Considering two dual-tankless options - feedback requested

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mcdavis

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I'm in the market for a new hot water system for a large home (5000 sq ft).
- 6 bathrooms, 4 full
- spa jacuzzi
- 2 dishwashers
- 1 shower with two lines, 3 heads + 2 body wash
- dual laundry rooms
- total of 13 sinks in the house

Water heaters are quite close to the gas source and exterior wall (4-6' vents). Thinking we will upgrade to 2lb gas. Considered running a dedicated line to the tankless, but there are a lot of BTUs in this house - close to 1,000,000 if everything is going at the same time...

I'm going to be here a while and am looking for the best possible design.

I looked at stainless tank vs. tankless and decided on tankless. I need ~150gal. Current have 1x75gal and 1x50gal. Both power vented. Pretty small space - maybe 5x5.

I'd like one control system for both tankless heaters, so I want them to communicate. If they can load shed/alternate to maximize longevity that would be a great bonus.

Considering two options:
1. 2xNavien NPE240A + external re-circ.
2. 2xRinnai RUC98i's + HTP everlast stainless 10gal buffer tank (1500w electric so it will work with 120V 20A) + external re-circ. Cannot use two RURs or RUC+RUR if you want just one control system for both units - silly limitation.

It is a difficult decision. Option 2 is about $1k more expensive. The Navien has two SS burners and a built-in buffer tank. I'm leaning toward option 2 - the Rinnai design - because:
a.) they appear to have the best reliability
b.) they have the longest track record
c.) they are the most commonly used design and as a result could be easier to have serviced
d.) best venting (UBBink concentric)
e.) I like the remote 195T control.

Location is the Pacific NW - water hardness = 1 - it is very good water.

Thoughts?? Thanks, -mark
 

Jadnashua

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What is your wintertime incoming water temperature? That will help dictate what and how many tankless systems you may need. You will need substantial gas lines, and LOTS of BTU's to raise water high enough in the potentially large simultaneous hot water demands.
 

mcdavis

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Water temp varies between 38 and 75. Typical winter avg. may be around 50. I like my water at 130, so the temp rise in winter is likely around 80.

Anyone else hear about new Rinnai tankless options coming in the spring??
 

mcdavis

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I should add that I am ok with the sizing of the units. If I get 9.6gpm (2x4.8 per 80 degree rise), that is fine with me...don't often have a lot of simultaneous use.

I'm more interested in feedback on the design, components, etc. Thanks! -mark
 

Dana

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Water temp varies between 38 and 75. Typical winter avg. may be around 50. I like my water at 130, so the temp rise in winter is likely around 80.

Anyone else hear about new Rinnai tankless options coming in the spring??

Why do you "like" your water at 130F ? That is a DANGEROUSLY high temperature, with a significant & rapid scald potential if sent to the distribution plumbing at that temp. Typical tub-fill temperatures (after the tub faucet's mixer) are ~110F or cooler, typical showering temperatures are ~105F.

While 130F is a reasonable storage temperature to limit pathogen growth an a tank type water heater, the output is typically tempered to ~115F prior to the distribution plumbing. With a truly tankless or tiny-tank tankless water heater there is no potential for stagnating water growing pathogens, since the water temperature is no maintained at that temp, and the water is 100% purged with nearly every hot water draw.

If the house is heated with a hydronic boiler (pumped hot water) you may be better off supplying a least half the hot water with an indirect tank off the boiler, depending on the boiler, and the distribution plumbing topology.

Independtly of your hot water heating solution, if you have at least 5' of vertical drain downstream of the multi-head shower, you can get quite a lot of additional showering performance out of a drainwater heat exchanger without adding more burner, and it will pay for itself in fairly short years on fuel savings alone if you actually USE that shower regularly.

The fatter & taller they are, the higher the return efficiency, and it's generally true that the fattest & tallest that fits will pay for itself quicker due to the higher performance.

While drainwater heat recovery may not always be economic for single low-flow shower heads & parsimonious showering times, for gusher showers like yours it always pays off. In your case it if saves you the cost of another tankless it pays for itself up front.
 

Jadnashua

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If you really want 130-degree water in the winter, you will not get the flow rate you described! It would be closer to 3.50gpm. ANd, unless you cascaded them, that would mean a really slow tub fill. I run the numbers in one of the tutorial posts - you can run them yourself to double-check. They'd work fine in the summer, giving you lots of simultaneous hot water use, but could be disappointing in the winter. That assumes you periodically clean out the mineral deposits - even with your relatively soft water, it will still be necessary, just probably not as often as those living where it is harder. My other major point to people is finding someone who both stocks parts and understands how to actually repair them. If you're lucky, that may not be an issue, but you may not have a lot of choices. Pretty much anyone can change out a tank, not everyone can reliably fix a tankless system, nor will they likely have parts available readily to them in the middle of the night or on a weekend.
 

Dana

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BTW: Unlike most states, Oregon even offers a tax credit subsidy for drainwater heat recovery (called "Waste water heat recovery" on that document).
 

Jadnashua

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The calculations aren't all that hard with some basic math. 1BTU will raise one # of water 1-degree F. That's the output of the burner, not the input as they're never 100% efficient (well, an electric one could be nearly 100%). Say you had a 200K BTU burner that was 75% efficient, you'd use 150K BTU as the output. The spec sheet should give you an idea of the efficiency. A gallon of water weighs ~ 8.35#. So, then the #gallons desired/min*60minutes will give you per hour. Then, multiply that by 8.35#. Multiply that by the number of degrees you want to raise that water and you'll have your BTU output requirement. Tankless systems look wonderful if you have mild water temps, but can be really problematic if you have cold (mine nears freezing after a cold spell - I've actually measured it at 32.5 after running for awhile like you'd get taking a shower in my home) or you need lots of volume. A tank's output will be nearly constant until you've used about 75-80% of it's capacity, depending on the burner if it starts out at set temperature. A tankless' output will start to drop (some will throttle the volume) as you exceed its burner capacity.
 

mcdavis

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Thanks for all the responses folks!

A few things:
1. I like 130 in the tank for pathogen control...the 105-115 range is fine at the faucets...I assume that significant loss will occur in the pipes, even with re-circ. It is literally 20 degrees outside right now - we see 2-3 weeks of this a year because we live at 1000'.
2. The drainwater heat exchanger is something I will look into, thanks for the recommendation.
3. Yes, I will do a regular cleaning of mineral deposits. Thanks Jim. I also have an installer who is very knowledgeable about service...he gave me a list of all the problems he sees & fixes with Navien vs. Rinnai...he installs and services both.
4. Thanks for the tax credit info Dana!
5. Jim: calc's - 2 x 199k BTU burner x .95 = 378k BTU output. 378,000lbs or water by 1 degree. My rise is ~80 degrees. So, 4725lbs of water by 80 degrees or 565gal of water....which seems like a lot?

I'm really interested in the show later this month to see if Rinnai releases anything new...rumors are that they will.

Any general feedback on the idea to use 2xRinnai + HTP 10-gal stainless? Seem like a reasonable system? Know of anything better??

-mark
 

Jadnashua

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A couple of things to recalculate...you're using the INPUT value of the system, you need to find out the outlet which will be somewhat lower - they are not 100% efficient! Plus, you're assuming both, and if you have each one supplying things on its own, while yes, you might get that amount, if one of them needed more than 1/2, you wouldn't have it. That's a bit under 5gpm based on your inlet gas use on each branch. A tub could use all of that on one branch. A multi-head shower could easily use all of that as well. The saving grace would be that any water use (except maybe for say a dishwasher or washing machine if you wash anything on hot), you won't be using full hot - you'll be mixing it with some cold, which improves your numbers. What most people do, though, is lower the tankless outlet to just above the max they want for their big users (showers/tubs), which means you may be nearly all hot going in and not mixing it with cold. There is really no big deal with using a lower temperature when you have a tankless system, since the water fairly quickly returns to below the critical temperature range unless you're running recirculation or have a buffer tank in the device. If you insulate your pipes, with recirc, the water temp will be quite close to that at the heater way at the other end.
 

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I'm going to bump this tread, hoping to get some more feedback.
I too have a house of similar size and water appliances. My original set up was; 2-50gal electric water heaters with a dedicated recirculation line/pump. One of my water heaters died about a year ago, I bypassed it, and am currently running on one 50gal tank.
I too am looking at 2 units; the Rinnai RUR98iN and the Navien NPE-240A, I'm wanting to utilize my recirculation line with one of these 2 units. My utility room is located in the center of the basement, plenty of room (6'x6'x12') power and natural gas. My challenge is going to be ventilation. I've got 24' to the exterior wall and all 3 heat pumps are located on that side of the house. I know with my water appliances that I have I should cascade 2 units. At this time I'd like to install 1 but be prepared to install the second. Researching both units, there's about a $500 cost savings with the Navien over the Rinnai, plus approximately $600 cost difference in using PVC with the NPE-240A over the concentric vent pipe the Rinnai uses.

My questions are... what would y'all do? Which units would you go with. Support seems to be good for both units in my area. If i went with the Navien and plumbed for a future unit, what do I need to step my PVC vent and exhaust pipes to?
Thanks in advance
Glenn
 

Dana

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Rinnai (a first tier Japanese vendor) is the world's largest manufacturer of gas-fired appliances and has a good reputation, and would be the safe bet. Navien (a first tier Korean vendor) has some good stuff, a spottier record in the US, but much improved from a decade ago. If local support is good for both I'd personally go with the Navien and pocket the difference in upfront cost. YMMV

You don't HAVE to go with the concentric venting for the Rinnai, BTW.

The quality & tankless of the installer is more important than the product itself. This guy bought the RUR98iN, but the installer was (apparently) a hack who didn't hook up the air intake, over-flexed the flex gas connection, and undersized the gas line which was teed off with another big burner appliance rather than a home-run of right-sized pipe back to the regulator:

index.php
index.php


Some (all?) of the new Naviens internally compensate for undersized gas lines, making them less sensitive to some of these issues.
 

GlennM

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Thanks Dana, I read thru the install manual for the Rinnai last night, I see there's a couple of venting options.
I saw the post on that install, I think I would be a little disappointed if I had paid for that.

I think I found what I'm looking for in the Navien installation manual.
Cascading 2 heaters, if I go with 3" PVC:
2-90's = 10'
24' of 3" =24'
1-45 = 2.5'
1- concentric vent = 5' (big difference in the Rinnai, I think it said add 15')
Grand total of 42.5 feet
When installing more than one heater, I see I've got to add back flow dampers to each unit, I don't see a drag calculation for those, thoughts?
IMG_0418.jpg
 

John Furbee

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I'm in the market for a new hot water system for a large home (5000 sq ft).
- 6 bathrooms, 4 full
- spa jacuzzi
- 2 dishwashers
- 1 shower with two lines, 3 heads + 2 body wash
- dual laundry rooms
- total of 13 sinks in the house

Water heaters are quite close to the gas source and exterior wall (4-6' vents). Thinking we will upgrade to 2lb gas. Considered running a dedicated line to the tankless, but there are a lot of BTUs in this house - close to 1,000,000 if everything is going at the same time...

I'm going to be here a while and am looking for the best possible design.

I looked at stainless tank vs. tankless and decided on tankless. I need ~150gal. Current have 1x75gal and 1x50gal. Both power vented. Pretty small space - maybe 5x5.

I'd like one control system for both tankless heaters, so I want them to communicate. If they can load shed/alternate to maximize longevity that would be a great bonus.

Considering two options:
1. 2xNavien NPE240A + external re-circ.
2. 2xRinnai RUC98i's + HTP everlast stainless 10gal buffer tank (1500w electric so it will work with 120V 20A) + external re-circ. Cannot use two RURs or RUC+RUR if you want just one control system for both units - silly limitation.

It is a difficult decision. Option 2 is about $1k more expensive. The Navien has two SS burners and a built-in buffer tank. I'm leaning toward option 2 - the Rinnai design - because:
a.) they appear to have the best reliability
b.) they have the longest track record
c.) they are the most commonly used design and as a result could be easier to have serviced
d.) best venting (UBBink concentric)
e.) I like the remote 195T control.

Location is the Pacific NW - water hardness = 1 - it is very good water.

Thoughts?? Thanks, -mark
Is best to locate the tankless, on demand water heaters close to where the hot water is needed. You don't need to waste energy heating long water lines or contentious circulation or central control system. The on demand water heaters will save you a significant amount on your energy bill for gas or electric. You will only be using energy when you use hot water. If you are away zero energy use. A tank system continusly keeps water in tank hot. A central system, tank or tankless means you need to heat the water lines from the tank to need & more delay getting hot water plus wasted water. Distribute on demand water heaters close to need.
 

GlennM

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I went with the Navien NPE-240A. It's been up and running for a week now, couldn't be happier with the set up and hot water delivery. I vented with 3" PVC added a sediment filter and a water softener while I was at it. I'm looking fwd to comparing my electric and gas bills in the coming months

IMG_1421.JPG
 

Dana

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I'm glad it's working for you!

I noticed that the foundation has no interior-side insulation- is the exterior side insulated (to IRC 2015 code-minimum of R10, in your climate zone 4A location? Even if you're not actively heating the basement it doesn't take much above-grade exposure for the foundation heat losses to be a double-digit percentage of the heating bill. If you're interested in taking that on there are multiple threads on this site on how to do that on the cheap without creating a mold-farm (or you can start your own thread in the remodel section.) Hopefully there's enough room between the studs and wall on the framing supporting the Navien to be able to do that properly.
 

GlennM

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I'm glad it's working for you!

I noticed that the foundation has no interior-side insulation- is the exterior side insulated (to IRC 2015 code-minimum of R10, in your climate zone 4A location? Even if you're not actively heating the basement it doesn't take much above-grade exposure for the foundation heat losses to be a double-digit percentage of the heating bill. If you're interested in taking that on there are multiple threads on this site on how to do that on the cheap without creating a mold-farm (or you can start your own thread in the remodel section.) Hopefully there's enough room between the studs and wall on the framing supporting the Navien to be able to do that properly.


Thanks for the observation and input its actually an interior wall.
 

GlennM

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5 month update. Couldn't be happier with the Navien NPE-240A. Its met and exceeded all of my expectations!
 
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