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Need Advice on Basic Gas Condenser Boiler Set-Up for Radiant Heating & DHW

SpiderSpider Member Posts: 4
edited July 2019 in Radiant Heating
Based on current advice, I am choosing between a combi condensing boiler TT Prestige Excellence or a TT Prestige ACV MAX 110 with Lochinvar 67 gal indirect water heater. The set up needs to provide hot water for a radiant heating system of 1250 sf in a concrete slab and another 1250 sf under a wood floor of Pex with fins. DHW is for three showers, washer, sinks, kitchen etc. The climate in oceanside San Francisco is very mild with winter temperatures in the 40s and 50s. The house is sealed tight with stucco and double pane windows. I am looking at two zones, up and down, with separate thermostats. The main question is whether a premium combi will do the job or add an indirect water heater that pushes the cost up by several thousand. Any additional advice greatly appreciated. - Tom


  • STEVEusaPASTEVEusaPA Member Posts: 4,742
    I’d add the indirect, but the boiler you are considering is too big. A proper heat loss would determine which size boiler you should install.
  • SpiderSpider Member Posts: 4
    Thanks for the quick response and the suggestion re downsizing the indirect tank. This morning I heard from a potential contractor who suggested Viessmann Vitodens with an indirect called Vitocell. Or a Viessmann combi Model 222-F with built-in 26 gal tank. I had researched Viessmann earlier and determined I did not want to go 2 weeks without hot water waiting on a spare part. I know this level of analysis is pretty broad, but I am trying to do due diligence as a homeowner. - Tom
  • STEVEusaPASTEVEusaPA Member Posts: 4,742
    I didn't recommend downsizing the indirect. I recommended a proper heat loss to determine the proper size of your boiler. I add an indirect to the system, with no upsizing of the boiler.
  • SpiderSpider Member Posts: 4
    Thanks for the clarification. I thought you were referring to the size of the tank, but now understand you are referring to the size of the boiler (TTP Max 110) that may be more than I need on some level. I will revisit how to calculate heat loss addressed elsewhere on this site. So far none of the contractors have provided this information. I will start asking. - Tom
  • SpiderSpider Member Posts: 4
    For further advice, there is one more set up or configuration brought to my attention. That is a gas condensing boiler for the radiant heat and a separate on demand tankless heater for DHW. I am not sure if this simplifies things or makes the job and operation more complicated. At least if one went out, I would have the other, which seems like a plus.
  • This homeowner in San Francisco used the "Find A Contractor" feature to track me down and arrange for a jobsite visit on July 24th for a quote on a replacement boiler. He accepted our proposal and we start the work at the beginning of September.

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  • Erin Holohan HaskellErin Holohan Haskell Member, Moderator, Administrator Posts: 1,531
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