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Thread: Radiant heating with tankless water heater

  1. #1

    Default Radiant heating with tankless water heater

    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. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Very few of those are designed for continuous operation...I think you are much better served with a small condensing HE boiler.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Senior Member CHH's Avatar
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    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 by CHH; 07-25-2007 at 11:54 AM.

  4. #4

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    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!

  5. #5

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    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 itís in the middle of winter.

  6. #6
    Engineer Furd's Avatar
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    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. #7
    Master Plumber master plumber mark's Avatar
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    Talking 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

    http://www.weilhammerplumbing.com/houseofhorrors/

  8. #8

    Thumbs up 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. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    Homeowner geniescience's Avatar
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    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.

    david
    Last edited by geniescience; 07-25-2007 at 08:11 PM.

  11. #11

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    Quote Originally Posted by lionz1fan
    Ver: Lions training camp started today...get on the bandwagon NOW! 10-6 this year!
    You really are their #1 fan.

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

  12. #12

    Default 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. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  14. #14
    DIY Member cattledog's Avatar
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    Default boiler vs tankless water heater

    Fan--

    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.

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