Combination DHW and Space Heating
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I’m interested in learning regarding the feasibility and advisability of using a hybrid “small tank” condensing or tankless condensing hot water heater to provide both DHW and space heating for new construction in a heating climate. Examples include Navien America 180/210/240 (EF .98-.95), Eternal GU195 (EF .96), Takagi TH2 (EF .93), Rinnai RC98HP (EF .93), Quietside ODW199 (EF .92), or Noritz NCC199 (EF .92). Alternative configurations could be desirable: a)circulator with a fan coil in the HRV duct; or for higher heat load b)circulator with baseboard hydronic distribution (easy to zone); or c)maybe both with priority to the HRV fan coil. First priority goes to DHW and no potable water used for space heating. Advantages could include high efficiency, avoid the cost of a separate furnace or boiler, and compact form factor. Any advice or experience re is this possible, initial cost, long term reliability, availability of aftermarket service and parts, longevity?
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Replies
John Siegenthaler is one person who writes quite a bit on the topic your asking about.
Here's a recent article, search back on the bnpmedia site for more of his articles.
http://www.pmengineer.com/Articles/Column/BNP_GUID_9-5-2006_A_10000000000000726274
Jan,
You will probably be interested in this research report:
http://www.toolbase.org/pdf/fieldevaluations/Tankless_Hot_Water_Heater_EvaluationSWD.pdf
The Navien looks like an impressive and well-engineered piece of equipment, though I have no personal experience with it. I have some experience with Triangle Tube condensing boilers and I may use their new combo unit, the Prestige Excellence, in my next project. I would suggest adding that unit to your list, as their materials and engineering is quite good.
We've been using the Quietside DHW and the Rinnai but the Navien has too much pressure loss at comparitively low flow ratesd to make this work well. One solution can be viewed here: http://www.chandlerdesignbuild.com/files/waterHeatersLatest.pdf
But it's important to caution your clients that the cost of propane is so high and volatile that this is a more expensive way to heat a home unless you have a very good envelope and a good solar thermal contribution. The potential for piping errors in these assemblies is very large, subtle on-site re-designs can totally cripple the unit. (also I have a better wiring diagram that I should be posting soon that reflects some inspector transparency issues)
All the best Michael