Tankless water heater. Need some data

[Sjsmithjr]

In an effort to make a meaningfull contribution to the topic of sizing tankless water heaters, I'm working on compiling a typical GPM deliverable map (maximum capacity, single unit) for the contiguous United States based on 120F and average water supply temp. I'm using DOE's fixture demand values to get a reasonable idea of available simultaneous demand.

I've looked at the supply demand curves readily available on the major manufacturers websites but, quite frankly, the scale is too coarse as to be of use. If anyone has ready access to supply demand curves, say plus or minus one-half gallon per degree rise, I'd appreciate it.

Thanks,

 

[Redwood]

The silence is deafening....

 

[Sponsor]

 

[Master Plumber Mark]

They make it up as they go......

I've looked at the supply demand curves readily available on the major manufacturers websites but, quite frankly, the scale is too coarse as to be of use. If anyone has ready access to supply demand curves, say plus or minus one-half gallon per degree rise, I'd appreciate it.

Thanks,

the situation and scales you are looking for simply do not
exist for accurately sizeing a tankless water heater...

their are simply too many variables to factor in...

with any normal plumbing system, usually when you ar not sure about something you normally just increase the size up a notch to cover any shortfalls...

my atitude when doing a plumbing system, is I would rather be too large than too small to accomidate either the water system or drain lines.... and its a simple matter of
just increaseing the size of the system a tad bit....

but with the tankless you are trying to make it perfect to
be both economical and efficient .....


to put in a tankless, I would rather oversize one that I know will do the job without any complaints in every situation , rather than undersize one and know it will not
work properly..

pullling your hair out is all you are going to accomplish
trying to size one up


go big or stay home......

 

[SewerRatz]

I agree with Master Plumber Mark. The biggest variable is how a house hold of people use their plumbing system. Yes there are people out there that is happy with the ability to run fixture all day long and get hot water. by the way "Hot water" is defined by Illinois plumbing code to be 120 Degree Fahrenheit or hotter. That same house hold turns on more than one fixture the temperature now becomes "tempered water" which by Illinois code is defined as "Tempered Water": Water ranging in temperature from 85°F to, but not including, 120°F

Now to use a gas fired tankless water heater to supply a whole house with hot water the system does need to be sized properly.. To do this a plumber needs to know how many fixtures in the home are capable of using hot water, and the flow rates of each fixture. Once the flow rate is determined, then you need to figure out the water incoming temperature, which will give us our desired temperature rise. here in Illinois its pretty much 50°F so to get our 120°F temp we need a 70°F rise. so now we have our rise temperature and total flow rate we can select a properly sized unit or units for that home.

This is where sizing it one size bigger comes in handy though to handle the unforeseen variables. Like water temp dropping down to 40°F, or the home owner feels they they do not need to follow the code because its such an inconvenience, and it wouldn't hurt anyone if they remove the flow restrictors from their faucet, or shower. And please does of you that just got the urge to argue that no one would say that they do not need to follow the code, read other posts within this forum. I see that a whole lot. As well I had home owners ask me if I could remove these water restrictors which of course I tell them I can not.

So as Master Plumber Mark has said it all depends on the variables, and once you figure it out make it one size bigger to handle the unforeseen variables.

 

[Ladiesman271]

Tank type water heater sizing per USDE


Tankless or Demand type water heater sizing per USDE

 

[SewerRatz]

Tank type water heater sizing per USDE


Tankless or Demand type water heater sizing per USDE

See the trouble is they do not take into account the variables. and assume that people have low flow faucets and shower heads. Just read my post about the variables. I really hate chewing my cabbage twice. http://www.terrylove.com/forums/showpost.php?p=185058&postcount=4

 

[Sjsmithjr]

To do this a plumber needs to know how many fixtures in the home are capable of using hot water, and the flow rates of each fixture. Once the flow rate is determined, then you need to figure out the water incoming temperature, which will give us our desired temperature rise. here in Illinois its pretty much 50°F so to get our 120°F temp we need a 70°F rise. so now we have our rise temperature and total flow rate we can select a properly sized unit or units for that home.

That was how I was going to approach it. I already have an isopach map of the average (so I've already introduced bias) incoming water temperatures for the United States; they range from 37F to 77F. Assuming a 120F output temp would then require a temperature rise of anywhere from 45F to 80F. It shouldn't be that hard then to determine to GPM deliverable by incoming temperature zone. From there you could get an idea of how many simultaneous fixtures you could supply base on typical fixture demand. (Most of don't have 0.5 GPM lav faucets in our houses). You also have make some assumptions regarding water pressure and the overall condition of the plumbing system.

For example, a single Noritz 9.3 might deliver anywhere from 5GPM to 9GPM within the above incoming temperature range. Of course, on the consumer side of their website they tell you can a 9.3 can deliver "up to 11.1 GPM" with the 9.3 GPM rating almost as an after thought.

Here's where it get's maddening. The gpm curves that I've seen are based on 105F output, the units come preset to a higher temp, and if you can find any information from a manufacturer regarding the "cold water sandwich" effect it'll be based on 120F. (If you dig enough, you will find that all manufacturers acknowledge temperature stabilization issues and for some the temp swing is dangerous to your health.) In other words, it's all apples, oranges, and pears. Add to that the many variables inherent to any installation as noted by Mark and SewerRatz and sizing these things properly is not a task for the faint hearted. I believe Mark noted some thime back that noted that if you really wanted to DIY a tankless correctly, you'd know more about you plumbing and plumbing codes than you ever really wanted to know.

In the end, it's almost a moot point to even be discussing these things in terms of a DIY installation. Electric models don't even warrant consideration. Homeowners aren't supposed to be able to buy Bradford White. Noritz and Takagi are explicit that to get a warranty their products have to be installed an authorized licensed contractor or approved and qualified plumber or contractor according to pertinent federal, state and/or local regulations. To date I believe Jakura is the only DIY're that has acknowledged the warranty issue for homeowner installations. He also subjected his installation to inspection by the appropriate AHJ. His tankless experiment thread is probably about the best by a Homeowner/DIY'er.

In the end that leaves the Bosch 2700ES (4 to 7GPM) as valid option for DIY'er looking for a whole house unit, but not too many people give that unit a thumps up.

 

[Ladiesman271]

See the trouble is they do not take into account the variables. and assume that people have low flow faucets and shower heads. Just read my post about the variables. I really hate chewing my cabbage twice. http://www.terrylove.com/forums/showpost.php?p=185058&postcount=4

There are two issues here.

1. How to "properly"size a water heater based on normal hot water usage. All you have to know is the normal hot water usage.

2. Code requirements for water heater sizing. Why don't you post the specific Illinois Code section that deals with this issue as you have for other issues?


I have never seen any definitive Code requirement for water heater sizing. In Massachusetts it fits in the general term of "adequate". In addition, the Massachusetts Plumbing Code refers you to the Sanitary Code with regards to hot water. There is also a link to the MA Plumbing code at the bottom of the Sanitary Code page.

Massachusetts State Sanitary Code


"Hot Water Facilities"

"Facilities for the heating of water must be provided (i.e. supplied and paid for) and kept in good working order by the owner. The owner must supply hot water in sufficient quantity and pressure to satisfy the normal use of all plumbing fixtures which generally require hot water to function properly. The temperature of the hot water is not to exceed 130° Fahrenheit (54° Celsius) nor fall below 110° Fahrenheit (43° Celsius). Under certain leases, an occupant may be required to provide the fuel for the heating of the water. [410.190]"





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

"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."


State of Minnesota Department of Commerce - Water_Heaters


Regarding tank type water heaters, the size reference notes that for gas hot-water heaters, "the most common size
natural gas water heater is 40 gallons". Just how long will "hot water" obtained from a 40 gallon tank water heater last with the simultaneous use of all water fixtures which generally require hot water to function properly?


You should also note what it says in the Minnesota summary. The "plumbing instructor" and his "friends" should revisit the so called and undocumented simultaneous use requirement of "the code".


Summary

• When looking at a new water heater, be sure
to compare the energy efficiency of different
models by checking the Energy Guide label.
Choose an EF of at least .64 for natural gas
and propane, and an EF of at least .93 for
electric.

• Buy the smallest size you can. Don’t try to
buy a water heater so you can shower, and
wash clothes and dishes all at the same time
without running out. Instead, plan your hot
water use. This is especially important if you
have a large family.

• Locate the water heater as close as possible to
where the largest volume of hot water is
used. Since heat is constantly lost through hot
water pipes, the shorter the pipe runs the
lower the heat loss.

• Insulate the water pipes and install heat traps
if your water heater does not have one.

• Take easy, low-cost or no-cost measures to
avoid waste in using hot water.
.

 

[Ladiesman271]

That was how I was going to approach it. I already have an isopach map of the average (so I've already introduced bias) incoming water temperatures for the United States; they range from 37F to 77F. Assuming a 120F output temp would then require a temperature rise of anywhere from 45F to 80F. It shouldn't be that hard then to determine to GPM deliverable by incoming temperature zone. From there you could get an idea of how many simultaneous fixtures you could supply base on typical fixture demand. (Most of don't have 0.5 GPM lav faucets in our houses). You also have make some assumptions regarding water pressure and the overall condition of the plumbing system.

I think that Mr. Rat says that you should determine usage first, then select your heater.


For a trial run this weekend I did a load of laundry & ran the dishwasher at the same time. Along with the simultaneous use of those two units I ran the hot water fixture in the kitchen. I measured 120 degrees at the kitchen fixture with the fixture set to the all hot setting. I still have 42 degree cold water coming into the house!

You may want to look at the minimum water pressure that is required for use with a tankless water heater. The more fixtures that you open (hot and cold) at the same time, the lower the water pressure goes.

In order to minimize water pressure drop, my washing machine alternates between hot and cold water when in use. The "cold" wash / rinse setting runs tap cold water for 75% of the the fill time, and hot water for 25% of the fill time. There is no simultaneous use of hot and cold water for any temperature setting.

 

[gregsauls]

We seem to be stuck on this idea that if one fixture is "on" in a house and getting 120 degree water from the tankless then if another fixture is turned on then the temp will go down and stay down. Not true! At least in the case of MY unit that is not true.

When water flow is detected the burner adjusts the flame size (aka BTU) in order to get the desired temp on the output side based on the flow demand. No mixing of water is done to get the right output temp at the tankless unit, only burner temp is adjusted. If the unit is raising the temp of water from 50 to 120 degrees and is flowing 2 gpm using one fixture and another 2 gpm fixture is turned on then MY tankless unit will raise the BTU rate of the burner to maintain the 120 degree output. What am I missing here?

I do agree that some degree of oversizing is needed. You don't install for "today" but rather for the "future" as is foreseeable.

 

[Sjsmithjr]

If the unit is raising the temp of water from 50 to 120 degrees and is flowing 2 gpm using one fixture and another 2 gpm fixture is turned on then MY tankless unit will raise the BTU rate of the burner to maintain the 120 degree output. What am I missing here?

Nothing. For your unit and your location you could even turn on a third fixture with a 2 GPM demand and, assuming the pressure drop isn't too much, handle the demand. After that, flow and pressure would drop off to what most would consider unacceptable levels.

 

[Ladiesman271]

Nothing. For your unit and your location you could even turn on a third fixture with a 2 GPM demand and, assuming the pressure drop isn't too much, handle the demand. After that, flow and pressure would drop off to what most would consider unacceptable levels.

The cold water feed pressure from the meter coming into the house is also simultaneously dropping off to "unacceptable" levels. Kind of hard to take a hot "high" pressure shower when simultaneous hot and cold water use is at high levels!

 

[Jadnashua]

When water flow is detected the burner adjusts the flame size (aka BTU) in order to get the desired temp on the output side based on the flow demand. No mixing of water is done to get the right output temp at the tankless unit, only burner temp is adjusted. If the unit is raising the temp of water from 50 to 120 degrees and is flowing 2 gpm using one fixture and another 2 gpm fixture is turned on then MY tankless unit will raise the BTU rate of the burner to maintain the 120 degree output. What am I missing here?

What you are missing is IF the demand is greater than the ability of the unit at its maximum BTU rating is exceeded, the temperature of the outgoing water will decrease. If you are within the limits of its capabilities, you should not, in a properly set up unit, notice. But, there is no magic here...it is an easily calculated situation to determine if you can get the volume required with the BTU input you have available. I don't care how much it can modulate the output, if there is not enough heat, it can't magically maintain the output - and, the response is not immediate. I might be fast, but not immediate; and, when exceeded, the temperature WILL drop, or, you'll experience a decrease in pressure as you exceed the flow restrictor (if present).

 

[gregsauls]

What you are missing is IF the demand is greater than the ability of the unit at its maximum BTU rating is exceeded, the temperature of the outgoing water will decrease. If you are within the limits of its capabilities, you should not, in a properly set up unit, notice. But, there is no magic here...it is an easily calculated situation to determine if you can get the volume required with the BTU input you have available. I don't care how much it can modulate the output, if there is not enough heat, it can't magically maintain the output - and, the response is not immediate. I might be fast, but not immediate; and, when exceeded, the temperature WILL drop, or, you'll experience a decrease in pressure as you exceed the flow restrictor (if present).

I agree with all of this... so we are back to "right sizing" the unit in the first place, just as you would with HVAC, electrical circuits, etc...

The one neat thing with some of the better tankless units is the ability to grow the system by adding another unit and have them communicate with one another. I do agree that "upsizing" will pay off tin the long run. We could have "gotten by" with the R53 but since the dealer didn't stock that size and the cost diff was only about $175 then "go big, or go home".

 

[Sjsmithjr]

I submitted my reply before I was finished. Jadnashua has hit on the rest of "equation". Greg has a TK3 and, on average needs 50F rise to obtain 120F water. He can get 6 GPM at 50F but if he pushes it to 7 GPM he can only get a 40F rise and will experience maximum pressure drop.

Install that same unit in North Dakota and it will give you, on a good day, 3 GPM at 85F rise. The end result will be the same 40F rise at 7GPM, but in this case that means no hot water to any fixture.

Yes, it's all about proper sizing. I simply wished to illustrate in a simple way what proper sizing might actually mean because after reading numerous online reviews, it would seem that a lot of people buy first and size later. Possibly due to the hype and possibly becausing purchasing a water heater used to be a no brainer.

 

[gregsauls]

I submitted my reply before I was finished. Jadnashua has hit on the rest of "equation". Greg has a TK3 and, on average needs 50F rise to obtain 120F water. He can get 6 GPM at 50F but if he pushes it to 7 GPM he can only get a 40F rise and will experience maximum pressure drop.

Install that same unit in North Dakota and it will give you, on a good day, 3 GPM at 85F rise. The end result will be the same 40F rise at 7GPM, but in this case that means no hot water to any fixture.

Yes, it's all about proper sizing. I simply wished to illustrate in a simple way what proper sizing might actually mean because after reading numerous online reviews, it would seem that a lot of people buy first and size later. Possibly due to the hype and possibly becausing purchasing a water heater used to be a no brainer.

Nope, don't own a TK3... I have a Rinnai R75lsi. Could have "got by" with a 53 but was advised to "upsize"... so I did.

 

[Sjsmithjr]

Nope, don't own a TK3... I have a Rinnai R75lsi. Could have "got by" with a 53 but was advised to "upsize"... so I did.

That's right, must be Jakura that has a TK3. The Rinnai R75lsi can also deliver 6 GPM at 50F rise but maxes out a 7.5 GPM and a 35F rise. Was the recommendation for a 53 based on Rinnai's online "tankless assistant" that you referenced in another post? Seems to me it's recommendations are a bit undersized; I entered my zip code and one bathroom a got a recommendation for a unit that could handle a bath group and little more.

Seems the advice you got to upsize was good. Where have I heard that recommendation before?

 

[gregsauls]

That's right, must be Jakura that has a TK3. The Rinnai R75lsi can also deliver 6 GPM at 50F rise but maxes out a 7.5 GPM and a 35F rise. Was the recommendation for a 53 based on Rinnai's online "tankless assistant" that you referenced in another post? Seems to me it's recommendations are a bit undersized; I entered my zip code and one bathroom a got a recommendation for a unit that could handle a bath group and little more.

Seems the advice you got to upsize was good. Where have I heard that recommendation before?

My plumbing supply guy will not stock the R53 as he says they are undersized for most installs in the area I live in. His reasoning, why stock something that will give problems, refuse the sale.

 

[Ladiesman271]

I submitted my reply before I was finished. Jadnashua has hit on the rest of "equation". Greg has a TK3 and, on average needs 50F rise to obtain 120F water. He can get 6 GPM at 50F but if he pushes it to 7 GPM he can only get a 40F rise and will experience maximum pressure drop.

Install that same unit in North Dakota and it will give you, on a good day, 3 GPM at 85F rise. The end result will be the same 40F rise at 7GPM, but in this case that means no hot water to any fixture.

Yes, it's all about proper sizing. I simply wished to illustrate in a simple way what proper sizing might actually mean because after reading numerous online reviews, it would seem that a lot of people buy first and size later. Possibly due to the hype and possibly becausing purchasing a water heater used to be a no brainer.

The typical tank type water heater is sized at what 30 to 40 gallons? At your specified 7 gpm, you are going to run out of "hot" water after at most 4 or 5 minutes of use. What good is tank type of water heater?


The water pressure in the home will also drop when 7 gpm is drawn through the water distribution system. Have you ever drawn 7 gpm and observed what happens on your own home system?

 

[Ladiesman271]

Noritz flow rate reference





Noritz sizing reference