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The Heat Is ON. Using Electricity

Posted By: Muniac

The Heat Is ON. Using Electricity - 10/12/12 08:32 PM

Introduction:

This article makes a case for using electricity as a heat source for mobile living spaces. After wintering for three seasons in Park City, UT I've learned a few things which I'd like to pass onward with the hopes it will help others gain knowledge about heating systems. The information presented here is a distillation of a larger two part article on surviving winter. You don't need to be embarking on a cold winter stay to need heat, either. In August up in the mountains of Colorado our heat will click on in the early morning hours. It takes the chill off things.

The three most common energy sources for heating are diesel, propane and electricity. Many vehicles can use all three of these for heat. The more common energy sources for heat are diesel and propane, however. In our case, I'm not comfortable with using a pressurized fuel inside a small space. Plus its combustion creates moisture and consumes oxygen which needs to be managed. So propane is included here for comparative purposes only. Our vehicle has a 40 gallon propane tank and remote filling isn't practical. Most propane delivery services won't fill it either. Sometimes you can cut a cash deal but that's iffy at best. Almost always they require you to use one of their tanks, instead. Plus you'll discover that propane is the most expensive of the three energy sources. We use about 5 gallons of propane per year.

Heating Fuels:

To compare propane, diesel and electricity we need to select a common unit of measure that can be applied to all three fuels. Diesel and propane are sold by the gallon while electricity is sold by the kilowatt making direct comparison impossible.


Propane, diesel and electricity normalized for comparison.

If these fuels are used for heating then their heat output can be measured in BTUs. Using common formulas it's easy to determine the cost for those BTUs for each fuel based on how it's commonly sold. The table above is one I worked up from November 2011 using average pricing and the current electricity rate in Utah. Below are the constants I used to crunch the numbers for the chart. As an academic exercise, feel free to update this with current pricing. I don't think you'll find the outcome and conclusions presented herein to be vastly different.

1 gallon of diesel fuel produces 157,300 BTUs
1 gallon of propane fuel produces 63,450 BTUs
1 kilowatt of electricity equals 3,412 BTUs

For convenience I arbitrarily selected 10,000 BTUs as the energy chunk to keep from getting into fractionally small amounts of money. It's all relative so the idea is to make comparisons easy to see and understand. Boiling these fuels down to cost per BTUs works perfectly for a heating application. We know in advance the fuels are being consumed for heat and it's important to know what that will cost.

The graph is somewhat sterile in that it assumes 100% of the energy will get converted to heat. In reality, this isn't the case and we'll explore this in more detail. That said, you can see pretty clearly that propane is the most expensive of the three fuels. At the pricing I used, diesel and electricity are very close. This was an interesting piece of data since many people believe (including me) that electricity is just hands down more expensive for heating. With diesel fuel at a nominal $4/gal this just isn't the case. As the price of Brent Crude fluctuates along with other economic factors energy costs just aren't stable. Back in the Spring of 2009 I was paying $2/gal for diesel. Now diesel fuel has more than doubled in price. Should electricity do the same thing, this comparison will change. And I haven't a clue what to expect.

Converting Fuels To Heat:

From the above (at current or near current pricing) we have nailed down the cost per 10,000 BTUs. Well almost since the conversion process influences how much heat you'll get. Propane and diesel convert through the heat of combustion while electricity requires a resistance element. Propane and diesel require some sort of burner. Propane is a clean burning fuel while diesel is not. Electricity can't be burned so it needs to be converted instead. Let's take a look at these energy sources in more detail.

PROPANE - As mentioned I'm not going to go into much detail with propane. For us, it's just a cooking fuel which burns essentially clean. Meaning it can be combusted inside a living space without a flue or exhaust system. For all intensive purposes it converts 100% to heat. Making this heat source efficient. Burners for this fuel are relatively inexpensive as well. But it's a pressurized fuel requiring special safety considerations. And it's the most expensive of the three energy sources. In an extreme "all else failed" emergency we would lite a burner for heat.

DIESEL - Diesel is the most energy packed fuel of the liquid lot. But it doesn't burn clean. A special burner nozzle or furnace is required to extract its heat. The combustion requires a flue and/or exhaust system. That means valuable heat goes out the tailpipe as it were. The equipment required to burn this fuel is expensive. And it's maintenance is expensive as those with a Hydro-Hot system know first hand. Exhaust gases on the Hydro-Hot system run in and around 600F which is a lot of heat blowing out into the air. Residential oil burners (closest thing to a Hydro-Hot system) run around 80% efficiency. My educated guess is the Hydro-Hot system is well below this number. But let's give the benefit of the doubt and use 80%. We'll also exclude the $8K purchase price and another $2K worth of repairs you'll need to perform to keep it running. The 80% efficiency makes diesel more expensive than the chart shows. That's because 20% of the fuel cost is lost heat out the tail pipe. There is also an overhead required to keep the heat transfer fluid at 190F. This occurs regardless of whether the system is called upon to produce heat. With hydronic heat (known as hot water in a residence) the heat transfer fluid is pumped through hoses. Heat energy is lost here too. The transfer of this heat energy to air at the heat exchanger isn't 100% efficient either. As mentioned, the 80% efficiency number is giving this system much more credit than it deserves. The take away is every $1 you spend heating with diesel plan on at least $0.20 or more getting wasted. But it's a necessary evil, so to speak, because we need it for hot water.


A Hydro-Hot system. An expensive option that's expensive to fix.

ELECTRICITY - Our third energy source and the centerpiece of this article is electricity. Like everything else prices vary more than they should. In 2012 we paid about $0.10 per kilowatt in Utah and $0.12 per kilowatt in Colorado. I can only image what it costs in California. If electricity costs in and around this amount and diesel is close to $4/gallon than electric heating makes obvious "cents". Beyond a lesser expensive fuel, the equipment to convert it to heat (compared with that used for diesel fuel) is both simpler, cheaper, safer, smaller, easier to take care of and 100% efficient.


My small space heater at 650, 1000 or 1500 watts. A Grainger product made in China.

The conversion of electricity to heat is done through a resistive element. The power dissipated is as follows:

Power in watts = Current in amps squared times the resistance in ohms.

In your toaster and hair dryer naked nichrome wire is typically used. Today's space heaters (like that shown above) use a ceramic element. Which is nothing more than nichrome wire encapsulated in a ceramic material with integrated fins or a radiating surface. The conversion is 100% efficient meaning there is no wasted heat. From my constants above, converting watts into BTUs is just arithmetic.

BTW - A BTU stands for British Thermal Unit and is the amount of energy required to raise 1 pound of liquid water 1F.

Heating Systems And Heating:

With diesel you'll burn the fuel in a chamber that's vented to the outside air. A blower or turbine atomizes the fuel for ignition and provides an airflow (oxygen). The flame inside heats the walls of the chamber which transfers its heat to some other medium. In the case of the Hydro-Hot, that's a propylene glycol solution aptly named a heat transfer fluid. The hot fluid is then pumped into heat exchanger zones where the heat energy of the fluid is transferred to forced air. A zone is an independently controlled living space area. Vehicles typically have two zones (front and rear) and 2-3 heat exchangers. A temperature sensing device (one per zone) monitors air temperature in each zone and cycles the heat source for that zone on/off through a thermostat module as required. An increase in temperature will be realized when the heat put into a space exceeds that which is lost. Less heat lost means less heat energy required to facilitate a temperature rise. That's why insulation, double window glass, gaskets and caulk are used. These things save on energy and money.

A typical Hydro-Hot system is rated at 50K BTUs. But you won't see that much heat at the radiators (or heat exchangers) owing to losses and inefficiencies. If you are running domestic hot water, you won't see any heat at the radiators since the unit shuts them off. As outside temperatures fall, more interior heat is lost. Thus maintaining a comfortable temperature inside your living space simply requires more heat. That means a bigger heating system or running the existing one for longer periods of time. There's something known as an acceptable duty cycle but let's not go into that here. The idea is you don't want a heating system to constantly run trying to keep up with the demand at the thermostat. When the heating system is on (read Hydro-Hot) that puts wear and tear on its components to include turbine motor and pumps to name just a few.

When temperatures fall to between 30F and 35F a stock vehicle (no extra insulation added) will begin exceeding the Hydro-Hot's capacity to comfortably provide heat. Windy conditions will only make matters worse. As previously stated, Hydro-Hot diesel heat is expensive. Costs include both fuel and repair bills. You can reduce said costs by supplementing your heat with electricity. I've done this for the past two seasons with very good results. Refer to my Surviving Winter articles for much more detail.

Using Electric Heat:

A good supplemental electric heating system is more than just purchasing a cheap Walmart heater and plugging it in. In fact, I don't recommend any sort of a cheap system at all. Do the job properly from the start.


Electric space heater on dedicated 20A circuit. Note the control box.

What I would suggest is installing front and rear dedicated 20A circuits. This is easy to do by running 12/2 (no splices) from the circuit breaker box out to residential outlets. Parallel off the front/rear A/C breakers since heat and A/C won't ever be on at the same time. Put these wasted circuits in the winter time to good use.


My front dedicated 20A heater outlet. It's a split circuit.


My rear dedicated 20A heater outlet.

There is a difference between the pins in 120 and 240 volt outlets. Make sure to use the 120 VAC version. This is easy to screw up! DO NOT use cheap plastic RV outlets. The biggest challenge with running in dedicated outlets is running the wiring. Always a pain in the ass with RVs. I used metal junction boxes for the outlets. Make sure to select the ones capable of mounting in a blind hole. Use good quality Eagle or Leviton residential outlets.

The trick to making a space heater work properly is controlling it from a remote thermostat. To do this you'll need to build a heater control box. It's nothing more than a power relay, 24 VAC transformer and remote thermostat. I used a plain simple wall mount thermostat. If you want to go digital and programmable that's fine too.


My heater control box within a metal enclosure.

Most electric space heaters have an integrated thermostat inside them. Cold drafty floors will cause poor performance, short cycling and what's referred to as "cold 70" in the living space. Having a remote thermostat with an anticipator will allow mounting on an inside wall 5' above the floor and provide proper control of the heater. This makes a HUGE difference.


Thermostat mounts at 5' on any wall you like. Simple.

Thus control of the heater isn't from the floor but a more appropriate spot within the heating zone.

Performance:

Earlier I referred to this as supplemental electric heating. As it turned out for us, the electric heat became a complete replacement for the Hydro-Hot system. It's also important to note our vehicle has radiant electric floor heat which remained on throughout the winter season. As for the Hydro-Hot it provided hot water and plumbing bay heat only. The higher electric bills ($0.09/kW) were still hundreds of dollars cheaper than buying diesel fuel ($4+/gal). Plus we avoided beating the crap out of our Hydro-Hot system while it struggles to satisfy the thermostat in 0-4F weather.

20A will provide 2400 watts of power. We were able to run twin heaters on each outlet. One is set to 1500 watts and the other is set to 650 watts. We left the Hydro-Hot system set to 65F on both zones as a backup should the power go out. This would keep the living space above freezing until power returned. Make sure your space heaters don't use a digital control. Most of these won't restart after a power failure. In other words the heater control defaults to off. No good.

Conclusions:

For motorhome applications, electric heat is cheaper and more reliable. Our Hydro-Hot system puked several times during the winter. Not a fun repair in the cold of winter. If you winter in climates that require using heat, the savings in diesel fuel will pay back the investment in electric heat in a couple of seasons. Proper use of electric heat isn't plug and play. You'll need to do some wiring and build a couple of control boxes. Also, electric space heaters take up floor space. Perhaps not practical for some life styles. We didn't find this a problem. The system components aren't permanently mounted so seasonal storage is possible. About 4300 watts of electric heat plus our floor heat kept us warm throughout the winter months. For really cold climates, plan on installing extra insulation. I cover all this in Surviving Winter. Dropping cloths, jackets and towels on the heaters isn't a really good idea. Make sure your heaters have a safety tipping switch and over temperature cutout. The one shown here has all those safety features.

When I made my heater control box, I used a 20A rated accessory socket. To my surprise this melted. I replaced this with a residential outlet and that has performed fine. Again, make sure all your wiring, contacts and outlets can handle a full 20 amps. Don't always believe the ratings on stuff made in China. They do things differently over there. Stay warm during the winter and have fun. laugh
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