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Need Recommendation for an Electric Heater

jasonmann | Posted in General Questions on

I’m building a home office that includes two separate spaces, separated by a sound-proofed wall. (i.e. no penetrations in that wall if at all possible. It’s framed with double 2×4 studs separated with a 2″ air gap.)

The two spaces are 12’x13′ feet and 12’x20′. Ceilings are angled, but average about 8.5′. My walls and floor will be roughly R-33 (dense-packed cellulose) and ceilings will be roughly R-45 (also dense-packed cellulose).

I thought about using mini-splits to heat these spaces, but after talking to a heat pump installer about it, it seems like overkill. He recommended I just use electric baseboards. I’ve seen lots of mentions here of other electric heat options, including radiant cove heaters wall-mounted, oil-filled radiators. I’ve also seen “infrared” ceiling panels, which seem to be just electric resistance heat mounted overhead, unless I’m missing something.

So… which would you pick for these spaces?

Bonus points: can I get away with skipping an ERV setup for this building, or is that a bad move? (I am air sealing to the best of my ability, so I’m assuming it will be pretty tight when I’m done.)

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  1. Patrick_OSullivan | | #1

    > The two spaces are 12’x13′ feet and 12’x20′. Ceilings are angled, but average about 8.5′. My walls and floor will be roughly R-33 (dense-packed cellulose) and ceilings will be roughly R-45 (also dense-packed cellulose).

    What climate zone are you in? The ceiling being angled makes it sound like it's the roof plane, and if so, you should be cautious about air permeable insulation without ventilation. Perhaps you do have ventilation above the cellulose, in which case, ignore me!

    1. jasonmann | | #2

      No, you're right, it is a hot roof (i.e. unventilated, dense-packed.) I am in coastal Maine, Zone 6.

      I'm pretty confident that with 13" of dense-packed cellulose, there won't be any significant moisture that makes it to the underside of the sheathing. (In part, that's because this isn't a home, with a kitchen, bathrooms, etc.) You're right that we need to consider it carefully. And frankly, it's one of the reasons that I think an ERV might still make sense for such a small space.

  2. charlie_sullivan | | #3

    The first step is to do a "manual J" calculation of the heat load, and then consider what size heating system you need. A ducted minisplit might be the way to go, with very short ducts to feed both spaces.

    Yes, you will want ventilation. Without moisture sources like showers and dishwashing, you'll probably want ERV rather than HRV, to avoid driving the humidity too low for comfort and health. (You might even need a little active humidification.)

    If you succeed at keeping the humidity at say 30%, you then should think about roof construction to avoid moisture problems. Here's an article that lists some good options:

    1. jasonmann | | #4

      Thanks, Charlie. I arrived at 9,773 BTUH after doing a Manual J calculation here:

      And what you said about ERV rings true. In my current house, (double-stud 11.5" walls, dense-packed) we use a Zehnder ComfoAir 350, and I actually swapped from an HRV core to an ERV core after the first winter for that reason. I still have to use some small humidifiers in bedrooms, but the house is noticeably more comfortable since that change.

      I appreciate the help.

      1. charlie_sullivan | | #5

        This is actually a tricky one. A ducted minisplit, mounted between, feeding both, would be perfect except for the fact that you are going all out on sound isolation and that would hurt that some. A minisplit or multi-head minisplit could work finding a system that can modulate down to to low level is hard--maybe someone else can help with that. My favorite solution for delivering low levels of heat separately to two spaces is a air-to-water heat pump, but the smallest ones aren't really small enough. They'd still work fine with a buffer tank but by the time you buy the heat pump and the buffer tank it's a very expensive solution. The smallest I know of is the Sanden, which is isn't meant for heating use, and getting high COP out of it in heating use is apparently tricky.

        Two suggestions, both a little outside the box:

        1. Ducted minisplit with a super sound isolating duct system.

        2. I gather this is an outbuilding, and presumably you have some kind of nice high-efficiency heating system in the main building. Put a high performance heat pump water heater in the main building, and then run hydronic lines from that to mini slim wall mount fan coil units in the offices.

  3. onslow | | #6


    I would lean in favor of baseboard or cove heaters unless your summers are warmer and more humid than I recall from my time growing up back east. Being in a coastal environment usually meant shifting winds, cooler summers and warmer winters than further inland. However, if you do need air conditioning to survive, the trick will be to find a mini-split set up that will dial down far enough. That and dealing with efficiently removing humidity during the summer.

    Your Manual J of 9700 BTU/hr seems a bit excessive unless you are speccing more windows and doors than I am guessing. That or possibly a distortion of total heat loss from an excessive ACH rate. Depending on what goes on in the office, a full on ERV might be foregone. If you are receiving visitors keeping the air neutral would trump any savings from not having the extra losses derived from too aggressive air exchanges. Given the relatively small area and volume with very excellent insulation, I think you will find your own presence along with computers and printers will produce more BTUs than you might expect. This will be especially apparent during spring and fall so maybe an ERV would end up a wash energy loss-wise.

    I am guessing that the 9700 BTU/hr rate is for a design temp of -5F, which could happen more days than I recall. Still, that is a peak load and the problem will be heating (or cooling) without short cycling a minisplit for the much greater part of the year when temps are 35-40F. (or above 75) Especially since the load will be split across two rooms. A middle load value that you would see most might mean each room only requires 2000 BTU/hr, or less if you have multiple monitors and desktop computers. Lower still if the ACH assumption is off.

    At these load levels, baseboard or cove heaters may offer a final cost edge over minisplits. One 8' cove heater per room can easily provide the peak needs of 5000BTU, or 2 4' for better placement options, at a cost of ~ $300 for each room with thermostats. I favor the cove heaters over the baseboard type for ease of placing furniture in the room. They are essentially dead silent and I am anticipating many years of no maintenance use. Cat hair doesn't go that high.

    Operationally, if you are not paying Massachusetts electric rates, a cold day rate may pencil out to be $ 5-7 a day, which is definitely more than a minisplit will cost to run. Should be less, I just heated north of 3000 s.f. for a month for only twice that and we have been below zero several nights. That guesstimate is also for high load periods, which unless the arctic has moved south won't be most of the time. While the minisplit might always beat the cove or baseboard method on consumed electricity, maintenance and noise might play bigger part when dealing with a small office. You might find the minisplit might wins based on available electric panel space unless you are running a separately metered drop to the building.

    I have lived exclusively with cove heaters for 5 years now in a CZ6B because the spouse did not want any fan noise. We lucked out on not having AC due to local environmental conditions that makes it possible to survive summer without. I do expect that climate warming will force our hand in the decade ahead, but meantime we are good. The option of a minisplit was available to us, but at an absurd cost. I am still paying my heating costs with the money not spent on the mini.

    The local power company is now offering rebates to induce moving to minisplits, so do remember to factor in any potential rebates. Hopefully, any rebates will apply to the very small size unit you would need to find.

    1. jasonmann | | #7

      Thanks for the detailed reply, Roger. You're right that our climate is tempered by the ocean, both in summer and winter. (We're on an island, so we're literally surrounded by a large thermal mass.)

      The heat pump guy I talked to (who is very good and is installing a job for a friend this winter) also recommended baseboards. Even with our higher than average electric costs out here, he thought the payback on a minisplit would come long after the life of the equipment. And it would have all the issues you and others have described being oversized. He said that, if climate impact is part of my concern (it is), he'd also avoid the potential release of refrigerants in this situation, and would put the money saved into offsets and/or more solar panels on my roof.

      On the ERV, do I understand you correctly that you think it's worth it, or did you mean you thought it might not be necessary. FWIW, I expect to be able to crack a window and use fans for most of the year, when it's pretty mild here. So the real question is whether I'd need the ERV during the winter. For the most part, I will be working alone in one office, with my wife working alone in the other. Visitors/clients come by occasionally, but it's not at all the norm.

      I'm trying to do a really good job air sealing, so I don't expect a lot of fresh air coming into the building through the shell, but maybe my opening and closing the door every few hours will be enough?

      Again, thanks to you and the others here for your help.

      1. charlie_sullivan | | #8

        On the ERV, the fact that you can ventilate with a cracked window with less energy penalty than doing that at night. But if you are heating with electric, the energy cost is higher, so the benefit of ERV over unconditioned ventilation is more than with cheaper heat. An option would be to plan for the ERV, maybe put in the ducts, but wait on installing it until you try it out maybe monitoring CO2, and see if you need it.

  4. onslow | | #9


    The heat penalty versus the installed cost of an ERV with ducting is again an issue. The limited volume of air you are dealing with will be against you just like the small heat load. Running the ERV may exact a higher energy cost than the small amount of heat recovery provided. The dry air issue could be addressed with a few plants or an ultrasonic humidifier. The code people may inflict their opinions on you over anything offered on GBA.

    In theory, for your ~400s.f., you need 0.4 of the 1000 s.f. value of 50cfm suggested for ventilation . That is 20 cfm, which many a small bath fan can deliver outbound. At 10 cfm per room it won't need much of a crack to keep up with the fan. I vaguely recall encountering windows in the UK fitted with small sliding vents at the base of the window, much like the adjustment bit on your refrigerator produce drawers. (had to check the spelling thanks to New England being my original language) Not sure if any of the Scots used them, since they flung the whole window open regardless of season.

    I am calculating your total volume of 3,366 cu. ft. based on a 12x33x8.5 box. So 20cfm x 60 is 1200 cfm per hour of run time. Unless lobster burritos are as dangerous as the non-lobster variety, then I can't imagine why you would need or want to flush the space that aggressively. Then again, if you are an artist or someone using stuff that gives off fumes you might need to.

    The Lunos wall gizmo that utilizes a thermal exchange core and fan set up might be better suited to the very low demand. There are less expensive versions, but I can't recall the name. Might want to look to reviews of their success.

    Once I get my numbers straight on the btu per cfm per degree rise, I will advise.

    1. jasonmann | | #10

      Thanks again. I'm considering this Lunos e-go device: Is that the one you meant? It seems better suited to a space this size.

      Although I may just embrace my inner Scot and open the windows...

  5. onslow | | #11


    In the process of checking my numbers before I spoke further, I had to re-learn that the commonly used factor (1.08) in HVAC discussions regarding required BTU/hr input to achieve a particular delta T is only accurate for 0% RH - 70F air at sea level. Not terribly revelatory to most I suspect.

    I was a bit surprised to find the difference between 0F and 70F air density is 15%. So, it bringing in 0F air at 20cfm for one hour (1200CFH) and raising it to 70F will cost you 1736 BTU of "new" energy load, or 15% more BTUs than the 1512 BTU one would get by using the 70F start point. The effects of RH on the load are surprisingly small despite the higher specific heat of water moisture. Still more surprisingly the effect is downward on the load about 1%. A bit hard to get one's head around even knowing humid air is less dense than dry air.

    For the simplest method of literally throwing heated air out via a bath fan to be made up by fresh new outside air; every 1200cu.ft. of air exchange occurring during the largest delta T period (70) requires 1736 additional BTUs or approximately the energy provide via resistance heat available from 1/2kwh. 10 cents if your cost net out at 20 cents kwh being on an island. One also needs to factor in the bath fan electricity, which can be remarkably modest judging from this random fan from (about 1 cent an hour)

    shows 3 watts for 27 cfm at <0.1 sone, which should be favorable to your quiet space plans.

    The Lunos E2 seems to have a similar watt to cfm rating and the additional advantage of 90% heat recovery. 1 sone is equivalent to 28db if the internet is to be believed. The Lunos specs say low to high speed is 16-26db. The demo video below is not too instructive, though the viewing options that appear on the side bar might prove illuminating. Googling - Lunos noise - might be beneficial as well. It is worth noting that one site offers a extra sound hood that reduces levels by 6db at an upcharge.

    The Panasonic FV-04VE1 WhisperComfort ERV might be a good cost compromise though the stated efficiency seems to be 60%. The youtube link is an interesting quick view for installation considerations. I think Martin might take issue with some of the magic air arrows in the explanatory animation on the Panasonic site.

    It is important to note the 20F cutoff for air intake on the Panasonic, didn't look at the Lunos for temp limits. Equally important to keep in mind is that physics demands lower heat recovery efficiency for lower delta T's. The recovery value of the heat will shrink as you hit shoulder seasons. Of course so do your heating requirements, so I will let you do the calculations with your local weather data. Let's just say diminishing returns.

    In any case, take the anticipated air change levels you foresee and then figure if the savings derived from a full on ERV or HRV will really be worth it. The Lunos E2 at $1100 a pair solves the two room issue and the 90% recovery sounds attractive though confirmation of how fast it goes down relative to delta T range would be a good thing to know. Sound-wise the general fan and radio noises I have in my own office would likely drown out the Lunos E2 fan noise. The E-go is quite a bit more for a pair and noiser.

    Then again, a 100 bucks for a low volume fan and a cracked window might suffice for a low volume situation.

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