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Author Topic: HVAC blower engages on 80m TX.  (Read 4660 times)
KI6IQO
Member

Posts: 15




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« on: August 15, 2017, 03:11:54 PM »

Hello,
I am seeking a solution to prevent furnace blowers engaging during transmit. The system is not mine, so I don't have much room to experiment or tear things apart. Ideally I'd like to install a simple pre-made or DIY filter behind the thermostat to stop the issue.
This is a gas heater systems with outdoor AC compressor.

In my limited research it seems the switching for these systems typically run on 24V AC with potentially separate 24V transformers for both the indoor furnace/blower and outdoor compressor. I'm assuming that RF on these long low voltage lines creates a potential inside the digital thermostat and makes one of the transistors driving a relay conduct. But maybe there is some solid state switching inside the furnace??

It seems to me if I put .1uF across all the wires before some inductors to the thermostat, I would remove any RF potentials inside the thermostat.
This seems a little overkill to me- and maybe unwise if dual 24v systems. Perhaps all I need is .01uF across one 24V supply, fan, and common line (if it has one).


Any thoughts or similar experiences?




 
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KH6AQ
Member

Posts: 7718




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« Reply #1 on: August 15, 2017, 03:36:54 PM »

I would do two things:

1. Contact the HVAC manufacturer to see if they have an RFI kit.
2. Add clamp-on ferrite chokes to the suspect cables. DX Engineering sells ferrites.

To see if you are making headway it can be good to threshold the issue. To do that key down at the lowest RF power and slowly increase it until the problem occurs. Repeat several times to see how variable the trip point is. Then when you apply ferrite chokes you repeat this to see if things are improving.
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KI6IQO
Member

Posts: 15




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« Reply #2 on: August 15, 2017, 10:26:08 PM »

1. I can try this route, however these type of requests often get a "huh? RF what?" response from the operator wanting to send a technician.
2. I have some type 31 pieces, however these are rather expensive and also too bulky for tight places (IE behind thermostat). Bypass capacitors are cheap and small.

Thinking about it more, it may not be the thermostat. When the thermostat engages the blower, the blower will maintain an on and an off delay. If the blower is instantly turning on and off, it suggests some type of electronic switch in the furnace itself.
« Last Edit: August 15, 2017, 10:28:20 PM by KI6IQO » Logged
W1AN
Member

Posts: 18




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« Reply #3 on: August 16, 2017, 08:29:33 AM »

Please be very careful! This is a safety issue for which the the manufacturer should take sole responsibility! I would not touch or suggest the owner or anyone else do anything on a gas appliance until instructed in writing by the manufacturer! And this should only done by authorized party under manufacturers direction!! The communication between thermostat and the rest of system may or may not be a simple thermostat. It would be hard to explain that your fix did not cause a future problem.
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KH6AQ
Member

Posts: 7718




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« Reply #4 on: August 16, 2017, 09:52:48 AM »

A simple possibly fix is to reduce the RF field at the HVAC.; the antenna can be moved farther away, the transmitter power can be reduced, or both.

The problem is probably common-mode RF current and ferrite cores are the weapon of choice. Yes they are expensive and on 80 meters it will take many of them. I would arm myself with a dozen or more type-31 cores.



« Last Edit: August 16, 2017, 09:56:13 AM by KH6AQ » Logged
KI6IQO
Member

Posts: 15




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« Reply #5 on: August 16, 2017, 12:53:08 PM »

Antenna can't be moved. That is avoiding the problem, not fixing it.
I'm also not adjusting natural gas lines, burners or otherwise.

If the problem is where I now suspect, chokes may make the most sense. The furnace closet should have enough space for chokes and extra wire.

I don't think it should take more than one or two #31 ferrites, as long as they are used correctly with many turns. The part that I worry about is if the excitation can manifest as differential mode between different control wires, particularly with the remote compressor. It would be most convenient to take the multi lead wire and wrap it all on one core. The concern is if equal but opposite current is induced on certain wires, necessitating individual cores for each wire in the bundle. The advantage to adding a bypass capacitor, is that it prevents differential mode.
If I can conclude the frequencies in use on these control cables, and the source/termination impedance there is no risk to adding a suitably sized and rated capacitor.





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KC1GCG
Member

Posts: 126




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« Reply #6 on: August 17, 2017, 01:58:28 PM »

I hope you fix it. If its common mode on 80 try a ugly balun? That was my solution for an end fed that was causing tvi for me.
Also consider heeding the advice of most on this issue and do not touch other peeps stuff. To many stories of " u touch it u own it" and when that hvac ever breaks down it's going to be ALL your fault regardless of facts. I remember reading a funny but sad one where someone tried to blame a ham when thier washing machine died! I think it was my maytag died the day after u put up that big ugly antenna.
I think the arrl has some good info on this to educate the owner. And you are right you may get a dumb tech or worse one that is reading what the lawyers said to say. Look for the story of the kidde co detectors. But in spite are your best and noble intentions only try stuff on your end otherwise when that hvac dies on Xmas eve you are going to be the one getting the call. Or heaven forbid if the place burns to the ground after you modify it guess who is going to be knocking on your door? Fire inspectors,..insurance and a bus load of lawyers. Oh newsreporters too....
Be a nice guy (to yourself) make sure your install in clean and just give them the arrl brochure. Good luck either way.  John k1jrf
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KM1H
Member

Posts: 2224




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« Reply #7 on: August 18, 2017, 03:42:29 PM »

Mouser has the best prices for the large 31 and 43 mix ferrites plus snap on types.

On 80M you need about 1000 Ohms minimum impedance to be an effective common mode choke and the 31 mix is better there. I use a 5 stack of the 2.4" diameter ones with an easy low C 7 turns of RG-11 since I run all 75 Ohms here. For 50 Ohms and RG-213/LMR-400 has the same results.

That combination offers 5000 Ohms reactance and more from 160-40M. For the higher bands 43 mix is better. For milder common mode problems 43 mix can be used 80-10.

This is the best paper available on the subject and I suggest all read it several times and keep a copy in the bathroom for those relaxing times Grin
http://audiosystemsgroup.com/RFI-Ham.pdf

Jim also has other papers that get into de RFIing home electronics and lots of other subjects.
http://audiosystemsgroup.com/publish.htm

Carl
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KI4TZX
Member

Posts: 3




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« Reply #8 on: August 27, 2017, 05:38:52 AM »

I have been an HVAC tech for 40+ years, but don't claim to know what is causing your problem. But more info on the type, brand, and efficiency might help. Basic systems just switch 24v to turn things on and off. Gas heater blower motors are 120v normally. But the new type X13 motors use 24v at a module on the motor for speed select and 120v to run. Then you have the digital communicating systems for higher efficiency systems. These use variable speed motors. I would lean to the motor if it is an X13 type motor. They have been a pain in our butts since they came out. So in short, unless you can spend some time with it, your guess is as good as mine.  

As a side note....I have a digital communicating, variable speed Heat pump system that sits less than 10 feet from my shack......never has caused a sound on any band or turned on on it's own.
« Last Edit: August 27, 2017, 05:40:53 AM by KI4TZX » Logged
N3QE
Member

Posts: 4819




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« Reply #9 on: August 30, 2017, 04:54:03 AM »

But the new type X13 motors use 24v at a module on the motor for speed select and 120v to run. Then you have the digital communicating systems for higher efficiency systems. These use variable speed motors. I would lean to the motor if it is an X13 type motor. They have been a pain in our butts since they came out. So in short, unless you can spend some time with it, your guess is as good as mine.  

We had a completely new HVAC system put in about 10 years ago. About 5 years in, the blower motor stopped working, completely dead. I went into the attic and found out they had put in a fancy-pants five-speed X13 motor, maybe in some attempt at improving SEER rating, but only ever programmed (via the 24V line to one of the five speed wires) to a single speed. The electronics inside the X13 motor was fried and cost of replacement was astronomically high. So I just replaced it with a plain old capacitor-run AC motor - and it has been working fine ever since.

There is a difference in wiring between X13 and regular old AC motors. In traditional AC motors, a relay is controlled by the 24VAC from the thermostat, and the relay applies/removes 240VAC power to the motor. In a X13 motor - and I'm guessing this is true for any fancy electronically commutated motor control - the 240VAC goes to the motor all the time, and the 24VAC signal electronically (not via a relay) selects the speed and activates the motor electronics. I'm very confident that stray RF in my house HVAC is not going to close a real electromagnetic relay. But I would not be surprised at all, if stray RF could activate an electronically commutated motor.
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WB4SPT
Member

Posts: 456




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« Reply #10 on: September 09, 2017, 06:48:24 AM »

Hello,
I am seeking a solution to prevent furnace blowers engaging during transmit. The system is not mine, so I don't have much room to experiment or tear things apart. Ideally I'd like to install a simple pre-made or DIY filter behind the thermostat to stop the issue.
This is a gas heater systems with outdoor AC compressor.

In my limited research it seems the switching for these systems typically run on 24V AC with potentially separate 24V transformers for both the indoor furnace/blower and outdoor compressor. I'm assuming that RF on these long low voltage lines creates a potential inside the digital thermostat and makes one of the transistors driving a relay conduct. But maybe there is some solid state switching inside the furnace??

It seems to me if I put .1uF across all the wires before some inductors to the thermostat, I would remove any RF potentials inside the thermostat.
This seems a little overkill to me- and maybe unwise if dual 24v systems. Perhaps all I need is .01uF across one 24V supply, fan, and common line (if it has one).


Any thoughts or similar experiences?

 

First step is to determine if the issue is with the thermostat or the air handler.  ie;  remove the thermostat and then test. 
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