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thankless hot water heater for heat

azl | Posted in Energy, Heating & Insulation on November 11, 2008 05:36am

Can I replace my old high efficency boiler with a equivalent rated tankless hot water heater such as a renee or takagi?


Edited 11/17/2008 8:24 pm ET by azl

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  1. User avater
    McDesign | Nov 11, 2008 05:45am | #1

    It'll be a thankless task, either way.

    Forrest - no help for the newbie.  Anyone really know this?

    1. bobguindon | Nov 11, 2008 07:17am | #3

      Forrest - no help for the newbie.

      Shouldn't that read 'Forrest - no help for the newbie by the used-to-be?'

      Sorry, couldn't resist...

      Bob

      Edited 11/10/2008 11:21 pm ET by bobguindon

  2. shellbuilder | Nov 11, 2008 06:22am | #2

    Yes, its done all the time. domestic hot water works better not being combined with it.

     

  3. davidmeiland | Nov 11, 2008 08:31am | #4

    I don't think most experienced radiant designers/installers would spec a tankless water heater as a heat source. You can probably get the highest level of efficiency with a top-of-the-line modulating/condensing boiler, as well as cool stuff like integrated controls.

    But to answer your question, yes you could use a tankless. Personally I think their greatest appeal is for heating domestic hot water in situations where space is limited or endless flow is needed (i.e. huge tub). 

    1. AlanRoberson | Nov 11, 2008 08:36am | #5

      JLC had an article a few years back about a guy doing this - a how to in the magazine

    2. azl | Nov 18, 2008 04:44am | #13

      Thank you for answering. The tanksless hot water heaters claim over 90% efficency. Also,the newer fancy boiler have a flame on continuously so why should they be more efficient.

      1. davidmeiland | Nov 18, 2008 06:35am | #16

        From what I understand, the tankless heaters are most efficient when running at full burn. The slow flow rate of a radiant system combined with the high incoming water temps would absolutely kill the efficiency of the system. You want a high end boiler like a Veissman, Burnham, etc. Do some lurking at heatinghelp.com and get a feel for what the pros are installing... look for the "Wall" forum.

        1. azl | Nov 20, 2008 06:07am | #26

          Thanks

  4. Tim | Nov 11, 2008 09:10pm | #6

    You can. By "old", how old? And by "high efficiency" what do you mean high efficiency? The biggest question is: "Why would ever consider doing such a thing?"

    A tankless water heater, operating as a small boiler in a typical IFRH application (15 degree dT) is about as efficient as an old cast iron boiler from the 1950's, maybe 60%. Any efficiency rating for these things is based on a 60 degree dT. Compare the size of the heat exchanger in your tankless to that in your boiler.

    I know this because I use a left over tankless water heater to heat my garage. If you have one and you're not planning heating your living space with it, go ahead. If this would be your sole source of comfort heat, don't do it.

    1. azl | Nov 18, 2008 04:49am | #14

      Thank you for answering. The tankless heaters claim eficiency of over 90%. Why do you feel they are such a step back?

      1. Tim | Nov 19, 2008 05:41pm | #25

        As others have mentioned, the 90% number is the very best case, 100% full fire with low temperature (60 degF) water comining into the heat exchanger and probably 120 going out.

        The effectiveness of any heat exchanger is directly related to the temperature difference between the exchange fluids, in this case water on one side and hot flue gases on the other side (as well as other factors such as mass flow rate, surface area and the conduction/convection qualities of the materials of construction and the surface area details). The hot flue gases are going to be at essentially a constant temperature, though the mass flow rate will change based on the firing rate. In a domestic water heater application, heating cold water to make hot water for use at the faucet/shower, the incoming water is in the 55 to 65 degree range and the supply temperature is in the 110 to 130 degree range (depending on preferences and applicable codes). Obviously the water flow rate changes with demand as well.

        In a radiant floor heating application, common designs utilize 1/2" tubing in 200 to 300 foot loops that have an approximate 15 degree temperature drop. Depending on the details of the floor and the installation, the supply temperature to meet the design heating load could be anything from 85 to 160. My insulated bare concrete garage floor/slab system works well at 85 degrees, an under the subfloor (staple-up) system might require upwards of the 160, or even higher in some instances. The higher the return temperature to your heat exchanger (water heater, CI boiler, condesnsing boiler, etc) the less effective it is. The less effective the heat exchanger is, the less efficient the overall heater and heating system is. Assuming a typical supply temperature of 105 and a return of 90 (vs the rated 60) you can see where the difference comes from. High temperature differences equate to high(er) efficiencies. Lower dT's = lower efficiencies. So if you cut the dT by 75% (from 60 to 15) you will have to expect a significant reduction in efficiency, though not 75%.

        Also, as some others have mentioned, a water heater, with or without a tank, is not the ideal choice for comfort heating. They are not designed or intended for that service, though they are frequently use for this application. I use tankless water heater (that I hated as a source of domestic hot water, so removed and replaced it) to heat my garage with an infloor heating system. It works well, but if it fails, my car will be a little frigid in the morning or my butt gets cold when working on the snowthrower. No biggie. Frankly, the demand is small and the efficiency was a secondary consideration, in my case. I had it, so I used it. I would never have purchased a new tankless water heater for this application. For nearly the same price I paid for this unit (Bosch AquaSatr LP with power exhaust, about $750) I could have bought a small wall hung boiler (at a wholesale price of course ;>). And when it fails, that is exactly what I will replace it with.

        Edited 11/19/2008 9:47 am by Tim

  5. BillHoover | Nov 11, 2008 09:19pm | #7

    Two points:

    1.  Some tankless products do not like warm water inputs and hence when recirc systems are used, their warrantee is voided.  Make sure you check on the warrantee for the one you buy.

    2.  Most tankless water heaters are about 82% thermally efficient.  If you buy a high efficiency boiler, they will be much higher than 90% efficient, hence, if you aren worried about your fuel bill, you will buy a high efficnecy boiler.

     

    Bill

     

    1. Tim | Nov 11, 2008 09:47pm | #8

      Bill, be cautious of using that 82% thermally efficient number in regards to comfort heating. See my previous post. Those units are rated with cold water (60 degrees) and and a high dT (these make heat exchanger more efficient). There are no ratings at 15 to 20 degree temperature differences, but the efficincy is much less than you expect.

      1. BillHoover | Nov 12, 2008 12:46am | #9

        Absolutely!  There is no reason to believe that a tankless water heater will perform any where near as well as a high efficiency boiler which is designed for this application.

         

        Bill

        1. snap pea | Nov 12, 2008 04:57am | #10

          But a regular water heater with a tank can be very suitable for a radiant system, is that correct?

          1. rich1 | Nov 12, 2008 06:54am | #11

            They are used, but the same arguments against them are the same as against tankless. 

            You also need to check with your AHJ.  May not be legal in some areas.

          2. DaveRicheson | Nov 15, 2008 09:21pm | #12

            Can't remember if it was JLC or FHB that did an article on just that. IIRC they used a heat exchanger and circulating pumps that kept the RFH and domestic HW seperate. I believe it was done with a standard domestic WH because the stored water is maintained at a higher temp. and the RFH loop is generally much lower.

    2. azl | Nov 18, 2008 04:57am | #15

      Thank you for answering.

      Why do some of tanks less heater not want warm water returned to them.

       The tankless heater also claim over 90% efficiency and in theory, since the only go on when there is a call for water and not continuously they should be more efficient. 

      1. woodman53024 | Nov 18, 2008 04:36pm | #17

        You asked two questions:

        First, the warm water input issue.  Tankless water heaters must adjust the firing rate of their burner to match the heat needed in any given situation.  The heat needed is determined by the flow rate and by the temperature rise needed.  If the temperature rise needed is very small, the burning rate must be very small.  The problem is that if you have a 200,000 BTU/hr burner which is what most whole house tankless units have, they can not turn down the burner below 17,000 or 20,000 BTU/hr  (typical turn down ratios for burners are 10:1 or 12:1).  Hence, if the heat needed is lower than the minimum firing rate, the burner will not come on.

         

        Secondly,  efficiency:  I don't know what makes you think that most tankless water heaters are 90% efficient.  Most of them have an Energy Factor of 0.80 to 0.82.  For a tankless heater, the energy factor is very nearly equal to the thermal efficiency (since there is very little standby loss).  Hence, most tankless units on the market are 80 to 82% efficient.  There are only a few condensing tankless water heaters on the market and they are very expensive and quite rare.

        So back to my original comments, based on the warm water return issue and the fact that you can easily buy much more efficient boilers (or storage water heaters), using a tankless heater for hydronic heating is not a wise choice.

         

        Bill

         

        1. DaveRicheson | Nov 18, 2008 09:43pm | #18

          Would he also need a mixing valve to temper the water down to RFH temperatures?

          I'm just thinking out loud here. I thought most radiant floor heat was in the 90 degree range, maybe as high as 95 degrees, but I not up to speed on RFH. If a tankless water heater is used, how low can you set them?

          1. woodman53024 | Nov 18, 2008 11:38pm | #19

            I do not see a problem with yhe low temperature. Most tankless heaters can handle a low output temperature IF the burner requirements are high enough. But as I mentioned before the low flow rates and small temperature differentials are likely to make a tankless heater a poor choice. On top of these issues is the low efficiency of tankess compared to a good boiler or a condensing storage water heater.In fact, I can not see a good reason to even consider a tankless for this application. Why would one want to us one for rfh?Bill

          2. DaveRicheson | Nov 19, 2008 02:27pm | #24

            Thanks

        2. NRTRob | Nov 19, 2008 12:53am | #20

          Woodman got this on the head.Basically you have one of two situations:1. You really need the output of the boiler you've got (answerable only with a heat load calculation or analysis of your fuel bills vs degree days, perhaps). In that case, a mod/con boiler will have payback over a tankless heater in any heating application unless you live in a climate with very few heating degree days.2. You don't really need the output of the boiler, which is vastly oversized for your heat load, in which case a tank water heater will perform VERY nearly as efficiently as a tankless in a heating application, for MUCH less money and less electrical use to boot. If the tankless ever did achieve parity due to a percent or two efficiency boost, it would be eaten up in a single service call.... and if it did, you are in a cold climate, probably with a real heat load, and you should get a mod/con.Basically, there is no common situation I can see or think of where a tankless water heater is the right choice for a heating application.Anything over 90% efficient will get you in a similar price range. The question is whether it will function properly, and what features it has. For that kind of money, you should at least get outdoor reset, and no tankless water heater I'm aware of will give you that.-------------------------------------
          -=Northeast Radiant Technology=-
          Radiant Design, Consultation, Parts Supply
          http://www.NRTradiant.com

          1. User avater
            jonblakemore | Nov 19, 2008 06:20am | #21

            "2. You don't really need the output of the boiler, which is vastly oversized for your heat load, in which case a tank water heater will perform VERY nearly as efficiently as a tankless in a heating application, for MUCH less money and less electrical use to boot."How many heating degree days would be the max to fit within your second scenario? Are we talking Mobile or Knoxville? 

            Jon Blakemore RappahannockINC.com Fredericksburg, VA

          2. NRTRob | Nov 19, 2008 06:29am | #22

            Heating degree days measure volume of heat needed over the course of a large period of time.. it's not a way to size a boiler, just to choose a cost-effective heat source. BTUs/hr under "design conditions" is the way to size an actual heat source.to put it another way, imagine two houses that both need 50,000 BTUs/hr to keep the house at 70 on the coldest day of the year, but one is a modest home in maine and one is a sprawling McMansion, poorly built, in Texas.Both might need the same amount of output from a heat source, but the maine house would have much faster payback on a high efficiency heat source (more heating degree days) than the texas house would (less heating degree days).But, both houses still need the same "size" boiler or water heater.. that is, they must both be capable of generating 50kBTUs/hr.I haven't done the math for everything, but up here in maine (7500 HDD or more) generally anything over 20kBTUs/hr should be looking at higher efficiency for any normal price differential I regularly see.-------------------------------------
            -=Northeast Radiant Technology=-
            Radiant Design, Consultation, Parts Supply
            http://www.NRTradiant.com

          3. User avater
            jonblakemore | Nov 19, 2008 07:03am | #23

            Thanks. 

            Jon Blakemore RappahannockINC.com Fredericksburg, VA

        3. azl | Nov 20, 2008 06:08am | #27

          Thanks

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