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Tankless diagram needed

Brule Member Posts: 4
Have new, well insulated two story, 2000 sq ft passive solar house with slab first floor, cast iron radiators on 2nd flr. This first winter, didn't have a boiler hooked up--just heated the house with a wood stove and passive solar. Now I want to hook up radiant floor and upstairs radiators. Was thinking about a non condensing boiler with mixing valve. However, have a Rinnai 75ls tankless for domestic hot water and an not sure I want to live with the delay for hot water going forward. The kitchen sink provides only enough flow to reach 130 degrees. If the tankless is set at 140, it doesn't start. The shower has enough flow to start the tankless at 140.

Am wondering if I can utilize the tankless for the radiant heating. I don't think I need high temp water for the upstairs radiators, as I didn't have them connected this winter, and the bedrooms stayed warm.....

Reliability and cost is important. I think I would be ok with a 40 gal propane for domestic hot water. Thanks.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Not recommended...

    In your own words, "Reliability and cost is important." Doing this will not give you years of trouble free service. It will result in heat source failure, and eventual complete replacement. The tankless was made for a very large differential in temperature. Radiant floors can not provide that necessary differential, and consequently, it will short cycle and die an early death.

    Every tankless heater I have seen that was misapplied in this manner suffered a horrible death within 5 to 7 years of having been installed. I would suggest in stead that you look into one of the fine modulating/condensing appliances on the market, and use a indirect or reverse indirect DHW heater to satisfy your hot water needs.

    You have one chance to do it right, or you can do it again, and again, and again, and end up spending more money than you would have spent to do it right the first time. ANd then there is the inconvenience of unreliable heat source....

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jack
    Jack Member Posts: 1,047
    First off,

    Rinnai will void your warranty if you hook thie R75 up to your heating system. Lots of very good reasons for doing so. I cannot imagine why you are running your system at 140, or 130 for that matter. I run mine at a max 120 and the system performs better than at high temps. Thinking that higher temps will "improve" your hot water systems performance is "thinking like a tank". What you need to do is reduce the set temp. That will increase the hot water percentage of delivered water. That purges the hot water pipe more quickly, giving faster hot water delivery. If you look at the temps you actually use, max 105* for a bath and 110-112* for a shower you can see that higher temps will delay delivery. Beyond that, if the delay is to much the unit is improperly located or not piped correctly. Look at a Metlund design recirc system.
  • Brule
    Brule Member Posts: 4
    Hot water solved

    Thanks for the replies. While gathering more information, my disappointment re long wait times for hot water from the tankless has been resolved---I plumbed a shower in a future sauna space with just the test cap---not the mixing valve. I was getting cold water mixing with the hot.......

    Now that I know I need a boiler and not a tankless hot water heater, what advice can you give me re supplying the pex in the first floor slab and the cast iron radiators on the second floor? If I use a modulating condensing boiler can I use cast iron pumps?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Glad you found the cross connect...

    I know how frustrating THAT can be...

    So long as the PEX tubing you use has an oxygen barrier on it, you can use ferrous components in the closed loop. If it is non barrier, then you need to look at the boiler manufacturers warranty closely (some don't warrant if used with non barrier PEX)

    Due to the fact that you have two extreme temperature demands, you will have to set a mixing valve to limit the supply hot water temperatures going to the low temp call during a simultaneous call situation.

    I prefer the use of the Lochinvar boiler, which does allow for two different (actually up to 3 different) calls for heat of differing values.

    With your tankless water heater, there is still a good chance of you experiencing what is called a "cold water sandwich", which should be self explanatory. Tis the nature of the beast... The usual "fix" is knowing when to or when NOT to jump in the shower after you've turned it on. The basic problem is a lag in logic, and over/under reaction of the heat source. Once it is "locked in" to a constant load, hot water remains constant. It just takes a few minutes to wake everything up and get it moving in the right direction, under tight control.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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