Thursday, September 5, 2013

The Scary $5 Water Heater / Gas Grill Propane Tank Emergency Expedient

Ol' Backwoods has an emergency preparedness item for you today.

Suppose there's a short-term to medium-term disruption of your natural gas to your home, and you really need hot showers and clean dishes.

This hack shows you how to run your natural gas hot water heater for short periods from a propane grill tank,  and do it safely.   By building a simple $5 pipe adapter BEFORE the emergency comes, you can be ready to go when there's an emergency disruption of the natural gas supply.  At the bottom of the post is the engineering behind the hack, and why it works.

I know this works, because I heated three tanks of water with my water heater using this technique.

WARNING!  THIS IS AN EMERGENCY EXPEDIENT, ONLY TO BE USED IN TIMES OF DIRE NEED.   I am NOT LIABLE for what you do with this information; it is presented for educational purposes only, in the field of emergency preparedness.  If you do this expedient, it must be MONITORED CONSTANTLY and only be used for short periods.   It requires GOOD VENTILATION, as the expedient may increase CARBON MONOXIDE output from the water heater.  This expedient is not a replacement for natural gas service, and may damage the burner of your water heater if used for a long period of time! 

In what situations might you do this?  Any emergency that causes the interruption of natural gas service for a few days or more.  Here are some examples; all have occurred before, in the US or the rest of the world.  How do I know?  I was an engineer in the natural gas industry.
  • Backhoe operator does not call utility before digging, and breaks open a residential gas main;
  • Accelerants in burning building near gas meter burns through natural gas pipe, causing the pressure to be too low to operate your water heater;
  • Train wreck or chemical plant explosion nearby to natural gas pipeline breaks pipeline open;
  • Earthquake causes a gas line break;
  • Terrorist attack against natural gas pipeline or pumping station;
  • Economic collapse.

Let's Do It!

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You've decided to prepare for this emergency?  Even after my warnings?
Okay, what do you need?  4 things:
  1. Special pipe adapter you will build below;
  2. Full propane tank from your gas grill (or buy a dedicated one);
  3. Gas grill hose & regulator off your gas grill (or buy a dedicated one);
  4. (Optional) carbon monoxide monitor (CO monitor), which everyone should have near their gas hot water heater anyway.

The Pipe Adapter

To the right is a picture of the special pipe adapter you will assemble and test.  The pipe adapter allows regulated-pressure propane from a gas grill tank & hose to flow into your water heater.    If you build the pipe adapter ahead of the emergency, you will be ready to make hot water with your grill tank.

I'm not going to tell you each and every step to build and install the pipe adapter.  If you haven't ever done basic plumbing before, this is NOT the project to start with!  If you are in a country that uses metric plumbing parts, you will have to figure out your own sizes.  Below is what we use in the US.

The lower part is a 2" long, ½" diameter black pipe nipple, designed for use with natural gas.  I purchased it from a local hardware store for about a dollar. Buy the black iron pipe, not galvanized. The upper part is a brass flared male 3/8" to female ½"  pipe adapter, purchased from the same hardware store for a few dollars.

The two parts of the pipe adapter need to be put together with PTFE ("Teflon") cream, to prevent leaks.  Use your vise and a pipe wrench to get them very tight together.  YOU DON'T WANT LEAKS!  And we will be checking for them.  DON'T PUT PTFE CREAM ON THE MALE END of the brass piece THAT GOES TO THE GRILL HOSE!

Hooking The Grill Tank to the Water Heater


Using a pipe wrench, remove the pipe that connects between the output of the natural gas value and the input of the thermostat.  If you aren't sure, you shouldn't be doing this!  Usually, there is a tee where this can be accomplished easily.  The bottom of the tee goes to a capped vertical pipe, which is designed to catch condensation in the gas.  The top connection of the tee is where your pipe adapter will go.

After putting PTFE cream on the threads of the pipe adapter, screw it into the top of the tee.

Hook up the gas grill hose to the top of the pipe adapter.  DO NOT USE PTFE CREAM HERE!

Okay, here's a picture showing the FULL hookup to my water heater.  PLEASE, PLEASE pay attention to the notes in the picture.  Click to embiggen, or right-click to save it and view in your picture viewer.

Don't EVER Do This!  (Unless you have to)

Procedure to Light and Burn Water Heater

FIRST, turn off propane tank valve (should already be closed), and turn off the pilot light valve AND the thermostat on the water heater.
  1. Lather 50/50 soap/water mixture over all your connections, even on your gas grill hose.  A sponge is good for this, as shown on the right (thanks WikiHow).
  2. Turn on the gas grill valve, SLOWLY to avoid triggering the leak restriction device built into the regulator.  If you are getting expanding bubbles, STOP!  TURN OFF THE GAS GRILL VALVE, and work on tightening your connections. I was very careful making the pipe connections, and used enough PTFE cream on the joints, that I didn't have any bubbles at all.
  3. Light the pilot light on your water heater. Watch it burn for at least 2 minutes. Be sure you are getting a mostly-blue flame, with some yellow edges. (YES, YES, I know, using flame color to determine fuel mix is dangerous! This whole thing is an emergency expedient, remember?)
  4. Now, turn the water heater gas valve to ON. Be sure you have no leaks at this point! More soapy water on the joints, please!
  5. No leaks? It's time for the moment of truth. Turn the thermostat up to about 120°F (more on this later).  Listen and watch for the burner to light.  Be sure it seems to be burning nice and hot, with at least 50% of the flame blue. We expect the upper part to be yellow, because, remember, this water heater wasn't designed to be run this way. (YES, YES, we know, using flame color to determine fuel mix is dangerous! This whole thing is an emergency expedient, remember?!)

 Remember, this hack is for emergency use ONLY.

Run it for ½ an hour, be sure the flame is still burning good and heating water, then shut it off and go take your shower, or have someone else watch the flame and the CO monitor for you.  TURN THE DARN THING OFF WHEN YOU'RE DONE WITH HOT WATER for awhile! Remember, it's an emergency, and we are conserving what little propane we have stashed.

The Tech Behind the Hack

First, we are heating stuff, so we should talk a little bit about the energy density of fuels.   A BTU, or British Thermal Unit, is the amount of energy required to raise one pound of water one degree Fahrenheit.

Different fuels have different energy densities, or BTU values. A gallon of liquid propane contains 91,502 BTU's. A pound of liquid propane contains 21,591 BTUs. A hundred cubic feet of natural gas (a so-called CCF) contains approximately 102,000 BTUs, but a hundred cubic feet of propane will yield 224,880 BTU's – almost 2½ times more heat– because propane has a higher energy density.

How hot does the water have to be, then? 170°F water (77°C) is hot enough for sanitizing dishes, if kept immersed for at least 30 seconds. 140°F water requires many minutes (like a long dishwasher cycle) to sanitize dishes, but takes much less energy, as we see below. An even lower temperature, like 120°F, is more reasonable if you use some kind of chemical sanitizer when washing dishes. At 120°F (water weight: 8.25 lb/gal), it takes roughly 5 minutes for a scald burn, so you are less likely to get burned at a wrong shower setting.

Alright, how much water do we need to heat?  My hot water heater has a 38-gallon capacity, which is 315.86 pounds of water at year-round earth temperature (about 50°F here in North Carolina). The weight of a gallon of water varies between 8.312 (50°F) and 8.125 (170°F) pounds.

Let's calculate the amount of energy in BTU's to raise my 315.86 pounds of water from 50°F to

BTU = water weight * temperature difference = 315.86 * (120°F-50°F) = 315.86 * 70 = 22,109 BTU.

Okay, we have the energy required to heat the water from cold to the desired temperature.   This is a straightforward application of unit conversion, since 1 BTU will raise 1 pound of water 1 degree F.

But how long will it take to heat the water? Depends on your burner.

My hot water heater burner, running natural gas, can supply 40,000 BTU per hour. Assuming 100% heat transfer to the water (not true, but close enough), we can heat a tank of water in:

Hours = BTUs required / BTU /per hour = 22,109 / 40,000 = 0.57 hours x 60 min/hr = 33 minutes.

But the burner was designed for natural gas, not propane.

If the propane's BTUs were used as efficiently by the water heater burner as the natural gas BTUs are, then the water would heat about 2.5 times faster on propane, because propane has about 2.5 times the energy density of natural gas.

But it won't burn propane as efficiently. The orifices (little holes) in the burner are sized for efficient mixing of natural gas and air, not propane and air. Propane orifices are usually about 60% of the size of a natural gas orifice, for the same BTU rating (I looked this up in a paper design table I have). That means running propane through natural gas orifices cause too much gas to be mixed with too little air, causing incomplete burning, increased carbon monoxide and soot production, and in general less energy.  Chemists call this a non-stoichiometric mixture, and the gearheads among us would say, "she's runnin' too rich."  And they would both be right.

Just how far off are the orifices for burning propane?  Not far.  A 0.129" orifice for natural gas would yield about 59,000 BTU per hour. The propane orifice size to rate the same BTU's should be between 0.089” (55,000 BTU/hr)  and 0.094” (61,000 BTU/hr).  To yield the 40,000 BTU/hr that my hot water heater puts out on natural gas, it would need a 0.075" orifice to run on propane.  But it doesn't; its orifices are likely closer to 0.129" in diameter (nearest natural gas orifice size that corresponds to 40,000 BTU/hr), allowing too much propane through.

So, keep in mind, it could take longer to heat the water than it does with natural gas.  In the case of my water heater, it was really pretty close; about 45 minutes vs. the about 30 minutes it takes normally.

What other considerations are there?  The pressure of the gas at the input of the regulator on the water heater.  My water heater burner is well-matched to the gas grill regulator I have.  The water heater is rated for natural gas pressure at between 5” and 14” of water column. I bought a “Type 1” gas grill regulator (commonest type in the US), which delivers a pressure of about 11” of water column. This hack likely wouldn't work for a much larger burner that requires a higher pressure. By the way, 7 inches water column is equivalent to 4 ounces per square inch pressure (1/4 PSI). 14 inches water column is 8 ounces pressure ( ½ PSI). 1 PSI equals 27.7 inches water column.

Even with the correct pressure, my orifice is the wrong size, and my flame color is only about 75% blue, with the rest yellow. This means I am getting soot and CO, so be careful! I wouldn't have even attempted this hack if my water heater wasn't next to an exterior wall in my attic with PLENTY of ventilation.

No, it's not optimal. But, for a temporary expedient in time of emergency, IT WORKS. I know it works, because I heated three tanks of water with my water heater using this technique.

I want to emphasize again: this is NOT something I would want to do all the time. If I was running off propane all the time, I would buy a propane water heater. And maybe at your bugout location, you do or will. But if you live in an area where earthquakes or man-made disruption of the natural gas system is possible— even a misplaced backhoe – this is a viable technique to get you hot showers and clean dishes in a short to medium-term emergency.

Again, DON'T DO THIS if you aren't sure!

Ol' Backwoods would to hear feedback from anybody who builds their own pipe adapter.  Leave me a comment.


  1. Erwin,

    Sounds like English may not be your first language, but I'll make the attempt, anyway. In the situations that this modification may be useful, THERE WILL BE NO PLACE TO BUY A READY-MADE WATER HEATER. If there is no natural-gas service in your area, switching to propane, and buying the proper water heater, may be a useful option. But then again, by buying and storing the US$5 in pipe adapters to make this little hack work, you can ensure that your family has hot water during a short- to medium-term interruption of the natural gas supply.

  2. Hmm, looking at this conversion reminds me of another heating question I have. My propane heated house uses fuel like its free. Someone (who heats with natural gas) mentioned maybe the connection from the propane to the heating system is "wrong" and uses too much gas. They suggested having a plumber not the gas company check this. Could this be, I have lived here 6 years and just put up with it in the winter. Any thoughts if this could be? Would a plumber know ?

  3. I wonder if leaded solder would make a better seal between the brass and the iron of the adapter. They have two different thread types? So have you tried solder instead of paste or ptfe. Would preheated air improve propane rich combustion? Catalysts?

    1. Mark,
      It's hard to get solder to wet iron unless the iron is plated. The paste works great.
      Preheated air might improve the combustion, but remember, this is intended to be an expedient. If you're running propane that long, start looking for a propane water heater.

  4. Thanks for the tip! So, I could use the teflon gas tape instead of the paste?

    1. Windy, that's up to you. I'm not a pipefitter, but it seemed to me that the paste would make a better seal than tape.

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