cleaning around furnace and water heaters....

I would not use aerosols around that gas-fired equipment. They are using propane as a propellant in many aerosols. Might not reach dangerous levels but best not to take the chance. Same goes for gun scrubber type products.

I used to have the same problem in my old house. I just made sure I was not within 8 feet of the gas-flame appliances.
Watch out for gas dryers as well. I was using mine as a table-top while spray blasting some parts. The wife pushed the button to start it up before I could stop her. It blew the metal cover panel across the adjoining two car garage. Kinda cool....

I use Ed's Red, which may contain up to 25% acetone, which could be of concern. I was cleaning the oil out of bullet casting molds by scrubbing with acetone (pure acetone, not Ed's Red), and then flaming it to dryness. I was decanting some acetone from a 1 gallon metal can (1" metal screwcap) onto the mold, reached over and deposited the can at arm's length away from the mold, then lit-off the mold. The flames followed the trail of acetone vapor from the mold to the can, setting it on fire. I was lucky! I leapt up, dropped the cap onto the burning can, and got is screwed down before the acetone got hot enough to bulge to can. That was a valuable learning experience!

M-Pro 7 and Hoppe's Elite are nonflammable and low odor.

Don't play with any aerosols around any live flame. It's too risky. Even if you are 8 ft. away from the flame air currents can carry your fumes right up to it. Get the right ratio of air and fumes and you will have combustion. You should have some kind of fan moving the fumes out of the house even if there is no flame present. A box fan set on a window sill will do the job. Better yet, don't buy aerosol gun cleaning products. Use a liquid and apply only what is needed and where it's needed with a brush or Q tip. Aerosol cans are messy and waste half of the product. A bottle of CLP and a M 16 brush and some rags are all that is really needed to clean a firearm. That's all I have ever used for 40 years. Just mop it on and let it sit for a few minutes and brush and wipe it off. You're done.

I’m 15-20 feet away using CLP and Ballistol and both are always bubbled short and into something. The Hornady One Shot is the only thing shot onto a patch but even that spray is seconds and allowed to disperse. Powder is locked in its wooden case when I am cleaning. I have never had a problem and don’t expect one.

[In the basement I discovered I had to be more concerned about lead dust from dry tumbling on the edge of my workbench. Never again. Now I wet tumble with pins.]

Is your furnace a high efficiency model, 2 pipe system? If so it's a closed combustion unit and shouldn't be a problem when using common sense. The water heater is most likely a open combustion system and more prone to problems with vapor.
If the furnace is an open combustion system the heat exchanger can be damaged by off gassing of products in the ambient.
Most furnaces installed within the last 10 years+ are a 2-pipe system.
If your not sure post a picture and I can tell exactly what you have.
My furnace is in my shop.

You have a 2-pipe furnace and the water heaters are open.
The water heaters are more of a concern to me if you have strong or heavy fumes.
The room should have a combustion air intake for the water heaters.

Any of 'em have a pilot light? Re-lit pilot lights using a match myself for 5 years with no fuss anyway. Highly unlikely you'll ever have any problems other than the things being in your way.
If there were any issues with any kind of solvent or combustible being used close to any of that stuff, there'd be written warnings all over those appliances.

I dealt with paint thinners and a wide variety of solvents for most of my working life. IMO, you'd pass out from the fumes long before the concentration got anywhere near explosive levels.
FWIW
YMMV