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FPHXer dumb question (you know you love dumb questions!)

Sorry, I can't seem to find an answer for this.
So if I have a ci boiler with an output of 38k BTU's and a design loss of 28k btu's and a flat plate x-changer between the slab and the boiler. How much of the 38k is lost in the transfer of heat to the slab side? (btu's that never make it to the slab). Ballpark. 1k btu's? 5k btu's?

Basically am I still going to be way over the design loss or will I be closer because of the xchanger?
Or am I confused and this doesn't matter?

(I have found a stellar deal on a 4 month old minigas-50 (derated to 45k input for elevation) only 300 bucks!
Am I barking up the wrong tree?

Thanks!

Comments

  • Ironman
    Ironman Member Posts: 7,076
    To know the answer to that, you would have know the size, make, model of the HX; the flow rates on both sides and the temps (in and out) on both sides.
    All their manufacturers have formulas and programs for sizing them. In other words: it requires some engineering.
    I use a lot of flat plate HXs and if you give the width x leng x number of plates, I can ball park it for you.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • toasty-toes
    toasty-toes Member Posts: 17
    Ironman,
    (now I have Black Sabbath going through my head!)

    Yikes! I wouldn't have a clue at this stage what size the HX would be.

    Basically I am trying to figure out if I should pull the trigger on this boiler deal or walk away. I know its a bit over the top in output, but I would imagine there are not a lot of perfectly matched load to boiler output installations out there.

    If there are losses I am not accounting for, that make this boiler suitable for my needs, I'd sure hate to pass up the deal.

    So yes there are losses in the HX (just not easily ballpark-able)

    hmmmm what to do? what to do?


  • Ironman
    Ironman Member Posts: 7,076
    That's about the smallest btu boiler that you're gonna find and it should be fine. It may need a bypass installed depending upon the return water temp.
    Give me the dimensions of the HX and I can help you. Also, a diagram and/or some details on the radiant floor:
    1. In slab or staple up?
    2. Square footage of the floor?
    3. Tubing lengths, spacing, number of loops?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,972

    Ironman,
    (now I have Black Sabbath going through my head!)

    I think more like Tony Stark....IMHO Bob is a brilliant guy.

    steve
  • toasty-toes
    toasty-toes Member Posts: 17
    Well I just bought it!...so either way I have a shiny, almost new Boiler for super cheap!
    Ok Ironman, Thanks.
    (still have Sabbath in my head)

    (you guys should set up a beer fund we can all contribute to!)
    KickStarter BeerStarter! Good thing I am far too lazy to actually create an "app" and make a bajillion dollars!

    960 sq feet of slab on grade
    One zone
    4 circuits
    Non O2 barrier pex (1/2") in 230 foot loops spaced at 10" - 12" apart.

    This is the boiler:
    http://alliedboilers.com/products/mini-gas-boiler/
    (MG-50)
    Still need to determine pumps, but I would/like to stick with very low power models like Alpha's if possible.
    As far as HX I am clueless. I just know I need one to isolate my boiler from the devils Pex.
    Not looking at domestic water at this point as I have an On Demand that works well.

    ok, I should get back to actually working so I can pay for all this!

    Thanks everyone.
  • Ironman
    Ironman Member Posts: 7,076
    "Ironman" is a monicker for someone with a lot of endurance. When I was 17, four of us, including the service manager, were carrying a very large cast iron rad. I never had a grip on it and consequently lost it all together on my end. The service manager kept riding me about it. After a few days, I'd had enough and told him to put his arm up on the sales counter at the shop. We locked up and in about 10 seconds or less, I slammed the back of his hand down. He was stunned since he out-weighed me by at least 60 pounds. He started calling me "Ironman" and it kinda stuck.
    When I joined the army and volunteered for airborne duty, I learned that it was also used there to describe those who had gone through advanced training that required great endurance.

    @toasty-toes
    Do your have any insulation under the slab and around the perimeter?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    RobGSWEIZman
  • toasty-toes
    toasty-toes Member Posts: 17
    edited September 2015
    Ok, so now I have an image of you arm wrestling while Black Sabbath plays in the background!
    Oh...now you are jumping out of a plane. DA-na--na-na-na!!!!!
    That's cool you have a legit username! Now I want to light my feet on fire so I can claim mine! hahahaha.

    So, yes the slab has 2" foam board under the slab and around the perimeter 2' down the 7' frost wall. Yes I live is a frozen wasteland.





  • Zman
    Zman Member Posts: 7,434
    Bob has you more than covered on this on but I can't help but point out you cannot lose BTU's in a heat exchanger (anymore than you would lose to the air in a similar piece of pipe). You can however missize the exchanger and end up not being able to transfer the energy you need at the temperature you desire. If you download the calculator from these guys it will give all that plus the resistance the circulators will face.
    If you don't want the headache, I am certain that Bob's ball park and humming some Black Sabbath will also work just fine. ;)

    http://www.gea-heatexchangers.com/products/plate-heat-exchangers/
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    IronmanSWEIRobGBobbyBoy
  • Zman
    Zman Member Posts: 7,434
    edited September 2015
    It is a bit clunky because you have to go back and forth until you get the program to give you the results you are after.
    It was obviously written by engineers for engineers.
    I do wish there was a better one.
    At the end of the day it just comes down to the surface area and the flow on either side...
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    BobbyBoy
  • Zman
    Zman Member Posts: 7,434

    The results you typically desire are the two outlet water temperatures once you provide the two flow rates and the two inlet water temperatures.

    No further data should be required.

    However, you must also input the BTU transfer rate to get those two outlet water temperatures. Note that you have no control over that transfer rate. You can set it at whatever you wish and get whatever result you wish..............none of it relevant.

    I agree completely.
    You would make a crappy engineer. When you explain it like that, people can understand it. You would be out of a job in no time. :p
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    IronmanCanuckerBobbyBoy
  • toasty-toes
    toasty-toes Member Posts: 17
    hmmm.
    You guys amaze me. I pop in to see whats what and there is so much info I need to ingest it and let it digest.
    I will get back to you. I need to do some reading now and get up to speed.
  • toasty-toes
    toasty-toes Member Posts: 17
    Ok, I read a bunch and I think I have a grasp on most of the whats and hows. (in a rudimentary way)

    So, is there a diagram somewhere of this sort of set-up?
    I have scoured the web thinking I would find a near boiler piping diagram, that matched this, but have come up blank. The CI boiler, hx, mixing valve, seems to be an odd duck maybe?

    Also, I have seen wild variations on HX prices. Not looking to skimp on things, but also don't want to over do it if it can be avoided.
  • Zman
    Zman Member Posts: 7,434
    The I valve would be a good product for your system.
    If you use the piping shown for "3 way valve" and replace the closely spaced tees in the middle with your heat exchanger you would be pretty close.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    BobbyBoy
  • Zman
    Zman Member Posts: 7,434
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    BobbyBoy
  • toasty-toes
    toasty-toes Member Posts: 17
    Ok, I like the price way better than the 1389.00 dollar one I looked at but, are 10 plates enough?

    Doodled this up. Sorry it is not done to normal Plumbing drawing specs, but I don't know what those are.
    Is this more or less correct?
    If yes, I would imagine I need expansion tanks for both sides. Various filter/clean outs, gauges, drains, etc.










  • Ironman
    Ironman Member Posts: 7,076
    That looks good.
    Did you get my P/M?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Zman
    Zman Member Posts: 7,434
    With the smart valve, you don't need the mixer on the boiler side. The smart mixing valve on the radiant side will measure the temp on the boiler side and adjust accordingly.
    That exchanger will work fine. I will post the calcs if I have time.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • toasty-toes
    toasty-toes Member Posts: 17
    Awesome, thanks Zman.
  • Zman
    Zman Member Posts: 7,434
    You really could go smaller. I like the price and availability of this one. Plus it has very little head loss, perfect for radiant..
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • toasty-toes
    toasty-toes Member Posts: 17
    That is super nice of you...appreciated.
    This plumbing/heating stuff sure involves a lot of reading!
    ;)