Nest issues with dual systems
I have an AC for cooling and boiler for heating. The boiler is controlled by a Taco relay, model SR506. My current setup is as follows:
Rc, G, Y, C all connected from the AC handler to the matching Nest terminals. Rh, W connected to the Taco relay. When installing, the Nest warned me of error n72 - "Power wire Rh detected. No connection to the equipment". Despite this error, both cooling and heating control seems to be working, so I ignored it and moved on.
Unfortunately, I recently discovered that this setup suffers from "phantom" heating - the zone will intermittently activate on the Taco despite the Nest not calling for heat. At one point this heated the house to nearly 90 degrees.
After some more research, I decided to plug one of the "Power Stealing Thermostat Resistor" (SRTR-001RP) into the R and W terminals in my Taco and I'm crossing fingers that'll solve it. I'm still seeing error n72 if I go to the equipment menu in the thermostat, so that makes me skepitcal. Also, I don't understand why the thermostat would try to "power steal" from the Taco if it's getting a C wire from AC handler?
I've also come across a few posts that mention using a relay. I have multiple zones so it would be a wiring nightmare at the heating board and frankly I just don't understand why it's needed. My very naive understanding of how the system should work is that the Rc and C wire power the Thermostat, and then it closes the loop between Rh and W to turn on heat. In practice, it seems something far more complex is happening which I don't understand.
Comments
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Try switching R and W at the Taco panel and removing the thermistor.
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You need relays to make it work like it or not. The Nest is telling you the heating and cooling is not separated. It needs to be separated by relays.
I have grown tired of trying to explain this so maybe @EdTheHeaterMan will have more patience.
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I'm pretty sure the Nest take is power from the Rw side. So if there no C on the heat site, it will steal power through the W terminal. This is probably why it turned your heat on. I won't do this until its internally battery runs out so that is why it worked for a while.
Taco relays come with a set of load resistors you can install specifically for this case. Install this between the W and C terminal inside the relay module. This will provide extra power for the Nest to steal without turning the circulators on.
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Honestly my opinion is you should just replace the Nest thermostat. In my experience they are terrible for dual fuel systems and cause nothing but problems. A Honeywell or Ecobee thermostat is a much better choice.
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The relay module only has a single COM, and every zone only has R and W, no per-zone C.
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Thanks. Is there a post you can reference where you previously explained this?
Would using the Nest C Wire adapter also solve this problem?
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Google NEST issues
Then replace it.
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electronics is a mystery to me. I'm an analogue guy stuck in a digital world.
That said, in the world of electronics there are switch contacts that can be opened and closed by manual handles, temperature adjusting bi-metal, or relay coils that click and mercury switches that are silent. I can see that stuff in my minds eye. You can imagine it too. if not you can actually look at it on any Frankenstein movie. The big switch handle makes the switch close and the electricity can move thru it to the next thing it is to operate.
The nest does not have any actual contacts that open and close. Someone told me what it was called one time but I can't remember it right now. But in my minds eye I think of it as a virtual switch that opens and closes, when in reality it is some electronic device that somehow becomes less resistance when a computer chip says to act like a closed set of contacts and then become magically more resistance when that computer chip days act like an open switch.
The bottom line is that the Nest is not what your logical mind believes it should be doing. With this information in hand, every once in a while, we present the Nest with a puzzle that it can not solve and those virtual switch things get confused and are half way between an open set of contacts and a closed set of contacts. Not really closed enough to turn something on, and not really open enough to turn things off. This never happens when you connect it to a system that the Nest programers designed it for, like a gas furnace with central Air Conditioning, or a Heat Pump with electric resistance AUX heat.
You happen to present a unusual system for the nest to figure out and if you don't talk the Nest Logic Language, then you may get a translation error from what a analogue thermostat with ctual contacts might be saying a and that of what the Nest is saying. that is how you get 90°+ when you are set on 70°
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Wire like this. I think the resistor is 1kOhm 1/2W. You can get it from Digikey if you don't have any. If there are multiple Nest thermostats for each zone, you need a seperate resistor form the C terminal to the W terminal of each one.
If the Nest is still turning on the zone, add another resistor in parallel to increase the amount of power it can steal.
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simply put NEST steals power. Power stealing was fine in the analog world but not when computers are in the circuits.
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The Taco resistors won't work with Nest t-stats. Nest suggests using a 220ohm and up to 5-watt resistors.
If your SR506 has a metal cover, make sure the zones that are using a com wire your red wire needs to be on the right T and white wire on the left T.
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Tell us how you really feel. the Damn NEST is nothing but a thief!
anyway… You can fake out the NEST by adding a heat relay to the air handler. the heat relay has real analogue contacts that the SR500 relays can understand an you only need to use the Rc contacts.
One of my favorite wiring diagrams comes from @EBEBRATT-Ed
How to wire a RIB U1C relay to be used as an isolation circuit. It is generic and shows a thermostat that requires a C terminal connection, and the NO contacts of the relay to operate the boiler, but the NO contacts from the Yellow and Orange wire can operate a zone valve, or furnace, or switching relay, or what ever needs operating that the direct connection is having problems with.One of my favorite wiring diagrams comes from @EBEBRATT-Ed
How to wire a RIB U1C relay to be used as an isolation circuit. It is generic and shows a thermostat that requires a C terminal connection, and the NO contacts of the relay to operate the boiler, but the NO contacts from the Yellow and Orange wire can operate a zone valve, or furnace, or switching relay, or what ever needs operating that the direct connection is having problems with.
To make this diagram work for you, just add it to the air handler like this
If there is no wire from the air handler to the zone control relay and it is not easily done, you can run it thru using an extra wire from the Air handler to the thermostat then from the thermostat to the one control relay using one extra wire from the thermostat and the wires from the thermostat to the zone control, put the relay at the zone control.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan I don't think you really want @pecmsg or I to tell you how we really feel about the Nest thermostats. This probably is not the appropriate venue for that type of language
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So which resistors would work? It sounds like the relay might actually be easier given that I only have a single COM on the Taco but multiple Nest thermostats, one per zone...
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That is interesting…. you have the older SR 500 relay? Metal Cover?
How many wet heat zones?
How many zones with both wet heat and AC?
I can draw you a complete system wiring diagram that you can post in your air handler(s) and at the Taco Control.
This is the modified diagram for one air handler where you cant get a wire from the air handler to the boiler room.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Nest suggests for their t-stats a 220ohm up to 5watt resistor. If you have more than 2 Nest t-stats, you'll need an external transformer.
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Just to confirm - if I just swap out the Nests for Ecobees with the same wiring, my problem goes away?
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I have a SR506 newish I think with plastic cover. Here are the photos. Zones 2,3 are Nest thermostats.
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But they are separated though. How does the relay separate them? The diagram I've seen looks like it merges them.
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I'm confused. There is no jumper from Rc to Rh on the thermostats. Or are you referring to the Taco?
Also, wouldn't removing the HVAC C wire create an opposite problem where the Nest won't detect the Rc instead?
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Edit: Looks like Nest changed design since I used one. The only option is Ed's suggestion with the relay now.
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You should be ok with two Nest if connecting to the Taco control. On any SR control, anymore that 2 Nest will require an external transformer.
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Are you saying the presence of the C wire currently is doing nothing? That is, it's connected but Nest won't pull any power from it?
Here's my current wiring, btw.
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See my correction earlier. Nest changed the design since I wired one up, so only option now is what @EdTheHeaterMan suggests with a relay to trigger the SR.
This avoids any of dual transformer issues that they don't like. I can see why installers hate them, way too cranky.
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Yes you can trick it MOST times but not always. The computers decide.
sorry but
NESTSUCKS
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Do I need a relay for every thermostat?
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Although the Nest has RH & RC terminals, and you can have separate heat and cool hot wires to the Nest I believe (and this is just a guess) that they (RH & RC) connected internally somehow.
Why do I suspect this? My nephew installed a Nest for a 2 wire heat system and he used RC & W to run the boiler and hooked it to TT on an oil burner cad cell relay.
It worked fine until the battery went dead and I installed a separate relay and transformer and moved the hot wire on the Nest from RC to RH.
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Will an Ecobee work with my existing setup and wiring, no relay needed?
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Wouldn't it be simpler to power the relay via the 24V on the Taco?
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@backshift_dividers Said: Wouldn't it be simpler to power the relay via the 24V on the Taco?
NO. the 24V from the Taco is Limited to 15 VA I believe. and you want to save that power for the other zones. The Air Handler has a 40 VA transformer (more powerful) with extra amperage for additional loads like the computer in the NEST.
@backshift_dividers Said: Will an Ecobee work with my existing setup and wiring, no relay needed?
NO you need the C wire for either thermostat. This is your best way to do it.
Probably not. Only the Air Handler thermostats. How many zones do you have that are heat only and have Nest thermostats?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I did not get the information about how many heat only thermostats you have with NEST thermostats and how many zones with Air Handlers and NEST thermostats. So this illustration shows
- One air handler with hot water zone that has a nest thermostat
- One hot water only zone with a NEST
- Two hot water only with thermostats that do not require a C wire.
- The Air handler transformer operates the relay for the NEAT that is both heating and cooling, using a relay that you need to add to the air handler
- The other NEST thermostat needs no relay and can use the Com from the Taco SR506
- There is an additional one that can use the SR506 for a Com to C terminal in the future.
- Do not use more that 2 smart thermostats that require a C terminal on that SR506, because you will over tax the transformer if all zones call for heat at the same time.
That would be a total of two NEST thermostats.
The Air handler thermostat does not need the Rh terminal connected to anything so the E72 error will vanish.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Thanks Ed!
A few follow ups:
- Regarding the Ecobee - I was referring to keeping my existing wiring, with the Rc, Y1, G and C all coming from the air handler, and the Rh, W coming from the Taco. This is my current setup that the Nest apparently can't properly support without a relay. If my understanding is correct, Ecobee supports this setup without a relay. Is my understand correct? Their documentation seems to suggest this as well.
- Isn't "power stealing" from the air handler's transformer to power the relay also problematic? Since it's now powering both the thermostat and the relay. I guess the relay uses very little power though.
- I actually have 2 air/heat Nest thermostats (both have C from the air handler), and one heat only. So if I understand correctly, I need 2 relays and the heat-only Nest I can power from the 506 COM.
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You can try the Ecobee for minimum of $150.00 Each on http://www.amazon.com Or you can try the relay for under $20.00 each.
@backshift_dividers Said: Isn't "power stealing" from the air handler's transformer to power the relay also problematic?
Actually that is not power stealing anything from the air handler. (power stealing refers to charging the battery on a device when if is not calling for heat or cool). That air handler has a transformer that is powerful enough to operate the load of the outdoor unit contactor, reversing valve, defrost board and the fan relay on the air handler along with the circuit board (if any) and up to 4 stages of electric heat relays. (that's 10 loads)
Your set up is only supplying 24 volts to the outdoor contactor, the cooling fan relay. I think there is enough power left for the RIB U1C (Heat) relay. (that is only 3 loads)
Earlier you asked how the relay "ISOLATES" the two systems. you seem to think ot merges them. HERE is the isolation line
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I've got tons of thermostats out there using separate Rc and Rh without needing extra relays and transformers. None of them are Nest however. One is Ecobee.
I have removed enough Nests to fill a 5 gallon bucket because of W1 switching issues. Usually go to a Honeywell 9000 and W1 issues dissappear.
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One thing I don't understand - are dual transformer setups THAT uncommon? How is this setup not supported by Nest? Isn't it one of the top selling smart thermostats? There must be both an Rc and Rh terminal on it for a reason. Otherwise why have both if only one is supported?
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The Nest is a top selling thermostat for one very simple reason. The majority of houses have forced hot air heat with an air conditioner, or have a heat pump. They were installed in the last 20 to 30 years, and they have at least 4 (often 6 or 8) wires from the heating system to the thermostat. The Nest was designed to control those systems, and it actually works pretty well for them.
The Nest was never designed nor intended for systems — of any kind — which require only a simple circuit closure from the thermostat to turn on the heating system or the air conditioner.
The problems we see here come from trying to use the Nest on those systems. Bluntly, the Nest is the wrong tool for the job, but as in most situations where one has to use the wrong tool, there's usually a way to make it work.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
Are there any wifi thermostats that use relays? I'm not opposed to replacing the Nest. The Ecobee looks great and well supported but I believe it's also not using relays. Coming from the software world I have similar views on companies like Honeywell which may have the know-how to build thermostats but their software is garbage and I don't trust it to not have a crap ton of exploits.
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I took a quick look at the circuit board pictures on-line. There doesn't seem to be any local isolated power supply between Rc and Rh, so when used on dual transfomer setup as you have it, it still needs to steel some power from the heat side to run the electronics there.
This is not the full power needed to run the thermostat, it gets that from your AC. This is just a very small amount for the local smarts. I think my original suggestion of a resistor inside the SR to W of each Nest might fix your issue.
P.S. If this is a modcon, your reset curve needs to be adjusted. A properly setup system should only be able to heat the house up to 77F to 80F at the most.
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There is a fundamental problem with the resistor trick. You have no, or very little, knowledge of the rest of the circuitry to which the Nest (or other power stealing thermostat) is attached. Nor do you have any control over it.
These thermostats — particularly the WiFi connected ones — need a surprising amount of power to operate. That power is intended to be supplied by a steady 24 volt AC supply between Rh or Rc or both to C. If C is not connected, the internal circuity allows the thermostat to be powered through W and whatever W is connected to. Now when the thermostat switch is "on" it connects Rh (let's just talk heat) directly to W. This shorts out the power to the thermostat. There is an internal battery or supercapacitor in the thermostat to keep the electronics alive while it is calling — which is just dandy if the call is short, but if the call is long, such as coming out of a setback or in extreme weather, that battery dies and the thermostat stops operating. So does the heat.
If there is a C connection back to the power supply, the above doesn't happen. Rh and W are still connected, but — just like an old-fashioned thermostat — all that does is complete the circuit on the controlled device, allowing it to operate.
Now various controlled devices may be able to operate with something less than a completely shorted signal. On these, the various resistor kludges which have been suggested may allow enough voltage difference to remain to keep the thermostat alive — at least for a while.
For reliable operation, however, there is no substitute for a power supply connected to Rh and C, and for optimum independence from variations in the controlled device there is no good substitute for a relay with the coil connected between the W and C, which will thus be powered by the thermostat's power supply. The normally open contacts of that relay become the signal contacts to the controlled device.
I know this sounds clumsy and old fashioned to the computer types, and there is some merit to the notion that everything should be solid state and programmable (except that longevity and reliability is questionable) — but the problem is "everything". And you don't have that luxury with a heating system.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
What's a modcon? i didn't realize the system has other limits in place. I thought that as long as the Taco relay is closed it'll continue to run the pumps which will continue to generate heat...
Also, for the resistor trick - I did get a pack of those with the Taco but my zones don't have individual C terminals. There's only one shared COM from what I can tell. So I can't even connect the resistor between the W and COM since it's too short. I guess I could use an 18 gauge wire to extend it?
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