and 200 BTU/hr * (95+30ft)ft are much lower than 36,000 BTUs/hour. I believe this falls under your "TERRIBLE -the minimum fire output is oversized for the radiation" scenario? (again, thanks for your response)

I don't have to go with a combi. The contractor strongly recommended it over separate units to save money on maintenance costs over time.

.. perhaps its time to get a few more quotes.

 

[Dana]

You have enough baseboard on your single zone to emit ~19,000 BTU/hr at condensing temperature, which is GOOD! (Just not good enough for a Bosch combi.)

The HTP EFTC-199W combi boiler has more domestic hot water output than the Bosch, but throttles back to a minimum input of 19,000 BTU/hr minimum output of 18,000 BTU/hr which works just fine. Since HTP's national headquarters are in MA with good local support this is a strong candidate. Their smaller combi unit is not a good candidate, since it's minimum input is 28,000 BTU/hr, which would force short cycling if running it at highest efficiency.

Any of Navien's NCB-xxxE models would work with 95' of baseboard too. They are quite popular in MA with many local installer. It would be nice to have at least 140K of burner for the domestic hot water, so the NCB-180E would be a nice fit for a 1 or 2 bath, as long as you don't have a huge spa tub to fill. With a minimum-fire input of 14K the 180E would modulate a bit more than the EFTC199W. Either would be fine to use with outdoor reset control raising/lowering the temperature in response to outdoor temperature, if your actual design heat load is more than 18,000 BTU/hr.

Any contractor who would recommend a combi boiler with a min fire output over 30K for a system with only 95' isn't worth spending any more time talking to. They either haven't a clue what they're doing with low-mass heating system design, or they don't really have your interests at heart.