Help Sizing a Tankless Water Heater

[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/S...verhotRes.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.

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, 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, 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/show...&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, 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. 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!"

[sjsmithjr]

Missed that...did you edit ?

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

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!

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.

[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.