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Dual Purpose Instant Water Heater

Post in 'The Green Room' started by velvetfoot, Sep 5, 2012.

  1. velvetfoot

    velvetfoot Minister of Fire

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  2. Jags

    Jags Moderate Moderator Staff Member

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    So basically it is a boiler that also produces DHW? Each temp controlled independently. I like the basic concept.
  3. velvetfoot

    velvetfoot Minister of Fire

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  4. Jags

    Jags Moderate Moderator Staff Member

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    At 7000 bean pods you are getting into gassifier w/storage territory.
  5. velvetfoot

    velvetfoot Minister of Fire

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    I know. But, hey, I'd free up a flue for it! :)
  6. btuser

    btuser Minister of Fire

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    I looked at them and from what I found out they've got a terrible track record, but it may have been oil models that I was looking at.

    It wasn't a Samsung product when I was looking at them (3yrs ago or so) and I think they're Korean. Not a bad thing but may help you sniff it out.
  7. woodgeek

    woodgeek Minister of Fire

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    For <$7k you could get a munchkin modcon gas boiler and a marathon tank and get better DHW (steadier), a unit properly sized to your space heating load (which might be smaller than your max DHW load, if you have two showers), higher overall efficiency, also a PVC vent and likely a better service record/life.

    You got hydronic radiation or radiant ??

    IOW, I no likey demand systems.
  8. velvetfoot

    velvetfoot Minister of Fire

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    Hydronic, alas.

    I'm just crazy about standby loss.
  9. woodgeek

    woodgeek Minister of Fire

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    If you have a cold start boiler with a well insulated tank, then the standby can be negligible (and doesn't matter in the heating season). Or am I missing something. Gas fired tanks have lousy EF, but the standby losses are all in the flue.
  10. velvetfoot

    velvetfoot Minister of Fire

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    It's heating up the mass of the boiler and its water just to warm up the indirect tank.
  11. woodgeek

    woodgeek Minister of Fire

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    And the thermal mass of the 'on demand' heat exchanger?? Seems comparable to that of a small modcon, and runs at lower AFUE.
  12. velvetfoot

    velvetfoot Minister of Fire

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    They also seem more complicated, or 'foreign', to me, anyway.
  13. woodgeek

    woodgeek Minister of Fire

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    Oh yeah, you are also cycling and heating up the demand system every time there is even a small call for DHW. A low mass boiler filling a well insulated tank will cycle a lot less (if properly installed).
  14. velvetfoot

    velvetfoot Minister of Fire

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    I guess the key is low mass.
  15. Thomas Anderson

    Thomas Anderson Member

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    I use a Takagi TK-Jr on-demand propane water heater for both hydronic heat and DHW. It's technically a backup system to my solar thermal, but since I don't have my solar thermal tank running yet, the TK-Jr has been on primary duty for several years now. It works like a champ. Total installed cost (venting, extras included) was about $1,000. I have the unit's thermostat set at 167 degrees, which works great for the hydronic heating. And I split off with a tempering valve to deliver 120 degrees to the taps (except the kitchen/dishwasher, which I keep at full temp). I couldn't imagine buying anything more expensive than this when this solution has worked so perfectly. I have a 100lb propane tank which I fill about 3 times a year, once in Nov, once in Jan, and once in March. That will go down to probably just once a year as soon as I get my solar thermal system fully set up.
  16. johnny1720

    johnny1720 Member

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    Just curious about the size of your house? I have a Paloma tankless that is only used for DHW and I use about 180 gallons a year. Before we had babies in the house we used less than 100 per year. When you say you have a 100lb propane tank do you mean you take it back and fourth to the store or does the propane company come fill it up?

    I love your setup, we did a similar one at work with a Navien System
  17. Thomas Anderson

    Thomas Anderson Member

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    Oops, I misspoke, I meant 100 gallons, not 100 lbs. Yes, the propane company fills it. I just call them whenever it gets down to about 20 gallons or so.

    My house is 3,200 sqft plus a 720 sqft heated garage (set to 40::F) . However, it is a super-insulated and tight house and uses passive solar, so it doesn't need tons of BTUs of heat. My TK-Jr probably only fires for about 2-3 hours per day in the middle of winter, plus whenever someone calls for hot water. That's like 1.5 gallons of propane per day during the winter, including cooking. We also don't keep the house super warm either. Mid-60s is fine. We fire up the wood stove if we want to get warmer than that. Though when the TK-Jr does fire up in the morning and the evening, it makes our kitchen and foyer floor (tile over a 4" concrete slab) a toasty 90 degrees. That thermal mass is then able to keep the house warm for hours.

    However, if you need more heat than that, what's nice about these on-demand units is that it's pretty easy in a small space to put 2 or 3 in parallel if just one doesn't provide sufficient BTUs for your house. I would much rather have two TK-Jrs for $2,000 than a single $2,000 unit. It'd be more bang for the buck plus redundancy to boot. The docs say that the TK-Jr can put out a max of 140,000 BTUs/hr (more than twice as much as my wood stove, which is rated to heat 2000 sqft). It has a direct vent and outside air kit with an exterior vent box which integrates both of them and pre-warms the intake air with the exhaust chimney. Another nice feature is that it has an anti-freeze heater inside to keep water in the coil from freezing if the unit should be exposed to those conditions (which mine occasionally is because it's in an insulated shed outside the house -- electric pipe tape keeps the rest of the pipes from freezing).

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