Part 3: Heat Distribution, Air Quality, Safety, Sizing
If you haven't already read part 1 or 2 of this post, you can start here.
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Heat Distribution
In a small enough space, heat distribution isn't an issue. But long and skinny spaces or spaces with multiple rooms might need some extra help to provide even heating. And some heat sources like hydronic or ducted heaters require a distribution system to get the heat into your space.
Ducted Heat
Many heat pumps and furnaces are designed to work with ducted heat. In addition to more evenly heating separate areas of your living space, ducted heat can be helpful for maintaining plumbing in utility ares.
Ducted heat requires both a supply and a return. Supply ducts blow warm air into your space, and returns pull cold air back to the heater. Cold air returns should generally be located on the far end(s) of your space, away from the heater, and close to the low point of your space. If you're using ducted air to heat a separate utility area, there should also be a return in the utility area.
Distributing Heat from Non-Ducted Appliances
If your heat source is not designed to work with ducting, you can still add ducting to your space to help distribute your heat more evenly.
For a 120 volt electrical system, use a 120V fan system mated with a 4" dryer vent. You can even add an air filter. Hide the duct inside of a cabinet to run air from the warm area of your house to a colder area. Wire it with a thermostat for a fully automated system.
If you're using a 12 volt house battery , use a 12 volt blower fan with 3" dryer vent and a 12 volt thermostat instead.
Either of the above systems could be used with a household thermostat, but you'd need to add a relay. The internal wiring of the thermostat is not designed to handle the amount of current needed to drive a fan. So, instead of running the fan's power through the thermostat, the fan's power runs through the relay and the thermostat switches the relay on and off.
Hydronic Heat Distribution
If you're using a hydronic heating system, you'll need some sort of radiator to distribute the heat into your space. The heating loop should be filled with glycol to prevent freeze damage to your equipment. The glycol loop needs a small expansion tank to allow for thermal expansion of the fluid, and a pressure relief valve for safety.
Some hydronic heating systems can work passively by convection, but some need a circulating pump if one isn't included in the heater already. A small hot water circulation pump will keep your heating loop moving. This model will work with either a 12V or 120V system.
If you're interested in doing some tinkering, it should be possible to drive your 12V water pump using a few thermoelectric modules driven by the heat source. This would be a great project for an off-grid wood stove feeding a hydronic heat loop.
In-Floor Hydronic Heat
The most luxurious type of hydronic heat is in-floor radiant heat. The principle is the same as electric in-floor heat, but instead of an electric coil, you either sandwich a PEX coil between the finished flooring and subfloor, or mount the PEX coil to the underside of the subfloor with a heat transfer plate, then insulate below.
Natural convection won't adequately circulate glycol through a hydronic in-floor heat system, so you'll need a pump if your heating appliance doesn't already have one.
Hydronic Radiators
There are countless styles of hydronic radiators available. More traditional solutions include baseboard heaters and bulky cast iron radiators.
Modern designer radiators are available in a dizzying array of traditional and transitional designs—tall, short, vertical or horizontal.
Indoor Air Quality
Indoor air quality is a concern in any space, but it can be an especially difficult challenge in small spaces during the Winter. Dust, humidity, VOCs (volatile organic compounds, i.e. nasty man-made chemicals), and mold spores tend to build up very quickly in small spaces. Mold spores in particular can be exacerbated by added moisture from appliances that produce water vapor like unvented propane, butane, or ethanol heaters or cooking appliances. Building a relatively airtight space can be great for energy efficiency, but it can also make for an unhealthy environment.
Air filtration using a HEPA filter can vastly improve air quality in small spaces by removing particles. If the filter is equipped with a carbon filter, it can also remove VOCs from the air.
HRVs vs ERVs
The best way to maintain good indoor air quality is by regularly exchanging indoor air with the outside air. Opening windows or allowing drafts isn't the most energy efficient strategy for exchanging air in the winter.
A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) is a device that exchanges indoor air with outdoor air while keeping heat inside (or outside) of the living space. These devices are essentially counter-current heat exchangers that transfer the heat between the intake and exhaust without mixing the two air supplies.
While an HRV retains heat in your living space while exchanging indoor air for fresh outdoor air, it does not retain moisture. Humidity will pass through an HRV along with the intake and exhaust air supplies. This works best for a small space in a cold climate, where you want to retain heat inside your space and exhaust excess moisture buildup from inside your home.
On the other hand, an ERV has the ability to retain both heat and humidity levels inside the living space. That's important in hot, humid climates where you want keep your home cool, but avoid adding more humidity from outside.
Since tiny spaces typically have the problem of too much humidity inside, rather than too little, an ERV would not usually be the best choice for tiny house or RV in a cold climate.
Warning: Some ERVs, like the popular Panasonic WhisperComfort ERV, will go into "exhaust only" mode during freezing conditions to protect the core. In a cold climate, that means your ERV will stop working on the days when you need it most. If you have a relatively airtight house and you're heating with natural draft heater like a wood stove or direct vent propane fireplace, the ERV in exhaust only mode could draw carbon monoxide into your living space. An HRV is generally a better choice for a tiny space in a cold climate.
Safety
Safety is a critical consideration for any heating system. If a particular heating option cannot be used safely in your space according to the manufacturer's instructions, you should choose a different heating option.
Tiny homes are typically not subject to building codes, but that does not mean you can ignore them. Safety requirements are there to protect your life, the lives of your family members, guests, and the lives of first responders. The best way to avoid burning down your house is to follow the manufacturer's instructions for installation and clearances.
Electrical Safety
For electric heaters, make sure your wiring is rated for the load of your heater plus anything else on that circuit. You should try not to rely solely on a safety device like a breaker or a fuse to avoid burning your house down. Know the rating of your circuit and the load of the devices on that circuit, and don't exceed the rating. If you're using an extension cord to hook your house up to a generator or shore power, make sure the cord is rated for the total electrical load of every device running in your house.
An electrical management system like the Progressive Dynamics EMS is cheap insurance for protecting your home, appliances, and life, on top of the usual safeguards like circuit breakers, fuses, and common sense.
Proper Clearances
Most heating devices have requirements for clearance from combustibles. Before you install or plug in a new appliance, find out how far a it needs to be from combustible materials like upholstery, curtains, sheetrock, and wood, and don't violate those clearances.
Most commercially available heating systems are very safe if used properly, but homes burn to the ground every day with and without families and pets inside of them because of simple clearance violations.
Alarms
Always maintain at least one smoke alarm and one CO alarm in your living space. Modern smoke and CO alarms are available with batteries that last the full lifetime of the alarm. Smoke alarms should be positioned high on a wall or on the ceiling near the sleeping area. CO mixes freely with air (it doesn't sink), and detectors often have a screen with a CO level reading. So, the best places for CO alarms are on the wall somewhere that they're easy to read, preferably near sleeping areas. Keep alarms away from heating or cooking appliances to avoid false alarms.
Propane tends to sink and pool in low areas, so if you use propane in your house, you should have an LP alarm in your living space, located near the floor. LP alarms are available for 120 volt AC or 12 volt DC. You will most likely smell gas before the alarm detects it, but an alarm can save your life or home if you're sleeping, in a loft area, or if a neighbor hears the alarm while you're away.
Sizing Your Heater
Some manufacturers like to claim that their product is suitable to heat up to a certain square footage. But there's a lot more that goes into heating than the size of the heater.
How well insulated is the space? Are the doors and windows tightly sealed, or are they leaky? What's the temperature outside? What temperature do you want it to be inside? Don't take the manufacturer's square footage number for granted.
BTU Ratings
The only way to know how much heat you need is to do some math. Find a few online BTU calculators, plug your space in, and play with the numbers. There's a little guesswork involved in things like the insulation variable and the climate, so check a few different calculators before you settle on your numbers.
Bigger is not always better. A forced air system that's too large won't be able to cycle through the air in your house before the thermostat turns off, resulting in hot and cold spots in your home and high energy bills. A wood stove that is too big can be difficult to keep under control, either making you open your windows in the middle of winter to cool your space down, or having to choke down the fire and fill your chimney with creosote, risking a chimney fire. The best heating system is one that precisely fits your needs and your space.
Look up the BTU rating of the appliance, and compare it to the needs of your space. Talk to the manufacturer of your primary heat source, and see if they agree with your numbers. While you're at it, how responsive are they to your questions? Do you get the feeling that they'll help you out if you have trouble with the install or need warranty repair? Do they have any promotions or coupon codes available?
Consider Multiple Heat Sources
Remember, you don't have to choose just one heating method. For cold climates, a combination of heating methods might be the safest bet.
You might choose a propane hydronic heater for domestic hot water and in-floor heat, and add a propane furnace to keep your space toasty on the colder Winter days. You might use a mini-split electric heat pump for primary heat, but add a small wood stove for ambiance and backup heat for when the power goes out.
