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Community and Q&A

Heat load numbers for small elevated porch

johns3km | Posted in General Questions on

I have a 80sf (8’x10′) back room that is currently unheated. It is elevated so it is exposed underneath, only 2 exposed walls, and lots of glass/door (probably 60%+ on those 2 walls)…basic fiberglass insulation on walls, floor and ceiling when I renovated it. Angled ceiling from 6′ to 7.5′. I came up with about 2500BTU/hr using a heat loss workchart at 60F differential in Massachusetts.

A 750w baseboard would cover the load but at 5700HDDs that seemed pretty pricey to run over the winter for such a small room. Now I’m planning on tapping in to a steam run and putting a small radiator there instead…cost to run should be marginal.

That said, if we took off the entry door from the main house 32″ x 80″, how would that affect my load numbers with 70F air allowed to enter…is there a way to account for those gains? 


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  1. Expert Member
    Dana Dorsett | | #1

    If you insulate the floor walls & ceiling to MA code-minimum the load will probably drop by quite a bit. What did you install for R-values for the "...…basic fiberglass insulation on walls, floor and ceiling..."?

    What do you have for windows?

    A 15-18 square foot door might be good for 100 cfm at a delta-T of 5F, maybe 150cfm at a delta-T of 10F. So if it's 70F in the main area and 60F in the unheated room, you'd get 150cfm x 60 minutes/hr x 10F delta x 0.018 BTU/cubic foot per degree-F = ~1600 BTU/hr of parasitic heating. If you heat the room to 65F with baseboard that parasitic heat drops to ~1000 BTU/hr.

    The loss characteristics of the room are probably very different from those of the rest of the house. Rather than tapping into the steam, running a hydronic loop off the same boiler would be easy to implement as a micro-zone.

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    3. johns3km | | #6

      Last attempt for now

    4. johns3km | | #7


  4. Expert Member
    Dana Dorsett | | #8

    The R19 floor is below code and might be worth fixing if it's easy. If it's 2x6 joists installing 2" of reclaimed roofing polyiso (cheap and available in MA) added onto the bottom sheathing would pretty much do it. From a cosmetic point of view and to protect it a painted layer of half-inch OSB might be "worth it". The perimeter of the foam and sheathing-sandwich can be covered with 3" drip edge properly lapped behind the WRB.

    For the local heating, it's fin-tube baseboard or 9-10" tall cast-iron baseboard an 8' section on the long side of the room could deliver the full 2200 BTU/hr with 135-140F water. When the boiler is at full 215F temp it would satisfy the thermostat pretty quickly, possibly delivering the hot-flash overshoot followed by the chill. Mixing it down to 140F with a thermostatic mixing valve would deliver longer, more comfortable cycles, and avoid the hot-flash / chill situation, but with the door open to the main house that finer touch may not be necessary. If it's a play room for the under-5 set going with cast iron baseboard and cooler water temperatures would be a real comfort factor relative to cheap fin tube, due to the thermal mass of the water volume and radiated fraction of the heat. (Scrap yards often have the stuff for $20-25/foot. It cleans up and paints up easily, and is literally bulletproof against the kind of ammo your kids will be using. :-) It's pretty hefty- an 8 footer would be ~120-125lbs.)

    Since it's not a closed pressurized loop the pump has to be able to handle the head of pumping water uphill, assuming the boiler's water level is below the floor level of that room (pretty much guaranteed, eh?) but that's not really a problem. With a hydronic loop operating off a steam system you'll want a good crud filter ahead of the pump, since the condensed water return usually picks up some sludge & scale. Even though it's an open system the pump doesn't have to be oxygen-tolerant- the condensed water coming back from the steam rads isn't anywhere near the oxygenation levels of fresh potable water- it's about as "dead" as fully closed pressurized hydronic systems.

    1. johns3km | | #9

      Thanks, that’s some good info to look in to.

      The room maintained 65/66 overnight with the door open and it’s 13 degrees out...guess I should have tried that a long time ago, we were just used to it being a freezing room so always kept it closed.

      If I only need to heat it a couple degrees, does that make electric more cost efficient? What could the operating costs be? Trying to balance installations costs with cost to run. Thanks!

      1. Jon_R | | #12

        Your numbers are very close to my calculation - about 1700 btu/hr through an open door with a 5F differential (almost 5000 btu/hr at 10F).

        Unfortunately, if you want the room as warm as the rest of the house, the electric heat will have to provide 100% of the load (ie, not just a small boost).

        1. johns3km | | #13

          Ah gotcha- so as it is heated closer to 70, the less of an impact the house heat will come in to play.

          I think my best bet is to throw a killawatt meter on my space heater for a week and see how much I consume and go from there. Thanks for the help and I’ll be sure to update.

          1. Jon_R | | #14

            Correct - at 70F in the room and 70F in the remainder of the house, the house provides no heat to the room.

            Measurements beat estimates, but every 12 hours or so, I'd record the delta-T (indoor to outdoor) and the watt-hours/hour for that period. Then graph the data and fit a line through the upper points (wind and sun will cause this data to be somewhat noisy).

  5. Expert Member
    Dana Dorsett | | #10

    An electric blanket on the floor when in use would probably be enough to keep the toddlers happy. (In Japan purpose-made electric floor pads/blankets are common, but I don't know where to get them in the US.)

    A 1500W 2- stage 600W/900W oil-filled radiator type electric space heater used only when the room is occupied would also work. The 600W element and one conscious human would pretty much cover the design load, the 900W could be switched on when you need to bring it up to temp quickly. (Leaving the door open even when unoccupied would keep the room at a moderate enough temperature that it CAN be brought up to temp quickly with the full 1500W active.) They all have an on-radiator thermostats to control the radiator's temperature, and when dialed low the surface temperatures are well below the peak temp of your steam rads. These are widely available for $45-75. The oil filled radiators are quiet, and much harder to start a fire with than anything with a fan or visible glowing orange elements. If it's only on when the room is being actively used it's not going to add much the operating cost.

    1. johns3km | | #11

      Interesting you mention rugs- that’s an attractive idea. There are a couple sellers with 365w or 500w under carpet floor heating options.

      I guess I figured hardwired baseboards would be safer than something that plugs in to an outlet (we have a wall mountable flat panel heater we have used). The door will come off for now so want to treat it as part of the house and heat as needed without worrying if something was left on when we leave the house.

      Last question- As far a operating costs, if it kept 65 degrees unheated on a 56 HDD/15F day, the actual load must be much lower than 2200BTU/hr no?

      Thanks for all your help.

  6. Expert Member
    Dana Dorsett | | #15

    Keeping it 65F degrees with the door open for parasitic heating off the main house is not a good way to measure the heat load. The air flow estimates through open doors are very squishy numbers. That's a very different from keeping it 70F with the door closed using a an electric space heater. With the door closed a space heater on a Kill-a-Watt and some very careful temperature tracking (both indoors and out) would give you a much better handle on it.

    I hadn't seen the under-rug pads before- that's different from the Japanese products I've seen, but works about the same. The 5.5' x 8' version would fit in your room, and at 500 watts cover the lion's share of the load even with the door shut.

    If going for a hard-wired solution electric panel radiators are much more comfortable than baseboards, and low voltage mesh radiant floors are even more comfy (but would require re-doing the flooring. The heat load is about 650 watts, but 900 watts would be the ASHRAE recommended 1.4x oversized factor. Installing more than 900W results in lower comfort, due more cycling & temperature swings on cold days.

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