Help Sizing a Tankless Water Heater

[schmede]

I h ave decided to go with a tankless (that is not my question) but I need help sizing it.

I live in Salt Lake City. The winter water temp averages 46 degrees but can be as cold as 34. I like HOT showers, and will have two showers going at once every day. So probably a max of 5gpm most of the time.

I decided on the Noritz 931 since it can raise the water temps about 75 degrees when 5.6 gpm are flowing. It is 84% efficient

BUT...that model does not qualify for the $300 tax credit. Do you think I will be happy with the Noritz 841 which will raise the temps 75 degrees when 5.0 gmp are flowing? It is 93% efficient. Do you think that will raise the temp enough for a HOT shower when two showers are going? Also ss the efficiency rating between these two units (84 vs 93) enough to notice a difference in my gas bill?

Thanks in Advance. I have to decide this week so any help would be great!

[jadnashua]

Well, let's see...34+75 is 109, now run that through maybe 20-30' of pipe or more, and you'd be running straight out all hot water that is likely at least a few degrees less than that 109 ...maybe a lot. You might want to set the shower temp where you like it and run it into a large glass and then stick a thermometer in it to see what temp you really like. My guess is it may be a bit more than 109-degrees if you like a hot shower. I don't think you'll like either one if you like a hot shower...and, if anyone opens another fixture - say to wash their hands, the temperature will either drop OR it will decrease the flow (can you say anemic shower?) to try to maintain the desired outlet temperature. Is there a way to make this work? Yes, even if you want to stay with tankless...put two in series and you should be able to not only take two showers, but maybe run the dishwasher or run a load of clothes at the same time...it won't be cheap. Easier to use a tank, though, and probably cheaper in the long run.

As the thing ages, you need to delime it or the temperature rise goes down (the minerals act as insulators). how often you have to do that would depend on your water and use.

[master plumber mark]

. Do you think that will raise the temp enough for a HOT shower when two showers are going? Also ss the efficiency rating between these two units (84 vs 93) enough to notice a difference in my gas bill?

Thanks in Advance. I have to decide this week so any help would be great!

No it wont suffice..
if you take one shower at a time you might be ok with the temp of the water as low as you claim it gets...

you will not be able to take two showers at the same time with temps that low...

it has got to do with flow rates...

good luck with the whole experience



people will walk through the fires of hell for
that 300 dollar tax credit..

and spend a fortune for it...

they will drag their bodies across burning sands towards
a mirage....telling themselves its all worth it in the long run....




grab that brass ring.....

[nhmaster]

I think that after spending close to 3 grand to have it installed you will try very hard to convince yourself and everyone else that it is the most wonderfull thing in the world while secretly wanting to bash the thing to hell and put in a decent water heater. :eek:

Did I say that?

Why yes, yes I did. And I'm stickin by it:rolleyes:

[master plumber mark]

I think that after spending close to 3 grand to have it installed you will try very hard to convince yourself and everyone else that it is the most wonderfull thing in the world while secretly wanting to bash the thing to hell and put in a decent water heater. :eek:

Did I say that?

Why yes, yes I did. And I'm stickin by it:rolleyes:

I have seen quotes as high as 5 grand for the tankless water heater in our area...

they will drag themsleves across burning sands
just for that 300 dollar rebate,
just to say to their freinds that they have gone green.


then next year
you could shove bamboo shoots under their fingernails
and they still would not admit that they screwed up.......





.

[SewerRatz]

Install two units. The second unit will handle the higher flow rates as well as the really low water temps you are talking about. Now that you spent the $$$ on the install of two tankless units and have to maintain them both on a regular basis, I think the wonderful $300 tax credit doesn't look so wonderful.

[schmede]

Guys...point taken. But I am not trying to be trendy and green I am just trying to add some space to my little bungalow and a tankless will allow me to do so, plus I already have a 3/4 gas line and I only have like a 2 ft run to the outside wall. Basically I have the ideal set up for going tankless. SO...

If I run the numbers on the Noritz 0931... 34 degrees (yes, I called the water district and they told me since it is run-off water it can get that cold...although the average in winter is 46, I want a HOT shower everyday in the winter)

plus, based on the Noritz chart an 80 degree rise at 5.3 and an 85 degree rise at 4.9 means my shower should be over 110. I just want to know from a practical standpoint if those charts are to be trusted? Also, the run to the master is only a few feet.

P.S. Based on a little more research, I believe the Noritz 0931 will qualify for the tax credit. I will make a phone call tomorrow to find out for sure.

[SewerRatz]

Another thing to remember after you install the tankless. Some tub and shower valves(if its with in 10 years old or newer) have a temp limit stop installed behind the handle. They are set to ensure your max water temp will not go over 110 or 115 degrees. Since your old heater was making 120 degree or hotter, that limit stop will need to be readjusted to your newly supplied 110 degree water supply from the hot side.

Just remember you have been warned. I have installed over a 100 of these not because I sold the people on the idea. They where already sold on it, and I tried to show them all the pros and cons of owning one of these. Some of the people did end up having me install the second unit due to the high flow rates and low temps. Others like to call me and complain they spent all this money and it works like crap. I just tell them I forewarned them.

[Gary Swart]

Let's not start another tankless water heater debate! The gentleman is obviously convinced the tankless is the solution to his problem and really isn't interested in hear us preach the facts of tankless life. He's going to do it anyway, so let's back off and do what he will.

[master plumber mark]

[quote=schmede;181456]Guys...point taken. But I am not trying to be trendy and green I am just trying to add some space to my little bungalow and a tankless will allow me to do so, plus I already have a 3/4 gas line and I only have like a 2 ft run to the outside wall. Basically I have the ideal set up for going tankless. SO...

I am just having a little sport, no big deal and no offfence meant..

If you absolutely must have one, I would suggest that you dont skimp on the size, its simply better to over size one rather than get out the graphs and flow charts and try to cut it down to the bare min...


it tis certainly better to oversize the unit to do a three- four bathroom home and be fairley content with the outcome, especially if you wish to have two showers going at once


rather than having luke-warm pisswater raining down on you for a shower next winter...becasue you undersized or put in what you thought should work..

[schmede]

I have decided to go with the Noritz 0931. In a worst case scenario situation I think it will serve its purpose well.

34 degree water
2 showers (5gpm)
rise up to 80 degrees

should mean a hot shower and a super hot shower when the conditions are less harsh.

My take away from this thread is...if you go tankless, don't half ass. By the way guys I am a woman not a gentleman and that is probably why I never half ass anything and why I have been more than happy with all the remodeling decisions I have made over the last year.

P.S. My dishwasher and washer both have internal water heaters so should not draw much from the tankless.



The dishwasher and the washer will still draw hot water from the tankless when running.
Terry Love

[jadnashua]

On either the washing machine or dishwasher, (at least with the dishwasher), why you are running them, they pull all hot - if, and only if, the temp is not high enough, they'll raise the temp. Now, some washing machines I've seen (mostly overseas) only have cold coming in, and if you want it hot, it heats it; but, I've not seen any of those here - they may exist, though. A washing machine has nearly unlimited flow, and could likely try to draw 12-15 gallons per minute. A DW often has a smaller line, and would be limited, but not that much. Either could easily overcome the flow available from a tankless in the winter-time with cold supply water.

[schmede]

On either the washing machine or dishwasher, (at least with the dishwasher), why you are running them, they pull all hot - if, and only if, the temp is not high enough, they'll raise the temp. Now, some washing machines I've seen (mostly overseas) only have cold coming in, and if you want it hot, it heats it; but, I've not seen any of those here - they may exist, though. A washing machine has nearly unlimited flow, and could likely try to draw 12-15 gallons per minute. A DW often has a smaller line, and would be limited, but not that much. Either could easily overcome the flow available from a tankless in the winter-time with cold supply water.
There are a few models that have internal heaters on the market in the US. I bought the samsung silvercare.

http://www.samsung.com/us/consumer/detail/spec.do?group=homeappliances&type=washersdryers&subtype=washers&model_cd=WF218ANW/XAA&fullspec=F

I don't know how much hot water the washer initially draws from the house water, but I know it can raise the temp significantly for sanitizing

I really don't use the silvercare function, but I bought it because it is very quiet and works well in a laundry space just off the kitchen



All plumbing in the US uses hot water for the dishwasher. That is how the plumber will connect them. So when the dishwasher fills, it will pull water through the tankless heater. Terry Love

[nhmaster]

http://www.aceee.org/consumerguide/waterheating.htm

[Cass]

To the original poster on this topic.

A properly sized and installed tankless water heater will serve you well. Look at what your max hot water flow rate needs are, your lowest inlet temp historically and the proper sizing of your water and gas lines (3/4" typically needed). Then look at your typical needs & average inlet temp and base the size of the tankless on something in between or... nearer the max serviced needs if you don't want to "think about it" during worse conditions.

Do descale your tankless once a year.... just like you would drain a tank type unit every year or so due to sediment.

Getting a "fair" answer from some on this board will not happen here. There are a few that simply hate tankless systems for their own reasons. There are also some of us here who have them, installed them ourselves and are quite happy with our purchases. Those of us who have them will continue to advocate them despite others "noise".

Since you seem educated enough about the OP to suggest he purchase a unit...can you tell me if he has hard or soft water...and if hard what his TDS are and his GPG is.

[Bigtruck]

I h ave decided to go with a tankless (that is not my question) but I need help sizing it.

I live in Salt Lake City. The winter water temp averages 46 degrees but can be as cold as 34. I like HOT showers, and will have two showers going at once every day. So probably a max of 5gpm most of the time.

I decided on the Noritz 931 since it can raise the water temps about 75 degrees when 5.6 gpm are flowing. It is 84% efficient

BUT...that model does not qualify for the $300 tax credit. Do you think I will be happy with the Noritz 841 which will raise the temps 75 degrees when 5.0 gmp are flowing? It is 93% efficient. Do you think that will raise the temp enough for a HOT shower when two showers are going? Also ss the efficiency rating between these two units (84 vs 93) enough to notice a difference in my gas bill?

Thanks in Advance. I have to decide this week so any help would be great!


I installed an electric Tankless and it works perfect. I've had it for about 7yrs.

I can take showers in two showers and use the dishwasher at the same time. No change in temp.

My sister installed a gas tankless. Her water temp fluctuates dramatically.

Ray

[jadnashua]

You will have to provid some proof for those bogus flow claims. I don't think I can even draw those flow rates in a bathtub with the hot valve full open.

Anyhow, have you ever done the laundry? Most people use cold wash and rinse. When I tried the hot setting (one time), the tankless had no problem keeping up.
A 3/4" rough-in valve I looked at had a spec of 18.5gpm at 45psi - this was for a tub. Some washing machines have small internal solonoids, so that limits the volume as can the hose you use and the shutoff valve you have. But, it could be the equivalent to a hose you water your yard with.

Depending on your pressure, a 1/2" pipe can free-flow probably in the area of 8-9 gpm, 6-7 more common.

On a tankless, some of them just restrict the flow rate to maintain the temperature rise, so yes, if yours worked that way, you'd be able to fill up the washing machine fine, but it would take longer to fill.

If the tankless doesn't have a flow restrictor built-in or one added on, then the faster you flow the water through it, the colder it gets. Most have some means of slowing the flow.

[ingeborgdot]

I love mine. I would never own a big tank heater again. The only thing for me is you have to wait a little longer for the hot water. After that nice long showers. We went with a nice big size, not even the biggest but almost. It is a rinnai.

[sjsmithjr]

I don't live in a municipality

Sorry, I thought you were in Austin. Does Texas still use the 2000 IPC? Do some counties in Texas, like Bowie, not have a permitting or inspection process for gas, electric, and plumbing work? Some of the very rural counties in Tennessee only have electrical and septic permits and inspections.

At any rate, for anyone under the IPC that would like to know how to properly size their water distribution system I believe the correct table to reference is 604.3, Water Distribution Design Criteria Required Capacities At Fixture Supply Pipe Outlets.

If I'm mistaken I'd appreciate a correct reference; we're under the IPC where I live as well.

Bradford White's Tankless sizing tool is great if you just want to play with various configurations: http://rightspec.bradfordwhite.com/Sizing/EverhotRes.aspx

I tried it out myself. Plugging in an overly optimistic water temp of 60F with and output temp of 130F, my house's configuration (one shower, dishwasher, clotheswasher), took the default for fixture flow rates (who's to say I'll always want that 1.5 gpm shower head), checked simultaneous use and was spec'd two 180000 BTU units with a maximum hot water delivery rate of 8.6 GPM.:eek:

If I change my evil ways, ignored code or got a variance, put a lockout on the washing machine, and told my wife we couldn't add that second bathroom she's been wanting (or plan on using them both at the same time) I could get by with one unit with a maximum hot water delivery rate of 4.3 GPM.

[sjsmithjr]

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 (I'm assuming too they thought it was too high) if I wanted to supply a shower, a dishwasher or kitchen sink, and a clothes washer simultaneously (peak demand). I understand that the total gpm given by the BW calculater is a function of the units spec'd and temp rise required as opposed to the calculated demand. Given that these are off the shelf items, it is almost certain some over demand capability will available.

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 (http://www.pinellascounty.org/build/PDF/WHGPM.pdf), 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.

...and before anyone says it, it is not inferred by this form that you could get by with sizing the system to not provide simultaneous use of some fixtures. The minimum gpm allowed is 3 GPM @50 degree rise. Why 3 GPM minimum you ask? I'll leave it to you to figure out.

[SewerRatz]

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 (http://www.pinellascounty.org/build/PDF/WHGPM.pdf), 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.

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.

[SewerRatz]

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.

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. http://www.terrylove.com/forums/showpost.php?p=184396&postcount=217

[Scuba_Dave]

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 (http://www.pinellascounty.org/build/PDF/WHGPM.pdf), 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.

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

[sjsmithjr]

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

Work with me here, Dave. :D 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. :rolleyes: 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!"

[sjsmithjr]

Missed that...did you edit ? ;)

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


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

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! :eek:

Sewer Rat noted on another thread (http://www.terrylove.com/forums/showpost.php?p=184396&postcount=217) 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.

[SewerRatz]

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.

[Scuba_Dave]

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 :(

[SewerRatz]

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.

[SewerRatz]

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

[sjsmithjr]

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

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

[sjsmithjr]

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

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."

[sjsmithjr]

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

Not at all, before we go any further, however, may I suggest you read the following re decorum
(http://www.terrylove.com/forums/showthread.php?t=23307).

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.

[master plumber mark]

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.


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 (http://www.terrylove.com/forums/posthistory.php?p=185485)

[jadnashua]

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.

[jadnashua]

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.

[jadnashua]

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.

[sjsmithjr]

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

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".