When it's below freezing outside and my propane furnace in heating our house there is ticking / knocking sound in my walls. What could be causing this and how do I fix it?

Are you in Siberia? How old is the unit, how cold is it outside, is your unit sized correctly for the cubic meters of space in your area, and how thick is the insulation in the walls and ceiling of your building? Here in the U.S., I am in West Virginia. My electric central heat / A/C unit is 4 tons in capacity, my unit is 18 years old but I maintain it myself, my house is 170.5 meters squared in floor area with vaulted ceilings and my ceiling insulation is about 30.5 cm thick, my walls are a bit on the thin side but my floors have been stuffed full of blown fiberglass. All in all, we have mostly mild winters here but when the temperatures get to about -6 C.or below for several days at a time, my electric heat would run 24 / 7 to keep the house at 21 C. if I didn't kick on my propane gas logs. I usually keep these in reserve for when the power goes off as my 7 Kw generator won't run that dinosaur consumer of a heater. I tried to say dinosaur k___er but Fixya considers the "K" word to be profane even if the victims have been extinct for 65 million years. Oh well. Anyway, I run the logs off from two 7 kilo propane tanks with gas grill regulators connected to a "T" on the soft copper line leading into the fireplace. I know this wasn't much help but maybe the comparisons gave you something to think about. Good Luck.

Heatpumps are more complex than an AC unit. Yes, there is a difference between a heatpump and an air conditioner. Where the real difference lies is out side in the condenser unit. There's a defrost board out in the unit, and depending on the brand and model, one of the wires will need to be changed so that the gas furnace will not come on. The problem with this, heatpumps use the heat that is available from the outdoors and transfers it to indoor. When the temps outside drop below 50 degrees, there's not enough latent heat to transfer, thus calling the propane furnace to operate. Without the addition of the furnace, you will not be able to heat the area properly. The only way to remedy this, is to remove the propane furnace and replace it with an electric airhandler with a heat strip for "emergency heat". The emergency heat term is used when the temps drop below the temps I mentioned above and calls for the heat strips to come on.

If it has a position for natural gas/propane then it needs to be changed to the position that the gas source is either propane or natural gas..

Sounds like ice coming of the coil. That would explain the ticking. The rubbing could be ice buildup before the defrost cycle or it's building on the blades of the fan throwing it of balance a bit.

Freon is odorless. A sewer odor could be caused by an unused sink, tub, or floor drain trap where the water has evaporated. A gas odor is a very serious issue that should be checked immediately by the gas company or the fire department. Gas can seep in from a leak outside the house or accumulate in a wall if a fitting fails.

what wires land where on your tstat?

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yes, sounds like a stuck relay. RAY YOU ARE A GENIUS!!!

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well lets start with the 2" pvc pipe. that is a combustion air intake it does not put air into the room, i hope. this is a 90% furnace and they should have installed it that way. there should be 2 pipes to the outside the other is combustion gases. the dust problem maybe caused by the air return pulling the air to that room. the bedroom normaly does not get the trafic that the rest of the condo does.so it would not have as much dust.

With the description of when this noise occurs, indicates that there is a gas pressure and gas volume issue. The size of the gas line running to this heater may be the problem. It should be a minimum of 1/2" diameter. Especially, since you have two other major heating appliances sharing the same supply line.

The other thing to consider is does this occur when the outside temps are below freezing and is your Propane Tank buried or above ground? Also do you have a high pressure two stage gas system? If so, the secondary regulator at the house may be freezing and not allowing enough gas into the lines.

If the regulator is freezing ... soak a bath towel in hot water, ring it out and wrap it around the regulator as quickly as possible to thaw it out. Once it thaws it should remain thawed as long as gas is flowing. If it doesn't, your LP Gas Provider may need to add methanol to the tank.

Good luck! Hope this solves your problem.

Really not efficent to run a heat pump at those temperatures. Below freezing is not good for the compressor in the outside unit. The humidity in the house with the low temp. is causing the condensation on the windows to freeze. Kinda like an air conditoner does in the summer without the freezing humid air from the house goes over the cold a/c coil and the moisture condensates on the coil running down to drain pan. raising the temp. in the house and lowering the humidity should help. I would also recommend running the electric heat in low outside teperatures.