Trying to calculate design heat load. found calculater for buildings but not for dhw. Can someone Give me the formula or refer me to a calculater?
how to figure dhw heat load?
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There isn't any formula - it depends on how often and long showers are, dishwashing technique, DHW tank insulation, and a bunch of things that are hard to measure. A rule of thumb is that an average family of four uses around 60,000 BTU/day, but there's a lot of variation.
The "industry" rule of thumb is 20 gallons per person per day. I thnink that is quite a bit high. Maybe 20 gallons for the first two, 15 for additional. It really varies from family to famiuly. Teenagers cab skew the calculation a lot 
As for the amount of energy required. Heating 80 gallons from 55 °F to 140 °F
8.33 x gallons X (140-55)
So 56,644 BTUs required.
Most residential water heaters have a 35-40,000 BTU/hr burner. High recovery units 60- 75,000 BTU/hr burners just for an idea.
A 4X8 solar collector can provide roughly 15- 20,000 BTU per DAY depending on location, time of year, delta t, etc
hr
As for the amount of energy required. Heating 80 gallons from 55 °F to 140 °F
8.33 x gallons X (140-55)
So 56,644 BTUs required.
Most residential water heaters have a 35-40,000 BTU/hr burner. High recovery units 60- 75,000 BTU/hr burners just for an idea.
A 4X8 solar collector can provide roughly 15- 20,000 BTU per DAY depending on location, time of year, delta t, etc
hr
AMEN on the teenager skew. I have a customer who called about a year ago claiming that the 80gl indirect was not performing as it once did. We had installed it about 7 years ago when the 4 kids in the house were ages 5-11. Worked fine then but seemed to not "put out" like it used to.................Hmmmmmm........I wonder why?
Here's a site that has a wealth of hydronic info on it and everyone should bookmark. Check out pages 39-42 for the DHW calulations based on occupancy and other household factors. Add at least 30% for teenagers.
http://www.burnham.com/PDF/htghelper.pdf
PS: If you find it useful, please take a couple minutes to drop a note to the good folks at Burnham Boilers to say thanks for sharing all the knowledge in that little book on the www . You can also order a hard copy from them directly for nominal cost.
Here's a site that has a wealth of hydronic info on it and everyone should bookmark. Check out pages 39-42 for the DHW calulations based on occupancy and other household factors. Add at least 30% for teenagers.
http://www.burnham.com/PDF/htghelper.pdf
PS: If you find it useful, please take a couple minutes to drop a note to the good folks at Burnham Boilers to say thanks for sharing all the knowledge in that little book on the www . You can also order a hard copy from them directly for nominal cost.
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