Strange sound from furnace

Max Death · January 3, 2015

A few background notes first before I get to the question.

Staying in Wyoming and it has been cold here. Been in this fifth wheel for over three years. Have an Atwood hydro flame, not sure of model. Had propane supplier fill external propane tank which we have been using. Supplier filled tank to over 90 percent.

Now to question.

Since supplier filled tank furnace runs as normal, but midway through cool down cycle there is a distinct hollow metal pop. Only does this once at at same point in cool down cycle. Never heard this before. Is this something to be worried about?

Not really wanting to pull everything out of basement in this cold weather to access the heater if there is nothing to worry about.

Appreciate any thoughts. Thanks

Kirk W · January 3, 2015

Are you sure that they filled the propane tank to 90% as tanks in use today are required to have a stop valve that closes at 80% to prevent more ? Even so, I don't see any way that such would cause the noise you have since the problem that comes with an over filled propane tank is liquid leaving the tank and that will damage the pressure regulator.

The only thing that I can think of to explain your sound is one caused by contraction of the metals that expand each time you use the furnace. If the outside air temperature is lower than it has been in the past, that could cause the contraction of cool down to take place at a more rapid rate and so make sounds that were not happening before. Most anything that has changed or shifted could also cause that to happen. Where is the sound coming from, the fire box? If so, my guess is that it has something to do with cool down rate and is not anything to be concerned about. Of course, how loud that noise is could also be a hint.

Dutch_12078 · January 3, 2015

As Kirk said, the expansion/contraction noises will vary somewhat with the outside temps. Each of our two identical Atwood coach furnaces sound a bit different as well, so even comparing the sounds of one furnace to another isn't always an accurate diagnostics tool.

SIBERNUT · January 3, 2015

That only happens when a mouse is (was....) inside.

Pat & Pete · January 3, 2015

You might check the screws that hold the burner in the fire box .

But , it does sound like contraction noise , like some motors will ting a bit when cooling ...

I found a fried mouse on top of the fire box once when checking the sensors , but we never noticed any different sounds .

kathydavidb · January 3, 2015

Yep, like others said it seems to be metal contraction noise. In residential ducts & plenums they put crimping or creases in to offset the "clunking". When not so cold out it may quit doing it, but if not then you may try to trace down the source of the flexing metal. Best of luck, Dave.

Max Death · January 3, 2015

Thanks everyone for your replies. The cooling was the first thing I thought of but have been in cold temps before and never heard this popping before.

Kirk this is a 100 gallon house type propane tank that has a gauge. The guage was pegged out past 90.

Will have to get into the basement as soon as it warms up. Since the consensus seems to be that there is no major worry.

Once again thanks again everyone

Kirk W · January 3, 2015

Kirk this is a 100 gallon house type propane tank that has a gauge. The guage was pegged out past 90.

The tank should still stop fill at 80%, as is the requirement for all propane tanks. I have a 200 gallon tank at our home-base and the maximum put into it is 160 gallons and the gauge reads 80%. I would be very cautious about letting them put 90% in in warm weather although it might not be a problem in winter. It is also possible that it was filled on a day with very low ambient temperatures and you checked it when warmed up as that expansion could cause it to go up...... I'm not sure how much is reasonable but you may want to check that out for safety reasons. You don't want the pressure relief lifting on warm days, which used to happen before the "stop fill " valves were required, but an operator doing what they called, jamming a tank.

This is taken from the American Energy site:

Q. WHY ISN’T MY TANK FILLED TO 100 PERCENT CAPACITY?

A. Your propane is delivered and stored in liquid form. Propane liquid, for example, will expand nearly 17 times as much as water over the same temperature increase. As a result, tanks and cylinders are never completely filled with propane-gas liquid. Tanks are filled to about 80 to 85 percent of their capacity. This leaves a space above the liquid, which allows the propane liquid to expand freely due to changes in temperature.

There are several important characteristics that you need to understand about LP gases when they are stored in containers. First, heat added to LP gases in a tank or cylinder is transferred directly from the air surrounding the container. Hot days, cool nights, rain and snow are a few of the many factors that affect the temperature of the liquid. Because of these temperature changes, you may see fluctuations in your container gauge.

Why was my tank not completely filled at tank installation?

When tanks are transported on service trucks the Department of Transportation only allows the propane installation company to fill the tank to a maximum of 5 percent, for safety. We then have our delivery truck stop by a few days later to fill your tank.

Wrknrvr · January 4, 2015

I did see an Atwood combustion chamber once that had a crack in it. It was near the outside edge close to the corner. I was not working on the furnace so did not know the outcome of it. Nor did I hear why they had the furnace out. Just happen to see what someone else was working on. That maybe what is happening as the metal as it cools at a different rate.

I have also seen the exhaust tube rust off of the chamber. Maybe the tube broke where it is welded to the combustion chamber it self.

These are rare but what you are describing is rare also. If you are going to remove the exterior exhaust tube it might be rusted tight. I had one once that took about 45 minutes just to free it loose and remove it. The exterior tube is slid over the combustion chamber exhaust tube.

Keep us informed as what is happening.

Vern

Bill B · January 4, 2015

Em Kirk.... The OPD is required in the smaller tanks, up to the 40#'s 'portable' tanks but not in the 100#+ tanks. They still have the old POL fittings (legally). That said, 80% is still the correct fill and such, but that is done by the filling operator, not by an OPD device.

Your 200 gal tank at home is a liquid fill but a vapor draw, although, it may also have a liquid draw port for a generator or other high use appliance. Again, operator controlled.

Good idea, bad idea, just the way the NFPA did it. Probably has to do with the fill rates of the larger tanks. Like the 5 gal per minute on the passenger diesel pumps and the dual 20 gpm large truck pumps.

I agree that the operator overfilled the tank, not a good thing, but in his case, for the popping, I'd wonder about the fire chamber. I just hope that it wasn't a liquid carry over into the propane jets in the burner. Remember that this is a HYDROhot - big water heater and he has very little duct work, just the flame chamber.

cornwellkidd · January 4, 2015

The tank may have a bad gage our house tank is off 15% our tank empty reads 95% when it takes 800 gals it's a 1000 gal tank

Kirk W · January 4, 2015

.... The OPD is required in the smaller tanks, up to the 40#'s 'portable' tanks but not in the 100#+ tanks. They still have the old POL fittings (legally). That said, 80% is still the correct fill and such, but that is done by the filling operator, not by an OPD device.

It did cross my mind after the post that the bulk tanks may not require that over fill protection, but one would think that they should since there is no means of knowing how full they are other than the gauge. I have looked a bit but so far have not found the regulations for those larger tanks. There must be regulations for them somewhere, but I sure have not been able to find them. The potential for abuse would seem even greater with those bigger tanks............

And no, my tank does not supply any liquid but works exactly the same as one in an RV, except it is larger. Most of the 100 gallon tanks today are the vertical ones but my 200 is a horizontal tank, like most that size and up.

jpatrickc · January 4, 2015

Most of the bulk tanks have a bleeder screw that is attached to a dip tube that goes down in the tank to the 80% fill point. When they start filling the tank they open the bleeder screw and vapor escapes. When the tank is full to the dip tube then liquid starts to escape and they stop the fill and close the bleeder.

Kirk W · January 4, 2015

Most of the bulk tanks have a bleeder screw that is attached to a dip tube that goes down in the tank to the 80% fill point. When they start filling the tank they open the bleeder screw and vapor escapes. When the tank is full to the dip tube then liquid starts to escape and they stop the fill and close the bleeder.

Also true for the small tanks and bottles found in RVs.