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Community and Q&A

Heating system for small space / intermittent use

drat | Posted in Mechanicals on
Hi all, I have a heating system question that I’ve been mulling over. I’m building a workshop/studio for a weekend house (a separate bldg, zone 6a), and it has a full bath. The building will be insulated to code (wood frame on an insulated slab on grade). The studio/bath/utility is approximately 210sf. The workshop is approximately 300sf. 
My original plan was to put a wood stove in the workshop and electric baseboard in the studio and bath. I would let the building go cold and heat the studio or shop on demand.  This means that I winterize the plumbing in the fall and just use the bath in the house. If we had winter guests that turn it to guest room use, we could turn the bath on and then re-winterize it when they leave. 
I don’t love this plan — I think the electric baseboard will be slow to heat, and I anticipate that at some point I’ll decide to carry the studio/bath over for some amount of time and not winterize. To this point, I’m insulating the studio/workshop separation wall and putting a thermal break in the slab. Another heating option is to put in a mini split, but the size seems awfully small for just the studio/bath/utility block (I get approx. 12,000 btu).  Perhaps that means that I omit the wood stove and heat the shop with the mini-split too (again, about 12,000 btu). 
There are other options out there I’m sure, and mechanicals are not my strong suit. My primary criteria are:
1. Quick to heat if I let it go cold.
2. Comfort. 
3. Installation and operating cost. 
Appreciate any insight. Thanks!

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  1. walta100 | | #1

    I do not think a wood stove is a good choice for a small building or a tight building. For fire safety most stoves require about 3 feet of clearance making 1/3 of unusable for its intended purpose. For a wood stove to work the air escaping in the flu must be replaced with air from outdoors. You would need to open the window or build a leaky building. Please note many insurance companies will no longer building with wood heat.

    Did you say if city gas was available?

    Is cooling a requirement?

    How large of an electrical service is available? Code will require a panel in this building if you have more than one circuit. Code will require 36” x 36” in front of the panel remain clear and open.
    Many fire codes will require this building to be 10 feet from any other building while avoiding any setbacks and easements.

    A 25K BTU electric heater like would warm the place up fast if you can feed it 30 amps of 240 volts. With some creativity you could control it with a phone app.

    “1. Quick to heat if I let it go cold.
    2. Comfort.
    3. Installation and operating cost.”

    The electric heater is great for goals 1 and 2 goal and the install part of 3 operating costs depends on you usage and electric rates but generaly high.


  2. onslow | | #2


    You have designed in a cold anchor relative to item one. You will never have a quick to heat after letting a slab go cold. Plan on area rugs in the office and thick rubber mats on the floor of the shop to ease things. That slab will take an enormous amount of BTUs just to get to 70F and still feel cold to touch. Maybe cork tile on the floor would provide a bit of relief, it is very unconductive.

    The only thing you can heat quickly to comfort levels is air. Once you let things go cold they will have to soak up enough heat to catch up to air temperatures. Adjustments in the form of material choices can speed the apparent comfort levels. Wood will not conduct heat quickly and thus will feel warmer to touch sooner than tile or metal, both of which will actually feel cool to touch until at least 8oF.

    Don't want to harsh your buzz, but the idea of winterizing the bath is frankly a perilous path. Maybe if you always do a full shut down, line purge and glycol the traps in September you won't end up with a cracked toilet, burst waterline or cabin full of sewer gas. Do you have a plan for a shut off that is completely frost protected and an air coupler downstream of that? Do you have a way to empty the tank of a toilet as well as the bowl. A plan to glycol all the traps in toilet, sink, and tub? Clear every single valve? Talk to someone that winterizes the big RVs or a caretaker of second homes in your area and see what is required. Then imagine doing this in February after the guests leave and you need to get back to work. Worse imagine the results of a "shoulda done that yesterday" that busts some line located under the slab or cracks the tub/shower valve.

    Planning on going with a 50 to 70 range via the minisplit with two heads would probably be a safer long term choice. Try to find one a bit smaller that will support two heads. The shop could stay at 50 and the office at 70. The set up should be less than a disaster brought on by frozen valves that only reveal themselves after you restore water should you still want to go with the winterizing idea. That is provided you have assurance the drain lines are good below ground. No sense keeping the top warmish and then getting an ice plug down below.

    You haven't said what activities you plan for the work shop, but do consider the fumes you might create. The fastest heating wood stoves also have very hot exteriors. Will little ones be about? If you have something fall against the stove will it melt and cause a problem of toxic gases? When you aren't in the shop, will your supplies be happy going cold? Would going between 50 and 70 be better for the supplies and make using them more productive. If you are a painter, do acrylics behave well when cold. I imagine oils would. They seem to be a good number of possible reason for narrowing the range you let your shop/office get to. Carrying the shop at 50 for the heating season should be fairly low. Upping your insulation beyond code would help tremendously.

    I will agree that resistance heat would be slow and expensive to run. My whole house is heated that way with cove heaters. I do however keep the temperature stable all heating season. I don't have natural gas and didn't want the propane monster tank above or below ground. Mostly we didn't want fan noise after so many years of noisy furnaces.

    We are fortunate that even being a 6B climate with summer temps in the 90's now, our overnight temperature drops 30-40 degrees. We open up and use the mass of the house to "store" the cool. This is how I know the slab will prevent you from having a fast heating environment. The mass of the house walls and floor easily carry us through the day with no AC as designed.

    The one catch comes during the winter months. If we have a door open too long (like one does in a Midwestern goodbye) the cold air hangs for a long while. The radiant cove heaters don't heat the air, they heat objects. That is why I know you will feel more comfort from warm air faster. A design flaw on my part, but we love the silence. And we don't really get many visitors out here anyway.

  3. drat | | #3

    Good comments Roger & Walter. To clarify a few things:

    1. the slab as a cold anchor is definitely an issue that I’ve thought a lot about: in the shop, I was planning to put down rubber mats as you suggest. I don’t expect to spend days on end in there in the winter, so I’m not overly bothered by that prospect. In the studio, I’m planning on a wood fitness floor (7/8” wood floating on 7/8” closed cell polyurethane). That room will be a fitness room amongst other things, and that gives resiliency as well as insulation from the slab below. I think that’ll allow it to respond more quickly to heating.

    2. Ok, I hear you on the winterizing. As a kid, that’s how we handled our summer place, and it never seemed to be that big of a deal. Smart valve locations (honestly, they weren’t really all that smart) and some RV antifreeze did the trick. But I’ve heard enough people say what you’ve said that it gives me pause. Maybe we had a string of warmish winters during my formative years. Maybe we just got lucky.

    3. So I hear the comments about wood stoves, but I will offer the experience of our house at the same location. It is straw bale, plastered inside and out, with a wood floor over slab on grade. Heat is propane above-slab radiant supplemented by a smallish wood stove. There is a LOT of mass in that house and the heat is very slow to respond, as you say. We set it back to 50 when we’re away. But once we build a fire in the wood stove, the entire place is toasty within half an hour. Not saying it’s the right thing here, but the radiant heat of a wood stove does huge things for comfort. That house isn’t passivehaus right, but it’s pretty tight, and we have no problems. All that said, the wood stove might not be the right thing for the other reasons you noted. And it’s not cheap either.

    4. My options for heat source are electric or propane. The building will have its own panel, so I can get the electric service I need.

    Thanks for the comments so far. Some good things to consider.

  4. ssnellings | | #4

    Forgive me if I missed it, I didn't see what the workshop is for. If you're working with flammable solvents or cutting wood products (producing air-borne wood fibers) you may want to avoid a wood stove from a safety perspective, especially given the small size.

    The risk is minimal, but the chance that your insurance company disagrees is a bit higher :) Mine did, so I junked the idea. Give your agent a call before install.

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