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New Mod-Con boiler with indirect HW sizing

GaryB Member Posts: 2
I am looking at getting my boiler replaced with a Triangle Tube Solo.

Based on heat loss of my house I am at about 38 MBH which puts me in the range of a Solo-60 (12-47 MBH).  However I get my hot water using a Triangle Tube Smart 40 indirect hot water heater.  With the Solo 60 I calculate I can recover about 1 gal/min of hot water which seems low to me.  Moving to the Solo 110 (22-82  MBH) r will allow me to recover about 2 gal/min, but then I have a boiler that is "oversized" but since it is a Mod-Con it will actaully modulate down to 22 MBH which is 40% below my heat loss.

Running out of hot water is not an option :)

Thoughts?  Advice?


  • Sizing

    I'm just installing a TT Solo 60 with a Smart 50.  Your combination should give you a couple of showers before you run out of hot water and then you have to wait an hour to recharge the heater.  Do you need more hot water than that?

    If so, you can extend the capacity of your Smart 40 by increasing the temperature to 140 or 150 degrees and adding a tempering valve at the outlet to reduce the delivery temperature to 120 or 125 degrees.

    Oversizing the boiler would be a mistake.

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • hot_rod
    hot_rod Member Posts: 20,809
    define your DHW load

    first then build to that.

    How much do you have now? Is it a dump load all in a short period of time, or a continous slow draw. If you need a lot of hot water in a short period of time, either the boiler needs to provide the power to do that, or you could add additional storage.

    As Allan mentioned boosting the temperature extends the drawdown also. I see solar tanks set at 160F or more, just be sure to have a quality, listed, code approved mixing device at the tank and or shower valves.

    Or plan a hot water conservation program DHW :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GaryB
    GaryB Member Posts: 2
    Need hot water for at least 3 showers.

    Well I currently have a vastly oversized steam system (150 MBH), which has the indirect running off of it.  I've lived with running out of hot water before and that is not really an option.  

    The new system will be a combination of radiant and panel/baseboard plus the indirect hot water.  The house has been majorly insulated + all new low E windows since I bought it which has droped the heat load considerably. 

    2 showers and then an hour recovery for hot water does not seem like enough to me.  (Guess who will end up with the cold shower).

    The price of the Solo 60 vrs 110 is a wash, but the 110 will probably allow for 3 showers before the water temp  drops, plus cut the recovery time by about a 1/3 (I figure a 40 minute recovery) .

    With  the modulating the 110 will still go down to about 22 MBH which in my thoughts would still make it better than a 50-60 MBH non modulating boiler.

    Am I missing sonmething here.  Putting in a Solo 60, and then having hot water issues will cause me nothing but grief :)
  • Paul Pollets
    Paul Pollets Member Posts: 3,649
    edited September 2010
    Use the larger appliance!

    Use the larger mod-con. You'll get much faster DHW recovery and still use the lower firing rates for your heating load. Make sure the indirect tank HX has a min. of 1" piping to the boiler.

    If you're feeling guilty for upsizing the boiler to the DHW load recovery, Alan's method certainly works. I like to see at least 60-100K btu's available to recover the tank. A tank that replenishes within 10-15 minutes makes for a happy customer.
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,827
    edited September 2010

    Yes, go with the larger boiler if you have one zone for your whole house.  If you have more than one zone and especially if you have any small zones, your larger boiler might start complaining if it can't unload the heat it's generating, even on low modulation.

    The trouble with this scenario is that you are trying to re-use your Smart 40.  In a perfect world, you could sell the old heater and get a Smart 80 or 100.  With your DHW requirements, I'd opt for a totally separate on-demand heater like a Noritz, Takagi or Rinnai.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083

    From the TT rep's mouth: The 60 is a tad small for anything more than a one bath/condo type indirect situation.

    Which nixed my spec for one that would have suited a ranch with a 30K radiant heat loss. Two baths. 50 gal Smart. I put in the 110. 
  • hot_rod
    hot_rod Member Posts: 20,809
    you really need to

    size to the largest load, in your case the DHW.

    More and more we will see the DHW load surpassing the heating load. Smaller more efficient homes will have small heating load, that's a good thing.

    Folks tend to use as much hot water as you give them. Install 50 gallons and they will draw that down the lowest useable temperature. Give then 120 gallons and they use all of that.

    I've never had a customer complain that I provided them too much hot water. But I have mis-judged a few on the other end.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
This discussion has been closed.