Tankless with recirc and possibly storage

Jason Quinn

I am involved in a project at the tail end. Job consists of a Takagi TK-3 unit unit for dhw. The homeowner is complaining about the extended time it takes to get hot water 85-90 ft away to the furthest fixture. BTW, no return line was installed and all the walls are now closed.

Can I install a "comfort valve" (tee fitting) at the furthest fixture and recirc pump at the unit? I do realize I must size the recirc pump to overcome the pressure drop of the tankless if I dont have a buffer/storage.

Furthermore, after determining peak demand, the Takagi wont be able to keep up so storage must be added.

Any ideas on the best vessel to accomplish this: storage tank or electric water heater (venting of another gas appliance is a challenge here)? Also, can I install a simple "comfort system" recirc system where it only sees the headloss/pressure drop of the electric water heater and not have to overcome the pressure drop of the tankless (i.e. check valves strategically placed on tankless hw discharge)?

I checked out Rinnai's site and got a few ideas from their installation diagrams. Thanks for any/all input

Ironman

Point of Use

I'd suggest you use a point of use electric heater or a 6 gal. electric tank. If you need a lot of storage, you could add a larger electric tank back near the tankless and re-circ between it and the point of use, not the tankless. Then pipe the hot supply from the tankless into the cold inlet of the electric tank.

meplumber

Ironman is right on.

We use a small point of use WH tank for recirc. Maybe Jack will chime in, he is the man on tankless application in my opinion.

Jason Quinn

Thanks for your input

I appreciate your insight, thanks. Sounds like a good idea.

Jason Quinn

On second thought...

I have a few questions. Regarding storage, the Takagi will do 7 gpm and I need 12 gpm at peak demand. Therefore, I was thinking a 50-75 gallon electric water heater as additional storage.

If I install the 50 gallon water heater in the basement, near the tankless, how will I interface the point of use on the 3rd floor with this (recirc) if the walls are all closed already? Am I missing something?

Ironman

Grundfos Comfort System

Install the comfort valve on the farthest fixture and the pump on the 50 gal.electric storage tank. Don't attempt to re-circ to the tankless. Just let the tankless pre-heat the water going into the electric storage tank. The electric will probably only supply 10 - 15% of the heat needed. The tankless will do the rest.



Re-circulating through the tankless will cut it's life expectancy to about 1/3 of it's normal life, increase scaling and require a larger circ.

Larry Weingarten

How close...

...can you get a bigger electric heater to your third floor use?  If you could get it close, I'd simply feed it from the tankless and not do recirc.  An option would be to use demand controls (like Metlund) on a recirc back to a storage heater in the basement.  That has the benefit of eliminating any piping redo.



Yours,  Larry

Jason Quinn

Thanks for your support guys... another ??

I'm still not seeing in my "minds eye" how the tankless is not seeing the recirc flow with a comfort valve.

The comfort valve bypasses water from the hot to the cold side. The cold water is supplying the tankless. The tankless feeds the electric water heater inlet and the pump is on the outlet of the electric water heater. How does flow not go through the tankless? Forgive me, I want to fully understand before presenting and installing this.

Ironman

You're Seeing Correctly...

As far as the flow path goes. But remember, the Comfort valve will close off the flow when it senses hot water. There is 1.84 gal. in 100' of 3/4" in pex. It takes a min. flow rate of 1/2 gpm for the tankless to fire. It probably would not get through its pre-purge cycle before the Comfort valve sees hot water and closes flow to the cold side. It may not even see enough flow to fire at all.

Jack

Ok, here goes!

I just checked the specs on the TK-3 and at a 70F rise you are looking at 4.6 gpm +/-. you say that at peak flow you need 12gpm. Do you mean peak is with everything operating? What is everything? What fixtures and flow rates do you have? Coming in late on a job can be a killer, so where ever you go from now you must be right. Otherwise the first guy who screwed up will be forgotten and you hold the bag. Ask Obama about that! Oops, don't go there;)



For recirc I am a big fan of the Metlund design system. I prefer a dedicated return line as I had a contractor put in one of the cross connects and because of the piping lay-out everytime the pump came on the toilet became a bidet. THAT was a head scratcher, but at least they were getting their bottom washed with warm water;) Given that the place is enclosed I would do the compromise fix on recirc and pump the mains in the basement. Run a 3/4" return line. That should take care of the majority of the water. Perfect, no, but it will, depending upon piping lay-out improve the over-all performance at a moderate cost.



With out the flow info in the first paragraph, it is difficult to help design, but let's look at a few things for now. Rather than adding storage right out of the chute, I would analyze the whole structure and see if you could add a second tankless handling the far side of the house. Attic installs can work really well here. Don't overlook it.



IF you go with storage, get a storage tank. Ideally you want at least 4 tank penetrations. Cold in, hot out, tankless supply and tankless return to storage. I've done a lot of these and they work really well, but I would go first with another tankless. Get the answers on the flow questions and we can go further if you'd like.



MAKE SURE YOU LOOK AT AND UNDERSTAND THE TAKAGI FLOW SPECS BEFORE YOU RECOMMEND TO YOUR CUSTOMER. EVERYTHING HAS TO FLOW (SO TO SPEAK) FROM WHAT IS THERE AND YOU ARE NOT GOING TO GET 7GPM OUT OF THAT UNIT.

Jason Quinn

Thanks Jack

I appreciate your input and creative ideas. The client does know this tankless will not keep up with the peak demand. The peak demand consists of a rainhead shower head at 6 gpm, 3 body sprays at 1 gpm each and another shower head at 2.5 gpm. So, its actually 11.5 gpm peak demand.

Jason Quinn

Storage tank suggestions...

I would like suggestions on the make and model of a storage tank that you guys have had success with. Thanks for any/all input.

Jack

How big a tank?

AO and BW have "storage tanks". A discussion to have with your customer is how long does he expect to shower. At 12gpm this is an important question, both for your choice of storage tank and his happiness factor. Again, by the time you buy and pipe the tank, you are in the range of another tankless. Of course you have to have the gas capacity to carry it. Generally, for an 80 gallon tank I'd want two tankless anyway.

Jason Quinn

Exploring an indirect or reverse indirect

There is a WM CGi-7 handling the hydronics onsite. At this point, I'm exploring the feasibility of abandoning the tankless for an indirect or reverse indirect water heater driven by the boiler. Thoughts?

Ironman

Ah, Now the Full Picture Comes to Light

The indirect would be my first choice over a tankless any day. As Jack mentioned, you need to find out how long the customer is going to take a 12 gpm shower to determine sizing. Even with a reverse indirect, you're probably looking at a 50 gal min. I would also set it up with priority and pipe it so it's not getting water temp that's on outdoor reset. In other words, right off the boiler at 180* supply.



P.S.



After doing a little math, you might be looking at a 119 gal indirect. With a 12 gpm draw it would be depleted in 10 min. After that, you would need about 420,000 btu's from the boiler to maintain a 70* rise in heating the potable supply. You might consider keeping the tankless and use it to pre-heat the potable before it enters the indirect.

Jason Quinn

Thanks Ironman

To clarify, my peak demand is 11.5 gpm; but I believe I need to multiply this by .8 to get my actual hot water peak demand as the cold and hot mix at the thermostatic valve. So, in this case its approx 9 gpm hot water. The client, is also doing his research and is suggesting we install an electric water heater downstream of the takagi as storage. He also mentioned they take 15 min showers... now i need to figure out the size of the tank. Any further input? Thanks again for everyone's help.

Jack

At 9gpm, I'd suggest

another tankless paired with the first. If you consider the load, the customer wants a 15 min shower. What if he decides to go longer? You can make a system big enough to handle it with a boiler and indirect but the fact is you are designing a system that 24 hrs a day will work to provide 15 min worth of high flow. Two tankless twinned together are ideal for this application as the system will modulate on the actual flow whether from a single lav or the mac-daddy shower. As well, once the max flow is achieved the system will continue to make that flow all day or until the septic system blows up. You have the "potential" for the shower but you consume no standby energy.

Larry Weingarten

This sounds like...

... a prefect application for a shower heat exchanger!  Have a look at http://www.renewability.com/ and http://www.retherm.com/ .



Yours,  Larry

Jason Quinn

Thanks to all

for your input. I'm going to advise my client to twin (2) Takagi's together. There are 2 onsite, but one is for his parents on the first floor with a separate gas meter. Nevertheless, this makes the most sense economically. I will also install a small electric water heater as a buffer for a grundfos comfort recirculation system. Thanks again.

Jack

Ah, Jason...

Here is something else to consider. Years ago I had a guy install two Rinnai's with a 10 gal elec buffer tank. This place was enormous and the plumber installed the water heaters in the most remote corner of the building from the load. He then pumped the tank. This was early in the game for me with the tankless and was a real head scratcher. I finally had one of those 3am EUREKA moments.



What was going on is that the pipe lengths were so great that the electric elements in the tank could not keep up with the heat loss of the piping, so the tank just never $hut down. The owner would get in the shower in the morning and run the piping and the tank capacity before getting hot water.



I offer this as a caution with the tank. Personally, I don't care for those systems.

Jason Quinn

Thanks Jack

Aside from "pumping the mains" in the cellar, what other options do I have when the owner is not willing to open up walls to run a dedicated recirc line? A larger electric water heater?

Ironman

Ah, Jack

You're missing something here. The Grundfos circ. has a timer that you set for the periods that the farthest bath will be used. Then the "Comfort Valve" will only remain opened until hot water reaches it. It this case there's about 100' of 3/4 pex to clear. That's 1.84 gals. or about 1/3 of a 6 gal. tank and the the Comfort Valve closes and there's no heat being wasted.

Jack

Yes, I did consider that...

I do understand that the Grundfos has the timer. I just wanted to relate about the worst case , or one of the worst recircs I've seen. It was one of those that I still wonder why it took me so long to figure it out and then I spent a bunch of time there to prove I was right. It is a cautionary tale:)



Jason, from a cost/benefit to you and the consumer, I think that pumping the mains is about your only option. Do run a 3/4" return line.



I would still look at the Metlund with the motion sensors. More money perhaps, but a better more efficient system.

Jason Quinn

Act Metlund D'mand System

Just ordered this to address the recirculation issue- the 02T model with motion sensor. Will follow up with my results after its installed. Thanks again.