Rinnai/Takagi/Enter name of tankless vs Indirect WH....cosmo

Cosmo_3

I have a prospective customer who wants to install a tankless wall mounted water heater such as the Rinnai #2532FFU to take care of domestic hot water in his new 3 bathroom house. main water draw is a three shower head master bath shower approx 7.5 gpm, 45 deg F entering water temp in winter, well system (assume water treatment for mineral). Fuel is propane. House is heated with a boiler, maybe a Vitodens 62 handling a mix of radiant floor, and fancoils.

My personal preference when it comes to residential hot water is sell them a V-300 Stainless indirect 80 gallon. This water heater loses a fraction of a degree of hot water per hour, and I set them up to maintain 155 degrees F, and mix using a thermal tempering valve before going to house fixtures. This gives me the highest possible output from the water heater, and is a very reliable way of doing things.

The other side is the Rinnai water heater. I was wondering how much $ energy efficiency savings there are. Don't forget that now we have a lot of extra gizmos, heat exchangers, and gadgets to maintain, clean, etc compared to the indirect.

In this application I don't see the tankless as being worth the extra maintenance/ etc. I however am completely open to anyone's suggestion otherwise.... make your case!

Thanks,


Cosmo Valavanis

Dependable P.H.C. Inc.

Paul Pollets

Flow

It's all about flow. The tankless will usually provide only 4.5gpm. You're way in the red zone...you'd have to buy 2 and twin them together to meet gpm fixture loads.

The V300-79 works much better in this application. Just add the DHW exp. kit and pipe 1" to the coils on the tank.

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JackFre

I'd vote the other way

I've had a lot of success with multiple Rinnai unit installs either fully on demand or to storage for larger systems. One of the advantages of the independent HW system is tht you can bring your boiler sizing to a "finer point" to handle its primary job of heating the house. In my own home, with my Buderus/Buderus 40 gal indirect my gas consumption dropped 65% when I turned off the boiler and turned on the Rinnai. You would need two Rinnais to get the flow you are talking about. The advantage of the on-demand is that when the systme is off...it is off, no stand-by losses. Check out www.rinnaisolutions.com

Scott Gregg

Some Math...

A three shower master requiring 7.5gpm. An average 20 minute shower…150 gallons of hot water for that draw. I don’t think the 80-gallon tank is going to look very good considering the forst draw is probably more like 70 gallons before it runs out and has to recover.

Put in the twin tankless system. I personally like Noritz the best. The twin Noritz N-069M hooks up with one simple cable connecting the two computers instead of the more expensive multi-unit control of the Rinnai. It becomes a “lead/Lag system where you have full modulation. (If someone is only washing their hands, one unit is on low fire!) The “lead” unit rotates to keep one from doing most of the work and if one develops a problem, it will be locked out and valves off and the other will still produce hot water to its capacity.

Don’t get too hung up on all the scary looking stuff inside these units, including the Rinnai. These are extremely well made products that have been around for a very long time. They are built with the kind of Japanese quality that is associated with their automobiles and made to be “cheap” like some other products. Other than cleaning the filter a couple times a year, there should be no maintenance. Rinnai has been here for a good 8+ years now, Noritz about 4 years. Ask around of folks who have them or install them about the maintenance and you see…there just isn’t much needed.

One more thing; with this system, when they get out of the master shower they are still not out of hot water! In fact the twin system will give them enough capacity to not only be in that shower but to run another 1-2 fixtures in the house depending on the time of year. (If their water piping will support the flow of course)

80 gallon indirect = 60 gallons first draw
Twin Noritz N-069M = 492 – 828 gallons! (75*-45* rise) No contest.

This is certainly a job for the tankless. Look into Noritz, if you choose to stick with Rinnai, you and they will still be very happy.

I hope to finish my Tankless tutorial as soon as I get the tax man behind me:

http://profitableplumbing.com/_wsn/page5.html

Bob R

In a 3 bedroom house I'd opt for the indirect. You already have a boiler which is exhausted, pulling combustion air from the outdoors and has gas pipe to it. Adding the indirect and piping will give your customer a neat, clean efficient system that 99.99% of the time will satisfy all of their needs. Recirculation is also easier with an indirect.
Bob

Ron Schroeder

Hi Cosmo,

I used to lean toward the tankless route but tend more toward the indirect route now. But mostly if the indirect is part of the "system" and is also being used as thermal mass for heating too. If the tank (preferably a reverse indirect)is run at a higher temperature with a wide differential and has a high mass compared to the boilers thermal mass, you can really reduce short cycle ineficiencies thru out the year. I prefer to pipe the tank as if it was the boiler (heating loads piped off of the tank) and the boiler just recharges the tank when needed.

[Deleted User]

National average...

of shower time is 7-1/2 minutes in duration. Don't ask me how I know this data...:-)

Makes a BIG difference in your final number.

Also, the average "at shower head" temperature of water is around 112 to 115 degrees F.

So, assuming the above, and a worse case inlet temp of 40 degrees F, then the numbers would be as follows:

7.5 GPM times 7.5 minutes = 56.25 gallons. No problemo for the tank at 80 gallons.

As for btuH capcaity,

60 gallons (56.25 rounded up) X 8.33 X 75 = 34,785 btu's.

34,785 divided by 7.5 minutes X 60 (minutes per hour) = 278280 btuh output.

Assuming you're running at sea level, 278,280 divided by .80 (combustion efficiency) = 347,450 btuH input.

Now, you have to step back and ask yourself, "What are the chances of having to cover three showers running at the same time?" and you also have to ask "What did these people do wit the 40 gallon 40K water heater they had before?" and the answer is "Human Load Diversity and Training". In other words, you have to explain it to them in terms THEY can equate to. "Mr and Mrs Smith, you will have a virtually endless supply of hot water provided that no more than one tap or one appliance is running at the same time. In other words, you can take as many back to back showers and NOT run out of hot water. But if you all decide to jump into the shower at the same time, you can all expect a LOT of luke warm water..." It only takes one time to reinforce that statement. Suddenly, a unit that will handle one shower head flowing is MORE than enough.

Education of the end consumer is VERY important when dealing with these wonderful devices.

At this point, you could consider installing a flat plate heat exchanger to handle showers etc. provided you have enough fire capacity on hand to handle the 2.5 GPM load. Storage tanks are for DUMP loads, like large soaking tubs. If a person could pipe his home in dual DHW circuit, one for instantaneous loads, and the other for dump loads, the tank could be size to handle the largest potential single dump load with diversity time on its hands. Dump load divided by .8 to compensate for the piston efect and dillution at the hot/cold interface will do the trick.

Just my $0.02 worth. BTW, FWIW. I used to live with a tankless water heater of limited capacity, so this is not hear say... Now I have a Poseidon tank :-) Google poseidon tank along with my last name and you will see what I mean.

Enjoy!

ME

Paul Pollets

Question

when you were using the Buderus did you use the 2107 control with DHW priority?

Once 2 instantaneous water heaters are used, the cost is equivalent to the indirect application. Standby loss in Viessmann tanks is insignificant due to the high R values of the tank. I'm not sure I see any advantage of the IDHWHtr, unless the designed flow rates are under 4.5gpm, and that is acceptable to the homeowner.

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Constantin

Allow me to disagree...

.... for one, the Vitocell can be held at a high temperature and be mixed down.

Compared to a tankless water heater, it is very simple, requires almost no maintenance, etc. It is easy to integrate with other Viessmann products.

Unlike a tankless water heater, it has no lower flow requirement. That is, no issues with faucets that are only cracked as opposed to fully open.

Recirculation is also possible, with minimal disruption, just add a pump, the Vitotronic will take care of the rest.

The vitocell 300 is hard to scale up and easy to clean... even from the inside. How many tankless WH have an 8" inspection port to the inside?

One/two/three less flue(s) to deal with unless the tankless is mounted outside.

With a Vitocell, you have a choice re: what fuel to use to make DHW. A tankless requires gas. Moreover, the Vitodens referenced above ought to surpass pretty much any tankless water heater in terms of thermal efficiency. Considering how much a heating system runs on a given day, setting aside a little time to recover the IDWH ought to be a workable solution.

If a tankless and a vitocell 300 were presented to me side by side, I would probably have a hard time choosing the tankless. KISS, durability, etc. tip the hand in favor of the Vitocell. Besides, you can always use a GFX to recover waste heat from the drain to stretch your supply... I get a 15°F rise. You could also consider a dual-coil Vitocell with an AC desuperator to recover even more "free" heat.

Bob Forand

Hi Cosmo

Rinnai has a sizing tool on their website. You can use the tool to size units for your demand. It is a pretty simple tool. Good luck..

Roy_8

Efficiencies

One other thing to consider is operating efficiencies. Your boiler is 80 - 98% depending on the model. The Pacific Rim water heaters are 85+% on full fire but when modulated down drop off significantly. ( Has to do with how they modulate). Most of the hot water use is not going to run the units at high fire but at something less then 70% efficiency. Particularly when sized for a large dump load used infrequently. Gonna burn alot more gas washing hands and doing dishes so I can have that once a week romp in the George Jetson Shower.
Funny how they fail to mention that?

Soooo... like Mark always says IT DEPENDS on how the system is used and designed. There is no right system for every application -depending on what the homeowner wants to do one sytem or the other will give him the most hot water the most efficiently. You guys are the pros you need to ask questions and learn both systems and where to use them.

hvac-tech

Constantin ???? for you

A Dual-coil Vitocell / AC Desuperator? Tell me more. And size of the GFX 4" or 3" , PVC or ???

Cosmo_3

Thanks for your posts

Somehow I am still leaning towards the tankless, but as alsways it depends.

The one drawback to tankless heaters I have seen is when washing hands at the lavatory faucet, and the fact that on some jobs per homeowner request I have had to wire the hot water recirc pump to operate 24/7.

We insulate the heck out of the hot water mains, and recirc loop in these cases and it is not as bad as when they are not insulated. Also consider that this hot water loop actually increases the storage capacity of the hot water system as a whole. But I try very hard to explain to my HO's that a hot water recirc that is always on is a waste of energy, and also increases a/c loads in the summer!

How about this guys, a hybrid system using both the Viessmann V-300 indirect, AND a tankless water heater installed in series afer the hot water outlet of the indirect tank. This would help take over heating the hot water when the indirect can't keep up, and will stay off as long as it sees hot water.

Anyone try this?

Thanks for the posts!


Cosmo Valavanis

Dependable P.H.C. Inc.