DHW Storage tanks - How is the usable storage tank capacity determined?

[immsk]

Hi,

Two questions:
Easy one first: Do you have a recommendation for a DHW heater sizing resource?

Second question: I am trying to determine the capacity for an existing Domestic hot water system.

I've been reviewing articles and literature to determine the actual capacity of usable hot water in a storage tank. Most sources say 70% to 80% of a tanks volume is usable - two examples below:

1. 80% according to AO Smith here
2. 70% to 80% here by Aerco Link here

I am trying to understand how this 70% to 80% number is determined.

Here are my assumptions:
1. Storage tank size: 100 Gallons
2. Hot water storage temperature: 140F
3. Cold water supply temperature: 40F
4. Lowest acceptable temperature in the tank during use: 110F
5. Assume there is no internal/external source of heating
6. No storage losses

So starting with 100 Gallons of water stored at 140F, I start drawing hot water from the tank. Whatever water I draw is replaced with an equivalent amount of cold water. Based on the image above once 30% of the volume of hot water in the tank is used and replaced with cold water, the tank temperature drops below 110F. Things I have considered:

[ul]
[li]When we initially start with 140F, the water drawn from the tank will be mixed with cold water to supply the fixtures at 120F - so we will be able to produce more usable hot water than is drawn from the tank. However I don't think accounting for this mixing will get me much closer to the 70%-80% range.[/li]
[li] There is stratification within the tank - therefore as cold water enters the hot water tank, it doesn't fully mix - the hot water within the tank stays hotter than a fully mixed tank would - not sure if this is accurate or how much of an impact this has
[/li]
[/ul]

If any one has any ideas on how this 70% to 80% number is determined feel free to chime in.

 

[LittleInch]

Your second point is the one which gets you 70 to 80%

The cold water enters at the bottom and assuming it's velocity is relativity low, the amount of mixing is low. Therefore the hot water at the top stays hot until about 70 to 80% of the original volume had left.

Normally these tanks are heated with an internal coil and use convection within the tank to heat the water over time. This flow is quite low and not enough to stir up the whole tank. If you added a small circulation pump or mixer then you could get the entire tank to be at a uniform temperature, but domestically no one bothers.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[immsk]

Thanks LittleInch, always appreciate the input - do you size DHW systems? Any recommendations for sizing guides?

 

[LittleInch]

No I don't size these things but I understand that there are many guides to doing so.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TBP]

I'd check the plumbing and/or building code for whatever jurisdiction the system is in.

 

[bgtengineer]

Domestic water heating sizing is a somewhat inexact science. You are looking to estimate the peak water usage for a system, which can vary for any specific application, even for similar building types. There are solid rules of thumb and empirical data available though, the best are available through water heater manufacturers, but the ASHRAE Handbook - HVAC Applications (chapter 50) also has some useful information. I lean into past experience and manufacturer sizing guidelines with these systems.

As a quick example, for a hotel the demand load is entirely determined by peak shower use, and you would estimate # of rooms, guests per room, peak usage %, showerhead flow rate, and average shower duration to determine the gallons used during the peak hour. For an apartment laundry you'd assume a # of cycles per washer per hour. For commercial kitchens you typically calculate the dump load of all fixtures.

The two components to domestic hot water system sizing are recovery rate (gallons per hour) and storage (gallons). Recovery rate will determine your power input in Btu/h, storage factors in the storage temp versus mixed water temp and the tank draw factor.

 

[PEDARRIN2]

As someone who does size these, there is no real consensus other than rules of thumb and/or past experience. The 70-80% is a bit older when storage tanks were not well insulated. Current rules of thumb push it more to 90% usable.

ASHRAE has some good charts that show how to balance storage vs generation - mainly due to the floor space storage takes up and also whether you have a large dump load or intensive requirements, i.e. showers in a hotel or dorm.

I generally use a spreadsheet that calculates a BTU analysis with generation and storage. That eliminates the % usable storage quandary.

All that being said, most of these rules of thumb are highly conservative and are based on a lot of assumptions and statistical data from around the 1950's. Had a water heater manufacturer who had accounts with some major hotel chains (Marriott/Hilton) who put flow meters on their systems to get actual data about usage, over many occupant loadings over multiple months. Needless to say, their data indicated much smaller water heating systems should be used.

But, as the old adage says, "Nobody complains about too much hot water." Just have to find the happy medium.

 

[immsk]

@bgtengineer Thanks!

@PEDARRIN2 thanks for you your input, Do you know where I could find the storage VS generation ASHRAE charts?

I am on the owners side now, back in my consulting days I had access to a spreadsheet that designers used for DHW load calculation. Being on the owners side I end up doing these analysis from scratch, which I actually like because you end up learning the science behind what the spreadsheet spits out.
Anyway I am wondering does your spreadsheet always give you a storage requirement as well? What do you do when sizing instantaneous heaters?

 

[PEDARRIN2]

My version is 2015 HVAC Applications, Chapter 50 Service Water Heating, Figures 16 through 23. The charts basically enable you to use gal/person or gal/unit and compare with gph/person or gph/unit depending on different parameters. That is a bit too general for me.

I designed my spreadsheet around dump loads like showers or emergency showers. If I have a larger kitchen, I can add that to the mix as well. I calculate the BTU of the inlet cold water, the generated hot water, the mix water at the shower. I use the flow rate of the shower, an expected peak period, shower duration and not being used during the peak period.

Then I look at the water heaters and/or storage tanks I am designing around and look at the btu generated and the btu available in the peak period.

I subtract the two. If I get a positive result by about 5%-10%, my design is good. If it is close, I might go up to the next higher input water heater or I might add a bit more storage. Because I look at generation and storage separately, I can add storage or generation as needed until I get a positive result.

If I am looking at an instantaneous heater, I just do not have any storage - so not a big deal.

 

[nuuvox000]

http://www.hotwatersizing.com

is AO Smith's (manufacturer) sizing software. Comparing to my own calcs it seems to be calculated using ASHRAE chap 50.