# Tankless with dedicated Recirc Question



## Phat Cat (Apr 1, 2009)

Here's the situation:

Have a customer who has a large beach house (4.5 bathrooms and 2 kitchens). They had two gas water heaters (75 gallon & 50 gallon) and a dedicated recirc line. They were happy, but they want to go 'green' and never run out of hot water.

The gas company sold them three Rinnai tankless heaters. They don't do the plumbing, so we were called in.

Rinnai designed the system. The three units are all mounted side by side. There is a 20 gallon tank water heater too. Rinnai specified the tank be set at 105, so the recirc. is circulating 105 degree water. At the furtherest point, the HO is waiting 5 minutes for the 120 degree water. They are not happy. Plus, this presents a potential burn situation. Someone hops in a lukewarm shower at 105 and in five minutes, the shower is 120. 

Since the recirc line doesn't go through the tankless units, why not set the temperature to 120 degrees?

Also, Rinnai specified dropping a 3/4" line back into the recirc line after the 120 degree water leaves the tankless units? Why not eliminate this line and let it go through the recirc. line?


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## SewerRatz (Apr 25, 2009)

Here is the way I like to plumb in multiple units with a recirc line and a storage tank. http://a-archer.net/images/Noritz.pdf


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## Tankless (Jun 12, 2008)

I just got done dealing with this sort of thing.

Very large house with a recirc pump going 24 / 7.

Long story short, we were installing a Metland pump. I noticed a few odd things about the recirc....opened up a few more feet of drywall in the mech room and I see 3 totally different recirc zones all mani'd together. So I start cursing and let my customer know he was in for alot more than he thought. This was the second house like this I have seen this done to. First one another guy did the install and I had to fix it. Point is, the recirc system didn't work well and even after I tuned everything, it still didn't work that well.

For the standby tank system I think it's a stupid idea, but if your clients want to spend the money...it can work.

I don't know the impact on the warranty by reheating already tempered water.

I'll assume one heater does one portion of the house, and the other two does the rest?
Got a diagram of how it is plumbed?
You guys gas is right? That will be alot of fuel at least 600 kbtu's
What the manufacturer says and the way I do it is sometimes a little different. But with the stand by tank....2 heating systems.....at least one recirc system....multiple circ pumps....is not just a little "oh yeah, throw a pump on it" kind of situation...at least not the way I do them. I can figure out whatever your setup is or needs to be, but I need to know the plumbing layout and more importantly the return system.


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## Protech (Sep 22, 2008)

Higher standby losses. 

I wouldn't use a 20 gallon tank. I would use a 1-5 gallon tank. Why have 20 gallons?



PlumbCrazy said:


> Here's the situation:
> 
> Have a customer who has a large beach house (4.5 bathrooms and 2 kitchens). They had two gas water heaters (75 gallon & 50 gallon) and a dedicated recirc line. They were happy, but they want to go 'green' and never run out of hot water.
> 
> ...


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## Phat Cat (Apr 1, 2009)

> Tankless said:
> 
> 
> > For the standby tank system I think it's a stupid idea, but if your clients want to spend the money...it can work.
> ...


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## Phat Cat (Apr 1, 2009)

Protech said:


> Higher standby losses.
> 
> I wouldn't use a 20 gallon tank. I would use a 1-5 gallon tank. Why have 20 gallons?


Rinnai specified 6, 10, or more as determined by plumbing professional.


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## Tankless (Jun 12, 2008)

I cannot think of a viable situation where putting tankless heaters in series is a good idea. Sounds like Rinnai is up to their usual mindless designs. 

Here's some experience for you. Even if you follow their lame directions to a tee, they still won't warrantee it in 3+ years when things start to go foul. 

I would be more interested in making the system work well for the home owner, than trying to obay the "reps" at Rinnai. Sorry, but they are not known for their technicle expertise.

If ur going to use a stby tank, heat it to 120-125
You will have one recirc on the return dumping into the 20 gal storage tank. That return is on a stat with a cut out at the target temp..120ish.

There is a lot to this kind of system. Alot of things need to work error free for this system to deliver HW as expected. Sorry, I have just been down this road before and these dumb ass reps think they know all when they know nothing but what a diagram tells them. On paper and real world are two different things. If it was me, I would yank that storage, get rid of the series connections and reconnect in parallel deviding up accordingly. You will spend more than you think on getting something like this working for along time. How does it make sense to circulate 105 water? For someone to even mention that as a target temp......careful who you listen to....


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## nhmaster3015 (Aug 5, 2008)

And for all the thousands of dollars and time spent they get to save....... NOTHING!

But they do get a ton of service hassles.

Who designs these dumb assed things? Better yet, can you believe the dumb asses get paid to design these dumb assed things? I'm in the wrong end of the trade. I'm going to get into dumb assed designing for pay. :laughing:


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## Tankless (Jun 12, 2008)

I make alot of money re-doing designs like this.....

Tankless is not an investment, I don't understand why so many ppl think they are...


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## ILPlumber (Jun 17, 2008)

I can see the market for tankless.

This is not it. 

What about condensing style regular WH's for an application such as this?


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## user4 (Jun 12, 2008)

nhmaster3015 said:


> And for all the thousands of dollars and time spent they get to save....... NOTHING!
> 
> But they do get a ton of service hassles.
> 
> Who designs these dumb assed things? Better yet, can you believe the dumb asses get paid to design these dumb assed things? I'm in the wrong end of the trade. I'm going to get into dumb assed designing for pay. :laughing:


Multiple tankless units make a ton of sense for a lot of applications, you have to think outside the single family home box. I have installed them in restaraunts, motels, bakeries, etc.


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## Phat Cat (Apr 1, 2009)

Our customer on their own went and bought the three tankless Rinnai units. They contracted directly with the gas supplier to connect the gas only. They purchased this multi-million dollar home that was in foreclosure and decided to make all these 'upgrades.' They just moved in yesterday. I'm certain what they had was more than sufficient and worked. My guess is they wanted an endless, instantaneous supply of hot water. Per Rinnai's design and recommendations, there is no way to achieve that.

My original question - Why the differential of 15 degrees for the tank heater? What's the point of circulating 105 degree water.

The only thing we can come up with is that setting it at 120 would require more energy and Rinnai couldn't back up their energy savings?


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## user4 (Jun 12, 2008)

PlumbCrazy said:


> Our customer on their own went and bought the three tankless Rinnai units. They contracted directly with the gas supplier to connect the gas only. They purchased this multi-million dollar home that was in foreclosure and decided to make all these 'upgrades.' They just moved in yesterday. I'm certain what they had was more than sufficient and worked. My guess is they wanted an endless, instantaneous supply of hot water. Per Rinnai's design and recommendations, there is no way to achieve that.
> 
> My original question - Why the differential of 15 degrees for the tank heater? What's the point of circulating 105 degree water.
> 
> The only thing we can come up with is that setting it at 120 would require more energy and Rinnai couldn't back up their energy savings?


I have no idea what the Rinnai rep was trying to accomplish, we usually use a ten gallon elctric heater as a storage tank and don't connect the electric to it, it is used purely as a storage tank.


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## A Good Plumber (Jun 18, 2008)

PC, I believe the reason for the 105 setting is that most of the time the hot water needed for a shower or to wash your hands would be about 100-110 max. 

A person usually showers at a temp of 108. Recirculating 105 deg water is a good temperature to provide hot enough water for washing your hands or face and is a decent temp to get started at in the shower. You can certainly set the circ temp at 110 if you so desire. They usually suggest setting it at 10 deg below your water heater set point.

A 3/4" copper water pipe holds 2.51 gallons per 100ft. If the furthest fixture is 100ft away and the shower delivers 2.5gpm, it will move the coldest water out of the pipe in approx 1 minute. That to me is not a long time to wait.
Keep in mind there will be some heat loss, so definatly insulate the piping.

I've installed several systems the way you've described and each of the customers is very happy with the results and the way it works.


As for the heaters being in series, the will be connected with a Multi-unit "link" cable and controler that modulates the demand.


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## A Good Plumber (Jun 18, 2008)

Killertoiletspider said:


> I have no idea what the Rinnai rep was trying to accomplish, we usually use a ten gallon elctric heater as a storage tank and don't connect the electric to it, it is used purely as a storage tank.



Your killin me. This would only create a larger "sandwich" effect if it is piped the way PC indicated. You would have more "cold" water to move out of the system before any hot water arrived at the fixture.


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## A Good Plumber (Jun 18, 2008)

PlumbCrazy said:


> Here's the situation:
> 
> Have a customer who............There is a 20 gallon tank water heater too. quote]
> 
> ...


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## Tankless (Jun 12, 2008)

A Good Plumber said:


> Your killin me. This would only create a larger "sandwich" effect if it is piped the way PC indicated. You would have more "cold" water to move out of the system before any hot water arrived at the fixture.


 
You must not understand that the storage tank is which is at temp (125 for me) gets recirculated back to the tankless when the temp falls X amount of degrees. There is no cold water sandwich at all if the system is working correctly. 2 recirc pumps operating off two different stats. One recirculates the storage tank, the other recirculates the water in the return. This is how all the manu's say to eliminate the dredded evil sandwich...


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## A Good Plumber (Jun 18, 2008)

A Good Plumber said:


> Your killin me. This would only create a larger "sandwich" effect if it is piped the way PC indicated. You would have more "cold" water to move out of the system before any hot water arrived at the fixture.


 
Read again!

I believe we are trying to help PC solve the design she was givin.


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## tooslow (Jul 17, 2009)

*recircline*

I have installed Raychem heat tape on hot water lines more than once, put the system on a clock for intended times of use and wa-la problem solved.

You could also pre-heat the incoming cw to maximize the tankless flow rate in regards to temp rise charts.


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## SewerRatz (Apr 25, 2009)

The thing that botehrs me about storing water at 105º is you are begging for things to grow in that tank. Also when I tool the Rinnai training classes they said you never install the units in series.


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## Tankless (Jun 12, 2008)

tooslow said:


> I have installed Raychem heat tape on hot water lines more than once, put the system on a clock for intended times of use and wa-la problem solved.
> 
> You could also pre-heat the incoming cw to maximize the tankless flow rate in regards to temp rise charts.





> so the recirc. is circulating 105 degree water. At the furtherest point, the HO is waiting 5 minutes for the 120 degree water. They are not happy. Plus, this presents a potential burn situation. Someone hops in a lukewarm shower at 105 and in five minutes, the shower is 120.





> Since the recirc line doesn't go through the tankless units, why not set the temperature to 120 degrees?


I assume it's plumbed like the diagram Sewer posted.
What I don't understand is how she is keeping 105 in the 20 gal storage tank. Is she using a tempering valve? The heaters are in series, is the first one heating up to 90, the second to 105 the third up to 120? is she grabbing 105 off the second heater for the stby tank?

It sounds like she is tempering back down to a stby tank, which is about as INefficient as it gets with this type of setup. There are alot of variables and more than one way to do this. Based on the info given, I assume it's as simple as increading the temp in the stby tank, because that is what is getting recirculated. I don't personally agree that 105 from the return is a viable way to do this. I have never met a HO who was happy with that. HO's want hotel hot water...at temp, 24/7. I come close to that with my designs but without having water flowing through the heaters any more than I have to. 



> Also, Rinnai specified dropping a 3/4" line back into the recirc line after the 120 degree water leaves the tankless units? Why not eliminate this line and let it go through the recirc. line?


Can someone explain this last part to me? I am just not getting it....

Are they saying to pre heat the returnED water that is going back to the stby tank?


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## A Good Plumber (Jun 18, 2008)

View attachment Document1.pdf


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## SewerRatz (Apr 25, 2009)

Here is Rinnai's hot water design book. On page 26 is their recommended way of installing 3 units with a recirculation pump and with a water heater. They show the three units in Parrall, and no where does it say to keep the tank set to 105. http://www.rinnai.us/documentation/downloads/Hot_Water_Design_Manual_Rev_B_R-TRWH-E-02.pdf


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## Tankless (Jun 12, 2008)

A Good Plumber said:


> View attachment 2506


She said they were in series, that is parallel


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## A Good Plumber (Jun 18, 2008)

My mistake. :jester:

You are right again!:laughing:


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## Tankless (Jun 12, 2008)

SewerRatz said:


> Here is Rinnai's hot water design book. On page 26 is their recommended way of installing 3 units with a recirculation pump and with a water heater. They show the three units in Parrall, and no where does it say to keep the tank set to 105. http://www.rinnai.us/documentation/downloads/Hot_Water_Design_Manual_Rev_B_R-TRWH-E-02.pdf


 
Please tell me how that diagram would eliminate a cold water sandwich.


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## A Good Plumber (Jun 18, 2008)

Tankless, The point is, the temperature differential in the circulated piping is very nearly the temp of that needed and should not be a factor. I believe it makes more sense to circulate the water through a small 10 gallon electric tank, if you want instant hot water, than to cycle it through the tankless water heater causing it to unnecessarly turn on and off.


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## Tankless (Jun 12, 2008)

A Good Plumber said:


> Tankless, The point is, the temperature differential in the circulated piping is very nearly the temp of that needed and should not be a factor. I believe it makes more sense to circulate the water through a small 10 gallon electric tank, if you want instant hot water, than to cycle it through the tankless water heater causing it to unnecessarly turn on and off.


I don't dispute that a 10 gal won't work fine, but I personally think the entire cost of the system isn't worth it given other ways of doing it. As for the temp difference...are u meaning the 105 circulated and the 120 tankless output?

I understand what you are saying, and I sorta agree, for resi I do not install systems like this. Condo / apt. complex's - small hotels...yes

That sais, I am not a fan of continous water transfer. I have seen too many pin holes as a direct result. All's I'm saying is I don't agree with Rinnai's planned systems (most) and if there is going to be water recirculated, I will make it so the water cycled is at temp....give or take a few degrees. had that been done in Plumb Crazy's setup (going against Rinnai's wishes) she would have never gotten the call back. 

Systems like this, that cost what they do, better work damn well. 105 at the taps for 5 minutes, IMHO doesn't constitute a well working system.

BTW, thanks for the dialog - I actually really like discussing things like this:thumbsup:


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## A Good Plumber (Jun 18, 2008)

The temp dif I'm referring to is the difference between the circ water temp and the point of use temp (usually about 108 degrees). 

As for the pin holes, if the pump is sized correctly and the flow of the water is not excessive, you should not ever experience any pin holes.(Hard water conditions aside) 

I too enjoy the dialog. I still have a lot to learn, I've only been in the trades for 32 years and I'm learning every day.


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## Phat Cat (Apr 1, 2009)

SewerRatz said:


> Here is Rinnai's hot water design book. On page 26 is their recommended way of installing 3 units with a recirculation pump and with a water heater. They show the three units in Parrall, and no where does it say to keep the tank set to 105. http://www.rinnai.us/documentation/downloads/Hot_Water_Design_Manual_Rev_B_R-TRWH-E-02.pdf


The above is what I was trying to describe.

Tankless - forgive my lack of knowledge. I used the words 'parallel and series' interchangeably - which I know you cannot do. So caught up in accurately describing situation - ignoring the tankless part.

The only difference on our drawing is the tank is shown to be set at 105. Based on the drawing - it shouldn't matter what the tank is set at since this water doesn't go through the tankless units anyway. Just trying to understand why Rinnai would have specified the temp. I think we were giving Rinnai too much credit. There isn't a valid reason other than saving energy.


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## Phat Cat (Apr 1, 2009)

Tankless said:


> I don't dispute that a 10 gal won't work fine, but I personally think the entire cost of the system isn't worth it given other ways of doing it. As for the temp difference...are u meaning the 105 circulated and the 120 tankless output?
> 
> I understand what you are saying, and I sorta agree, for resi I do not install systems like this. Condo / apt. complex's - small hotels...yes
> 
> ...


She had two gas fired water heaters. Should have been plenty of hot water given the fact our cold water isn't even that cold. The house will have the most use in the summer when her grandchildren come to visit. Most of the time it will just be her and her husband.

In an effort to go green, she went and bought the whole set up. By the time we were brought in it was a done deal. What she had was a better system IMHO and a whole lot less expensive.

Thanks everyone for your help. I'll let you know why Rinnai specified 105 when I find out.


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## SewerRatz (Apr 25, 2009)

I was reading the first few pages of that Rinnai guide, they are saying that the tank water heater must be set at the acceptable amount of heat loss or greater. In one of their examples that the normal working temperature is 120º and 100º water at the last fixture is acceptable. Here let me cut and paste from the guide.

This is from Page 7:
Determine the acceptable temperature drop at the last fixture in the loop. For example, if the supply temperature from the water heater is 120 ºF (49 ºC) and an acceptable temperature at the last fixture is 100 ºF (38 ºC) then the acceptable temperature drop is 20 ºF (7 ºC).

This part is Page 8
The following applies when using a tank water heater (gas or electric) to provide heat for a circulation loop.

The heat *output* of the tank must be equal to or greater than the calculated circulation loop heat loss.​So with what is stated there I am thinking they are saying 105º water is acceptable. But it does say it can be set higher So I do not see a problem with it being set to 120º


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## Phat Cat (Apr 1, 2009)

SewerRatz said:


> I was reading the first few pages of that Rinnai guide, they are saying that the tank water heater must be set at the acceptable amount of heat loss or greater. In one of their examples that the normal working temperature is 120º and 100º water at the last fixture is acceptable. Here let me cut and paste from the guide.
> 
> This is from Page 7:
> Determine the acceptable temperature drop at the last fixture in the loop. For example, if the supply temperature from the water heater is 120 ºF (49 ºC) and an acceptable temperature at the last fixture is 100 ºF (38 ºC) then the acceptable temperature drop is 20 ºF (7 ºC).
> ...


Thanks SewerRatz! The above makes a lot of sense. We were only given a drawing and the temp. is clearly indicated for the water heater. Would have been nice if it said 'or higher).

Thank you everyone else for your assistance! :thumbup:


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## A Good Plumber (Jun 18, 2008)

Can we exhale now?

:laughing:


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## jw13472 (Mar 29, 2009)

guys, guys, guys. calm down a bit and lets get back down to the basics of what tankless is all about. you must install a storage tank in order to keep the factory warranty not just for us but for the HO. in a few years they will be thankful that it was installed properly and that they will not have to pay for replacement parts on top of service fees. 

Also if no storage tank is used with a circulating system then you will be replacing the units about 4 to 5 times as quickly if not more depending on the length of the run for return. in order to keep constant temperature no matter what you set it at the tankless will have to run nearly 24 hours a day since it does not allow for temperature drop and will run continously in order to stay at the set temp.

Also setting these units in a series is a good way to extend the life of the units because they rotate and share the load when needed so that one unit is not the workhorse and ends up wearing out prematurely. Keep in mind that this is only my experience and each tankless job has been a well thought out planned install which all of us should take the time to do.

I have also been on the repair of another plumbers install and it is the most frustrating job ever when you can tell someone had no idea what they were doing and decided to wing it. But I also make it known to my customers that they are buying endless hot water and not something that is going to save them a great deal of money in the long run.


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## jw13472 (Mar 29, 2009)

*oops*

sorry was a little late on post. chicken pecking is an art.


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## Tankless (Jun 12, 2008)

jw13472 said:


> guys, guys, guys. calm down a bit and lets get back down to the basics of what tankless is all about. you must install a storage tank in order to keep the factory warranty not just for us but for the HO. in a few years they will be thankful that it was installed properly and that they will not have to pay for replacement parts on top of service fees.
> 
> Also if no storage tank is used with a circulating system then you will be replacing the units about 4 to 5 times as quickly if not more depending on the length of the run for return. in order to keep constant temperature no matter what you set it at the tankless will have to run nearly 24 hours a day since it does not allow for temperature drop and will run continously in order to stay at the set temp.
> 
> ...


 
Are you trying to give me a heart attack?


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## Protech (Sep 22, 2008)

which part were you referring to tankess?


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