Two issues that you are missing:

1. A multi-head shower sucks a HUGE (read ridiculously wasteful) amount of water. Yep, they can empty a water heater right quick! I have a hard time feeling sorry for someone who dumps 140 gal of heated water down the drain for two people to take a shower. Why don't you just flush $10 bills down the john and be done with it.

2. As I mentioned in a previous post, you are mixing hot water with cold and so 70 gal of shower water @105 isn't equal to 70 gal of hot water in a tank (@140). Assuming the water heater is set to 140, the incoming cold water is 60 and the shower temp is 105, you would be mixing 1.1 gal hot water to .9 gal cold water.

1. Yea.. I know the multihead shower can suck huge amounts of water down. Thats why its hard to size the system correctly because the peak demand can be very high, while typical usage is "normal".

Flushing money down the toilet is what my reef tank is for :) Burning 2 kilowatts per hour just for the lighting, without chillers, pumps, controllers, etc. And RODI water production wastes countless gallons per gallon of purified water. Yea, somehow I think the shower usage is the least of my concerns :)

2. Thanks.. That is the issue I am having difficulty calculating. It doesn't make sense to assume that the entire showerhead output is just hot water yet it seems to be how a lot of website oversimplify the process to. So calculating how much hot water is used per gal of shower water is the trick.

Assuming the incoming water temp is 60 works for a typical day summer day, but you have to plan for the worst, which is 40 degree water in the winter. In which case, for a water heater set at 140, shower temp of 105, it appears that the mix is 65% hot, 35% cold.

Great ! Thanks a lot. You redirected me to thinking this through correctly. Something like an AO Smith Vertex 100 would work well since it can keep up with a single shower head no problem and then has the storage tank to deal with any peak usage.

"It seems the 1st hr rating is overly simplistic since it doesn't really express how much the water heater is replenishing on a per minute basis."

Assume that the 'ten minute rating' is simply the contents of the tank unless the input power for heating is VERY large (and that would give a VERY large first hour rating).

The energy cost will actually be about the same.
It takes the same amount to heat the water no matter how fast you heat it.

One hour at 5,000 W is 5 kW-hrs, the same as 20 minutes at 15,000 W.

In ten minutes the heater is not going to be able to heat incoming water very significantly.

After even more research, I figure the best solution is going to be to install a tankless water heater with a large capacity ~200k BTU burner that can provide 5gpm @ 70 degree rise. Its just the only way of getting this done. The only thing is to figure out how to eliminate the cold water sandwich by use of a recirc loop for heating the basement or something.

Or you could do what I did... but an Eternal Water Heater. It is a hybrid. Best of both worlds. High GPM (9.5 GPM rating), with no pressure drop and no small cavities to clog like typical tankless units. Check it out at www.eternalwaterheater.com Very expensive, however, but not much more than the Vertex, and doesn't take up nearly as much room.

Good luck.....

Here is a link that might be useful: Eternal Water Heaters

Brickeyee,

These hybrid units are 100,000 BTU @ 95% and would heat about 3.8 gal per minute at 45 degree rise or 2.5 @ 70 degree rise. That gives you a 30 minute rating of about 125 gal.

drtechno,
The 200k btu/h tankless won't supply your master bath shower, It would be enough to supply two showers at 2.5 gpm at 70 degree rise. I think the answer is more than one hybrid if you really need that amount of first 30 minute rating.

countryboyok1,
The Eternal are similar to other hybrid gas heaters except they have a higher BTU input - the 200k isn't going to produce 9.5 gpm though @ a 70 degree rise it produces 5.5 gpm.

"That is the issue I am having difficulty calculating. It doesn't make sense to assume that the entire showerhead output is just hot water yet it seems to be how a lot of website oversimplify the process to. So calculating how much hot water is used per gal of shower water is the trick."

Not hard at all.

If you have 140 F hot and 40 F cold and mixed them equally you would get 90F water.
The ratios just scale to other outlet temperatures.

"These hybrid units are 100,000 BTU @ 95% and would heat about 3.8 gal per minute at 45 degree rise or 2.5 @ 70 degree rise. That gives you a 30 minute rating of about 125 gal."

What hybrid units? The OP was talking about a plain old tank it appears.
The Âinstant units are very different.

Sorry Brickeyee, the OP mentions the Vertex 100 in his second post and other posts on the board. I just happen to be tracking with him.

The hybrid units in this case are a high input, high efficiency gas heater with a tank. Kinda like a condensing tankless with a tank.

I think that the point of the original post is that if the first 20-30 minutes is what's really important, how do you get to a number using "first hour rating".

drtechno, to answer another question... in the case of a tankless, you don't worry about calculating mixing ratios. You just assume an output temp of 105 and compare that to the input temp to get your temp rise. In practice, you will probably set a tankless to 120 or so and mix in cold, but the numbers work the same if you calculate the entire gpm at 105.

Thanks all !

Continuing to work this through:
"The 200k btu/h tankless won't supply your master bath shower, It would be enough to supply two showers at 2.5 gpm at 70 degree rise. "

From my understanding, a large tankless unit would be fine since a shower running @ 7.5gpm @ 110 degrees is only using ~5gpm of hot water (assuming 120 input temp and 40 degree base water temp). The larger units from Takagi, Navien, etc. can supply ~5-5.5 gpm @ 70 degree rise.

I like the idea of a hybrid system (like the eternal units, thanks for the link!) since it eliminates any wait for hot water at a lav faucet (since we will probably be hooking up a half bath to the same hot water circuit).

Is "delayed hot water" a moot point in a manifold direct home run system (our setup) ? I heard it decreases the wait time for hot water, but not sure if it reduces it to a non-issue.

"in the case of a tankless, you don't worry about calculating mixing ratios."
Not sure why. This ruins my analysis if you do this and eliminates the tankless option :)

drtechno,

How cold is the water at the coldest part of the year? If it is 35-40 degrees (I've seen mine get near 32) then a 70 degree rise would bring it to 105-110...not 120

On a tankless, you assume a shower temp of 105 as if the tankless was outputting 105 degree water and forget about mixing. So, a 200k btu/h tankless could heat ~5 gpm from 40 - 110 degrees (which would give you a little extra for heat loss in the pipes and the option to run more than 105 if you like hotter showers) which is enough to run two standard showers, but not your 7 gpm master shower... and certainly not all three.

The reason that you don't worry about mixing ratios on a tankless is because you aren't storing any water. On a tank system, it makes sense to store 40 gal of 140 degree water and mix it down to 105. There's no benefit to doing this on a tankless. Some of them even have bathroom controls so you can set your shower all the way to HOT and then set the water heater to output the exact temp you want.

In practice, you generally set a tankless to 120 or so for the dishwasher and washing machine - but it doesn't change the capacity of the heater. You are running less water through the heater, but it's at a higher temp rise so the math is the same.

The truth is that a 200k btu condensing tankless will supply XX gallons of hot water, regardless of manufacturer. As jake stated, no reason to mix the water, just use the temp you want coming out of the shower and it will tell you the flow rate you can expect.

In other words, 200k btu is 200k btu, regardless of how much mixing you do. Lets say you want 105F water. Let's say you take 140F water and mix it with 70F water. Your unit flows 5GPM at 140F. That means you will get 10GPM at 105F (half 140F and half 70F). Your unit will be rated for 10GPM at 105F and a 35 degree rise. Again, it doesn't matter how you mix it, 200k BTU is 200k BTU. You can't magically get more 105F water by mixing it.

Hopefully that all makes sense and my engineer brain didn't complicate things. :)

For the same reason, an Eternal 200k BTU condensing unit will flow the same as a Navien or Rinnai 200k BTU condensing unit. Only advantages to the Eternal are no cold water sandwich (since it has a 2 gallon tank), no small passages to clog or flush, and no pressure drop associated with those small passages. That's the reason I went with the Eternal. 200k BTU condensing tankless without the negatives of tankless.

Good luck

Ah thanks. Great explanation.. Now it all makes sense.

I love how only CO is considered in the 'carbon footprint.'

If the thing is burning fuel it is throwing out a lot of carbon as CO2.

Very amusing.

Yo, Brickeyee,

Where did that come from?!?
Natural Gas is mostly methane which produces carbon dioxide and water when it combines with oxygen.

Yes, anything that starts as carbon has a carbon footprint. Who said anything otherwise?