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Assistance in sizing indirect with boiler

smar Member Posts: 2
I've done a lot of reading but I want to confirm the size of the indirect tank I'm trying to pair with a new boiler I will be getting when I convert from electric heat to NG.

The boiler will be around 55000 BTU. My existing electric DHW tank is 60 gallons and maybe a dozen times per year we run out of hot water usually during the winter months when longer showers and more bathes are being taken. I'm looking for the boiler to run at 160 F and the indirect set to 140 F. Highest flow could be 5 GPM for 10 minutes with 2 showers going. Washing machine is usually run on adjusted cold water (little hot water used to regulate incoming cold to a set cold temp). Dishwasher is done overnight independently of any other usage.

I'm thinking a 50 gallon tank should eliminate those dozen times where we ran out of hot water. Would I be correct going with this size? Will I experience "cool down" on the house heating while the boiler is recovering the indirect tank (assuming all the hot water was consumed) or the time it would take should have little to no effect on the house heating?

If it matters, cold water from my municipality can be as cold as 40 F so 100 F difference to get to 140 F.

Thanks for any guidance on this.


  • mattmia2
    mattmia2 Member Posts: 9,469
    Some one should be able to do the math on this. 5gpm for 10 min is a lot. My initial reaction is that is too small. That boiler is only somewhat more recovery than the electric tank. The tables in the manual for the indirect will be for a boiler about 3x that size.

    You definitely will need to run dhw priority. You will need a thermostatic mixing valve on the indirect to get a consistent dhw temp.
    EBEBRATT-Ed Member Posts: 15,345
    5 gpm at 10 min.

    4 gpm of 140 deg hot water =4x 140=560

    1gpm cold water @40 =40

    560 +40/5x

    600/5=120 degrees at the shower

    so for a 10 min you would use 40 gallons of hot water

    Its usually condidered that a 50 gallong tank will be 80% hot when you are drawing on it so 50 x.8=40 gallons

    So if your usage is 40 gallons as above your tank is borderline. Of course the boiler will start heating as you draw water

    If your boiler is 55,000 output it will recover 1.1gallon/min or 60.6 gallons /hour

    You may want to consider a larger tank
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,248
    edited January 2022
    Hi, I agree with @mattmia2 on this that 5 gpm for 10 min is a lot if it's just for two showers. Normally about 70% of the flow from a showerhead is hot water. So assuming 2.5 gpm heads, that's 1.75 gpm doubled or 3.5 gpm of hot water. If you're filling tubs, that dump load is big and the storage tank needs to be sized to accommodate it. Most people use a tub two or three times per year however, so for that use, you can turn up the tank temp to get the extra hot water.
    It may be that by using lower flow showerheads, you can go with a smaller tank. Just get showerheads that put out large water droplets, so they carry the heat further and give a nice shower. I use 1.5 gpm Delta heads and everybody's happy o:)

    Yours, Larry
  • hot_rod
    hot_rod Member Posts: 21,878
    You could increase the tank temperature to extend the useable hot water. With a listed mixing valve, of course 

    That size boiler should recover a bit faster then your previous 50 gallon gas fired tank, those are typically around 30- 40,000 btu/ hr before efficiency derate. 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,550
    You will have approx 3x the recovery of your existing electric unit. With only 10 gallons less storage, I think you will be happy with the 50 gallon indirect.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein