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Flow rate measure

Running a three zone system, about 450 feet 1/2 pex per zone with three Taco zone valves in insulated basement slab.

Using a Taco 008-F6 pump rated at 0-14 GPM, which runs all three zones.

I'm using a 50 gallon water heater as source but want to switch to a tankless. Flow rate on tankless model (Rheem RTEX-18, 18 KW) is rated at up to 5 GPM.

The system works pretty well as far as heating but the water heater runs constantly. No air in system which runs around 18 PSI.

I'm confused as to how to size the tankless if the pump runs at say 14 GPM but the heater can only provide 5 GPM of hot water....

What am I missing? This Rheem was recommended on another radiant forum . Thanks


  • Zman
    Zman Member Posts: 7,467
    edited November 2021
    Your tubing lengths are way long. How many square feet does it serve?
    No matter what you do, you cannot add more resistance to that loop. The 008 is not running in a happy place.

    What size (BTU) is your existing water heater? Is it heating the space adequately? What is your motivation for changing the heat source? Where did you get the 18KW number?

    Now for the elephant in the room. Why would you want to use a tankless water heater as a boiler? Sure they cost less upfront, but you will buy 2 or 3 water heaters over the next 20 years instead of one boiler.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 21,526
    I think you will find that neither a water heater nor a tankless water heater will find much favour here as heat sources -- particularly for a radiant slab. Great for a shower. But they aren't boilers, and aren't meant to be.

    You would be far better off -- and find you used less fuel, probably a lot less fuel -- if you looked at a tool which was meant for the job: that is to say, a mod/con boiler properly sized to the heat loss of the structure.

    Ideally, your first step in sizing the boiler would be to determine the heat loss of the structure, and base the size of the boiler on that, remembering that mod/cons can turn down the burners to suit lesser loads when it is warmer out.

    Then figure out what temperature your slab needs to run at to take the heat from the boiler and provide it to the structure. For a properly designed radiant slab, that's usually in the 90 F to 110 F range, but it can go higher.

    The flow rate is usually adjusted so that the difference in temperature between the input lines and the returns is around 20 F. The piping is arranged, with a mixing valve controlled by outside temperature, so that the slab circulation is on all the time, and the boiler -- at a different speed with a different pump -- is also on all or most of the time, adding just enough hotter water to maintain the slab temperature where it needs to be.

    OK. That's how a nice efficient, long lived radiant system would work.

    If you insist on running a tankless water heater -- as I say, quite the wrong tool for the job, but perhaps you are sold on it and it's your system -- again, you need to go back to the structure heat loss and choose a tankless which can provide that much heat. This has nothing to do with flow rate, but with the power. Keeping in mind that tankless heaters don't modulate. The one you cite is rated at around 60,000 BTUh It is designed to maintain a constant output temperature up to the maximum flow rate -- which is determined by that power rating and the temperature rise from its inlet to its outlet.

    Now the flow rate through your system is determined by the power you want delivered and the temperature difference between the inlet and the outlet -- and has nothing to do with the heat source. Go back to the heat loss from your structure and use the standard hydronic formula to determine the total flow rate -- BTUh = 500 times delta T * gallons per minute.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jamie Hall
    Jamie Hall Member Posts: 21,526
    A further thought. I notice that you are contemplating using electricity for space heating. May I ask where you are located, and if you have checked the relative cost of heating with electricity vs. heating with LP? In most areas of the United States, electricity is anywhere from two to four times as expensive to heat with, so location matters.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • RadiantHeat
    RadiantHeat Member Posts: 2
    ZMAN, the system was professionally designed a few years back by Radiant Design Service. It has worked well as far as heating. The entire space was 1456 SQ FT and each loop was specified as NOT MORE than 500 feet. It was originally designed with a Takagi gas powered on demand heater but I didn't have propane hooked up at time so went with the electric water heater. Since it worked, I left it that way.

    My motivation for changing is simply cost, the water heater does the job but it must run continually to keep up. I've been in touch with TACO regarding the circ pump and they stated it is a good fit for the system and I have had no issues with it or the system as a whole. Just feel the water heater should not have to be running continuously, once the slab reaches a stable temp. (after seasonal start up)

    The system was calculated at 32, 032 BTU's. The water heater is 60,000 BTU's but never shuts down. Normally, I only have two zones on anyway as the 3rd is storage space.

    Without getting too far into the weeds, I was just looking for clarification regarding circ pump capacity vs, heat source capacity. Doe s the heat source have to be able to provide MORE GPM then the circ pump?

    Thanks for input.
    EBEBRATT-Ed Member Posts: 13,783

    I see your 32,000 btus do you know the supply water temperature and TD is was designed for?

    The water heater not shutting down would seem to indicate enough flow.

    For instance 32000 btu with a 40 degree drop in water temp you would need to move 1 gpm for every 20,000 btus or 1.6 gpm

    for a 30 degree td it would be 2.2 gpm

    You have to know the supply water temp and TD and gpm required and then compare that to the heat source you are using to see if it will handle the load

    Alan (California Radiant) Forbes
  • Ironman
    Ironman Member Posts: 7,173
    You’re confusing data about the pump, the tankless and how components are selected.

    The circulator can move up to 14 gpm, but that’s at 1’ of head (resistance). With the improperly long tubing lengths that you have (they should be under 300’ for 1/2” pex), you’ve probably got 8-10’ of head that the pump is seeing. That means it’s moving about 5 gpm at best.

    A radiant floor in a living space should be designed and operate with a 10-12* delta T (temperature difference). A 20* is acceptable for a non living area.

    The problem that you’re gonna have if you try to use a tankless (wrong appliance) is that it has an extremely high resistance to flow because it’s designed to have 60 psi coming in and an open faucet going out. Typical hydronic circulators can never overcome that kind of resistance plus that of the tubing in the floor.

    You’ve gotten some very good advice on here already. Please follow it and do the heat loss calc and then look at using a mod/con that’s appropriately sized.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Zman
    Zman Member Posts: 7,467
    I will stick with my statement regarding 450' loops and the 008. The circ is running at the upper end of its range which is bad for its efficiency and longevity. That fact and 450' is a poor design practice, professional or not. That being said, you do not want to add resistance to the loop, so be sure to pipe whatever new heat source you decide on in a primary/secondary configuration. Once you go to primary/secondary, the GPM on the heat source side is irrelevant.

    There are countless posts on here from folks that saved a few bucks and installed a tankless water heater instead of a boiler. Give that decision a great deal of thought.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GroundUp
    GroundUp Member Posts: 1,649
    How do you figure that an electric water heater is putting out 60,000 BTU? Is this a commercial unit with four elements all wired to the same stat? A standard 50G electric tank is putting out 15,354 BTU at any one time. If the stats were wired together would be 30,708 BTU and would make perfect sense as to why it's running constantly given your 32k load calc. Tankless are trash and you will regret changing it out. If you're okay with the electric bill, switch it to an electric boiler and do it properly. If not, use an LP boiler. Also your loops are far too long as has been mentioned, but obviously it works and cannot be changed so that's that.
  • SENWiEco
    SENWiEco Member Posts: 151
    If you are interested in calculating the true heating load of the dwelling, check out LoopCad by Avenir. It is very intuitive and easy to use by even an amateur like me, and puts out a very accurate and professional model of your dwelling with a room by room heat loss/gain calculation.
    Sean Wiens