Products > Other Equipment & Products

Problems with using shielded twisted pair for 24VDC power?

(1/2) > >>


I'm looking at getting a heat pump installed at my house that uses a communicating thermostat (specifically, the Lennox iComfort S30) that uses two pair of wires: 1st pair should be shielded (per manual) and 18 to 22 awg.  This is a flavor of RS485 I believe.  The 2nd pair is just 24VDC to power the system.  No mention of shielding or twisted pair for this cable.

Finding two twisted pair shield cables is pretty easy.  Finding one with a shield twisted pair cable and two other untwisted, unshield conductors is a bit harder.

So, the question is this: Is there any problems with running DC power (low power) through shielded twisted pair?  The only thing I can think of would be cost, but I am FAR from an expert.  The reason I am asking is I have seen people discussing issues with not using shielded cable for the communications pair and it sometimes causing problems.  I want to make SURE the correct cable is installed so I never have any issues down the road.  And no, I do NOT necessarily trust the contractors to install the correct cable correctly.  These are HVAC contractors after all that are still getting used to variable speed motors.



No, you can use whatever for DC as long as the cross section supports the current.

Not to nitpick too much, but you shouldn't have 24VDC on that system.  There is 24VAC to power it and 12VDC in between units.  Twisted pairs will work fine for 24VAC and 12VDC, but I'm not sure you want them in the same physical cable without shielding in between them.

Will it be a KNX cable maybe?

Won't the installer run appropriate cable for the system?

I looked at communicating equipment but decided I am not a fan of the systems that are currently on the market. I like the idea, but the problem is there is no standard for communicating thermostats so with one of those systems you are locked into one of the small selection of very expensive and completely proprietary thermostats offered by the vendor. The one I looked at was over $500 for the thermostat and it was less capable than the Venstar thermostat I have. There are only two main advantages of a communicating thermostat that I can think of, the main one is that it has full control over the output of modulating systems and the other is that status information and errors are displayed right on the thermostat. I'm not sure how great the energy savings are vs using a conventional thermostat with an algorithm in the furnace or heat pump to control staging, and the status messages you can get directly off the unit itself if something isn't working.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version