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Community and Q&A

Sizing a gas water heater for a bathtub

BioBob | Posted in General Questions on
I want to buy a 30-gallon gas tank-type water heater only to supply 2 isolated bathrooms.  How can I know if 30 gal. will be big enough for a hot bath in January?  Specifics; 
My bathtub is full to overflow with 30 gal of water plus 17 gal. of me. 
How hot would the bathtub water be if I turn on the bathtub fill at 5 GPM, and I run 30 gallons from only the hot tap into the bathtub, considering the mixing of cold into the HWH tank as I draw off all 30 gallons?  Assumptions; 
1. This is the only draw on this WH. 
2. Pipe run will be very short–about 10 feet; I’ll put this second WH in a closet next to the bathroom. I’ll use 1/2″ PEX, but I doubt that matters much (thermally) for so little pipe. 
3. I’ll set the thermostat to 140 F to balance legionella safety and standby loss.  Assume the WH has reached it’s set-point; all 30 gal. start out at 140 F. 
4. My cold water input is 73 F now in September, but I’ll plan for the water input to be as cold as 50 F in mid-winter (in Modesto-Fresno part of CA).

I want to know BEFORE I buy the WH how close it might come to being undersized for taking a bath.  I’m dumbfounded that none of the standard HWH metrics answer this question.  First-hour rating? Useless; I want to fill the tub and climb in in 5-7 minutes. GPH recovery rate? Again, useless for this.  Rule of thumb- “70% net usable capacity” for tank-type HWH?  I assume that means that the first 21 gal out of my 30-gal tank will be close to 140 deg. How close?  And what about that last 9 gal?  
Puzzling: this seems a simple number for a mfr to measure (at a standard assumed temp rise, e.g., 90 F rise), and it’s a crucial number for knowing what size WH to buy to assure an adequately hot bath. 

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  1. Expert Member
    Akos | | #1

    You generally fill a tub with around 105F water but you keep the tank at 140F. This means you can get much more out of a 30 gal usually 40 gal. You can even stretch this by increasing your tank temperature. Provided you start with hot tank, you should be ok to fill the tub.

    None of the manufactures numbers will tell you if you'll run out of water as none of them use a realistic test for this. The 1h rate is the closest but it assumes too much recovery, a tub gets filled in 15 to 20min, so you only have about 1/4 of the recovery time.

    However, a tank that small will run out not from tubs but from showers. It doesn't take that long to use up 30gal of hot water with two showers even with a reasonably low 2gal/min shower head. There is not much price difference for a 50 gal, which is guaranteed to hold up. Unless you are really tight on space, I would install the larger unit.

    If you must have a very small tank, look for ones with a big burner. Something like this can run a single bath filler indefinitely:

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