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Electric water heater for slab heat inefficient?

I wish I had found this site so much earlier than I did. I was just about ready to throw in the towel on my floor heat, but you all have given me hope. I have to admit, I'm painfully ignorant of the technical aspects of these systems. I thought I had done enough research before I started, but now feel like the proverbial DIY guy who is in way over his head.

Anyway, first a description of what I have: 24x32 pole building with 10 foot sidewalls, in central Illinois, with R10 in the walls and R25 in the roof. 5 inch slab with 3 inch eps foam under it and around the outside edges. Four 200' runs of 1/2 inch pex on 12" centers (layout included in pictures) running as 1 zone. Richmond 30 gal water heater modified with dual 6500 watt heating elements that are wired to run independently and, if necessary, simultaneously. Air separator and G12L water worker expansion tank. Grundfos UPS15-58FC pump with Taco SR501 zone control. Running a 40% solution of cryo-tek antifreeze (blue). (Heat board also pictured, constructed from 3/4" copper then wrapped in pipe insulation)

I was able to keep the shop at a steady 55-60 degrees, but it cost me an extra $200 month for the electric. To me that was not economical at all, and has me thinking something is wrong because everything says radiant floor heat is the most economical.
I had this system running with 140* water (didn't realize at the time that was MAX temp for concrete) but by the time the water came back out of the slab it was about 80-90 degrees. I do not have a mixing valve, and wondered if I should install one to mix supply water with the return water and bring its temperature up after it has come out of the slab and before it dumps back into the water heater so that it doesn't have to recover so many degrees, therefore keeping the water heater from running as much. Or after doing research on here I am wondering if maybe I'm not pushing the water fast enough through the lines. Causing it to lose too much heat before exiting the floor. Would a second pump on the return side overcome that and reduce my temp drop in the floor? Or is it just that electric (10.8 cents per kw) is just too inefficient to run a system like this and I should replace the water heater with an LP boiler? Any and all help is appreciated. I tried to include everything I thought might be relevant, but if there is something else you need to know I will try to get the answer.

Comments

  • HVACNUT
    HVACNUT Member Posts: 4,194
    Not sure about the whole electric water heater setup. You probably need more BTU's that a mod con could've given you. And I think 800 ft on one zone with the Grundfos is too much.
    IMO, the correct circulator, a 4 zone manifold with actuators, and make it a least 2 zones, but I would split it into 4.
    GroundUp
  • Zman
    Zman Member Posts: 6,981
    Your piping and distribution should work just fine. You have adequate slab insulation as well.
    At 13KW, that's 44K/Btu, you have more than enough for the shop.
    As far as fuel prices go, to save money with propane, you would need to buy fuel for less than $2.65/gallon.

    The one place you could save energy is to turn the water temp way down and perhaps invest in an outdoor reset control.
    When you send 140 degree water to the slab, you waste energy and reduce comfort by overheating (then under heating ) the slab and space. Try turning the water heater down to 100 degrees and see how it works.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    STEVEusaPAArgyleWoodworks
  • Gordy
    Gordy Member Posts: 9,514
    Turn supply temp way down like @Zman said. You will be surprised at what a 40 degree supply temp reduction will do.
    ArgyleWoodworks
  • Gordy
    Gordy Member Posts: 9,514
    edited December 2018
    Also glycol reduces heat transfer a bit.

    Another thought is how long this has been up, and running? It will take some energy to get the mass of the slab up to temp. Then it will coast along with some bumps in btus.
    rick in AlaskaArgyleWoodworks
  • HVACNUT
    HVACNUT Member Posts: 4,194
    edited December 2018
    OK I'll shut up now.
    And dont click "Agree".
    Zmanrick in AlaskaSTEVEusaPAGordy
  • DZoro
    DZoro Member Posts: 1,048
    A slab sensing thermostat wouldn't be a bad idea, either.
    GordyArgyleWoodworks
  • ArgyleWoodworks
    ArgyleWoodworks Member Posts: 4
    Thanks everyone for the comments. @Zman, LP right now here is $1.30/gal. So if the electric is going to continue costing $200 a month, the LP should be roughly half that. At that an LP boiler would end up paying for itself in about 3-4 years (assuming roughly 4 months of operation a year). If turning my supply temp down makes a drastic difference, then obviously the payoff for the boiler would be much longer and maybe not worth it at this time. @Gordy, i had the system running for a month. Starting to wonder now too if I had a huge charge because of the initial heating of the slab and didn't take that into account. Just got the electric bill, blacked out, and when i woke up I had turned off the system. Probably a huge mistake. I'm going to start off by lowering my supply temp. I ordered the stuff to install a mixing valve thinking that would solve a big part of my problem with the extreme temp drop. Should I hold off on that? It is a fairly cheap modification, but I can always return the stuff if I end up not needing it. Thanks again for the help everyone.
  • Zman
    Zman Member Posts: 6,981
    I don't see any benefit in having a mixing valve on an electric setup. I would seriously consider an outdoor reset control. You would need a control like a tekmar tied into a contactor on the water heater.

    Historically, propane prices have been all over the map. Recently, if you could avoid buying in the middle of the winter, the prices have been under $2.00. I would not use $1.30 as a long term price
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    ArgyleWoodworks
  • ArgyleWoodworks
    ArgyleWoodworks Member Posts: 4
    Thanks @Zman. Can you point me in the direction of an appropriate outdoor reset control? When I look them up I find tekmar thermostats with slab sensors. I assume that would be a better setup than my cheap standard honeywell, but I didn't find anything that adjusts based on the outside temp. Also, what is that adjusting? Just how long it runs based on outside temp? Because with my water heater it can't adjust the water temperature, that has to be done manually. Sorry for my ignorance, just want to do what I can to make it work.

    I lowered the SWT and it is still keeping it a toasty 55* in there. I hope it makes a big difference on operating costs. Thanks again everyone for the insight.
  • ratio
    ratio Member Posts: 2,923
    You want one of these (manual). Relay outputs to control the heat stages (through a contactor), outdoor reset, limits on SWT, scheduling, night setback, and more. Simple to program.
  • Zman
    Zman Member Posts: 6,981
    ratio said:

    You want one of these (manual). Relay outputs to control the heat stages (through a contactor), outdoor reset, limits on SWT, scheduling, night setback, and more. Simple to program.

    That is a good one.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,284
    Hello, It's a long shot, but is there any way to add insulation to roof and walls? The numbers you gave are about half of what I'd expect in see in new construction. :o

    Yours, Larry
  • ArgyleWoodworks
    ArgyleWoodworks Member Posts: 4
    > @Larry Weingarten said:
    > Hello, It's a long shot, but is there any way to add insulation to roof and walls? The numbers you gave are about half of what I'd expect in see in new construction. :o
    >
    > Yours, Larry

    @Larry Weingarten I agree completely. It is a pole building with 1.5 inch rigid foam insulation between the girt boards. And double stacked in between the perlins in the ceiling. I would love to double it, or more, but it wasn't in the budget at the time. And since the shop continues to be a "work in progress" so to speak I still hope to someday increase it.