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Old 06-28-2010, 11:33 AM   #1
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Hot Water Solution for Spa Shower


I am installing a Moen 275 Spa Kit in the master bath (3/4" supply lines). I was initially thinking of replacing the rental 50G hot water tank with a tankless solution. However, the flow rate for the shower and body sprays combined is 9.75gpm. I'm in Canada, so getting that flow rate with a 75F rise (water supply is ~8C in the winter months) seems to push me into a commercial 250K BTU tankless unit (mega $$$).

Anyone have any solutions? Am I better off just installing a high efficient 60G tank? Any recommended models?

Dave

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Old 06-29-2010, 09:05 AM   #2
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Hot Water Solution for Spa Shower


93* is not very much for a shower.... typically from 100-110 is what is used.

Tankless are generally a solution to conserve energy and water. Your spa system is the exact opposite. A huge water waster.

Are you adding a second water heater or replacing the main tank with a larger unit?

For 9.75 GPM a 60 gallon unit is only going to get you about 6 minutes.
And that is if nothing else in the house is being used.

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Old 06-29-2010, 09:48 AM   #3
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Hot Water Solution for Spa Shower


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93* is not very much for a shower.... typically from 100-110 is what is used.
I take it your 93* (that old fahrenheit scale huh ) is the 8C raised by the 75F of the tankless unit. I think the unit will just produce a lower flow rate at the higher rise. Of course that is bad for me!

Quote:
Tankless are generally a solution to conserve energy and water. Your spa system is the exact opposite. A huge water waster.
But it is only wasteful while I am in the shower. Even if I take a 15 minute shower, isn't that better than heating the tank for 60*24= 1440 minutes per day?

I would have to work through the numbers I guess recalling all my thermodynamic courses from 30 years ago. Anyone know the formulas for calculating the BTUs for heating a tank of water and keeping it hot for the entire day?

Quote:
Are you adding a second water heater or replacing the main tank with a larger unit?
I'm thinking of replacing the 50G with either a 60G or a tankless. There are some 50G rapid recovery units also that "work like a 75G" according to the manufacturers.

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For 9.75 GPM a 60 gallon unit is only going to get you about 6 minutes.
And that is if nothing else in the house is being used.
Agreed. But won't those 6 minutes be great! I may have to live without the body jets over the winter when the cold water is ~8C, or use them sparingly. Since my only son is moving out soon, there will just be the 2 of us in the house.
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Old 06-29-2010, 02:23 PM   #4
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Hot Water Solution for Spa Shower


With that kind of demand per minute, your out of the residential heaters into the commercial units. Anything you do will be costly. One option is a unit that has high storage and recovery rates with very high thermal efficency. For a 15 min. shower, your looking at 150 gallons per shower. You would need 100 gallons or more of storage with a 150,000 BTU gas input, which will recover 240 gallons per hour or 60 gallons in that 15 minutes. This setup you could run the shower for 15 minutes and get 160 gallons of hot water out of a 100 gallon tank. With these larger units you need to think about gas supply and pressures, commercial units require higher gas flow rates and pressures to keep up with the demand.

Also, you need to look at the drain size, a standard shower drain for a 2.5 gpm shower is not going to keep up with a 10 gpm shower head
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Old 06-29-2010, 02:56 PM   #5
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Hot Water Solution for Spa Shower


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With that kind of demand per minute..
Also, you need to look at the drain size, a standard shower drain for a 2.5 gpm shower is not going to keep up with a 10 gpm shower head
I think I am pretty much convinced that I'll be going conventional rather than on-demand and also taking shorter showers!

What would be the code requirements for the drain(s) on this type of spa set?

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Old 06-29-2010, 03:59 PM   #6
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Hot Water Solution for Spa Shower


You need to provide a trap and drain size capable of draining the 10 gpm of water. A standard 1-1/2" trap and pipe will not cut it, you need at least a 2 inch trap and 2" drain pipe pitched at 1/4" per foot. If I was building that shower from scratch, I would put in a 3" trap and 3" drain to allow for some slight buildup of debris such as hair and soap scum in the line without restricting the drain flow. Take a look at this link below, a 2" trap can handle from 5.7 gpm up to a 12.3 gpm shower w/bodysprays.

http://www2.iccsafe.org/states/virgi...20Drainage.pdf

I'm going to put a 55 gpm shower in my next house
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Old 06-29-2010, 06:30 PM   #7
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Hot Water Solution for Spa Shower


Quote:
But it is only wasteful while I am in the shower. Even if I take a 15 minute shower, isn't that better than heating the tank for 60*24= 1440 minutes per day?
you don't heat the water 60 minutes an hour 24 hours a day. You only replace lost heat. If your tank is well insulated and if you are not using water, the tank should stay hot for a long time without added heat input. The better insulated it is, the less energy consumed to maintain temp.

Quote:
Anyone know the formulas for calculating the BTUs for heating a tank of water and keeping it hot for the entire day?
It would vary greatly depending on how well your tank in insulated.

What I would suggest is 2 50-60 gal water heaters in series. That way you would have up to 120 gallons of hot water. Since you would use cold to temper the hot water, even using an arbitrary percentage such as 25% cold to 75% hot, that would give you 150 gallons of water for your shower. If you have a flow rate of 10 gpm, that would give you a 15 minute shower.

I would also set up the heaters to run with the last heater being primary and the first one being secondary, just as with a dual element heater where the upper element is primary and the lower secondary. That would mean if you ran out of hot water, the second heater would run first to heat water. That would allow you to use hot water as soon as it produces hot water without having to run the 60 gallons in that tank trying to draw hot water from the first heater.


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The Engineer

I'm going to put a 55 gpm shower in my next house
do you have a thing for cold showers or do you have a nuclear reactor water heating system?

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