Hi All,
Apologies for the long post. I'm newly registered here, but have appreciated your collective wisdom for years. Robert O'Brien's articles on boiler sizing have been especially helpful. I would greatly appreciate your advice as I specify my new system (and yes, I understand the install is more important than the equipment). There was a great June 2014 post titled "Modulating-condensing boilers," but I would love to hear six years more experience can tell us.
I am specifying a system for a new 3,500 s.f. home. Manual J indicates 67k BTU demand. Primary heat will be served by Trane air source heat pumps, with cutover to hydronic coils in the high 20’s. There will be 3-4 small hydronic radiant flooring zones (<400 sf total) in the bathrooms. Domestic hot water will be provided by an indirect tank. Fuel will be propane and the system will be direct-vented.
I am leaning toward the Viessmann Vitocell 300-V (EVIA-79) for its anticipated longevity. I would like to be able to handle 5 GPM at 110 degrees faucet temp for 45min+, with surges up to 11 GMP for 10-15 min, hence the large size. In my calculations, I assume DHW priority and storage at 150 degrees, mixed to 120. The system will have recirculation on the two primary baths.
I am trying to decide between two very different boilers: the Weil-McClain GV90+ (84k or 113k output) and the Viessmann Vitodens 200-W (102k or 127k output). I am drawn to the virtues of cast iron / traditional boiler longevity and (I believe) simplicity of the GV90+ vs. the modulating efficiency and intelligent capabilities of the Vitodens that will allow me to size larger for DHW while maintaining efficiency for heat. My goal is to buy a system that is quiet and, apart from routine maintenance, won’t have to be touched for 15+ years.
Thanks in advance for any advice you can offer, and happy holidays!
Getting to a few pointed questions:
a) Am I completely off track in any aspect?
b) Are there any strong recommendations between the two models re: longevity / maintenance / repairs, etc.?
c) Aside from the long recovery period (which I can tolerate) are there any concerns that this DHW tank is too large for the Weil-McClain 84k or Viessmann 102k boilers I listed? I am under the impression a thermostatic mixing valve will compensate for the declining temps as the reserve draws down.
Summary
New System; propane fuel; direct vent
67,000 BTU heat demand; served first by air source heat pumps, hydronic coil backup
2 x hydronic coils
3-4 x hydronic flooring zones (small, bathrooms and a foyer)
1 x Indirect DHW (family of 5; 3.5 baths; would like to support sustained flow of 5 GPM @ 110 faucet temp for 45min)
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