Impossible?

mrichard

Customer has 180 gallon soaking tub which was newly installed afew years back and only started using recently to findout his hot water source is undersized.



Along with 4.5 baths with a 75 gallon gas water heater providing domestic he is running out of hot water before the tub can fill. He wants to install an "on-demand" to only kick in when 75 gallon gets low to provide sufficient hot water to tub only.



Thanks for any suggestions or info 

Mark Eatherton

Run the numbers....

Unless the input to the 75 gallon water heater is ENORMOUS, i.e. 500,000 btuH, the 75 gallon water heater is not big enough for the soaking tub alone, much less compounded shower/washing machine loads.



You can expect to get 80% of the stored energy out of the 75 gallon tank before dilution kicks in, plus 80% (minus altitudinal considerations) of the rated input for the first hour. After that hour, it is strictly the thermal output of the appliance.



Even if you had a dedicated line going only to the soaker, I seriously doubt that the tank heater would be capable of a full fill, without running the water at an extremely low flow rate, like 1 GPM, and I seriously doubt the consumer is willing to wait three hours for the tub to fill :-)



Even if you put a dedicated tankless water heater in just to service the needs of the tub, at a maximum output of 5 GPM (@ 80 degree rise), it is going to take 36 minutes to top off the tub. That means, when the 10:00 news starts, you begin filling the tub, and when the news is over, you hop into the hot tub. Think they are willing to do this? I didn't think so.



The normal course of action would be to estimate the total combined hourly loads, including filling the tub, running simultaneous showers, and potential machine loads. You can then decide if you want to cover the loads with significant storage, and reasonable recovery, or reasonable storage and MEGA recovery. My vote goes for scenario 1.



Once you have determined actual GPH draw demand, take that factor and divide it by .8. THis will be the required water storage on site. Now, recovering that storage in one hour is a reasonable expectation. So, using your case, if 4 people per hour shower, and the national average time in the shower is 8 minutes, and the flow rate is 2.5 GPM, of which 80% is hot water, then the hourly shower load in GPH is 64 GPH (2.5*.8*8*4).



The soaking tub holds 180 gallons, so 180 plus 64 = 244 GPH. 244 divided by .8 = 305 gallons storage. Obviously, a 300 gallon tank would probably not fit in this location, so figure on installing 2 each 150 gallon tanks, or 3 each 120 gallon tanks, or 4 each 80 gallon tanks.



Assuming a 100 degree F rise (40 to 140 degrees F), then the required input to the tank would be 305 * 8.33 *100 = 254,065 btuH. If you were to use an 82% efficient copper fin tube boiler, and you were operating at sea level, then you would take the 254,065 and divide by .82 for an input of 310,000 btuH in order to get the required output.



At this point, you have to stop and ask yourself (and your customer) some serious questions. Is the existing mechanical services (fuel line and flue pipe etc) capable of handling this load, and how much money is the consumer willing to spend in order to guarantee he can maintain his current life style, and quickly fill the new soaker tub...



Plan B: Add storage with adequate capacity to service the needs of the soaking tub, leave existing fire power in place, and educate the consumer that he can only fill the tub if it is the first unit to be filled, and that he won't be able to take a shower until the storage has recovered from the load that was imparted. As long as they are only taking showers, the existing (obviously) water heater is capable of handling the shower loads with some time diversity (spread out the loads).



Plan C wold be to do plan B, and add a tankless water heater in series with the storage to allow showering immediately after the tub has been filled. Again, is there adequate facilities to handle these additions?



The consumer is driving the decisions here, You're just the tour guide.



But, to you and all other contractors reading this, let it be a lesson. Just because the architect drew a 75 gallon water heater in the mechanical room, it does not mean that it is going to work without issues. That is why I always ask for and get a session with the homeowner to ask them what their expectations are as it pertains to the DHW system. Do this early in the discovery process and avoid issues on the back end of the project.



HTH



ME

mjcromp

To add

my 2 cents. If you put a tankless in series like I have seen before you will still be restricting flow through the storage tanks. My opinion would be check needed GPM's and then spec out a few tankless and plumb them in series and connect them together. Rinnai and Noritz have kits to link multiple units together.  I have installed 2 units before to meet demands and if GPM is low then only one unit turns on and then when that unit can't handle the output it goes to the next. It also alternates which heater starts the call for hot water so your not running one heater all the time. I house I worked on had 4 Noritz heaters strictly for the master shower. Fed by a 100 gallon electric simultaneously wired heater.

mrichard

Thank you thank you!

Thanks for all the info and suggestions guys!



Some great info there..wow!

Rich Kontny_3

Tankless will work if.........

  You have adequate gas volume, a place to vent, good water quality and pressure Noritz has models up to 13.2 gallons per minute. Or you can indeed pipe two heaters in series.

Go to the Noritz site and see their applications.This would be a much cheaper option than what Mark is suggesting and it will take up far less physical space.There is an installation of three Takagi tankless that takes care of the laundry room at the Radisson here in Milwaukee (they never run out of hot water)

Mark Eatherton

You forgot to mention something Rich....

and it is not your fault, but I think it only fair that you should finish the statement...



13.2 GPM @ a 45 degree rise... I am not aware of anyone other than Eskimos who can shower/bathe comfortably in 85 degree F water. ;-)



Granted, 380 K input is LARGE, but the instantaneous efficiency at that point is around 78%.



Using my 100 degree rise, and the 78% thermal efficiency, 380K large btu's converts to 6 GPM @ a 100 degree F rise. Not bad, but still, is the homeowner willing to wait the time (~30 minutes) necessary to fill the soaking tub? And then there's altitudinal deration, which here in Denver, that 6 GPM would actually be 4.8 GPM at a 100 degree rise...



The problem is that these tankless manufacturers don't have a set of standards, i.e. 100 degree F rise, against which they can quote their capacities to, so they quote whatever makes them look good, and unless the consumer looks real hard at the small asterisks floating around, they have no idea what they are getting...



I believe it was a wallie named Ken Secor who said "Great marketing trumps good engineering every time"...



Not busting on you, but I think we at lest owe it to the consumer to divulge all the full information, no?



ME

PLUMLOCO

Impossible

I agree with ME data,but to simplify it, you have to the 10+/- minute storage (not 1st our ratings,divide them by 60 minutes) in the basement to fill the tub.

I don't care how you heat it.

I bet the builder never discussed this, just sold the tub,and never discussed with the "plumber"

Dan Joyce

















0

george_42

more hot water

An easy way to get more hot water out of a storage water heater is to raise the water temperature to 180 or so and install a thermal mixing valve at the heater. This give much more capacity and also kills bacteria in the tank. By the way , according to the new codes , a mixing valve is required on all water heaters , but who is doing that , George

mrichard

how well does that work?

Any comments on George's post above? Just how much more hot water are they gonna get? Obviously this is going to wear the existing tank that much quicker no?



Customer is looking at cheap options,yet willing to add onto existing storage tank.



Thanks again guys