How Much Will I Save By Converting A Propane Water Heater To A Natural Gas Fired Heater?

Tankless water heaters and hot water storage heaters using Propane or Natural gas express their energy usage in therms or BTU's of gas. A therm is equal to 100,000 BTU's (British Thermal Unit) of energy. Depending on its quality, natural gas typically contains about 1,000 Btu per cubic foot of gas. Therefore, 1 therm of natural gas would be more less equal to about 100 cubic feet of natural gas. 1 Btu is enough energy to heat 1 lb of water by 1 degree Fahrenheit

Propane is a component of natural gas that is separated and liquefied and is often called Liquefied Petroleum Gas or LPG. For this reason the cost of propane will always be more than the cost of natural gas. Propane is liquefied to be able to store quite large quantities in energy terms in relatively small pressure vessels or tanks. Propane is a very convenient energy source for remote areas and small portable heating units as used in motor homes, RV's or caravans.

If you look at the table below you'll see that propane costs about 55% more than natural gas for the same energy quantity

Cost Of Natural Gas and Propane Energy Units Used On This Website

The efficiency of LPG and natural gas heaters are about the same since both burn under very similar conditions and both suffer from wasted energy in exit gases. This makes the comparison calculation of running costs between the two types of fuel simple since the answer is a direct function of the difference in price. In other words a propane heater with the same energy factor as a natural gas heater will cost about 55% more to run than a natural gas heater.

The difference between worst and best gas heaters in my 2004 survey can be seen below ...

Let's take a look at a really good example of how lots of money can be saved if you have a similar situation t the example I'm going to discuss ...

My Current Hot Water Heating Situation

• My existing water heater uses a propane heated storage water heater to heat 1,000 lbs (about 260 gallons US) of water every day from 40 degrees to 80 degrees (ie 40 degrees Fahrenheit increase). Using the formula shown below the theoretical amount of energy needed to heat this amount of water is ...

• 1,000 x 1 x 40  = 40,000 Btu's per day

• Lets assume my present propane heater is an older model and is running at low end of energy factor range ... assume the energy factor is 0.46 (see above table) then actual fuel used will be around 40,000 divided by 0.46 = 87,000 Btu per day at a cost of $2.202 per 100,000 Btu's.

• Daily cost = $1.76 or $641 per year

Proposed Hot Water Heating Situation

• My NEW water heater will use natural gas to heat 1,000 lbs (about 260 gallons US) of water every day from 40 degrees to 80 degrees (ie 40 degrees Fahrenheit increase). Using the formula shown below the theoretical amount of energy needed to heat this amount of water is the same as before which is ...

• 1,000 x 1 x 40  = 40,000 Btu's per day

• Lets assume my new natural gas water heater is very modern and the best there is in fact and is not a storage heater but a tankless water heater. Then the energy factor will be 0.75 (see above table)

This Is Enough Information To Calculate Savings By Changing The Fuel and Hot Water Heater Type

There are 2 factors that will influence the saving ... and the same duty is required in both cases (ie 1,000 lbs per day of hot water)

1. Higher efficiency heater (factor of 0.46/0.75)

2. Lower cost of fuel per Btu (factor of 1.42/2.202)

• New system daily running cost = $1.76 (daily cost of old system) x (0.46/0.75) x (1.42/2.202) = $0.70 per day

• This amounts to $254 per year running cost for the new system compared to $641 per year for the old system

• As you can see the impact is dramatic and is reason why being able to calculate the different costs of all systems yourself is vital. If you can't do it you will be at the mercy of salesmen who use the numbers to suit their own products.

Quick Savings Estimate To Judge Hot Water Heater Decision

If you are thinking of changing your system you can make a quick estimate in percentage terms (ie what your new cost will be as a percentage of your old cost assuming load is same) of what you will save as follows ...

1. 100 x (new energy factor/old energy factor) x (new energy cost per million Btu/old energy cost per million Btu)

2. Using the above example % saving will be ...

3. 100 x (0.46/0.75) x (1.42/2.202) = 39.5

4. In other words the new water heating running cost will be only 39.5% of the old system hot water heating cost

Basic Formula For Heating Water In All Systems

Flow rate of water in any units (eg gallons per minute or litres per hour) x a constant number called specific heat x the number of degrees the water must be heated by .... interpreting this in simple terms it means if you double the flow and want to keep the temperature increase the same (eg from 40 to 80 degrees)you must double the energy supplied be it in the form of electricity or gas or any other energy type.
1 Btu is the enegy required to heat 1 lb of water by 1 degree Fahrenheit so if you work in lbs and degrees Fahrenheit calculation of energy for heating water is very simple indeed and the formula is ...
Number of Btu's required = mass of water x 1 x rise in water temperature in degrees F or if a continuous flow of water as in a tankless water heater system is required then  ...
 Number of Btu's required per hour or minute  = mass of water  flowing per hour or minute x 1 x rise in water temperature in degrees F

Common Terms used By the Hot Water Heating Industry ... Important

In any industry there is a degree of jargon and the hot water heating industry is no different. I've tried to define the important terms in a simple to understand practical way.

• Energy Factor ... This factor relates to how efficient a unit is and can be interpreted as follows ... if the Energy Factor is high then the annual cost of energy (gas, propane, oil or electricity)  to run the tankless hot water heater or the storage heater will be lower than it would be for a system with a lower Energy Factor. Energy Factors for Gas tankless water heaters range from 0.64 to 0.82 normally.

• Average Gas Energy Cost ... it is assumed that national US average fuel cost of 1.42 $/therm will be used.

• Electricity Energy Cost ... it is assumed that average national cost is $0.1178 per kiloWatt Hour

Take a look at how to choose a new tankless water heater

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