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I'm planning a 20x20' detached garage which will have a bonus room built into the roof trusses. I'd like to use this bonus room as my home office — so, since I live in the cold North, it'll need a heat source.

Given that the space will be thoroughly insulated, and that the only mechanical I can easily run from the house is electricity, how can I heat this space (12x20x7', just shy of 1700 cubic feet) in a reasonably efficient manner?

Considerations:

  • As it'll be used mostly during normal business hours, the temp can be allowed to dip at night, but must stay reliably above, say, 50° (for the sake of the computer equipment)
  • The faces of the gable roof are East and West (I'm open to insulated skylights and/or some kind of passive solar, if it'd help)
  • If needed, I can run water lines to the space (the detached garage will only be about 12' away from the house)
  • I'm open to some sort of burned fuel (pellet stove?), as long as the fuel itself is relatively inexpensive and widely available (I have access to as much chopped wood as I can burn, but something more compactly stored / consistently burned would be nice)
  • The trusses will be customized anyway, so I can use a heavy / reinforced bottom web, should it be necessary to pour some kind of underlayment layer (for in-floor radiant heating or just plain heat retention via mass)
  • The furnace for my home is fueled by our city's gas lines (may be possible to run a line to the office space, if needed)

Given all of this, what kind of heat source (or combination of sources) would get this home office through a cold Wisconsin winter in a reasonably efficient way (up-front cost can be higher, assuming the savings make up for it in less than, say, 5-7 years).

Edit: change text in last paragraph from "as efficiently as possible" to "in a reasonably efficient way", to clarify the scope of response I'm looking for.

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What is the price of electricity at the site? –  wallyk Dec 16 '13 at 8:50
    
So you're looking for a efficiency comparison between basically every available heating system? You'll also have to take fuel cost into account, so the local price of gas, electric, and solid fuels is important. –  Tester101 Dec 16 '13 at 12:25
    
@Tester101 It doesn't need to be the absolute most efficient means of heating (so, no — not a comprehensive comparison at all). I'm just looking for a good, reasonably efficient option, given the considerations. –  jasonmklug Dec 16 '13 at 14:43
    
@wallyk On my most recent utility bill, I'm seeing that I pay $0.12061/KWH. –  jasonmklug Dec 16 '13 at 14:44
    
If wood is free, that would likely be the most cost effective over the life of the system. Electric heat is 100% efficient, so that's likely to be the most efficient as far as fuel -> heat goes. Natural gas might be the cleanest (depending on where your electricity comes from), so it might be the greenest. This (as all efficiency questions) is difficult to answer. It will depend on availability of local resources, personal preference, and numerous other factors. –  Tester101 Dec 16 '13 at 14:52
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4 Answers

Geothermal Heat Pump

Efficiency

If upfront cost is of no concern, a Geothermal heat pump is the most efficient heating/cooling system. According to the Department of Energy, geothermal heat pumps can be between 300 - 600% efficient.

Financing

There may be government programs available to help fiance the installation of the system, as well as state programs.

Payback

Some sources estimate a payback period as short as 3 years, though actual results may vary. If you switch your main house over as well, use the system for hot water, and take advantage of government programs. The payback period could actually be quite short.

Electric Baseboard Heaters

Efficiency

Since electric heat is 100% efficient, every ounce of electricity used it converted to heat.

Financing

If there is adequate space and power available in the service panel, an electric baseboard heating system is fairly inexpensive to install.

Payback

Depending on the cost of electricity in your area, electric heat may or may not save you money over other heating options.

Natural Gas Room Heater

Efficiency

Since it's a smaller area, a standalone natural gas forced air system may not make sense. However, a smaller natural gas room heater (or a few) might make sense. Natural gas room heaters can be between 65 - 100% efficient (depending on the heater), so they can be quite efficient.

Financing

As with electric room heaters, natural gas room heaters will be fairly cheap to install.

Payback

Again, depending on the cost of fuel. A natural gas room heater may or may not save you money over other heating options.


More Info:

The U.S. Energy Information Administration has a Heating Fuel Comparison Calculator (Xls) that might be useful.

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You will save the most money long-term by far by ensuring that you insulate the area well, and only use the heat when needed (Nest thermostat of programmable ftw). ' Aside from that, given the efficiency of new systems you probably would see the payoff of upgrading to one of the most efficient systems vs a middle of the road same fuel source system in 10-20 years -- probably longer than the life of your system because of the high premium on those top-tier systems.

Radiant heat is very efficient but since the are under the office will be unheated (I'm assuming) I think that you would lose some efficiency to the space below. Electric and propane can both be very efficient but the cost of each of those products obviously differs. Since you can run electric easily, I'd suggest going with that unless you determine that the cost savings of propane or NG outweighs the installation cost (which may well be the case). With electric, I'd hazard to say that you can DIY the entire install if you are fairly handy.

Here's a cost comparison of heat sources, calculated from warmair.com

Cost per 100,000 btu of useable heat  

Corn Pellets:       $3.20  
Electric baseboard: $3.16  
Heat pump:          $2.07  
Oil:                $0.81  
Propane:            $1.23  
Natural gas:        $0.95  
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Barring large changes in relative fuel prices, gas is often the cheapest (wood can be, but if you start counting your labor, usually not.) Your local market may vary, but it probably won't vary that much. Electric is cheap to install, expensive to run unless you happen to have a good arrangement for a water/ground source heat pump (and those are expensive to install...) - air source won't work through a cold winter.

Run a gas line. You'll have plenty of options then.

For the "reasonably efficient" side of things, you're at the design phase, so put in plenty of insulation (yup, you did say that - just reinforcing), and consider things that cost a little when you build, but save a lot later, such as 2 inches of XPS under the floor and outside the foundation walls (little direct impact on your current planned use of only heating the second story, but...)

You mention running water lines, but you also mention that your house is heated by a furnace (ie, you didn't say boiler.) If you had hot water heat, well-insulated (and insulated from each other, which people often forget) pipes from a hot water heating system would generally be the best option in terms of requiring little additional heating infrastructure and making use of what you already have. But that does not apply if you have a hot air furnace (unless you want to do something with your (presumably also gas) hot water heater and a heat exchanger, which could work.)

Extracted from the Vermont fuel price report for Nov 2013 (more details if you follow the link above):

 Type of Energy  $/MMBtu
Fuel Oil Kerosene Propane Natural Gas Electricity Wood (cord, green) Pellets

$33.35    $37.86   $40.61 $18.28      $43.46      $14.65             $18.83 


Vermont electricity is relatively expensive at $0.15/kWh.

To expand a bit on my comments: since you appear to have enough of a predisposition to radiant floor heat to make me think "you'd really like some" the typical approach here would be a hot water source appropriate to the load (either a small-tank type water heater or a tankless heater are both common) simply because you won't have any use for the BTUs of a house-sized boiler. To run "quick air heating" and "slower radiant floor" off the same system, you make use of a couple of mixing valves so that the floor can run at a lower fluid temperature, while the baseboards or fan/coil units can run at a high temperature (the water heater makes high temperature water, and the mixing valve mixes some radiant floor return water with fresh hot water to make warm radiant floor water.) You can also, if you choose and your site seems good, add solar water heating to this type of system. Payback may be rather long, though. Then again, with adequate insulation, it may take a long time to pay for the amount of savings you'll get from being at 50F overnight, .vs. the cost of baseboard and/or fan-coil units you could do without if you just left the radiant floor heat at a somewhat higher setting.

Direct-vent wall-heaters or room-heaters offer the potential (you have to buy one that can) for heat during power outages, as long as the gas system is working - even with thermostatic control. Not all can, but some can - so if that's a priority, shop for it to see what your choices are, and if you like any of them.

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Assuming I use gas as the fuel source, what would the end implementation look like? Are you talking about a wall-mounted unit(s), a gas boiler for a fluid in-floor system, or something else entirely? –  jasonmklug Dec 16 '13 at 16:22
    
Whatever makes sense to you. For heating this size, well insulated, space hydronically, you'd probably want a water heater rather than a full-on boiler. Given the low night-time temperature, radiant floor might not be the best approach - it's better at "steady heat" than rapid temperature swings - but you could blend one with some baseboard or "fan coil units" to get the rapid air temperature response, while still having a warm floor. Under-floor is of course one of the places where good insulation will matter, no matter how you heat. TBC. –  Ecnerwal Dec 16 '13 at 16:33
    
A direct vent, sealed combustion wall furnace would likely be cheaper to install. You could also use a direct-vent "room heater" in the form of a thing that looks vaguely like a wood stove, if that suits your taste better. There are many options. –  Ecnerwal Dec 16 '13 at 16:35
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I would lean toward a high efficiency natural gas furnace.

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Please explain why you're recommending this type of heating. –  Niall C. Dec 17 '13 at 5:26
    
As other's have stated, it is often the most economical heat in terms of cost per btu. The air from a gas heater is warmer than that of a heat pump. With gas you have can it cool down when not in use and then bring it up to a comfortable temperature faster than with a conventional heat pump. With a heat pump, you can't drastically change the temperature without backup heat coming on. If power goes out, you can use a smaller generator to run the blower, or you might not even need a blower depending on the type of system. I would go with a vented high-efficiency forced hot air system. –  BrianK Dec 17 '13 at 17:21
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