Radiant heating with tankless water heater

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by lionz1fan, Jul 24, 2007.

  1. lionz1fan

    lionz1fan New Member

    I am wondering if anyone has had experience with trying to support a radiant heat system with a tankless water heater. 6.5 gallons per minute, tubing is stapled to the bottom of a floor system (well insulated and reflected), and located in Michigan. I am predicting about a 30 to 40 degree drop in temperature through a run.

    1) Is this a viable option to a boiler or water heater?

    2) What makes or models of tankless water heaters are recommended for this application?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    Very few of those are designed for continuous operation...I think you are much better served with a small condensing HE boiler.
  3. CHH

    CHH New Member

    Denver, CO
    First question is: why are you interested in tankless over a boiler? Is it a very small heat load?

    Second question: The temperature drop sounds excessive. Radiant systems are low temperature and most efficient at lowest possible temperature. How can the system accept a 30 to 40 degree temperature drop?

    To answer your question: no, I have no experience using a tankless as the heat supply for a radiant system.
    Last edited: Jul 25, 2007
  4. Verdeboy

    Verdeboy In the Trades

    The Takagi tankless units claim to have radiant heat applications. Do some research on them.

    Forget about the Lions for now. We have a pennant to win! :D
  5. got_nailed

    got_nailed DIY Senior Member

    You need to look at the recovery rates of the units and the GPM. You would be better off with a boiler sized to what you need. They have good recovery rates.

    (Electric) I have seen 2 30gallon hot water heaters used in series with high recover rates. But I always wondered about the power bills in the dead of winter. Just make sure your electrical service is sized right and you set up a close loop system properly.

    You need to find out how much heat you will need when every thing is on when it’s in the middle of winter.
  6. Furd

    Furd Engineer

    Wet side of Washington State
    Some water heater manufacturers will void any warranty if their heater is used in anything other than domestic (potable) water heating service.

    Your local plumbing or mechanical code may also prohibit using a domestic water heater of any kind for space heating applications.
  7. do the boiler

    recently I was looking at a wood burning boiler
    for my own home......the wife is not freindly about it yet

    It would the best of both worlds...you get the heat from
    the boier in the home and the floor heated too.

    I have heard of people doing the tankless
    but I dont trust them to be the main heat source
    for the home......

    why not just a plain old gas boiler
    or a wood burning boiler with the
    gas as a back up

    I would trust that over the tankless any day...

    I have a hvac company freind that gets callls every week
    in this town from people trying to find someone
    to repair their tankless systems....

    once its in , then you are stuck with it...good or bad

    Last edited: Jul 25, 2007
  8. lionz1fan

    lionz1fan New Member

    Why am I going tankless????

    Cost. I understand a boiler is the water heating of choice but at a 5:1 cost ratio it does not make cents. The rate of return between the two would take me 30 years.

    I started out with a water heater and calculated that there is not enough recovery time to have an efficient system. Tankless has the gallons per minute and BTU's necessary to support my system with a pump connected to a thermostat.

    I will check into local codes and make sure that I do not void any warranties if I do this.

    I was really hoping to find someone that used tankless in an application.

    Thank you all for your replies. I will take them all into consideration.

    Ver: Lions training camp started today...get on the bandwagon NOW! 10-6 this year!
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    There isn't a good temperature control on a tankless...there is, but it isn't in the same league as a boiler. A tankless is designed to take in cold water and raise it about 70 degrees or so. Depending on the heat load, the radiant floor heat supply will wander all over the place. You'll need additional controls to temper it properly, and on a really cold day, it might take a couple of passes through it to get up to temp. Depending on the flow requirements, you may not get anywhere near the desired temperature. A boiler is designed specifically for this situation. There is a reason a boiler costs more. But, you'll find a more simple, less expensive boiler might not cost much more than a tankless that would actually work (who knows for how long). A boiler should work for a lot longer than a tankless in a home heating mode, so you'd probably be replacing one much more often. Also, keep in mind that there is a minimum pressure most tankless systems are designed to operate at. This is much higher than most heating systems are designed for. the control valves, etc you might need are designed to operate at a nominal 1 atmosphere (14.7 pounds) of water pressure. This is probably below the minimum pressure required on the tankless. Problem is, the heat exchanger is fairly small, and you might end up with it flashing the water to steam, creating all sorts of safety and operational problems. A boiler's heat exchanger is desinged to keep things protected and to handle it at the proper pressures and volumes. Engineering a system that will work without big problems when it isn't designed for it is just asking for trouble.
  10. wow. System level overview, with systems engineering focus. To add a marketing input, I'll add that the engineering design people in all the firms making heaters (boilers, tankless, etc) have all studied these constraints a lot, hoping to do better, make better product, and sell more. It's a competitive market out there, and if it were really all that easy to use tankless many would have done it and many would be selling it.

    in other words, all the concerns jim raised are valid, imo, and prevent anyone from making a living with tankless fro heating. Good luck if you install it and tweak it to your liking.

    Last edited: Jul 25, 2007
  11. Verdeboy

    Verdeboy In the Trades

    You really are their #1 fan.

    Nothing like setting yourself up for a major let down... for the 50th year in a row. :D
  12. Budau

    Budau New Member

    Used a Takagi with limited success

    I have been using a Takagi unit for my whole house heating system. It crashed after three years, and I am putting in a Trinity boiler right now. I think the Takagi would have done a reasonable job under other circumstances. I live in Western Canada where it is not uncommon to have weeks of -30 to -40 degree temperatures. I have a 2000 square foot home with a basement walkout. The basement has in floor hydronic and the main floor is served by an air handler. The code up here doesn't allow a Takagi to be used in a closed loop system, so I was required to use a heat exchanger. The in floor worked excellent, and I would consider trying it again in a garage. The air handler was probably sized incorrectly and didn't work well at all. During cold periods it would run for 40 to 50 minutes of the hour. I really don't feel I was getting proper heat transfer through the air handler system. As a result I just finished pulling it out and am putting in a high efficient modulating furnace. I neglected to mention along the way the Takagi developed a problem that I don't know how to fix. I spend a lot of time away from home and need confidence in my home heating system. I thought I would bite the bullet and put in the correct equipment.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    Most of the tankless systems aren't designed for that kind of heating use. Ideally, for best comfort, a boiler should run continuously at the designed heat load - this produces the most efficiency and comfort. A properly designed boiler is often a much more reliable choice.
  14. cattledog

    cattledog New Member

    Portland, Oregon
    boiler vs tankless water heater


    If we take your predicted flows and temperature drops (6.5 gpm/30-40F) and use the rule of thumb that 1 gpm at delta T of 20F delivers 10Kbtu/hr it looks like you have a load of between 97.5Kbtu/hr and 130Kbtu/hr. These are quite large. I'm not sure about the size of the space being heated, the design day temperatures, the duty cycle you were anticipating from the tankless, and the insulation of the envelope, but with numbers over 100K you really should be sure of your heat loss calculations. It doesn't sound like you are just adding floor heat to a new addition or some bathrooms.

    With heat requirements like you are projecting I feel it would be far better to use a high efficiency modulating and condensing boiler designed for hydronic heating. The heat exchanger and controls are likely to be far superior to what you can get with a tankless. The difference in energy efficiency will also be significant.

    There are many choices of manufacturers for "mod/con" boilers and there are quiet, sealed combustion, wall hung units which can fit in quite tight spaces if that is the constraint driving you to the tankless heater.

    For your application, I would strongly advise you to change your thinking on heat source from a tankless water heater to a mod/con boiler.
  15. LaurelD

    LaurelD New Member

    The fact is, that Takagi units work very well. There are two problems. One, tankless hot water heaters as boilers--just not as efficient as a real boiler because, as others have said, it is designed to take cold water from 50 to 120 degrees, not 120 degree water to 140. Boilers do that better. And Two, the takagi just can't do this job longer than five to seven years without busting. And then you need a new one. HOWEVER, my opinion is that they are ok, and here's why. Math.

    Do the math: A new Takagi every five years costs about $700-800, whereas a new condensing boiler costs $3,000-$4,000 every ten to fifteen years. And during that 15 years, there is mucho maintenance and occasional repairs. With the takagi, it runs great till it breaks. So on that basis, it's a wash.

    I've used Takagis for about 15 years and changed them out three times one one house and four times on the other. I am financially ahead of all the other people who bought boilers, which are expensive to maintain and service. The new condensing models of boilers are extremely energy efficient, but they are not long term machines, very unlike the old school wasteful cast iron boilers that would easily last 40-50 years.

    I feel okay about switching out the Takagis because they pay for themselves, and I like gambling that every five years when it's time to replace, somebody would have invented something newer that would work better.

    I have hopes for the Navien And Rinnai combo boilers which produce hot water and also function as boilers, but so far, the reviews aren't great.

    So it's not an easy answer, but for the expenditure of money and eaze of a switch out, tankless units like Takagi are fine. You spend maybe 10-15% more on energy, but you save a bundle on repairs. It's just one woman's opinion. (A licensed plumber woman.)
  16. Dana

    Dana In the trades

    Do the math? Really?

    I did the math- you dragged up a 7-years-dead thread! (For this!? !)

    And here I thought Halloween was behind us! ;)

    But on the merits of the argument, it sounds like you are designing your tankless systems to run at low delta-Ts and high flow rates even with the tankless, which is a good way to toast 'em in 5 short years, but it doesn't have to be that way. It doesn't take much of a buffer tank to run 40- 50F delta-Ts (or higher) and l0w flow on a Takagi, even with 120-130 water in the tank from which the radiant is sipping (with the bottom of the buffer tank still in the condensing zone for a condensing tankless). A 1-1.5 gpm flow on a 40-50F delta-T is a high enough burn rate to cover most homes in the US, and it can be dialed down from there to get it into a modulating mode on some systems. Most only need 0.5-0.7 gpm to fire up, so it's possible to tune the burner output to match the whole house load and buffer the output, reducing the total numbers of burn cycles to something closer to what it would see in just a domestic hot water application.

    It's not really designed for the application, but it can work pretty well if you do the hydronic design math to optimally utilize the unique features of a tankless and avoid burnig it out early. Doing merely "disposable boiler" math, substituting a tankless for a boiler while keeping the same system parameters as if it were a boiler is potentially leaving quite a bit of tankless lifecycle and efficiency on the table.
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