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Community and Q&A

BTU Sizing for baseboard heating

cbut8995 | Posted in General Questions on

Hi All,

I understand a manual J will be the best to get the right answer but I need to put a budget in with my GC on the runtal baseboard radiators and was hoping the guideline would work for 40 BTU/sqft of space in Brooklyn, New York weather as said on the runtal radiator website. This is a new construction building, cast in place concrete foundation, with supermaxxx joists and steel decking with poured concrete floors. Insualtion will be mostly rigid but have been discussing with my GC and spray might be a better option for us. Large 8 foot windows. 

I attached a doc to show sizing of the apts window sizes etc. 

Space 1: 560 sqft Basement concrete walls below grade. 5 windows and a door. 8 feet ceiling height. Was looking at only putting in 10K BTU worth of baseboard underneath the windows which is significant less if we use the their guidlines which would warrant me to use around 20K BTU. 

Space 2: 220 sqft not counting kitchen hallways or bathroom. concrete block party walls, 120 sqft of double pane glass windows and doors. North West Facing. I have allocated roughly 10K BTU baseboard underneath the window which goes about in line with runtal’s suggestion.

Space 3: 130 sqft not counting bathroom (which I plan to add floor radiant heating since there is a window in the bathroom), and closet. South East facing and I have allocated 5.5 K BTU baseboard heat which is about in line with runtal’s suggestion.

Space 4: 520 sqft all together for each apt. All the regular 1 bedrooms will have the same features except 1 side facing North west and the other south east. Sizing recommendations would be around 20K BTU of baseboard heating but with the baseboards underneath the entire windows, I only have around 6K in the bedroom and 6K in the living room for a total of 12K which is significantly less. 

Space 5: 650 sqft total of floor area. Same features as the Space 4 1 bedrooms but these apts have an added mezzanine making the height of the apt approximately 18 feet tall in the living. Not sure if height makes a significant difference but for this apt given we have the mezzanine I was able to add another 5.5K of BTU of baseboard giving the total of 17.5K BTU for this apt which may not be enough for such a high ceiling apt and apt size. 

Please let me know what you guys think as runtal baseboards are the most expensive I have worked with but I have heard good things, they are reliable but cost wise I need to be close to what I allocate to it. 

Thank you.


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  1. Expert Member
    Dana Dorsett | | #1

    >"I understand a manual J will be the best to get the right answer..."

    No kidding?

    >"...40 BTU/sqft of space in Brooklyn, New York...."

    That has to be one of the crappiest rules of thumb for any house in Brooklyn unless it is a completely UNinsulated detached single family with only single pane windows (no storms), and even then it would be on the high side. For new construction apartments with continuous R10-R15 rigid wall insulation and code-min low-E replacement windows the design load would be realistically well under half that, probably under 1/3. Call it 12 BTU/hr per square foot of conditioned space, averaged over the building (not per room.) Without at room by room load calculation it's impossible to size the individual room radiation for good room to room temperature balance.

    Runtal baseboards are fine- if you oversize the radiation by 3-4x like that you'd be able to heat the place with a condensing water heater or boiler with 100-120F water.

    Given the cost of radiation, you'd be money ahead to hire an engineer to run a real Manual-J and size the radiation to be able to deliver the 99% load (+15F outdoors, 70F indoors) with 140F water.

    There are lower cost pretty-good commercial panel rads/baseboards out there. Myson's contractor grade series are a good value. EcoStyle panel rads are a step up, but still cheaper than Runtal or Buderus.

  2. cbut8995 | | #2

    Hi Dana,

    I get your point. I agree too I may just keep the same sizing so they look nice wall to wall and then set the temperature from the combi boiler down to 150 or 160 degress F for the baseboard heating to lower the BTU output. I have to use runtal after researching that they are the only ones that have a low enough profile to fit underneath my windows which only have a clearance of 8 inches. 2 inches from the floor with 5.75 inch wall mounted baseboard will still allow the window to full open.

    If the BTU from the baseboard heating is too high and warms up the room faster I am assuming it would just shut off completely or short cycle if the combi boiler doesnt reach its min run time for the heating portion of it. I have already discussed with Akos about adding a 20 gallon buffer to the combi boiler to prevent short cycling on the heating portion.

  3. Expert Member
    Akos | | #3

    High density residential is a different animal, you can't really use any rules of thumb. For example, any interior bedrooms would have a cooling load even in the winter time.

    Here, for multifamily, you need stamped hvac drawings for a permit, so you have to get the design work done anyways, might as well use that and size properly instead of guessing.

    Expect for costing more (Runtals are $150/foot, installing more than needed adds up quickly), there is nothing wrong with over sizing. You would just adjust the outdoor reset curve accordingly.

    It is better for efficiency as with over sized rads you can supply them with much colder water. Colder water means more condensation on your boiler, so higher efficiency. This all assumes that the outdoor reset is configured properly (which is not actually hard, just needs to be done).

    1. cbut8995 | | #5

      I agree Akos. I dont think in NY we need a stamped HVAC drawing except to show where it would be and brand and BTU. I have never had a much detail approved plans around the HVAC area as we only point to where the lines run and where the location of the indoor and outdoor units.

      Do you know of any reputable online companies that can do the manual J if I just send them floorplans, insulation and window specs and can turn it around within a few days quickly?

      For the basement which is already built, would you say 6500 BTU of baseboard heat at 180 degrees F is enough for a 500 sqft basement that will be used as an accessory space for like a media room or living room area. It can always be less if we configure the temperature to be lower but I guess 6500 would be the max at 180 since as discussed in another post this would be its own zone of heating in a 3 separate zone heating (living room, bedroom, and basement)

  4. Deleted | | #4


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