New furnace causing interference with AM radio?

I just wrapped up a quality assurance site inspection where I heard something unusual. The homeowner had a new furnace & central air conditioning system installed in November.  Since then he's been hearing a lot of interference on his AM radios in the house and his hunch is that it is somehow related to the installation of the furnace. I'm not well versed in radio frequency transmission, so I didn't have any clue about what could be causing this phenomenon.

Has anybody else come across this issue with new furnace installations? 

Cheers,

-Nick

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Replies

  • Thanks everybody! I will send this thread along to the homeowner and hopefully he solve this mystery. When I get additional information I'll be sure to update you all.

    Cheers!

  • Hi Dennis, nice job covering all of the issues. It has been many years since I used an AM radio to locate some electronic noise, and you are correct, the older ones are better. :-)

    Bud
  • The AM radio works well - generally the older the better :-)   Older radios tended to have larger ferrite rod antennas.  That internal antenna also was very directional.  It meant that you could easily pinpoint appliance noise INCLUDING electrical arcing.

    Older, less efficient furnaces typically use a split phase capacitor motors.  While they are not particularly the MOST efficient motors - they generate very little (if any) electrical noise.

    As suggested by others - an amateur radio operator may be more sensitive to the noise - because they are trying to pick up very distant stations.  The same can be true for a home owner that lives off in the fringe areas of a radio station.

    Electronically Commutated Motors (ECM) can easily be the source of the problem... but the manufacturer of the furnace should have addressed the noise problem as part of their design.  Virtually all electronic devices sold within the US must have FCC testing for emissions.  The furnace would have also.  The manufacturers normally solve this by making sure the motor is correctly shielded and grounded.  The furnace cabinet sides generally connect together (electrically) to provide additional shielding.  Finally the installation instructions should have required a good earth ground.  If the furnace isn't grounded - all the "high frequency" noise that is generated by the switching circuits in the motor and  the computer board on the furnace -- make their way along the power connection -- and the duct work is then used for the ground.  Duct work can make good antennas.  

    Amateur radio operators often check for loose, poorly grounded appliances and duct work.

    The other sources of noise - that could be related to the HVAC -- include electronic air cleaners - which would come on when there is air.. and possibly a UV sanitizing light.

    If the furnace was installed properly and it is indeed the source of the noise - contact the manufacturers tech support (or have the installer do it.) This becomes a bit tricky because you need to find someone to take ownership of the problem.  Unless the furnace installer has seen this often - they may dismiss it.  The factory will want the installer to check their work, etc.... 

    Easy way to get their attention is to use the AM radio - and see how it works outside the house.  If it has wiped out reception to just a radio station that is quite some distance away - there may not be a lot that can be done... replacing even an incandescent with a CFL would probably cause the homeowner the same problem.  

    But on the other hand it is quite bad in the house AND only with the furnace on... it was installed properly and it has a ground, then I would hope the manufacturer would want to know and see.  This is where digital cameras on cell phones are handy.  You can capture what you see and forward if necessary to the manufacturer.

    I'm not sure you would want to use a capacitor to solve the problem ... if the noise is from the motor  it might be just a simple snap on RF ferrite choke...  see: http://www.rfparts.com/fercke-aaa.html 

    But before I would even use the choke,  I'd put in a call to the manufacturer... it could be a known problem and they very well might have suppression kits that they want you to use.

    The same rules about checking for grounding apply to the electronic air filters and UV sanitizers.  It may even be possible (not likely) a new fancy communicating thermostat is causing the problem.  That could be fixed by making sure that the wire is either shielded (and grounded at the furnace) or is twisted pair.

  • I have put in a 30 MF cap in line( hot to hot com to com) and took care of this it. Clean up the power or add a ground  or both.

    If a ECM motor and has weak ground and the gas pipe is the ground  I have seen this make lines in TV's.    I took at 6' X 1/2" ground rod next to dirt floor crawl space.  Of if good ground at box run a new ground to furnace.  The new furnaces take a lot better ground- pre 1980 did not use ground , the flame rod uses earth ground to make the flame prove flame or  ground path is the flame so it takes a very good ground for new installs.    Test your ground with a thumper  or take a water hose and water your ground rod see it goes a way

    I did a install next to radio station and got the wire less controlls to turn on and off with out need 

  • We've seen interference created by furnaces that causes problems with short-wave radios, but never AM. However, it sounds like a similar issue. There is typically a section in the installation manual that discusses radio interference, and offers suggestions specific to the brand/model to reduce this. If that doesn't work, contact the installer, and have them work with the manufacturer to help resolve the issue.

  • If you try a battery powered AM radio and still have the problem, that eliminates the power as the source of the noise. All electronic devices have emission limits, so I would be talking to those who installed it.

    Bud
  • If you Google RFI aka radio frequency interruption, you will find a lot of good info on it. Like Bob says, is it all the time or just when the furnace is running - do the have fluorescents nearby or neon lights, was any other work done? Depending on the exact source & cause of the problem, they maybe able to install a noise filter, while in other cases that won't work

  • Does it do it all the time or just when the furnace runs? Can you identify when in the firing sequence the noise starts?

    • I believe it happens whenever the blower motor is running.

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