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Need about 18,000 btu's to heat small apartment any ideas.

jareedo_2
jareedo_2 Member Posts: 10
  I have a customer that needs about 18,000 btu's to heat a small apt. with domestic h.w. also. They want something in LP, Navien's has a combi that is 20,000 min. , I 'm just worried about short cycling. Any suggestions?  Thx

Comments

  • icesailor
    icesailor Member Posts: 7,265
    Short Cycling:

    It won't be "short cycling" when it is making domestic potable water. Every day of the life of the heater will have a period of domestic hot water use. There could be up to five months of no heating load and only a very short period of needing the 18,000 BTU out put (if ever).

    Some need to get over their obsession of sizing a combo unit to the heating load when the domestic potable heating load is far greater and it is 24/7.

    And easily provable.
  • nugs
    nugs Member Posts: 77
    Navian

    The surplus 2000 btu ain't going to make a big difference.  On the other hand, the average large dog puts out about 15,000 btu so I suppose you could get them a German Shepard.  Probably cost less to operate too.
  • meplumber
    meplumber Member Posts: 678
    18,000

    Do you realize that 3 people in a room, put out 15,000 btus/hr of body heat? The average 42" Flat Screen TV can put out as much as 6,000 btus/hr. So unless the apartment is completely unoccupied, then there may not be a call for heat at all.



    I agree. That combi should be fine. As ice said, the DHW load will run far more than the heat will call.
  • Zman
    Zman Member Posts: 7,543
    The key is Mass and Differential

    I don't know where the BTU numbers for humans and dogs are coming from. I have always figured 300-600 per person depending on the level of activity.

    The other 2 variables in determining the cycle length is the mass

    (usually determined by the volume of the water) and the differential in

    the boiler cycle at low fire. Many mod/cons will automatically increase

    the differential at low fire in order to prevent short cycle. Also look

    at the water volume of the boiler. Firetube designs usually have more

    volume. In the event you can't come up with enough volume, a buffer tank

    will solve the problem. I am assuming 18,000 is your design load on the coldest day. That being the case I would look at the cycle length on a "typical" day. If you system  can unload 10,000 Btu's on a typical day and is producing 20,000 btu's at low fire you are going to be producing 10,000 Btu's extra.If you look at a boiler system that has 10 gallons of water that runs at a 20 degree differential this is what it will do with the extra 10,000 btu's



    10(gallons) x 8.33 (weight of one gallon) x 20 (degree differential) =1666 Btu's the system can "store".



    10,000 Btu/Hr is 166 Btu/minute. This system would have a cycle length of 10 minutes 1,666/166=10



    Not bad!



    Don't worry about cycling on DHW. You should have plenty of storage to prevent that.



    Just figure system water and differential and voila.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Zman
    Zman Member Posts: 7,543
    TV

    A 42" flat screen lcd uses 130watts or 450Btu/hrs
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Ray_Frechette
    Ray_Frechette Member Posts: 27
    Man, I would hate to have to feed

    and clean up after any dog that puts out 15000 BTU's per hour.



    4 BTU per calorie.  3,750 calories per hour.



    White Bread has 1300 calories per pound. 



    A loaf of bread is 1800 calories.



    2 loafs of bread per hour...  And many many trips to the fire hydrant outside...
  • Body Heat

    Zman is correct.



    Under normal conditions, the body looses heat as follows:

    190 BTU's by radiation

    110 BTU's by convection and evaporation

    100 BTU's by exhalation

    400 BTU's per hour



    Thanks to Richard Woolsey Shoemaker in his 1948 book, "Radiant Heating".
    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
  • SpeyFitter
    SpeyFitter Member Posts: 422
    Lochinvar knight

    How about the Lochinvar knight whn05

    5 which modulates from 11000 to 55000 input?
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • Zman
    Zman Member Posts: 7,543
    DHW

    That knight is using the old triangle tube firetube exchanger. I like it.

    What are your Domestic hot water needs? How cold is the water from the ground? How many fixtures? Are they low flow? Which HX tank are you thinking. These questions will probably dictate the boiler size.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • bob eck
    bob eck Member Posts: 930
    18,000 btu

    How about a A O Smith Vertex gas water heater. This unit has tapping's on the side where you can install a heat exchanger to keep the domestic water and heating water separate. Use low temp copper baseboard like the heating edge made by Smith's Environmental.  www.hotwater.com

    Triangle Tube Prestige Solo condensing gas boiler model PS60 16,000 - 60,000 BTU input with a Smart 30 Gallon SS indirect water heater or their Challenger CC85 23,000 - 84,000 BTU input with 2 GPM domestic hot water condensing gas boiler / water heater all in one unit. www.triangletube.com
  • meplumber
    meplumber Member Posts: 678
    Alan and Zman

    Current guidance from ASHRAE is for 500 btu/hr heat gain per person. They changed from 400 to 500 in 2001. I guess it is because we are getting fatter as a nation. From the same ASHRAE Manual, Large flat screen TV's can produce as much as 6,000 btu/hr heat gain.



    I was simply trying to illustrate just how small of a load 18K is.



    I agree with your comments on thermal mass zman. Excellent point.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    I don't think so....

    500 X 3 = 1,500. Not 15,000. Your math is a little off...



    I have seen some passive houses that would only require a few bodies to heat, but thier loads were on the average of 5 btus/square foot per (wait for it..........) YEAR.



    These babies are going to change the way we keep people comfortable.



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Zman
    Zman Member Posts: 7,543
    Old numbers

    I think the TV numbers you are using are outdated. The TV you describe would  use 1750 watts or 14.5 amps. I used the actual wattage on a typical 42" set. That is assuming all the energy is wasted(turned to heat), which is not true. Some is turned to light, not heat. It would be impossible for a TV to produce more energy than it uses.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • nugs
    nugs Member Posts: 77
    Hot Dogs

    Dogs put out a lot of heat but you have to shave them first to get rid of all that insulating hair.   A few wraps of aluminum fin tube around the mid section would help to dissipate the heat a little faster also.



    All jocularity aside though, for 18,000 btu/hr I think that installing a mod con and all of the associated baseboard, piping, valves and controls is a little over kill, especially in an apartment.  Why not slap in a wall mounted gas heater and be done with it at less than a quarter of the cost?
  • Ray_Frechette
    Ray_Frechette Member Posts: 27
    All electric usage converts to heat by and large

    The light given off by a TV set or a compact fluorescent or an LED bulb is visible light and the photons convert to heat when it hits a solid object just as readily as the light form the sun coming in through your windows does when it hits the floor or walls.
  • Ray_Frechette
    Ray_Frechette Member Posts: 27
    All electric usage converts to heat by and large

    The light given off by a TV set or a compact fluorescent or an LED bulb is visible light and the photons convert to heat when it hits a solid object just as readily as the light form the sun coming in through your windows does when it hits the floor or walls.
  • 500 BTU's

    is the correct answer for gross output of a human body under normal conditions.  Of that, 100 BTU are used up by the body for maintenance.
    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
  • Jack
    Jack Member Posts: 1,047
    Rinnai EX-22

    Nets out about 17,500 btu's. This is one of the ideal applications for this unit. have to deal with hot water in another way. Other heating option is a 410a mini-split.
  • scott markle_2
    scott markle_2 Member Posts: 611
    Waste heat

    If (for the most part) the energy released stays within the building envelope...would it be correct to assume that any byproduct of electrical energy (light,motion, sound )will end up as thermal energy.



    I recently installed a mini-split for a home theater area, I asked for the operating wattages for the equipment and figured (based on the above assumption) that the heat gains would be equivant to the wattage of the equipment.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    99% true....

    i have seen it said by electrical engineers that 99% of electrical energy ends up as heat in the envelope.



    Home theaters are one of the BIGGEST problems that I have come across in the field as it pertains to human comfort. I have infrared photos of an adjoining wall that contained all the amplifiers, and other electronic equipment that was keeping a home theater (in a basement of course) at a mean radiant temperature of around 90 degrees F.



    The problem with trying to adjust a heat loss calculation for these potentials, is that they are there sometimes, but will they be there at design conditions?



    I know certain GSHP manufacturers who try and take these into consideration in reducing the net net size of their systems, and its a game of Russian roulette. If the lights aren't on, the gain is not there. Even with appliances (refrigerators and freezers) the load is not a constant, so the output is not a constant. Unless it is controllable (i.e. gas log fire place) you'd best not depend upon it for heat.



    As it pertains to cooling, you MUST consider it as a potential, because if you don't it will be there, and your equipment will be incapable of maintaing good human comfort at cooling design conditions.



    Also, figure worst case scenario for wattage. I realize that the incandescent bulb is going the way of the dinosaur, but if you figured CFL's and the consumer didn't like the light color and changes them to the good ol' Edison yellow light bulb, your wattage contributed from lighting could QUADRUPLE... There will ALWAYS be a black market for incandescent bulbs.



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • moey
    moey Member Posts: 40
    electric

    Did they consider electric? I would opt for that, 18000 btus is about 5000watts. Of course you would have to put in a electric hot water heater most likely. I guess it depends on the area of the country your in for electric rates.



    That probably would not make you any money though :)
  • SWEI
    SWEI Member Posts: 7,356
    strangely enough, YES

    Just ran through an exercise on something nearly identical in size (my architect's guest house.) Conclusion:  Thermolec B-6TMB, and this in an area where NG is significantly less expensive than kWH.    Inexpensive, high flow (direct piped) and built-in ORC.
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