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Final Decision: Indirect HWH or Turbomax Reverse Indirect/Buffer

D107D107 Posts: 1,431Member
edited December 2018 in Gas Heating
I have a contractor who knows his stuff; one last decision to be made. With the Buderus GC 144/3 CI boiler net output of 54Kbtu, Turbomax 23 first hour rating is let's say 95 gallons, (since it's 88gal @50kbtu I figure 8% more @54kbtu.) continuous 80gal? That's with boiler water temp feed 160, return 140. 80deg rise from 40 to 120º output. What I'm trying to do is keep max boiler water temp 160, mix down to 120 for house. Not sure if that's what is figured for first hour 95gallons.

Issues overall are 1)enough dhw? 2)DHW recovery interferes with heating cycle? 3) Do I need the buffering of the Turbomax to stop short cycling or is standard indirect the simpler way to go here?

1-Ordinarily the most water the most water my wife and I use would use is back to back showers, say 15 minutes x 2 = 30 x 2.2 = 66gallons. We use cold water for laundry, so if we had a dishwashing load just before our shower supposedly that's another 4-6 gallons per cycle?, so it seems Turbomax would easily handle hw load--currently supplied by stand alone 50 gallon 30-40K btu hwh mixed down from 140 to 120 that's never run out of water. Conversely, after a long heat cycle in cold weather, maybe first hour rating is no longer 95gallons?

2- contractor states that on a cold day and after a DHW load, the heat cycle might be delayed a as long as an hour. Our house is decently insulated so an hour delay sounds like a lot, but is it? (I know the Buderus Logamatic 2107 allows 30 minute max dhw priority cycle...--we won't have the 2107 here.)

3-Heat Loss 38K @10º design temp. 2 cast iron zones, basement baseboard. Note that at 40º outside temp, my overall house loss is 20K, Zone 1 8500Kloss/9200gain, Zone 2 4700Kloss/6600gain and Basement: 6900Kloss/4800gain. So Buderus net is 54Kbtu with smallest zone 2 calling for 4700kbtu loss. So when 54K net output his 3000-4000 btu small zone will short cycling be severe? Or on a more normal winter 20 degree day when smallest zone loss is 8100 when it alone calls for heat? And how much would the Turbomax 23 curtail any short cycling? Are we defining short cycling at more than three cycles per hour per zone?

(Note that this will be set up with ECM circ and three zone valves; piping is in series for each zone; also, contractor prefers hydrolevel control to Beckett Aquasmart? that the Buderus may ship with due to some reported issues with that.) Also my guess is that the 38K heat loss is really more like 30.


  • HVACNUTHVACNUT Posts: 1,909Member
    It's late and I'm getting up early so a quick comment.
    Turbomax or standard indirect, it needs a dedicated circulator size as spec'd for the water heater.
    A Taco ZVC-403 or similar with priority will allow zones valves and your ECM circ for space heat, and connections for an indirect with a separate circ.
    I don't know the supply and return sizes on the Buderus but the Turbomax is 1 1/4 in and out. And that's what it needs to achieve the stated ratings, as well as boiler BTU output.
  • D107D107 Posts: 1,431Member
    @HVACNUT Thanks, yes I'm sure the installer will put in the correct circ and piping. What I didn't realize when I first thought of using the turbomax is that in this situation it's basically an on-demand hwh which would usually require higher firing rates. Anyway aside from dhw capacity my questions are 1) Do I need the turbomax/buffer to avoid short-cycling; 2) with turbomax capacity can heat cycle be delayed as much as an hour on some occasions.
  • ZmanZman Posts: 4,693Member
    You are basically doing instantaneous demand DHW with some buffer to handle small spikes in usage.

    A buffer tank sounds like a good idea for your application.

    If you use significant amounts of hot water continuously for an hour, you will be in priority for that period. Not that big of a deal.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • HenryHenry Posts: 872Member
    I refuse to spec TurboMax as it is basically an instant hot water heater that looses some energy by the insulated jacket as a hot water tank. We usually replace them when it perforated copper tubbing just after warranty. One is much better off getting an HTP S/S indirect tank.
  • D107D107 Posts: 1,431Member
    @Henry Thanks for your comment. This is the first I've heard of issues with the Turbomax. Just after warranty would be 5? years? How many times has that happened? Were they your install or perhaps the result an improper install or untreated hard water?
  • D107D107 Posts: 1,431Member
    The Wall has a great thread on Turbomax from 2007 that was very helpful.

    What I was able to glean is that for a household that does not require simultaneous showers, the Turbomax 23's 90+gallons first hour rating should easily cover our dhw requirements. And the 26 gallon tank volume should be adequate as a buffer tank for our system. The warranty is ten years; I will check with the company to see if they've experienced the problems with perforated tubing at the ten year mark that Henry mentions. I would expect an indirect or turbomax to last 10-20 years compared to about 10 for a direct hwh, but I don't really know.

    Someone in the thread did express the opinion that hard water and turbomax don't mix in terms of the coils, but we would try to clean annually. Also with our direct hwh and our hard water, we are going on 11 years, and while clearly there is sludge at the tank bottom and probably the anode is not in good shape--with some rumblings going on--we have not seen any significant rise in the amount of therms used for hot water--as measured by our summer usage when the heat is off.

    Installing a separate buffer tank and indirect might be optimum, but of course that would increase cost, footprint and system complexity.
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