Help Sizing a Tankless Water Heater

sjsmithjr said:

I can't remember if it was on this thread or another related thread but the observation was made that plumbing codes didn't address tankless sizing. Another poster observed that 8 gpm (Bradford White calculator) was unrealistic if I wanted to supply a shower, a dishwasher or kitchen sink, and a clothes washer simultaneously (peak demand).

I then remembered that we had done some work in Florida a few years back. I humbly submit to you a Residential Tankless Water Heater worksheet, 2001 Florida plumbing code. Adjust as necessary for your minimum incoming water temp and required temperature rise; the physics and math will work across state lines. I think the worksheet would make a great sticky.

I'm done, here.

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Nice, thank you for posting that. I will check to see if Illinois has something like that. The last continued education class I was at they never did bring this up. I may take another continued education class early to pick the brains of the plumbing code writers.

sjsmithjr said:

I can't remember if it was on this thread or another related thread but the observation was made that plumbing codes didn't address tankless sizing.

I'm done, here.

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It was me on the other thread, after looking at that pdf I posted my findings with a bit more research in the Illinois plumbing code. https://terrylove.com/forums/showpost.php?p=184396&postcount=217

sjsmithjr said:

I then remembered that we had done some work in Florida a few years back. I humbly submit to you for your use a Residential Tankless Water Heater worksheet, 2001 Florida plumbing code. Adjust as neccessary for your minimum incoming water temp and required temperature rise; the physics and math will work across state lines. I think the worksheet would make a great sticky.

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I don't think a 50 degree temp rise will work across state lines
It certainly won't in the North

Bottom line is a properly sized system will supply enough hot water. I have 3 showers, 1 jacuzzi tub (has a heater), 6 sinks, dishwasher & washer

Scuba_Dave said:

I don't think a 50 degree temp rise will work across state lines. It certainly won't in the North.

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Work with me here, Dave. That's why I said to adjust the incoming water temp and required temperature rise.

The GPM demand won't change.

It should also be apparent that claims of supplying adequate hot water (read 120F) at the prescribed flow to all fixtures with a single tankless have been, in many cases, greatly exaggerated. Maxed out 7.5 GPM/120F; you can almost handle 4WSFU, which would be a single bath group, a kitchen sink or dishwasher, and a clothes washer. Max'd out at 2.1/120F; I guess someone could take a shower and someone could wash their hands at the same time but it's probably not an appropriate choice for a whole house water heater.

Anyhow, y'all go have yourselve's a nice day and Greg, don't go electrocutin' yourself. Pullin' a meter can result in a man killin' himself. Do me a tiny favor and update that blog of yours to include the hazards of pullin' an electric meter. Maybe remind the folks out there what can happen if that meter is under load and that the service feeds are still hot and such. Heck, might not even be a bad idea to let the electric company come out and pull it for ya. They'll do that, you know. Maybe have licensed sparky take a look at your work when your done. If not in person then maybe on one of those world wide web forums.

It's like my momma has always said: "It's easy if you don't know what your doing!"

Scuba_Dave said:

Missed that...did you edit ?

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Entirely possible. I have a horrible sense of time. My bad.

Scuba_Dave said:

I have 9 WSFU, do I calc based on all fixtures?
If so I am off the chart

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I imagine only the AHJ in your area knows for sure, but...

Your total demand for 9 WSFU is 13.7 GPM (IPC Table E102 goes beyond 5 WSFU). I believe you could use the "probability of simultaneous use" if you are allowed to not supply all fixtures and wanted some reasonable means by which to determine a probable demand load. The probablity that two general use fixtures will be in use at one time is 100%; four fixtures is 70%; eight fixtures is 55%; and, so on. Half of 9, round up to be conservative, that gives me 5 WSFU and using the Florida work sheet you'd need 9.4 GPM and the blessing of the AHJ. I'm not a plumber, however, and I have this sinking feeling I've got this wrong and NHMaster is about to send me to the dunces corner. Is that a C-130H I hear? Oh no, he hitched a ride with Redwood!

Sewer Rat noted on another thread that inspectors in his area require 120F at the kitchen sink with all other fixtures running; didn't specify what delivery rate, if any, had to be met.

Since we haven't placed half the licensed plumbers that contribute to these forums on ignore, you and I may very well learn something new today.

Well after reading through the code book again, there is no minimum requirement for gpm flow just for PSI The following is from the Illinois code book.

Minimum Water Pressure. The minimum constant water service pressure on the discharge side of the water meter shall be (at least) 20 p.s.i.; and the minimum constant water pressure at each fixture shall be at least 8 p.s.i. or the minimum recommended by the fixture manufacturer.

Oh and here is the part from the definitions part of the code.

"Hot Water": Water at a temperature of not less than 120°F.

"Tempered Water": Water ranging in temperature from 85°F to, but not including, 120°F.

One thing that drives me crazy about the Illinois plumbing code is they do not have all the information in a nice and easy to read sections. Like when they talk about water heaters, they talk about how its to be installed and what safety features need to be observed, if you want to find out about water temperatures, you need to go to the fixtures part of the code, and so on. So you bounce from one section to another just to piece together the code and hope you are interpreting it correctly. I can not wait for their next rewrite of the code, I hope the clean it up a bit.

So for someone to pass the 120 test they could install a .5gpm tiny flow on each shower & same or lower on the sinks. Then remove them after the inspection

120 degree water at the sinks the shower is to be set to a max temperature of 115, if I said minimum I am sorry I mistyped. Public showers are to be set at a max of 110 degrees.

To be considered "hot water" It suppused to be 120 degrees or more at the sink. Showers is max, sinks are min.

Ladiesman271 said:

Not sure what the logic there is to having a minimum water temperature of 120 degrees water at the kitchen sink.

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I'll give you hint. My wife is an executive chef and she know's the answer. 120F is the minimum.

Ladiesman271 said:

Here is part of the State of Minnesota version of proper water heater sizing.

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Your link isn't to any code requirment but is an energy guide. Care to share with the class what the State of Minnesota has to say about tankless water heaters in their energy guide? Here, I'll save you the trouble of cutting and pasting:

"Another major drawback is capacity. A tankless heater typically provides 1-2 gallons of hot water a minute. You may find this adequate. However, you may not have enough hot water for more than one use at a time. Before installing a tankless water heater, make sure its capacity will be adequate for your needs."

Ladiesman271 said:

Maybe you have trouble reading all of the words, or perhaps you have a problem with comprehension?

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Not at all, before we go any further, however, may I suggest you read the following re decorum
.

The Minnesota document referenced is dealing primarily with topic of energy conservation and makes recommendations for reducing energy demand. These recommendations may or may not be agreement with current best practice re sizing of water distribution systems nor is there any certainty that such recommendations will be in agreement with code requirements such as those promulgated by State of Massachesetts.

If you are truly interested in sizing hot water demands, I would suggest that you obtain a copies of the American Society of Heating, Refrigeration, and Air Condition Engineers Applications Handbook, the International Plumbing Code (IPC), the Uniform Plumbing Code (UPC) and the American Society of Plumbing Engineers handbooks that deal with this topic. You will find that each of these use probability theory to account for the various unknowns, which has been briefly mentioned in this forum. Depending upon your background, you may find it beneficial receive some formal instruction on the topic as well.

I'll be more than happy to have a discussion with you, but continuing to berate longstanding members while contributing little more than a series of incongruous statements, nonprofessional opinions, and weblinks does little to enhance your credibility on this or any other forum you participate in.

you simply havent got a clue

Ladiesman271 said:

The question is not how to "properly" size a water heater. The question is what is the minimum size required by "the codes" that apply to your area.

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codes are not written in concrete .....
and are usually open to intrepretation

Ladies man, you are truely an intellegent person
and I think that I speak for everyone here ....

we stand in awe of all your apparent knowledge....

or at least your ability to find info on Google



having a fun time
trying to pull everyones chain.. I dont understand why......

you are trying to make tankless something akin to
space shuttle technology.....

its simply a bathroom,
and everyone of them in the USA is different ......

get it???


ok lets try to factor in all the variables....THAT ARE NOT IN THE CODE..

lets try to equate the flow rate when grandma uses and flushes the toilet while you
are showering with the tankless...


Now lets factor in a well, and now lets factor in
pressure balanced shower faucets....


Now factor in grandmas bowel movement not going down
all the way with one flush...and she flushes three times
to get that to go down....


then she jumps into the other shower and attempts to
take a bath while you are in the other bathroom


so whats the pressure drop going to be..?????

how bout that flow rate through the tankless?????


man I can see that curve drop in my mind right now..

so whats going to happen to the guy showering if the tankless is undersized??? ....






When a simpelton like me sells a water heater

the first thing I ask is how many people are presently living in the home,
and how old everyone is living in the home....

if they have a few children about 9 years old....
I KNOW that the demad will be going up in about two years .
..


and I KNOW that the 40 gallon heater will not suffice.
..

so we usually go one size larger for the varaible of

larger increased demand when the childred become pre-teens in a few years

so they buy a 50 gallon..





Now all I state....

if you are putting in a tankless, I simply suggest
you oversize the unit for future issues and other variable factors....
like grandma and pressure drops....



but this is just too darn simple for you.....

isnt it???..
__________________
The reason greener pastures always appear to look greener is because of all the manure spread out in that pasture... its only upon closer examination do you realize what you have gotten yourself into....
L

Code is a MINIMUM standard, but that does not mean it is optimum for the way most people live. This is especially true if you look at those values they list for hot water. Also note that first hour draw and the size of the tank (assuming you have a tank) are not the same thing as it is heating as you are drawing water - may not keep up, but still it is extending the runtime. The code doesn't distinguish whether you are trying to fill a big soaking tub or take a shower with an eco, really low-flow showerhead. This all comes back to what do you expect out of the system.

My tank is rated at something like 180-gallons first hour draw, and if I wanted to (and do) fill a large tub fast, others could shower at the same time without experiencing flow or temperature problems. With your low-flow showerheads, that's essentially contant use by three showers. Can't do that with a typical tankless install (can be done, but at significant expense and complexity). A tankless can be set up to work with a recirculation system, but it becomes even more complex - a tank is essentially a no-brainer to perform the same task. Since most vanity faucets are already quite low-flow, washing your hands with warm water at most anything below full flow is impossible with a tankless. Easy with a tank.

So, tankless costs more to install, purchase, and maintain (most people never maintain their tank, and they still last years without problems), and saves a little on energy over a good tank. Tankless systems have their place, but the average American may not want to put up with their foibles or expense. If you do, fine. Climate and use patterns are the big enablers, and much of the US environment and users will have trouble adjusting to them.

You didn't ask, but I have an indirect tank, and calling people names and insulting their intelligence is not getting you anywhere. I may have confused things, but I thought it was you talking about around 1 gpm heads, or at least around that hot. My 60-gallon tank is run at 140, tempered to 120, and has a decent sized boiler firing it. It 'wastes' so much energy that it drops the temp while 'stored' at the rate of maybe 0.5-degrees/hour or less, so even if the power was off for a day, I could still have hot water available...try that on your tankless.

This is getting really old...as noted, tankless has both good points and bad. If you can live with the bad, and must have some of the good, then fine...but it is not, nor will it ever be the best solution for all, let alone many.

As an aside, try getting tepid water out of the tap and put it into say a measuring cup at full blast...it's hard, and may be impossible with a tankless. Simple with a tank since you get hot water at any delivery rate. Try filling a large garden tub in the middle of winter in the northern parts of the US in anywhere near a reasonable time with a typical tankles, and it won't happen. Your results may differ, but I've run the numbers...you'd need a huge tankless or several in series. A tank works much better and it's cheaper.

As opposed to the norm, I like a long hot soak in my tub any day I have the time, regardless of the time of year, or the time of day...

First hour starts with the tank hot, but no flow and the burner/heat exchanger not providing heat. Then, you start the flow and measure how much hot you can get before the temp drops below the test value.

Some places require hotter water to things like dishwashers, and with a tempering valve after then tank, you can reliably get that, any time of the year, regardless of the incoming water temp...this is not feasible with a tankless.

For most tankless systems, it gets quite complicated to run a recirculation system, otherwise, whenever the pump is running, your tankless will be too. Not very good for longevity or your budget.

Ladiesman271 said:

That FHR method does not follow government certification standards for water heaters.

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Well, if you want to go down that road, Table 5-1 of the UPC doesn't follow federal gov't certfication standards either. Oh well; good thing "the code" is a minimum and common sense (also known as best practice) can prevail.

As for the several paragraphs of advertising copy from a particular mechanical contractor's website. What's up with that?

FWIW I'm kinda hurt that I gave you the answer you were looking for re minimum water heater size based on "the code" and you never said "thanks".