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Small radiant job - need a heat source recommendation

We've got a kitchen/bedroom radiant job coming up; 500 square feet; probably 8,500 BTU's. Too small for a boiler, ideal for a water heater; preferably an electric one because venting is difficult and power venting is too noisy.

My question is how do you figure what capacity water heater you need for the job; 240 volts and high amp is available.

I can convert 8,500 BTU to 2,490 watts. Rheem has a 6 and 10 gallon, 3kW heater and a 15 and 20 gallon, 4.5kW heater. Am I on the right track here?
Often wrong, never in doubt.

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kcopprick in Alaska

Comments

  • Jamie HallJamie Hall Member Posts: 12,859
    Indeed you are. Like pretty much most heating applications, the best idea is to match the heat input (your 3 KW heater) to your heat demand (your 8,500 BTU). I'd probably use the 10 gallon -- but either one should be fine.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    Alan (California Radiant) Forbes
  • hot_rodhot_rod Member Posts: 13,315
    Most all of the tank style water heaters have screw in 1" elements and you can size the element to the load. 120 volt elements are available from 500W (1705 BTU) to 1500W.

    With 240V you can go as high as a 6,000W (20,460) BTU on a 30 amp circuit.

    Usually around a 30 gallon tank and larger, even a stubby you get two elements. So with two 30A circuits you get
    41,000 BTU/ hr.

    Johnstone Supply always had a wide selection of wattages and lengths available for electric tank elements.

    Shop around I've found the 6 gallon are often less than the 12 or 20, maybe a more common size around here.

    The 2 gallon Artisan under sink point of use are handy also, but they require a special element, not as customizable.

    Eemax has a good selection of point of use electric heaters also.

    Here is a 12VDC 200W element with built in stat. Off grinders use these connected to wind as a dump load, or battery based 12V systems. Or you can put one 12V and one 120 or 240V in a tank. Solar first, AC voltage as back up.
    A single 200 watt PV panel and some wire and you have solar hot water. something to think about for those fossil free cities ;)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    Alan (California Radiant) ForbesSTEVEusaPASolid_Fuel_Man
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    Why water? If there are no future plans for more floor heat, just put electric radiant in the floor.
  • SnowmeltSnowmelt Member Posts: 1,192
    I’d use the current water heater with a s.s pump
  • Why water? If there are no future plans for more floor heat, just put electric radiant in the floor.

    I though electric radiant was floor warming only, not full-blown heating.
    Often wrong, never in doubt.

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  • hot_rodhot_rod Member Posts: 13,315

    Why water? If there are no future plans for more floor heat, just put electric radiant in the floor.

    I though electric radiant was floor warming only, not full-blown heating.

    You're a plumber for crying out loud :), install pex and have a wide selection of energy sources.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    Alan (California Radiant) ForbesZmanGroundUpSolid_Fuel_Man
  • Larry WeingartenLarry Weingarten Member Posts: 1,801
    Hi @Alan (California Radiant) Forbes , I wonder if a heat pump water heater would do it? Use heat pump most of the time, but the backup element kicks in when needed.

    Yours, Larry
    Alan (California Radiant) Forbes
  • Jamie HallJamie Hall Member Posts: 12,859
    Reality check time. I realise that @Alan (California Radiant) Forbes is a plumber, and indeed one of the better ones from all I've heard. But his labour isn't free (at least I hope not). And he was asking about a heat source, which suggests to me that there isn't one currently available.

    Now I'm not usually keen on straight electric for heating -- on any large scale. But for 8500 BTUh? Somewhere around 3 KW? And I presume that the kitchen and the bathroom are not the same space? The more I think about it, the more I think I would go with straight electric radiant wire in the floor -- either prefab or lay it yourself -- with the bathroom and the kitchen on separate circuits. Just make sure not to damage the wiring when installing the floors...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot_rodhot_rod Member Posts: 13,315
    > @Jamie Hall said:
    > Reality check time. I realise that @Alan (California Radiant) Forbes is a plumber, and indeed one of the better ones from all I've heard. But his labour isn't free (at least I hope not). And he was asking about a heat source, which suggests to me that there isn't one currently available.
    >
    > Now I'm not usually keen on straight electric for heating -- on any large scale. But for 8500 BTUh? Somewhere around 3 KW? And I presume that the kitchen and the bathroom are not the same space? The more I think about it, the more I think I would go with straight electric radiant wire in the floor -- either prefab or lay it yourself -- with the bathroom and the kitchen on separate circuits. Just make sure not to damage the wiring when installing the floors...
    I suppose it depends on the installation details. Pex and plates can go on top or below if there is access. Are any of the mats or cables able to be stapled below the floor, if that is the method required,

    I like wire or mat in a tile set best.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 1,978
    I've installed many many Schluter electric cable systems. The cable snaps into their orange underlayment, this is generally for a tile floor. Never had an issue yet.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • The kitchen is a new pour. The bedroom has existing tile and I was planning on installing Viega Climate Panel with engineered wood on top.
    There's still a question of electric being only for floor warming, not full blown heating. We have cold winters out here. B)
    Often wrong, never in doubt.

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  • hot_rodhot_rod Member Posts: 13,315
    It’s all about surface temperature, regardless of tube, wire, or blowing air against the floor🌦
    You can melt snow with electric cable or mats, so heating a building is doable

    A load calculation and convert btu/ square foot to watts per square foot

    The mat system I used a few times was available in various wattage output from floor warming, to heating to SIM.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    Solid_Fuel_Man
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited June 8
    @Alan (California Radiant) Forbes said:
    I thought electric radiant was floor warming only, not full-blown heating.

    Heat is Heat. The floor is your radiator whether it is heated by water or wire. The Equivalent Direct Radiation (EDR) of the floor is the square foot of the heated floor. I don't think that your floor is listed in @DanHolohan book Every Darn Radiator (EDR) but the math is still the same. Delta-T of the radiator to the room temperature will produce the needed result. If the EDR surface is smaller, the temperature must be higher. If the floor is used as the radiator and a floor temperature of 85° is not enough to heat the space, then raise the floor temp to 90°, or 95°, and so on. BUT... There may be a point where the floor temperature is too hot to walk on. If that is the case, then the floor is not a large enough radiator for the room. Wire or water won't make a difference.

    I hope this sheds some light on your thoughts about Warming v. Full-blown heating.

    After thought: If the floor is not big enough for the coldest day (Design Temperature) then make provisions for another source before you close up the walls/floors. Electric resistance baseboard or panel... or ... some way to add radiator or towel warmer to the wall. We did a bathroom addition with a Buderus Towel Warmer as the only heat source for the room. I used a non-electric Thermostatic radiator valve for temperature control. Worked perfectly.

    In your case where there is no hot water source for space heating... put the extra wire in the wall for an electric towel warmer.
  • Dave H_2Dave H_2 Member Posts: 382
    Use the existing water heater and the the X-Pump Block; isolates the water heater from the radiant with a heat exchanger and two circs.

    Dave H.
    Dave H
    Solid_Fuel_Man
  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,333
    edited June 8
    Dave H_2 said:

    Use the existing water heater and the the X-Pump Block; isolates the water heater from the radiant with a heat exchanger and two circs.

    Dave H.

    Yes, that was an option, but the water heater would have to be changed for a larger one and there's no room. Also, there's only a 3-inch flue and a 75-gallon water heater would need a 4-inch flue.

    One of the water heater manufacturers used to make a 75-gallon water heater with a 3-inch flue, but I don't know if they're still around.

    And as far as installing electric radiant, I'm just not interested. I'm a wethead, not a sparkie.
    Often wrong, never in doubt.

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  • Dave H_2Dave H_2 Member Posts: 382
    why would you need to change out the water heater? 8,500 btu load (on a design day) should not break the bank on the DHW load unless they are already on the edge with the size of water heater there. That 8500 would only go down as the temp raises outside and there is not much more of a demand for heat.
    or
    You could raise the temp of the water heater, install a mix valve for the fixtures and gain just a tad more.

    Dave H.
    Dave H
    Solid_Fuel_Man
  • Sure, I see what you're saying, but dumbing down the system to a small electric water heater makes more sense to me right now. I like the X-Pump Block product and use it often enough to know that it is not trouble-free. Cost is an issue too.
    Often wrong, never in doubt.

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  • Jamie HallJamie Hall Member Posts: 12,859
    That's sort of a shame, @Alan (California Radiant) Forbes , though I can see your point. Straight radiant electric in floor will be more efficient and much easier to set up for control (try two zones, as I suggested) but... you do what you gotta do.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot_rodhot_rod Member Posts: 13,315
    Dave H_2 said:

    why would you need to change out the water heater? 8,500 btu load (on a design day) should not break the bank on the DHW load unless they are already on the edge with the size of water heater there. That 8500 would only go down as the temp raises outside and there is not much more of a demand for heat.
    or
    You could raise the temp of the water heater, install a mix valve for the fixtures and gain just a tad more.

    Dave H.

    Is the X Pump Block a lead free certified component? If only one side touches potable water is it required to be NSF listed?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • "Bronze casing for open or closed systems."

    I don't see any "Lead Free" designation, though.

    Dave?
    Often wrong, never in doubt.

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  • HVACNUTHVACNUT Member Posts: 3,357
    @Alan (California Radiant) Forbes , if you're tight for space, where do you plan to put the electric water heater?
  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 1,978
    How is the rest of the place heated? Scorched air?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • @HVACNUT The basement of a 3-story house is changing from a mechanical/storage area to livable space for adult children that have moved into their parent's house. The architect devoted some space for a boiler which will now be used for the water heater.

    The owners didn't realize that the direct vent boiler will make noise and steam outside the house and they didn't want a plastic flue running up the side of the building. And I didn't want to use a boiler either because I thought it was overkill for the small heat load.

    @Solid_Fuel_Man Yes, forced air in the rest of the house.
    Often wrong, never in doubt.

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  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 1,978
    What about losing the furnace and using a water coil in its place and then you have a boiler for the radiant, and install indirect or gulp...combi boiler.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • What about losing the furnace and using a water coil in its place and then you have a boiler for the radiant, and install indirect or gulp...combi boiler.

    ..............and the hits never stop.
    Often wrong, never in doubt.

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  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 1,978
    I'm not sure what this means?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • @Solid_Fuel_Man Your fan coil, Dave's X-Pump Block, Larry's heat pump..............all good solutions. But there are times when the cheapest solution is the best, no? A small, electric water heater to take care of a small heating load. Voila!

    Often wrong, never in doubt.

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  • Larry WeingartenLarry Weingarten Member Posts: 1,801
    Hi @Alan (California Radiant) Forbes , You just opened the door to an internal debate I have. It's the question of "first cost" versus "life cycle cost". It's sort of like buying a Tesla that has free charging vs getting a cheaper gas powered car that needs fuel. At some point the one that has free "fuel" will probably save money over the one with bought fuel, even though it costs more upfront. Is that a year out or twenty years away? Depends. Maybe it's like buying quality vs cheap tools.
    So the heat pump vs straight resistance becomes a question of how long the owners plan on being there, energy usage and resale valve... in addition to any rebates or incentives they might get for the heat pump. And then, I really like simple too as it's durable. Guess I have more questions than answers! :p

    Yours, Larry
  • Dave H_2Dave H_2 Member Posts: 382

    "Bronze casing for open or closed systems."

    I don't see any "Lead Free" designation, though.

    Dave?


    Sorry @Alan (California Radiant) Forbes I didn't see this until today.

    The X-PumpBlock is certified to NSF372 standards, which is the low lead certification.

    Thanks for asking.

    "See" ya later

    Dave H.
    Dave H
  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 1,978
    I'd go with the X-pump block off the water heater. However, their rebranded Tekmar controller is the weak point....ask me how I know!
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
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