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Radiant floor heating questions

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Fluke216
Fluke216 Member Posts: 33
edited January 2023 in THE MAIN WALL
I got some good info in a post from awhile back ill put a link to that incase anyone wants to see it. Right now looking for some input on best source to use to heat two zones I have setup. One is in slab two 3oo ft loops 1/2 and one is above it staple up two 300 ft loops with plates and insulation underneath. I don't have a propane tank or natural gas available here. I could probably get a smaller propane tank if that is the best option. I guess options include electric boiler or water heater/ propane boiler or water heater. Let me know what you think that would be great . I have some other questions on pump sizing and components but going to make that in another post once I figure out the best heat source I guess. thanks for any info much appreciated.
https://forum.heatinghelp.com/discussion/188933/radiant-heat-manifold-questions#latest

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  • Jamie Hall
    Jamie Hall Member Posts: 23,525
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    You seem to eliminate oil fired boiler and oil fired water heater. Why?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Fluke216
    Fluke216 Member Posts: 33
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    no reason that could be an option to there originally was an oil furnace here so there's an old tank but its side ways in a crawl space and probably dosent have much life left in it so could get an oil tank for an oil fired boiler or water heater.
  • hot_rod
    hot_rod Member Posts: 22,327
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    Do you know the total heat load? Do you need two temperatures?
    For really small loads, maybe under 8- 10,000 btu/hr I have used small electric water heaters. With a 240V 30 amp circuit you can get a 5500w element for 18,750 btu/ hr

    This small 2-1/2 gallon was for a master bath floor 4600 btu from a 1350W 120V element

    Two temperatures would require a mixing valve
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fluke216
    Fluke216 Member Posts: 33
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    Do you have a heat load calculator recommendation at all. Ive looked at some an it comes out to around 20k btu I could be wrong. Slab about 383 sq ft 2 in xps under concrete, 1/2 pex pipe about 3/4 down from top of concrete tapered xps on slab edge and r19 with 3/4 poly iso on outside of studs, 9 in spacing bathroom area has tighter like 6 in spacing. The Staple up About the same 380 sq ft aluminum plates cover almost all the pipe except bend at ends, then 2 in air gap foil barrier then r19 batt underneath that. The room would have r 19 walls. about 7in spacing same 1/2 pex
  • Fluke216
    Fluke216 Member Posts: 33
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    I think I need two temperatures because one is a slab application and the other is a staple up The one room above the other if that matters.
  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,186
    edited January 2023
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    Like Hot Rod said, electric water heaters are a very good reasonable choice.  If you have gas, The Bradford White Combi-Cor does domestic hot water and radiant. Used them often. Mad 🐕 Dog
  • hot_rod
    hot_rod Member Posts: 22,327
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    So a few steps needed for a proper design.
    The heat load of the rooms
    The type of heat emitter, radiant floor in your case
    Determine the r-value of the floor build up.
    Then you determine the required supply temperature needed to get the 20 btu/ ft design

    Software programs make it easy, but you can hand calculate all the steps also

    If you can find a RadPad slide rule, they are simple to use.
    Uponor has a nice design manual, possibly find an online version.

    Plenty of folks guesstimate and adjust until you get the result you want. Or not😳. In some
    cases radiant floor alone is not sufficient, with your 20 btu load you will be fine

    You are down to the last design step before you select mixing temperatures and components.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fluke216
    Fluke216 Member Posts: 33
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    alright so im going to run the numbers again and determine the best source of heat maybe a water heater. Would it make sense to size something larger to have the capacity to add another zone in the future.
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,074
    edited January 2023
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    ............deleted.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • hot_rod
    hot_rod Member Posts: 22,327
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    With electric water heaters, or electric boilers, to increase output you increase the element size

    typical 1” threaded WH elements are available from 250 watts to 9,000W

    of course the circuit, wiring, and breaker need to be upsized also 

    Typically  30, 40, and 60 gallon tanks are the least$$ as they are the most common.

    certainly no harm in larger size tanks if you have the room

    I leave the factory installed T&P and install a 30 psi pressure relief also

    on those small capacity tanks I didn’t even install expansion tanks. They run from room temperature to maybe 100F the tank is protected to 150 psi, 210F by the T&P

    No fill valves, Low water cutoff protects pump and element if the system runs dry
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fluke216
    Fluke216 Member Posts: 33
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    ok do you recommend a second water heater for a close system for this situation? that's what I was assuming I've also seen it with the heat exchanger to separate the radiant from normal house hot water supply. So say i get a 40 gal with two 4500 watt elements or I could something larger. would you install expansion tank on this size i assume just wanted to check.
  • hot_rod
    hot_rod Member Posts: 22,327
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    Definitely a separate tank dedicated to radiant. If you have enough capacity you could add a HX to your current tank. But the HX, stainless pump, etc would probably be more $$ compared to a separate tank. Plus it diminishes your DHW availability.

    A standard tank will have 4500W elements, so you get around 15,000 btu/hr. I think that would be close for those two areas assuming 20 btu/ft or so.
    Yes a #30 expansion tank.

    Box stores sometimes have scratch and dent tanks at reduced price. 30, 40 50 gallon tanks generally are all 4500W.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fluke216
    Fluke216 Member Posts: 33
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    ok is there any provisions i should make incase I change from a water heater to another source sometime in the future. Im planning on closed system water heater. Then I have two sets of 3/4 pex going about 20 feet away to two separate manifolds one feeding the in floor and one feeding the staple up they both stub up and down at the same location. This image has a setup i think is similar to what i need water heater would be different. and a 3 way bypass needed before the pump that goes to the slab heat with a bypass from the return to that mixing valve. Does that sounds correct. Any recommendations on components would be appreciated also. Thanks for any information guys



  • hot_rod
    hot_rod Member Posts: 22,327
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    No bypass needed unless you go to a conventional non condensing boiler some day

    probably want to look at operating cost between electric and other sources

    if your electric rates are high, it might pay to go with gas or Lp water heater

    coalpail.com has a good calculator for various fuels
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream