Electric Heating for a new bathroom addition 8feet by 5feet

Discussion in 'HVAC Heating & Cooling' started by annelisemcl, Jun 8, 2013.

  1. annelisemcl

    annelisemcl New Member

    Messages:
    12
    Location:
    Rose Valley, Pennsylvania
    Hello all & thank you in advance for any advice.

    Currently working on an addition 22ft by 22ft on slab concrete in Delaware County PA (southeast of Philadelphia). The walls & roof are SIPs panels. The ceilings are vaulted. Enclosed in this space directly to the right of the main front door is a full bath (on the N/E corner- 1 mainly east facing exterior wall 8ft long) at 8ft by 5ft which includes a velux permanently closed skylight(east facing) and a casement window (east facing). A primary electric heat source needs to be installed. My original thought was a thermostat controlled hardwired fin electric 3 foot baseboard heater. But I have been looking at the Runtal electric baseboards & Runtal Omni wall panels. They are just so expensive and I don't know if they are worth the 100-200% price increase over standard electric baseboard.

    Any thoughts or suggestions? (electric radiant heating under the tile was originally dismissed because of concerns of price & not being suitable as a primary heat source and also because I will be doing my own tiling over a ditra mat and didn't want to mess with self leveling flooring over the electric heating rolled mats as a relatively novice DIYer).

    Thanks,
    Ann Elise
    Last edited: Jun 8, 2013
  2. Dana

    Dana In the trades

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    If you're going to spend money on it, in a bathroom going with under-floor low voltage radiant under the tile has the best overall comfort factor, and beating any baseboard convector or wall radiator considerably. Next would be radiant ceiling (which doesn't have the floor leveling issues), followed by cove-radiant heating.

    Without a heat load calculation it's hard to say how much of ANY type of heater it really takes. Got some U-factors and square footage for the window & skylight? How about the R-value of the SIP & attic insulation,

    Cove heaters look like this:

    [​IMG]

    [​IMG]

    They work like this:

    [​IMG]

    Basically, a radiant cove brings every surface in the room up to about the same even temp. A cove heater that fits over the window would balance out the low radiant-temp of the window would be ideal, since the window is typically the lossiest & coldest surface in the room, and putting the cove heater on that wall directed toward room, it makes the average radiant temp of the exterior wall "feel" like the warmest part of the room rather than the coldest. And it makes the room also feel warmer than the air temp would indicate. Some people even control them with occupancy sensors, using the thermostat only to inhibit the heater from turning on when the room is already warm, rather than maintaining the room at a setpoint. They come on fast, and even a 60F room can feel pretty comfortable in under a minute, long before it's actually reached the thermostat's setpoint.

    They're pretty simple & cheap too- comparable to finned electric baseboard pricing- I can't imagine anybody opting for electric baseboards after experiencing cove heating. Same money, but world of difference on comfort, and cheap to operate too, if you take an occupancy sensor approach, with a line-voltage thermostat in series with the feed to the cove heater to keep it from overheating the room.

    In some ways it's better to oversize a cove-heater for the peak load if you run it in occupancy sensor mode, but with very low heat loads almost any cove heater would meet that description. A typical three-footer puts out about 1500 BTU/hr for about $75 USD. A four footer is good for about 2000BTU/hr at about $100. With 8' of exterior wall a 7-footer might be appropriate, which would be good for about 3500 BTU/hr, but is still in the $150 range. Estimates are from this vendor, but there are others. Odds are you'll get out for under $200 no matter what wattage or length.

    You have to pay attention to the wattage, and select an occupancy sensor that has some margin over the power draw of the heater. Most cheap box-store occupancy sensor wall switches are rated at 500 watts or less, and not appropriate for this application. Similarly the line voltage thermostat needs to be up to snuff, but even bottom of the line $15 versions usually have more than enough for what I'd anticipate here.
  3. annelisemcl

    annelisemcl New Member

    Messages:
    12
    Location:
    Rose Valley, Pennsylvania
    Dana,

    Thanks so much for your reply. I was hoping you would with all the past expert advice you have given.

    The SIPs wall panels are R-23 and the ceiling panels are R-38- no attic it has a vaulted ceiling. The floor will be porcelain tile over ditra membrane over concrete skim that is over 1 inch insulated foam board over a vapor barrier & crushed stone. The window is an Okna Awning with a .20 Uvalue (28 by 30 inches). The skylight is a velux non-opening skylight with a uvalue of .45 it is about 2.5 feet across by 3 feet height. The ceiling is vaulted from 7ft (exterior wall) to about 11ft (interior wall).

    In reply, are you stating that electric radiant floor heating under the tile would be sufficient to heat the entire space? Because my GC did not think it would be enough and more a comfort than a heat source?

    Also, the cove heater is interesting and I will research it (thanks). Why does it not create a heat loss at the floor with it's placement along the ceiling with the heat rising? How does the floor area stay warm?

    I had looked at the Runtal line but unless I can find their electric baseboard heater on **** or Craigslist or an Omni-panel there too; they are too rich for my budget. In your opinion, are Runtals efficiencies worth their cost or is it more the aesthetic appeal garnering such an expense?

    Thanks, Dana for your advice and help.

    ann
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,997
    Location:
    New England
    Radiant's IR (like the sun) heats what it is pointed at. INfloor heat via electric is generally limited to about 12W/sq ft (about 41BTU/hr) (you'd have to look at the specific one for their limits). So, without knowing your heating requirements, it's hard to say what is needed. Too late for you now, but Schluter is coming out with integrated heat+Ditra mats. Do not know when, but they've passed testing and now it's a marketing and supply thing, along with production.
  5. Dana

    Dana In the trades

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    I must have been asleep yesterday- just made the connection that this is the same house/owner as the question about the fan distribution of the woodstove heat... (DOH!)

    Run a heat load calc on the entire house, room by room, and see where it all comes out. If the mini-split is going to be the primary heat for the 22 x 22 space using the same fan for heat redistribution, the sizing recommendations on my post is still valid. Odds are the heat load for the whole addition will be coming in under 10,000 BTU/hr, and most 3/4 ton mini-splits can cover more than that at +15F.

    Jim has it right- it heats by heating up the objects (like humans) the room via radiation, rather than heating up a small object like a baseboard or panel radiator and distributing the heat via air convection. Like sitting in the sunny spot in front of a window, you can be quite comfortable even if the room isn't fully up to temperature, and in intermittent-use rooms like bathrooms not heating it to a setpoint when not occupied is fine for most people. Within 15 seconds of entering the room the radiant heater is giving you that warm glowy-feeling, even if the room temp is barely 60F.

    Using the U0.051 and U-0.029 estimates for the wall/ceiling and the +70F indoor setpoint & +15F design temp assumptions I made on the other thread...

    Awning window: 24" x 30" is about 6 square feet, so you're looking at U0.20 x 55F x 6'= 66 BTU/hr heat loss

    Skylight: 2.5' x 3' is 7.5 square feet of U0.45, so that's U0.45 x 55F x 7.5'= 186 BTU/hr

    Total glazing loss: ~250 BTU/hr ( which is about the output of a sleeping human, FWIW)

    You have about 40 square feet of U 0.029 ceiling for about U0.029 x 55F x 40' = 64 BTU/hr

    and about 125 square feet of U0.051 wall for about U0.051 x 55F x 125'= 351 BTU/hr

    All told it's only 700 BTU/hr, which could arguably be met with only 200 watts, but you'll definitely want to go bigger than that, 2-3 times bigger.

    Even with a window cracked open a hair and the fan on you'r probably only looking at 1500 BTU/hr, so even a 450 Watt unit (1700BTU/hr) could still heat the place just fine, but would take awhile to heat up if the room were starting under 60F. If used with occupancy sensor 600W (~2000 BTU/hr) still wouldn't be a total roast out, and would warm the room air more quickly if you're letting it run cool. Leaving the door open to the mini-split or woodstove heated place would keep it well within 10F of the main room temp, but if the door is normally closed for hours & hours it'll definitely cool off. At 1000W & up it would feel a bit "in-your-face" in such a small space, so bear that in mind. A 4-5' long 600W cove heater would likely fill the bill, but there's room to go smaller if the bathroom door is normally open. At 300W radiant coves would still work but would be so short as to start feeling like a point-source rather than beam of heat.

    Placement also has to be taken into consideration- when your face is only 18" away it can feel pretty hot. Your window isn't the same chill-factor code min window (or worse) that I was envisioning, and it might be better to place it a bit up on the higher wall to keep it out of your face, as long as it's directed to where you might bit sitting or brushing your teeth.

    Regarding efficiency: All resistance-electric heating runs at the same efficiency when running, but radiant systems are more comfortable when the room is still below the setpoint temperature. In bathrooms the bare-foot-on-warm floor is a nice bit of cush-factor. But any resistance heating that is used to maintain a setpoint even when the room isn't in use consumes quite a bit more power than those running off occupancy sensors. But the response time of radiant floors or baseboard/radiators is orders of magnitude longer than cove heating in terms of raising the average skin-sensed radiant temperature, and aren't suitable for occupancy sensor control. Coves are pretty much "instant on", but don't give you the cushy warm floor thing.
  6. annelisemcl

    annelisemcl New Member

    Messages:
    12
    Location:
    Rose Valley, Pennsylvania
    Thanks, Dana, again. I'll take your advice here on the cove heater at 600W or below tied to an occupancy sensor and your other threads advice on the mini-split (I ordered a Fantech inline FG fan today). I really appreciate your knowledge, expertise, and help. Thank you. Keep your eyes open because I will be back here & on John Bridge's tile forum with my DIY tub surround tiling advice and floor tiling over ditra questions soon. I already know I'll be ordering my 4th Toto toilet but many of the other decisions still need to be made.

    Ann:D
  7. Dana

    Dana In the trades

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    Even the Fantech FG 4 is kinda overkill, with 5x the needed air volume even at the temperature extremes, but it should do the trick. It's 20 watts, and you may need to isolate it with a bit of flex-duct for noise abatement if you're planning to give it a duct run- I couldn't find a loudness spec on it.

    A tiny 7 watt super-quiet super-efficient bath fan could still get you there with room to spare. But the FG-4 may be more suitable for how you intend to mount it, and I'm sure it's not a screamer.
  8. Dana

    Dana In the trades

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    See my math correction on your other thread- the Fantech FG 4 is in fact best choice here (nearly ideal, in fact.)
  9. annelisemcl

    annelisemcl New Member

    Messages:
    12
    Location:
    Rose Valley, Pennsylvania
    Thanks, Dana!

    The fantech FG 4 was ordered and arrived the next day. I passed on your recommendations to my GC for the split system. I am now sourcing pricing on the cove heater and the designated wire has been rerouted from the floor for a baseboard heater to the ceiling for a cove. Things are progressing with many thanks from your help and thoughts!

    ann elise
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