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Calculating Domestic Hot Water Usage
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Jacob Myron_8
Member Posts: 1
This method is based on an old Method used in the New York City Housing Authority.
It is Called the Shower Head Method. Unfortumately when one tries to anticipate what people need you have to assume the personal habits of the users at a worst case senario.
The average shower takes about 15 minutes. A water saving shower head uses about 3 gallons of water per minute of which 1.5 gallons of water will come from the hot water maker. A safe temperature of hot water is about 120 degrees F.
Typically the hot water maker is set at about 140 degrees and a mixing valve should be used at the outlet of the heater to temper the water to 120.
People will mix some amount of cold water to the hot water at the shower.
To supply one gallon of water per minute at a 100 degree rise 50,000 BTUH needs to be supplied to the hot water maker.
Most houses have two or more shower locations in them.
Now averaging occurs. How many showers will likely be used at the same time?
That depends on how many people live in the house.
A House with two occupants ( 1 Shower )
A house with 4 occupants ( 1 1/2 Showers )
A house with 6 occupants ( 2 showers )
A HOUSE WITH 8 OCCUPANTS OR MORE ( 3 SHOWERS )
Lets use a house with 4 occupants
1 Shower uses 1.5 GPM of hot water
the second shower will use 1.5 GPM of hot water for part of the time if two people shower at about the same time so we can cut the water production in half to 3/4 a GPM.
We have to supply 2.25 GPM of hot water for 15 minutes, that translates to 150 gallons of water usage by the hour or about 38 gallons of water in 15 minutes. The BTUH input is 50,000 times 2.25 or 112,5000 BTUH per hour.
We can supply hot water via a submersed coil or indirect heater attached to the boiler.
Assume a house needs 100,000 BTUH Net for heating and the hot water requirements are 112,500 BTUH net.
If we do not want to sacrifice heat the boiler size would be net 212,500 BTUH. That is a real oversized boiler because the shower will last for 15 minutes, heat recovery in the house will happen pretty quickly when the domestic hot water production is stopped.
Remember the boiler has energy stored. We need 100,000 BTUH
for heating and the burner will supply that. Of the 100,000 BTUH about 30% of that is used for a piping pick up factor.
By the above numbers a 100,000 net boiler will produce 2 GPM of domestic hot water and not supply heat for 15 minutes.
Add another 50,000 BTUH to the boiler size for the coil or indirect heater. The boiler will size out at 150,000 net.
Add another fly in the ointment.
Some one fills a bath tub while a shower is in operation.
The bath tub will draw about 3 GPM of water. So the safe minimum would be about 4.5 GPM of hot water.
To cover that senario or a washing machien and dish washer operating at the same time we need to supply 4.5 GPM of water or an 250,000 BTUH for domestic hot water production.
That means the boiler must supply 250,00 BTUH net at a minimum.
By installing a 4.5 GPM restrictor at the discharge of the hot water maker we prevent the boiler from dumping all the stored and inputted energy.
There are other methods of sizing down the boiler but we will need to use storage heaters and a circulating pump.
If we use a 75 gallon storage heater that wil store water at 140 degrees with a mixing valve at the discharge we can down size that boiler to about 150,000 net.
Remember we used in the worst case senario about 65 gallons of hot water and recovered about thirty gallons of hot water. Or, took out 65,000 BTUs from the storage tank. We need to put 65,000 BTUs back on a continuous basis.
The stored water in the storage tank dropped to about 115 degrees and we only need to by back 5 degrees to supply ou minimum.
The recovery period for the whole storage heater will be about 20 minutes and that will give you 75 gallons of water at 140 degrees.
Usually we will deal with about a 25 gallon draw at the same time.
HOt water supply has to allow for some amount of a worst case senario.
I hope I helped you a bit with calculating hot water needs.
Not every home is the same and the manufacturers need to cover them selves a bit.
We are stuck with burden of selecting equipment and the pricing. We have to keep the customer happy and be competitive. Some times we need to talk to the customer and explain to them what they are getting for the money.
IT DOES NOT MAKE A DIFFERENCE HOW YOU DO IT....
The bottom line is we need to supply 50,000 BTUH for every GPM or hot water supplied.
Jake
It is Called the Shower Head Method. Unfortumately when one tries to anticipate what people need you have to assume the personal habits of the users at a worst case senario.
The average shower takes about 15 minutes. A water saving shower head uses about 3 gallons of water per minute of which 1.5 gallons of water will come from the hot water maker. A safe temperature of hot water is about 120 degrees F.
Typically the hot water maker is set at about 140 degrees and a mixing valve should be used at the outlet of the heater to temper the water to 120.
People will mix some amount of cold water to the hot water at the shower.
To supply one gallon of water per minute at a 100 degree rise 50,000 BTUH needs to be supplied to the hot water maker.
Most houses have two or more shower locations in them.
Now averaging occurs. How many showers will likely be used at the same time?
That depends on how many people live in the house.
A House with two occupants ( 1 Shower )
A house with 4 occupants ( 1 1/2 Showers )
A house with 6 occupants ( 2 showers )
A HOUSE WITH 8 OCCUPANTS OR MORE ( 3 SHOWERS )
Lets use a house with 4 occupants
1 Shower uses 1.5 GPM of hot water
the second shower will use 1.5 GPM of hot water for part of the time if two people shower at about the same time so we can cut the water production in half to 3/4 a GPM.
We have to supply 2.25 GPM of hot water for 15 minutes, that translates to 150 gallons of water usage by the hour or about 38 gallons of water in 15 minutes. The BTUH input is 50,000 times 2.25 or 112,5000 BTUH per hour.
We can supply hot water via a submersed coil or indirect heater attached to the boiler.
Assume a house needs 100,000 BTUH Net for heating and the hot water requirements are 112,500 BTUH net.
If we do not want to sacrifice heat the boiler size would be net 212,500 BTUH. That is a real oversized boiler because the shower will last for 15 minutes, heat recovery in the house will happen pretty quickly when the domestic hot water production is stopped.
Remember the boiler has energy stored. We need 100,000 BTUH
for heating and the burner will supply that. Of the 100,000 BTUH about 30% of that is used for a piping pick up factor.
By the above numbers a 100,000 net boiler will produce 2 GPM of domestic hot water and not supply heat for 15 minutes.
Add another 50,000 BTUH to the boiler size for the coil or indirect heater. The boiler will size out at 150,000 net.
Add another fly in the ointment.
Some one fills a bath tub while a shower is in operation.
The bath tub will draw about 3 GPM of water. So the safe minimum would be about 4.5 GPM of hot water.
To cover that senario or a washing machien and dish washer operating at the same time we need to supply 4.5 GPM of water or an 250,000 BTUH for domestic hot water production.
That means the boiler must supply 250,00 BTUH net at a minimum.
By installing a 4.5 GPM restrictor at the discharge of the hot water maker we prevent the boiler from dumping all the stored and inputted energy.
There are other methods of sizing down the boiler but we will need to use storage heaters and a circulating pump.
If we use a 75 gallon storage heater that wil store water at 140 degrees with a mixing valve at the discharge we can down size that boiler to about 150,000 net.
Remember we used in the worst case senario about 65 gallons of hot water and recovered about thirty gallons of hot water. Or, took out 65,000 BTUs from the storage tank. We need to put 65,000 BTUs back on a continuous basis.
The stored water in the storage tank dropped to about 115 degrees and we only need to by back 5 degrees to supply ou minimum.
The recovery period for the whole storage heater will be about 20 minutes and that will give you 75 gallons of water at 140 degrees.
Usually we will deal with about a 25 gallon draw at the same time.
HOt water supply has to allow for some amount of a worst case senario.
I hope I helped you a bit with calculating hot water needs.
Not every home is the same and the manufacturers need to cover them selves a bit.
We are stuck with burden of selecting equipment and the pricing. We have to keep the customer happy and be competitive. Some times we need to talk to the customer and explain to them what they are getting for the money.
IT DOES NOT MAKE A DIFFERENCE HOW YOU DO IT....
The bottom line is we need to supply 50,000 BTUH for every GPM or hot water supplied.
Jake
0
Comments
-
Aha, but the
American Medical Association has determined that anything over 111 F is considered 'scalding' and has now opened you up for a lawsuit.
Be careful out there, put on a high quality mixer or start looking for a good liability lawyer, FACT!
1/2 of all of my currently open cases as an 'expert' are for scalding, BIG FACT! CYA, baby, CYA!0
This discussion has been closed.
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