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Nickdalzell1 · 07-20-2016, 07:08 PM

Yet another mod for my Behemoth! I've got this place so much like a house even the local campers are amazed the difference between the outside and inside!

this time a Nest Learning Thermostat. I, being a techno geek kept passing by the display and drooling wanting one but I knew it would be impossible seeing as my Dometic rooftop A/C (the only RV appliance left in this place) has no C wire. Only a couple of unused (white, blue) and just Y, G and Rc on the cooling bundle.

But after a Google+ thread saying that the 3rd Generation Nest can work without a C wire I figured it might be time to bite the bullet. Wiring it in seemed easy enough, but once I connected the thermostat and switched on the 30A, things went downhill fast.

Compressor immediately kicked on, but no fan. Thermo powered up, compressor freeze switch kicks compressor off, thermo then complains about Y not having power. Shuts down due to low battery before I can finish setup.

Tried so many combinations on Friday evening and gave up after I saw it was 4 AM (with same results, compressor tries to run without the fan until it shuts down and either trips the breaker or the thermostat complains. Google was no help, saddened my new gadget (which was a cool looking thing reminding me of a modern take on the 1970s Honeywell from days gone by, and knowing how convenient it would be to control my A/C from work if it got hot suddenly) was bordering on not being compatible I went to bed, as I had an event to go to at 8 Saturday morning and I needed sleep.

Saturday afternoon I figured my issue might be something called 'power stealing'. Nest was trying to pulse the Y to get power, but with the bad side effect of the compressor running without the fan. So now a new challenge was needing to run the damn C wire. I know enough about electricity to know it was requiring a 24V neutral and went to Lowe's to hack a means to make that possible.

They sadly don't carry HVAC transformers (if they do I wasn't able to locate any) so I got a 24V doorbell transformer for $14 to attempt this hack.

The idea is that since the thermostat bundle is not doing anything for the furnace (gutted long ago as part of the conversion from 12v to 120V) there isn't a heating transformer so nothing going to RH the other power connection. My plan was to rig the doorbell transformer to have mains powering it from a spare outlet that some genius figured was a good idea to place in the cabinet in the kitchen, right where the old Dometic thermostat was mounted. Then run the 24V hot output to the RH terminal, and 24V neutral to C. At first this hack did absolutely nothing but fry the trsnsformer.

Got another and realized my mistake. I had actually mistakened 'load' marked on the middle terminal of the transformer as neutral. Since this was a tri-volt gadget what happened was I had hooked 24V hot to 16V neutral and shorted the poor thing.

The two outer terminals were the 24V leads. So I wired transformer number 2 to that. Switched on power, first a loud buzz from the roof unit that spooked me, but then normal operation. Thermostat went to setup, set up wifi, set the default cooling mode and both the fan and compressor kicked on properly. After one all nighter and almost another day, success!

It is a nifty thing and far more pleasing to look at (and ten times more accurate) than the original.

Englishman · 07-23-2016, 05:50 PM

Nice work

Nickdalzell1 · 07-23-2016, 08:19 PM

It is, but I think the idea of a '30 minutes for most people' as listed on the back of the box was not intended for finicky RV roof Air units. Expect 8 hours and a few beers to make a Nest work in a TT. Especially one as old as mine is. The unit is over 20 years old, the A/C actually working is amazing enough. Also got two Nest Protect smoke/Co detectors to replace the dinosaur it had since 1995 that was getting false alarm happy.

Mynticelx · 07-27-2016, 02:05 PM

So two questions on this -
First, does the fan continue to run even though the A/C is not running? I like that feature with my current Dometic thermostat.

Second, do you have a more detail wiring diagram for this install? I might be interested in attempting the same install. I like the idea of wifi controlled and read thermostats.

jharrell · 07-27-2016, 03:09 PM

Surprised it works, I thought the relays in a RV rooftop air conditioner where designed for 12vdc and it sounds like your feeding them 24vac. Am I reading that correctly?

Normally a relay designed for DC will chatter and buzz if fed AC.

Nickdalzell1 · 07-30-2016, 05:51 AM

Actually the A/C relays are 12v but I'm falsifying the 24VAC to power the thermostat via a 24V doorbell transformer tied to the furnace circuit of the thermostat (RH and C) so it doesn't touch the A/C. The thermostat will properly control the 12v side for the A/C because all it does is bridge Y and Rc (it doesn't send power to those.) You can actually touch the Y and Rc wires together and everything powers up. Keep in mind that my Behemoth as I affectionately refer to my oversized overworked TT is a 1995 Sportsmen's and things might have changed in 21 years.

All I had to do was run a C wire and alternative 24V source to power Nest otherwise odd things kept happening and it complained about wiring problems.

I got a tri-volt transformer for a doorbell at Lowe's ($14) and some wire, 14AWG was good enough. I connected the 120V wires of that to a extension cord, and there was a power outlet inside the cabinet which the thermostat mounted there anyway, so I plugged the extension cord there when I was done. I ran the 24VAC wires to the thermostat RH and C (this would be the outer two terminals of the transformer output, doesn't matter which two are used for neutral and hot it seems, keep in mind I don't have the propane furnace anymore so if you use yours this trick could fry your furnace relay if you connect those wires) and plugged the transformer in. When you connect Nest to its base it should power up and go to setup, detect the roof unit and go through a series of tests.

Yes, you can run the fan all the time by setting it up that way via the app, or even set a schedule to run it at specific times. It will cycle the compressor as needed when calling for cooling

As for wiring itself, this can vary. On the bundle coming from the roof unit were five wires, only three seemed to be used however.

Yellow = Y, relay for compressor

Orange = G, relay for fan

White = unused

Blue = unused

Red = R, power from 12v relay, I connected to Rc terminal

jharrell · 07-30-2016, 07:45 AM

I missed that you left the 12v connected to Rc, thought you jumped it with Rh. Normally a RV thermostat just has a single red at 12v going in along with common(negative) and then the various heating/cooling leads going out.

Would love to do something like a Nest in mine, but I still have the 12v furnace so need Rh to be 12v. I have even thought about doing a secondary relay system where the thermostat runs 24vac and it's outputs just drive 24vac relays that switch the 12v to the a/c furnace relays.

Surprising to me how primitive RV thermostats are, I mean my 2016 came with a mechanical one!

Does look like Dometic is coming out with a blue tooth one with an app, wonder if Coleman will do something more advanced:
CT Thermostat - Dometic Corp

Nickdalzell1 · 07-31-2016, 08:22 AM

Mine had the Duo Therm comfort control center with the temp lever which thought a setting for 68 was actually the setting for 82. It promptly got replaced with a digital at first, then later a Go Control that pairs to Lowe's Iris to be controlled via smartphone like Nest, but is dependent on a hub which tended to drop the internet too often to be useful, which is one reason I liked Nest plus Nest can learn and looks nice compared to the utilitarian white.

You can still use a Honeywell smart wifi thermostat in your case because it uses AA batteries and works just the same as the stock otherwise. But I never liked the furnace and gas scares me and is expensive so I made everything all electric long ago. Thermostats like Nest and EcoBee like 24V to work properly.

My place is just a static trailer, it never moves. It's just cheap living for myself and my animal friends. So needing propane or 12v lights is pointless in my case, and limits my ability to customize.

Seeing as the CT Thermostat is Bluetooth it doesn't seem capable of being controlled when away from your trailer since Bluetooth only has a range of 15 feet, and using an app inside the coach seems lazy when the control is literally two feet away. That's another bonus of Nest. It's much nicer to ask my Amazon echo to turn the temp up or down than pull my phone and open an app. The app is nice when I'm at work and there's a sudden heat spell though.

jharrell · 07-31-2016, 08:50 AM

Yeh I have seen the upgrades with the thermostats that run off batteries, no thanks I don't want any more batteries to replace in my life or have my AC/furnace dependent on some AA's. Thought about maybe trying to buck down 12v to whatever the thermostat wants for battery input.

Its also not just 24v, that easy to get a boost converter for 12 to 24v its that it wants 24 AC rather than DC, need a inverter for that, they do make them, not very cheap.

I do have a Magnum 3000 hybrid inverter that runs the entire RV, will even run the AC for a little while on batteries, but don't want to have to have it on just to do 12vdc->120vac->24vac.

Funny thing I am sure the Nest just rectifies and bucks internally probably down to 5v, would probably be cheaper for them to make a 12v version for RVs, but the market is just too small I guess.

I will probably upgrade to a Coleman digital for about $50 at least to get more accurate temp control. Thought about trying to upgrade to their zone system thats RS-485 and figure out the protocol myself for remote monitoring and control.