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80gal indirect for family of 6 - overkill?

koselig Member Posts: 3
edited October 2019 in Domestic Hot Water
Hi all,

We are adding onto our 1bd/1ba cabin to make a home for our family of 6. Kids aged 5/8/11/12.

Currently believe it or not, we are getting by with a 40gal LP water heater running the basement slab hydronic as well as domestic hot water. This installation is original to the cabin and before my time.

I want the new system to be solar/geothermal friendly (we have plenty of space for a horizontal ground loop). The plan is for a mod con boiler (Lochinvar) to run the existing slab, as well as an addition (tuck under garage) slab and DiaNorm low-temp radiators in 4 bedrooms.

The new structure will have 2.5 bathrooms - the master with separate shower and 6' soaking tub, a powder room for guests, and a full kids' bathroom with tub/shower combo. With 4 kids I can easily imagine having a dishwasher, washing machine, shower, and tub all happening either at once or not far apart. For this reason, I have ruled out the 4.8gpm Lochinvar Noble combi. Would you agree?

Would an 80gal indirect tank be a better fit? Or would a 50gal heated to 140F mixed on output potentially work fine? My wife won't be running the tub every day of course, but I do want to make sure we have the capacity for her when she wants it. What do you think?

Thanks all!


  • Jamie Hall
    Jamie Hall Member Posts: 16,617
    If it were mine I'd go with the 80. It isn't that much bigger, and you have a much better chance of not having the last shower produce either an irate child or worse, an irate wife.

    But do hold it at 140 and mix it down. Not so much for the capacity, as to ensure that you kill any lamentable bugs which might get in there.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 6,793
    80 gallon for sure with a tub like that. The 80 will also have bigger heat exchanger allowing low water temps for the Lochinvar or heat pump to recover it.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • koselig
    koselig Member Posts: 3
    right on, thanks for sharing your expertise, guys.

    OK - 140F in the tank sounds great, I want to keep the boiler condensing as much as possible so my plan is to run the whole hydronic system in that 130-140F range.

    I did my best in loadcalc and came up with ~65k btu for heating load. The HVAC guy is suggesting doing the 199MBH Noble even if we do the tank so we have the on-demand DHW if we ever want it... since it will modulate down to 19.9MBH, will it be able to run efficiently throughout the heating season without short cycling if it is running two ~600sq-ft radiant slab zones plus the panel rads in the bedrooms and the 80gal tank?

    Is there anything about the single coil SIT080 tank that would preclude integrating solar and/or geo in the future? I know some have dual coils to integrate parallel systems. Would we set up the solar or geo to just "pre heat" the water going into the boiler in this scenario?
  • Zman
    Zman Member Posts: 6,793
    Is your HVAC contractor driving a tractor trailer to your job just in case he needs to haul a load of dirt someday? I don't get his reasoning at all. The whole point of adding the storage is to reduce the boiler size and increase efficiency. Keep in mind that your 65k is for the design day (1 or 2 days a year). With outdoor reset, the typical day will be a fraction of that. I am thinking that a Knight WHB 110 would be more than enough for DHW and will match you typical day load. What is presently heating your DHW?
    The dual tanks are nice but expensive. In less you have a future design in mind, you might be better off adding a flat plate heat exchanger if you do the solar/geo in the future.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mikeg2015
    mikeg2015 Member Posts: 1,183
    My daughter literally just took a bath in a smaller Koehler whirlpool tub and our WM 40 gallon indirect set to 140F using just 170-150F water kept up with no problem. I estimate in this situation, output from the boiler was 55,000BTU with a 10F delta T, 11GPM. It recovered about 2 minutes after the water turned off.

    Point being, that unless you use 180F water, you won’t get more than 80k in most conditions out of a indirect unless it’s really cold incoming water with a high flow rate. So the larger tank gives you a bigger coil and a taller total tank exposing more colder water to the coil.

    On recovery, my load drops ot 20-30k BTU with boiler water around 150.

    It’s a steam boiler so I’m trying to reduce standby losses by using as cool of water as possible, but still over 130F. Aquastat reads mainly return temp because of location, so longer draws result in higher water temps.

    Just one data point. A bigger boiler won’t make the indirect put out more hot water... without killing effeciency.
  • skalor
    skalor Member Posts: 13
    You mentioned it being a cabin. Are you on a well? If so, I would base dhw production on max flow rate of the well pump.
  • psb75
    psb75 Member Posts: 436
    With solar and geothermal inputs looming in the future it would behoove you to do a nice careful design in the boiler room to accommodate such additions--applying such principles as primary-secondary piping, hydraulic separation, "right- sizing" of pipes and tanks, and heat inputs ( esp. like the mod con boiler).
    And then put a lot of thought to the controls for the multiple systems interacting with each other. Generally consider: who has a "handle" on the system overview?
  • Jamie Hall
    Jamie Hall Member Posts: 16,617
    Several additional comments. Which don't change what I said about having an 80 gallon tank for this application.

    You have to consider -- and not confuse -- two different volumes and rates. First, what is the maximum demand on the system, and over what time period? 6 people … in my perhaps not so humble opinion you could easily soak up 90 gallons of hot water in a half hour. So you either need to have that in storage, or be able to recover produce that much that fast, or have some combination of the two.

    If you were to want to recover at that rate, you'd need an input on the order of 100,000 BTUh. You can get that with LP instantaneous units, but... do you want to? And you certainly won't get it from solar in the future unless you have a huge collector that would take 1600 square feet of collector in full sun.

    So... the obvious solution is an 80 gallon tank, either an indirect or stand alone LP fired unit, but with provision to connect to solar or geothermal in the future If you wanted.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England