Problems with using shielded twisted pair for 24VDC power? - Page 1
Forum section: electronics / general

author: RRRoamer
author_rank: Newbie
timestamp: December 06, 2021, 03:07:26 am
content: Folks,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.Thanks!RRRoamer

author: KaneTW
author_rank: Frequent Contributor
timestamp: December 06, 2021, 03:39:11 am
content: No, you can use whatever for DC as long as the cross section supports the current.

author: bdunham7
author_rank: Super Contributor
timestamp: December 06, 2021, 03:52:56 am
content: 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.

author: Tarloth
author_rank: Regular Contributor
timestamp: December 06, 2021, 04:02:35 am
content: Will it be a KNX cable maybe?

author: james_s
author_rank: Super Contributor
timestamp: December 06, 2021, 06:26:37 am
content: 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.

author: rsjsouza
author_rank: Super Contributor
timestamp: December 06, 2021, 03:05:06 pm
content: Quote from: RRRoamer on December 06, 2021, 03:07:26 amHere at my house they ran the two control wires on a shielded cable and the power is simply a pair of wires twisted around the outer jack of the shielded cable. I am on the fence to decide if this is the best solution or not, but it works well. I had no saying as it was already installed when I moved here a few months ago.

author: bdunham7
author_rank: Super Contributor
timestamp: December 06, 2021, 06:35:01 pm
content: Quote from: james_s on December 06, 2021, 06:26:37 amIf you're lucky. Do you feel lucky?QuoteThat seems to be true for a number of household systems, like HVAC and pool systems, where the choices seem to be expensive proprietary systems that require you to buy everything from one source or running a few more wires and settling for a bit more basic functionality. I also have a Venstar thermostat that does multi-stage operation and remote control and all of that stuff well enough. It just requires a few more wires but will work with almost any basic furnace or condenser. That way 10 years from now when I need to replace one of those I can chose whatever product I like.

author: themadhippy
author_rank: Super Contributor
timestamp: December 06, 2021, 06:42:25 pm
content: sounds like a job for scroller cable

author: james_s
author_rank: Super Contributor
timestamp: December 06, 2021, 06:45:36 pm
content: Overall I've been pretty happy with the Venstar, it supports my dual fuel system out of the box without requiring an additional fossil fuel board in the furnace. It does have one serious flaw that I find very annoying though, it supports only ONE scheduled program and the mode (heat/cool/auto) is set by the schedule so it is impossible to program it to keep the house the same temperature summer and winter. My old basic programmable thermostat had a hardware switch for heat/cool/off and separate schedules for heating and cooling and that particular aspect was superior. I'd simply flip it from heat to cool in the summer and back to heat in the fall. I never want my heat to come on in the summer even if it's chilly in the morning and I never want my AC to come on in the winter, even if I build a fire in the wood stove and the room gets toasty. What I've ended up doing is just turning off the schedule in the summer and setting cooling like an old fashioned manual thermostat but it's a shame because it's *almost* perfect and could be fixed by firmware that allowed selecting multiple programs. It could even store programs on the SD card it supports, which only seems to be used for loading photos onto it which is something I don't need. In a pinch they could even fix it all on the mobile app, allowing multiple programs to be saved and select which one is loaded onto it.

author: AaronD
author_rank: Frequent Contributor
timestamp: December 06, 2021, 07:22:37 pm
content: Quote from: RRRoamer on December 06, 2021, 03:07:26 amFor reference, PoE (Power over Ethernet) uses 48VDC on 4 unshielded twisted pairs. Not RS485, but it's enough to make me wonder if the two industries are even talking to each other. (probably not, because this kind of "professional isolation" seems to be universal, with a mix of innovative and brain-dead results)So, what if you got a shielded Cat-5 or better (dirt cheap), used one pair for comms, one pair for spare, and two pairs for power per the PoE pinout? 24V instead of 48V, but otherwise the same. The power would be inside the shield next to the comms, as it's just one shield around the whole thing, but maybe that's okay?