Domestic hot water load estimating

hot_rod

This question come up from time to time. With todays more efficient homes, the dhw load could exceed the heating load. So that needs to be taken into account.

Knowing how many gallons per day a family uses is one key. Rule of thumb loads for years have been 10- 20 gallons per person, per day. That varies based on how long you shower, fill a big tub, clothes washing, etc.
Simply putting a water meter on the supply to a water heater for a month or so could tighten up the numbers.

Lets use a family of 4 at 15 gallons per person per day
incoming water at 55°F heated to 120.

(4 X 15) (8.33) (120-55) = 32,487 btu/ day.

If you want a yearly number, subtract out days that you are not home. Water heater in vacation mode

(32,487) (335days) = 10,883,145, call it 10.9 MMBtu/ year

Next is the cost of the energy.
Electricity at .10Kwhr 96% efficiency (4% jacket loss)

(10.9MMbtu/yr) ($30.52) = $332.66 per year!

So you often hear estimates of 3- 400 dollars per year for average family of 4 for DHW.

Your mileage may vary.

at .30/Kwhr = $998.04!
At .30/ Kwhr it could be good time to consider a HPWH