Buffer Tank + Tankless Question

ragna_rOctober 13, 2009

I'm a homeowner who made the mistake 2 years ago of installing a tankless for my radiant floor heating. In the process of trying to 'make it work' using the tankless heater I already own, I've had the system re-piped into a primary/secondary loop system with a buffer tank.

My question with this setup is: should I be running the tankless at as high a delta as I possibly can? I've always had to run 140 deg. water in the stapled up tubing to get the rooms to come up to temperature with any kind of speed. Lacking the thermal mass of a slab or even the metal plates under the subfloor, I assumed that the higher the temp. differential the faster things would heat up. With my tankless I was thinking to set it at 158, and keep the buffer tank at 140 which would dump out to the floor loops. I'm trying to prevent the short cycling I was getting before and try to make the tankless run a little more efficiently.

Thanks for any help you can offer.

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Is this a condensing tankless heater? If not, you should be okay with the high delta. You are going to lose some efficiency as the temp goes up. My guess is that there will be some increase in the tendency for scaling to happen also. I don't know if this is a closed loop or if you are running domestic hot water through the tankless also.

Let us know how it goes.

I'd love to see a tankless heater tied to a 40-50 gal tank for domestic hot water. Seems like it would be very efficient and allow for the use of smaller, lower cost tankless units. It would also avoid the potential problem of increasing gas service.

    Bookmark   October 13, 2009 at 1:47PM
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Thanks for the reply. The tankless is a Takagi TK3, as far as I know it's not a condensing unit (no condensate drain that I can find). This is a closed loop, dedicated to the radiant floor heating with a separate tankless for DHW.

I always thought that tankless heaters excelled at higher delta-Ts but I understand the point about efficiency loss as the temp rises.

BTW, the DHW side of things works very well even given our cold well water and using multiple fixtures at once. I'm not sure a buffer would make things too much better than they are.
Thanks again.

    Bookmark   October 13, 2009 at 2:14PM
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Hi Ragna_r,
Please, please tell me why you said you "Made the mistake of installing a tankless for my radiant floor heating."

We are considering a very similar set up for our home - using a Takagi TK3 to power a closed loop system, with a separate DHW heater (50 gallon). What do you regret about using a tankless? Would it have been better to use a wall hung boiler instead? Fill me in!

I know you posted this 4 months ago, but I'm hoping you're still hanging around to help shed light on this!

Thanks so much!

    Bookmark   February 25, 2010 at 11:50AM
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Hello Marry what would you like to know. I am the takagi certified installer for NJ.
What would you like to know. and just so you all know it is a condesing boiler.

    Bookmark   February 25, 2010 at 10:16PM
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Hi wisehvac
Funny you should ask - it was actually your suggestion in the first place! :) Here's what you originally posted:

"Here is what I would do if I was you. You don't need that many btu's to heat up them two rooms. Plus you want to get hot water also. Have a certified takagi tankless water rep. come to your home. (this will be free ) tell him you want a tk-3 pro. (better heat exchange in the unit itself) it can fire up to 199,000 btu's and down to 11,000 btus. then you want to use a bronze bump with a heat exchanger on one side, plus boiler trim. (1/6 horse power. with a mixing valve for your domestic water. you want to then put two zones on other sided of a heat exchanger with a zone board for 2 zones with 2 pumps one for the baseboard. Ask for high trim baseboard so you can set the water temp down and get max btu's per linear foot. set the temp for 150 degrees during the winter cold month. Now for the radiant floor get a viega dealer in ur neighborhood. get climate panels that you can put cement nails in and the put a flooring on top. If you wanted to know more about this route just leave a post.

you can have all 3 coming off one unit. The best thing about this type of set up is that you don't need 180 degree water like a boiler. and in summer time you can set the water temp down to 120 or 115. and when your away on vacation you use no gas.

The down side is hat the climate panels can be expensive, the tankless water heater has to have a nice size gas line. the equipment to do this job can get pricy. I mean really pricey. With nice equipment comes a good price. Takagi & viega are my brand of choice. but you could use navien and wirlsboro if you wish. All 4 manufacturers will be happy to come to your home and sit down and talk to you. (I sell all four)"

Anything else you could elaborate on would be very helpful!

So, back to my original question, what would make someone regret using a tankless system like the OP did?

And the Tagaki TK3 - that's a condensing boiler you say?

Thanks so much!

    Bookmark   February 25, 2010 at 11:48PM
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I am not 100 percent positive but anything vented directly out of house is a condensing, that is why they don;''t go in chimney. your suppose to put a condensation tee with a damper on the vent. I would install tk's all day long. why he didnt like it is probably becuase he got a bad install and not educated enough to know better. If you take the info i gave you and go on the tk websight and look for a certify installer in your neighborhood then ur on the right track. The only thing I would make sure is that he is also a design and lay out with radiant. I would use a primery andn secounday line with this set up also.

    Bookmark   February 26, 2010 at 12:08AM
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Thanks! I've learned alot from the Radiant Floor Company website - are you familiar with them and if so, would you trust them to help with our layout?

    Bookmark   February 26, 2010 at 12:57AM
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you mean Radiant floor.com? Yes I have dealt with them on 3 different occasions. there system does work but personally there are a few things I don't like. The main thing I don't like is they really use a build you own manifold instead of a premade existing manifold. So you ask what is the difference. well look up wirsboro, Roth, or viega radiant floor. your able to adjust the flow rate going to each manifold with gauges and have very very good control with GPM. That is very important, That is called balancing the system. I just don't like there choice of material either. Instead of using a websight, why don't you call the companies i have mentioned and let them send a sales rep down to your house. That is there job! or even better call takagi ( I am there go to guy & they give me lots of work ) and tell them your interested in using there product (tk-3 pro) in a radiant floor application. They will get someoen there.
may I ask where are you from?

    Bookmark   February 26, 2010 at 8:44AM
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"Hello Marry what would you like to know. I am the takagi certified installer for NJ.
What would you like to know. and just so you all know it is a condesing boiler. "

Certified in what, went to a sales class?

NO, the TK3 is NOT condensing

The TK3 makes a lousy cheap wall hung boiler because it is a flash heater. Meaning water bypasses heat exchanger inside gets superheated and remixes downstream.

The TK-Jr would do a much better job with heating and radiant.

    Bookmark   February 26, 2010 at 1:25PM
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tk-jr doesnt have enough gpm and just so you know the tk jr , tk3 tm1 tk32 all condense water after 3 feet of run. if your using over 3 feet its recomended that you install a condensation drain. you can call it a flash heater. but they all condensate, some more then others. and no i didnt take a sales class, i actually demostrate them classes as well as give other classes at joghnstone supply.
th2 would be the best but its kinda pricey

    Bookmark   February 26, 2010 at 11:39PM
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We're in Denver, CO.
I've hesitated to call a rep because we probably will make our purchase online and I didn't want to take advantage of anyone's time. I have encountered some neighbors who have retrofitted their homes with radiant systems, and so we're going on a few "outings" to see them in action. I think this will be quite helpful!
Wisehvac, I've appreciated and trusted your input on this post and in other posts on the forum.

    Bookmark   February 27, 2010 at 11:59AM
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I suggest you go back for another certification class. The tk3 is not recommended for heating as the Jr is, read thier literature. So was the TK1S, both units not having a mixing valve bypass. Of course the Jr is piped primary/secondary so at 3gpm the potential of 30,000 btu's with a 20 DT per minute. Plenty of flow to draw full btu potential over an hour on the secondary. Just because it may have condensation issues in some installs doesn't classify it as a category IV condensing appliance.

    Bookmark   February 27, 2010 at 12:56PM
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peace rose, do me one favor call the takagi people 1 800 number from there websight and try to get someone local to come over. Who is going to install this if you buy on line? Sometimes when you buy on line it voids wareentee. Takagi is even trickier, if you buy it locally and its installed wrong they void warenntee.
zl100, your right in your last statement. Also servicing valves arent required. But how will flush out the system with out servicing valves. it may condensate thats why I always put a condensation drain in line. when useing a circ pump with the takagi its recomended taht you use the pro series or tm-32 or 50 wuith the better heat exchanger.
whats important is protecting the heat exchanger with spirol vents and anti scaling filters.

    Bookmark   February 27, 2010 at 9:19PM
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The poster was asking about tankless in a heating situation so Im not sure how a anti-scale filter would be applied since it is filled and a closed system.

Regarding the Spirovent, good air elimination as with all closed heating systems is important and spirovent makes one of the best, but not sure how it would protect the heat exchanger directly.

Right about one thing, buy it on-line and most tankless companies will leave you on the warranty side.

    Bookmark   February 28, 2010 at 5:51PM
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OK then I will tell you why scalling is a part of a closed system, do you ever look at your boiler and around some of the fittings wehere the green is. even like a gate valve and se it comming apart and start dripping at the nut. well that is scale. I am nott going to explain how it got there. But you have prevent scale build up.
What happens on a tankless if you don't use a spriral vent you will get air bubbles getting trapped in the heat exchanger and the air in the bubble will rott out the heat exchanger.
Those twwo things also void the warrnty.
before you ask me how I know. I read the installation manual, I toook the class. any other questions about tankless please ask me.

    Bookmark   March 1, 2010 at 2:12AM
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BTW, air bubbles don't trap themselves in a tankless heat exchanger. You have any idea the flow/velocity in the coils?

Scale doesn't come in with the air, but is part of the TDS in the water, some of which is calcium, lime.

Your confusing oxidation with calcification

Two different but sometimes part of the same issue depending on if it is operating as a true tankless DHW system, or an open or closed heating system.


I don't think I need to ask you anything, I work for a manufacturer.

One thing for all to remember, whenever advice is free, its up to the reader to disseminate what makes sense and is consistent, as we all know free advice if often wrong or bad.

    Bookmark   March 1, 2010 at 10:29AM
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When your using a circulating pump they want to use between 2 and 4 gpm so the air bubbles don't get trapped in the coil. That is what the tell me I don't make it up. If the flow is to slow it will oxidate if its to fast it will do something to the heat exchanger but I forget what it was.
Your right abut air and scale are two different things, maye I said it wronge. What I use is the de-scaling filter made from aqua-pure. I think the one post was usig it for domestic and space heat. when using the tankless you always have a possibility of hard water internally on the coils. Thats Why I as a service person always put a de-scalling filter before the unit and tankless servicing valves so I can de-scale the system.

    Bookmark   March 1, 2010 at 11:10AM
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