Tankless Inline With Tank For High Output / Demand

FerncreekjeepApril 3, 2012

I cant find much info on this. Has anyone installed a tankless that fed a 50 gal tank? I want to run a high output shower valve (moen io digital) where the capacity can reach 18 gpm. It will most likely not be run all the time at that capacity, but would like to have the ability to. I am also interested in only running the tankless when the demand is above 2 gpm. That way I limit how often the tankless kicks on, saving gas. I haven't found a tankless with that capability. Should I be looking for a volume specific diverter valve or maybe a control valve to switch the tankless on? Can the tankless lower limit be modified? If this is going to be too complicated, then what are my other options? I live in KY where the incoming water temp is an average 55 degrees. I also have a 1" gas feed, will I encounter any problems there?

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A shower that dumps 18 gallons a minute down the drain is beyond wasteful and gluttonous - even if you can afford it.

That said, someone contemplating such nonsense should be able to afford a plumber to design the system - including the drain, water heaters and gas supply -- to make it work rather than asking on a DIY internet board.

What you are proposing will definitely not work.

    Bookmark   April 4, 2012 at 9:06AM
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Mostly agree with jake, but...

Why not contact Moen or your local vendor directly? They've designed, manufactured, and marketed the Moen io units and are likely familiar with various hot water supply configurations to make them work up to their specification capacity. This will be an unusual configuration. You'll want to do it right. Get your info from people who have done it before.

    Bookmark   April 4, 2012 at 10:00AM
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I've done this. (tankless and tank in line) The tank water heater is small, like about 5 gallons. The tankless heats the water on demand, then the small electric heater holds a reserve. This reserve comes in really handy, it eliminates the "cold water sandwich" that occurrs every time the tankless heater cycles on and off. It keeps the output temperature from quickly fluctuating, which is a nuisance to deal with, and potentially a safety hazard.

Your idea about the tankless only kicking on above 2 GPM demand to save gas, is illogical. You want the tankless to do most of the water heating. Set the output temp high for the tankless and low for the electric tank. This minimizes the electric use for keeping the 5 gallon tank warm.

    Bookmark   April 4, 2012 at 11:51AM
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Among other issues, the tankless will limit the throughput based on the required temp rise. At no time will a residential tankless come close to the 18 gpm that is needed. A 200k BTUh (largest residential tankless) would produce about 7 gpm at the stated temp rise... It would take about 3 units on a manifold to do the job.

A 50 gal tank would be empty of hot water in about 4 minutes assuming 130 degree tank, 55 degree cold water and 105 degree shower.

We haven't even touched on the drain and the gas supply.

    Bookmark   April 4, 2012 at 12:16PM
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Thanks for the input so far. I have the drain squared away. I am installing a 5' linear drain with two 2" outputs that feed two 2" traps and then into a 3" main line. I also have a 1.5" vent within 2.5' of the drains.
I will most likely only be using 6gpm of hot water. I just wanted some kind of a reserve capacity (hence 50 gal tank) if i were to turn everything on and probably only for just a few minutes.
The idea of bypassing the tankless was to keep it from coming on during low volume draws (ie. shaving, washing dishes, washing hands and etc.).
I have spoke to four plumbers and no one seems to be familiar with this moen valve. That is why I am asking here, kind of a last resort.
Any other ideas?

    Bookmark   April 4, 2012 at 1:30PM
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Even assuming 55degF water supply it would be impossible with residential type tankless heaters, not to mention that your 55degF figure is an average, on the other hand, your water supply line is only 6" below your frost depth and in mid winter you can expect that temp to be in the order of 36 to 42degF.

Next let us consider Friction Head Pressure losses. 18gpm would no doubt nearly double the supply load to the house, and assuming the supply line was originally designed to meet the load the resultant increased load from your proposed shower would require increasing the size of the entire supply line from the well or municipal main all the way to the shower.

Your proposed shower alone would require a minimum of 1" lines and at 1" the pressure loss to friction head loss would be 34.3psi/100ft from the source of pressure, whether that is your well pump or municipal main all the way to the shower.

Now if we consider the velocity of flow, at 18gpm the velocity of flow is 11.79ft/sec, which is right at the code mandated redline for plastic pipe (12ft/sec) and it exceeds the maximum allowable velocity for copper pipe (8ft/sec) so in all likelyhood your local code would require installing an 1-1/4" line just for the shower and you would probably have to increase your house supply line to 2" or even 2-1/2", depending upon the length of run and number of other fixtures in the structure.

Now when we consider the DFU load on the sewer you would have to increase the drain line from 2" to 3" and when the load is combined with the existing load of the house you would probably have to increase the size of the building main drain & house sewer line.

Now regardless of what type of water heater you choose, tank or tankless, a 1" supply line will not be enough to satisfy that load and still supply the other gas fixtures in the structure.

    Bookmark   April 4, 2012 at 1:32PM
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I have a much better idea:

Make toilet paper out of real $10 bills glued end to end.

It would be more awesomely wasteful and wouldn't be as expensive.

    Bookmark   April 4, 2012 at 1:45PM
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We are confusing the rough in valve's unrestricted flow rating with the actual flow rate from the head.

Let me clear up some confusion here...The 18 GPM is the total designed capacity of the rough-in valve with unrestricted flow at 45 psi, with all ports open at the same time.

The valve has 4 outlet ports, each is designed to deliver up to 6 GPM, if only one is open at a time, with the unrestricted flow at 45 psi.

The shower heads and body sprays are still restricted to the EPA mandated maximum water usage requirements; 2.5 and 1.75 GPM, respectively. That rate of flow is based on the test conditions described above. If the actual unrestricted flow is less than the test conditions, the actual output from the fixture will also be less than the EPA requirement.

The shower system the OP wants is not out of line with other shower/body sprays. Rough-in valves are normally rated for a maximum flow of several times the rate of flow of the actual fixture.

I think the setup I have is quite adequate in this situation.

    Bookmark   April 4, 2012 at 5:38PM
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I think your setup is fine for what you are doing, but I don't want to second guess the OP's setup.

The OP needs to know that a single residential tankless will limit the water flow into (and therefore out of) a tank water heater. They actually throttle the output in order to achieve the set temperature if they must. Therefore, under average inlet temps, the OP would be limited to about 7 gpm - which is about 1/3 of his stated requirement (even if that requirement was likely inaccurate, as you pointed out). Note this only takes into consideration the shower - not the hot water needs for the entire home.

Lazypup brought up additional design issues.

I think a number of people get enamored with the idea of a "mega shower" without doing any of the math regarding what that actually means in terms of the plumbing to support it and the massive waste of both water and energy.

I stick by my sentiments that anyone looking to be that wasteful can afford to hire someone to design and install a system - It falls far outside the standard residential design.

Your point that the OP may not know what his actual GPM requirements are reinforces my argument that he needs a qualified plumber or mechanical engineer to design the entire system and/or give him a reality check.

    Bookmark   April 5, 2012 at 9:17AM
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I think the "mega shower" concept stems from a backlash against the low-flow craze. For me, 2.5 GPM is the absolute minimum flow I will tolerate in the shower. I've stayed at places that had such bad pressure in the shower, I just decided to wait until I got home to bathe.

As a habit, every time I install a shower head, I take out the flow restrictor and drill a 1/8" hole through it, then put it back in place. This prevents the flow restrictor from clogging up with debris over time. And I've measured the output, It is right around 2.5 gpm at full volume.

I can sympathize with people who live with a substandard shower, and have no idea how to make it work right. I can understand why someone would save up their money for the most wasteful shower they can find. Currently, they are hating every shower they take because it is a miserable trickle. The shower head is printed "2.5 GPM" and they associate 2.5 GPM with a trickle. When shopping around for a new shower, they emphasize the value for them is high rate of flow, hence the market for mega showers.

You, I, and many others here know that plumbing is a mechanical system, and it performs as a system. It must be designed as a system. Analyzing the efficency of a single component is useless out of context of the entire system.

And how wasteful is it really to spray yourself with 6 GPM of hot water for a couple of minutes? I can take a shower in 5 minutes with 6 GPM, and emerge feeling refreshed. Or use a miserable trickle of 1.5 GPM for 20 minutes, and come out all pissed off? I would rather save TIME and BE HAPPY! Same effect on the planet. Oh wait a minute, when I feel good and have more time, I can do other things to save the planet in more productive ways than depriving my personal hygiene.

    Bookmark   April 5, 2012 at 12:07PM
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Yup. Save me from nuclear war, please. I can figure out my shower.

I'm like aidan....I remove or modify my flow-restrictors. Doesn't everyone?

    Bookmark   April 5, 2012 at 12:15PM
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The next nuclear war that you want to be saved from will probably be over water or oil.

Places in the SW US are already having serious water problems - empty aquifers, empty reservoirs, fighting with farmers over who is going to get the water, etc. A 2.5 gpm shower hardly seems like an unreasonable sacrifice.

And although I can somewhat sympathize with your feelings about "figuring out your own shower" -- when there isn't any water, people look to the government for a solution.

I think there are several very large leaps of logic that you've made in your argument...but whatever.

    Bookmark   April 5, 2012 at 1:43PM
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Quote:"For me, 2.5 GPM is the absolute minimum flow I will tolerate in the shower. I've stayed at places that had such bad pressure in the shower, I just decided to wait until I got home to bathe."

This is a prime example of how most homeowners erroneously equate pressure with volume.

I have a 3/4bath off my mudroom so I can grab a shower as soon as I come in from work and not drag all my plumbers gunk through the house. On that shower I have a 2.5gpm chrome plated solid brass on a mixer that is fed with a line which averages 51psi, but when you stand under that shower head it feels like your under a pressure washer. In fact, my wife absolutely refuses to use that shower because she says it literally peels here hide.

We installed over 300 of those shower heads in a motel about 8 years ago and they got so many inquiries about where to get those shower heads that they actually sell them in their gift shop for $15. Not a bad markup when you consider they are in a two pack from the supplier for $5.49 a pack.

    Bookmark   April 5, 2012 at 5:03PM
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I am not equating pressure with volume. I know the difference. I do not want a HIGH PRESSURE shower. As I stated before, I need a flow volume of 2.5 gallons delivered every 60 seconds, to wash and rinse my body efficiently.

I am sure your 2.5 GPM pressure washer style shower is everything I would need to accomplish this small feat. The easiest way for me to determine if a shower head is up to my standard: fill a 5 gallon bucket in 2 minutes. If it takes more than 3 minutes to fill the bucket, I really can notice the lack of volume when I use the shower.

    Bookmark   April 5, 2012 at 7:50PM
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