Ecobee3 Review (9 units, net-zero building)

[Denwood]

I've referred a few times on the journal to a project building we took on 4 years back. The idea was to take an existing 9500 sq/ft warehouse, gut it, and turn it into a net-zero(ish) building. Phase 1 was the most important, establishing a tight, very well insulated envelope. Temperatures here hit -35C in the winter, and commercial buildings much smaller than ours can cost $2000/month to heat. Last February with an average temp of -20C (-4F) our building used $228 of natural gas to heat. On days where the temp is above 5C (41F) the building requires no added heat due to the insulation level, passive solar, and heat added by my staff/computers during the day. We did solar modeling to optimize window placement, added a passive cooling clerestory, installed overhead light sensors (almost no light switches in the building) etc. Our electrical use is pretty steady at $350/month in the winter, $450/month in the summer when AC is active. This is approximately half of what a comparable commercial building would use.

The main floor radiant over-pour. Existing slab was insulated with 2" EPS, thermal reflective, and 2" concrete added.

The mechanical budget was not trivial for the retrofit, including insulating the existing main floor slab, and over-pouring with 4 zone radiant. We also installed 5 air handlers, two HRV units, and one mini-split to handle heating upper floors, cooling and fresh air circulation. The wall system is a "furred airspace assembly) which has a thermal break and radiant reflective air space component. Walls are approx R45, ceiling R80. Dual redundant Triangle Tube on-demand units provide heat via hot water to both the floor and air handlers at something like 98% efficiency. There are nine thermostats, none of them smart, something I'm finally addressing.

I've been watching the industry for the last few years, and zero'd in on Ecobee as their thermostats do all the wifi bells and whistles, but also have a web portal (free) which provides usage reports/graphs etc. Unlike Nest, Ecobee tracks local weather and can intelligently handle set-back times and recovery as it learns your building. With the Ecobee3 integrated wireless temp/occupancy sensors, automation integration (Wink, Smarthings, IFTTT, etc.) it was time to pull the trigger. Best Buy (Canada) actually had three units in stock locally, and give me a nice discount on nine. Installation was very straightforward as I have "C" wires to the old stats. Everything you need, including a PEK wiring kit (addes a virtual C wire if you need it), larger base plate cover (if you need it), screws, etc. is in the box.

You can pair up to 32 of these remote occupancy/temp sensors with each Ecobee3 thermostat. The sensors use the same battery as computer CMOS, and last for four years. If the battery needs replacing, the Ecobee3 will alert you.

In this climate, the highest possible set back ,for the longest time results in maximum savings. This study using twin R2000 homes exposed to identical conditions showed savings as high as 17% on the coldest days with a 62F set back.

http://www.healthyheating.com/downlo.../nrcc48361.pdf

My goal is to minimize both electrical and gas use by conditioning space only when necessary, and eliminate staff/tenants doing silly things.

I've installed 3 of the 9 units so far (covering our upper floors) and so far have been quite impressed with the product. I'll post more here as the other six are installed. The Ecobee3 stats have some key features that will enable them to cover our four zones of radiant, five air handlers, and space that varies from a film studio to tenant rental spaces:

1. Programming is done on the unit, via the free web portal, or via the free smartphone app. You can also view indivual sensor temp and occupied status from any of the three sources. As you add Ecobee3 units to the web portal, they show up with basic info. Selecting them allows you to program, view usage graphs, sensor data etc. You can group your thermostats and decide what characteristics are shared. This lets you program a holiday on one Ecobee, and have it affect all nine (in my case) in the building. You can also toggle sharing of schedules, HVAC modes, alerts, (and more) when you group thermostats.

There are quite few options in the web portal, but here is a basic view of one of the schedules.

and sensor data/options:

2. Ecobee's web portal allows you to manage the thermostat, but also offers up "Home IQ" which is a series of reports to monitor your system, view motion events, etc. I'll need to wait for 3 weeks until data is gathered to allow viewing runtime, community comparisons & home efficiency reports. However, it immediatly starts charting quite a few parameters. The raw data can be downloaded for any period in .csv format.

3. The Ecobee stats are weather aware, so correlate learned recovery rates for a building/zone with the weather forecast. Ecobee has been doing this for a number of years and is in my opinion, the best implementations of smart stats I've seen. This smart recovery or cooling behaviour can be turned on or off. This is the plot of this morning (day 2) and you can see the thermostat is "learning" about the building's zone recovery. It was set to recover from 15C to 20C, at 9am. You can see it anticipated a longer recovery than was required (outside temps, -20C), bumping the temp at 5am, 6am, 7:30am, then a push at 8:15am. It will be interesting to note the behaviour change as data is acquired over the next week or two. My expectation is that the recovery will tighten up a lot.

4. You can pair up to 32 remote temp/occupancy sensors with each Ecobee3 thermostat to bias temperature in occupied zones, and intelligently activate a set back if no occupancy is sensed. One of our building zones (serviced by an air handler) has a common area where the thermostat is located, and two closed rooms occupied by the tenants. The business tenants don't have a regular shedule, so previously the "dumb" stat was set to condition this space 9-5. With the Ecobee3 and a wireless temp/occupancy sensor in each space, this air handler will only run if the tenants are present, and turn off after they leave. If they show up, the sensor in their room detects occupancy, and the ecobee3 switches into "Home" mode where the temp is set up to 68F, otherwise, the space is kept at 15C. Essentially the tenant behaviour will determine the schedule. Because you can specify which sensors are active during different comfort levels (including the thermostat itself), the system will bias temps towards the occupied space if you set it up that way. If no occupancy is detected over a 2 hr window, the system will revert back to "Away" temps and settings. With these options, if one tenant is having a meeting (generating more heat) then the Ecobee3 will see this and stop heating the space.

I had a real struggle to sort this behaviour out, with several calls to tech support, but learned a lot about how the "Smart Home/Away" and "Follow Me" modes work. This is the "Follow Me" sensor data from earlier this evening. You can the system was in "Away" mode at 7pm, but Tenant1 sensor detected occupancy, and "Smart Home" mode (the blue bar at 7pm at top of chart) was activated. The sensor shows about 16.5C when the tenant arrived, and about 10-15 minutes the data shows a 20C average. Because no occupancy was detected at the Ecobee thermostat, or Tenant2 space, the temp would have been biased to Tenant 1. This is good.

5. You can lock down the thermostats completely with a pass-code, or alternatively choose a few options in terms of what's allowed. I also really like that you can define allowable cooling and heating values, and lock these down as well. This ensures that kids/tenants (wife!) can only adjust temps up or down as you define. I had a staff member who was feeling cold this week adjust the radiant heat up 2 degrees on a 4500 sq/ft slab on the basic stat controlling it. No idea how long it was set that way..but this won't happen with the Ecobee3


Ecobee3 support in the automation world is still in my opinion a work in progress, but Wink, SmartThings, Homeseer, and Vera can all control the thermostats. I also tried out IFTTT.com (If This, Then That) a free web-based service which allows very simple automation tasks to be performed. I tried a few "recipes" with mixed results, but managed to get iPhone notifactions of "Smart Home/Away" events, and also had the Ecobee3 stats logging the same events in a Google doc. Given that Ecobee's site already does a great job of keeping data for you, there is not much point in this task, however it illustrates the possibities for simple automation. It only took about 5 minutes to get IFFTT.com working as a newbie.

I'll update this as we make our way through the installation.

 

[dfiler2]

This is some really good info. I am looking at putting wifi thermostats in some rental properties. The one building has 4 apartments and is connected to my shop. Last year I put a Honeywell in my home and my shop but was disappointed that there was no data logging available. These apartments are heated with off-peak electric and I have the heat figured into the rent, I think I could make these pay for themselves pretty quickly. I have been looking at the Ecobee but was having a hard time figuring exactly how they would work, I think your post here will answer my questions, Thanks

 

[Denwood]

You're welcome. The reviews I read online didn't get too "under the hood" so I figured at least a few folks would appreciate this review

I see you're in MN, so have the same concerns on efficiency. We have the same issue with on/off peak hydro, so part of the thinking here is to keep equipment use to a minimum where possible during those times. Being able to lock them down, while allowing some tenant adjustment is something I was looking for.

My intention is to integrate window sensors so that opening a window will turn off the AC to that zone. It's an issue..and I want to encourage passive cooling as this was designed into the space. AC adds about $100/month of electricity to our consumption over the cooling months.

 

[Denwood]

I've had to do some prep to sort C-wires for the main floor radiant thermostats as they are simple two wire units right now. The main floor has four radiant zones (about 4800 sq/ft), two air handlers = six more Ecobee3 stats. This is the equipment room, using redundant (they swap chores every day) 98% efficient Triangle Tube units. This room supplies the five air handlers with hot water, as well as the radiant floor. It's set up (at my request) so that there is very little power used, unless one of the zones calls for heat..so no constant running heating loop. The conventional heated loop is a monstrous waste of both BTU's and power in an efficient building.

I've been exploring the automation options to manage the building better for night cooling, HRV control etc. The Ecobee3 thermostats can operate indefinitely (and be programmed) completely without Wifi or a network, but when allowed to communicate with Ecobee's site will log data, auto-update new firmware, and allow "Cloud" based automation.

Once you authorize a service to access your Ecobee3's data channel from Ecobee.com, it becomes possible to do some neat stuff, with zero hardware expense. If you want to manage things further, turn "dumb" devices on/off, then you may need to purchase a hub and install that locally. There are automation hubs being introduced by nearly everybody, but quite frankly the entire automation industry is only getting more confusing, not less. Apple has their HomeKit, Google introduced Brillo, Samsung has Smarthings, HD sells WINK, and older solutions like HomeSeer, and Vera continue to offer more advanced solutions controlling Zwave devices. Another upstart is IFTTT.com which essentially connects devices via cloud (web based) channels to allow all kinds of simple automation programming. I tried the free WINK app (their hub does all processing in the cloud, so you don't really need to buy it for this application), and also IFTTT to get a taste for what they could do.

To use IFTTT, you set up a free account, and install the app on your phone if you want notifications to pop up. Simple recipes can be created in seconds integrating all kinds of hardware. You could for example put your Ecobee3 into Away mode if your phone location indicates you've left. These simple IFTTT recipes have worked very well, notifying me primarily if any of the thermostats have activated Smart Home/Away mode. This is very handy when setting up your sensor locations.

WINK offers a bit more in terms of automation logic, and is handy to summarize sensor status, and overall activity (of all of your stats). This is what the WINK app looks like on my iphone before you do any digging. WINK has only one control interface...iOS or Android, so definitely caters to those looking for simple automation.

This is the WINK activity summary, a nice compliment to Ecobee's data logs.

A simple Robot created in WINK (tested and it works!) to turn the air handler circulation fans on when solar gain in the south facing R&D area raises this area above its set point. This allows the South side warm area to pass off some heat to the North side of the building. Because all of the Ecobee thermostats log data, you can sort out how effective (or not) these strategies are.

Once the final six Ecobee3 units show up, we'll see how it all ties together.

 

[Denwood]

Here's a graph of this morning's Smart Recovery vs a few days back. It was warmer last night, but it looks like the thermostat is tightening up the recovery.

January 22nd:

January 26th:

 

[mooseracing]

We have a few Ecobees, I could see them ok for home use but not real impressed commercially. We are using multiple Ecobee's plus the extra sensors, their software doesn't seem to handle and manage the multiple t-stats together. It would be nice if they knew what each other were doing.

We are looking at more commercial software now, but the entry price is ~13k for 12 Furnaces. The Ecobees were more of a first run to see if they could improve our situation and they did for some points such as individual management ( i didn't have to go to each t-stat).

 

[Denwood]

Moose, there's a few routes I would take if wanting to automate further.

Grouping thermostats on the Ecobee site, you can decide if you want to have them share schedules, just vacation days, modes, etc. So in that sense you can have them working together. Different parts of our building require different treatment, so basically each stat will be set up differently.

You can use the sensor data and have the stats a lot better co-ordinated doing the following..as what I'd consider the most reliable, mature solution.

1. Purchase HomeSeer's HS3 software, a controller and the Ecobee plugin.
2. This would allow you to do just about anything with the furnaces, and using z-wave automation relays etc, manage HRV's, ventilators etc. just fine. One of the to-do list items for me is use our DSC security panel to indicate open windows, and shut down AC to that area when they are closed. DSC has a z-wave module that would "expose" all the window/door sensors and make this possible.

It makes sense to let the stats manage the schedule, but use automation to over-ride their mode, set point, fan operation etc. One thing I like about the Ecobee3 setup is they are fully autonomous if the web is down, and can work just fine without a controller..so no single point of failure. It's kind of like the internet

That said, I totally get why you'd want to go with a commercial controller. I've been on the fence going Bacnet full control modules, vs the Ecobee route for the last 3 yrs. Ecobee3 with the remote sensors and occupancy logic tipped the scale for me. I figured around $15-20K to do a commercial controler with relays etc.

 

[Denwood]

The install is complete. I'm still sorting out many tweaks to bias power use to off-peak times while minimizing gas usage. I've found the extra temp sensors very useful in our loading bay, and two entry areas, which only need minimum heat. Using sensors, I can run the radiant in those zones only when temps fall below 8C in those particular areas, whereas before I was guessing.

The largest radiant zone, our main floor work area is about 3000 sq/ft. On the old stats, this main radiant zone has never been set back since new. On the first few days, the slab showed 1.5C overshoot around 1pm, exacerbated by heat from the equipment room, computers and the Cinevate crew. You can also see that the outside temp climbed from -25.5C at 9am, to -6.5 at 5pm. That rather sudden climb is not typical...and it's a real challenge for a radiant system in a very efficient space. We'll see how the Ecobee3 for that zone manages this over the next week.

Here is some information I've gleaned over the last week on the Ecobee3 sensor behavior that I found very useful in terms of managing HVAC behavior.

Smart/Home Away Behavior Summary

1. When the thermostat switches from a Home comfort level, to an Away level, motion for the 1st hour will be ignored, so any occupancy detection in that hour will not trigger Smart Home mode.
2. Occupancy will remain in effect 2 hours after no motion has been detected during a Home comfort level. So the system will only toggle Smart Away mode after two hours where motion has not been detected, regardless if the sensors are excluded from that comfort level.
3. For the purposes of Smart/Home occupancy detection, all sensors are used, regardless of which are turned on for a given comfort level. You can "disable" a sensor's ability to detect motion, including the Thermostat itself, by taping over the detection window. You can also pop a remote sensor cover, and rotate it to cover the detection window. In one of our installs, I wanted the Thermostat itself to ignore motion, and have Smart Home/Away only triggered by the remote sensors. You can't do this by excluding the sensor from comfort levels. I accomplished the same with a piece of black tape over the stat motion sensor.
4. If Smart Recovery is enabled, Ecobee appears to be averaging the sensors active for the next comfort setting but using the current set point for temperature. This results in unexpected behavior. @Greencat posted this workaround in this thread: http://www.smarthomehub.net/forums/discussion/760/peculiar-behavior-smart-recovery-bug#latest

Finally, I hadn't noticed this graph under HomeIQ options previous, but it's very useful to summarize system run time as it relates to outside temperature, day to day. You can see the main radiant zone called for heat for a total of ~1 hr and 45 min on January 29th, with an low average temp of around -10C outside.

 

[Denwood]

We're still dialing things in, but my team at Cinevate is happy, and comfortable.

I set up our Hydro account online today and right away saw some benefits of the Ecobee3 thermostats and aggressive set backs.

You can see last weekend we used 36 kWh of power over both days, where previously there was as much as 65 kWh used. Looking over the last 5 months of consumption, we've never hit 36 on any weekend, regardless of temp. This is 100% due to the fact that the weekend set backs are idling the mechanical HVAC equipment in the building. My next goal is reducing overall as well as on-peak consumption (17.5c/kWh) during the week. In the graphs, it's the portion of consumption in red. Off peak pricing is 8.3c/kWh.

I have a few CO2 wall mount sensors coming from co2sensors.com to manage our Venmar HVAC unit by demand instead of just guessing on a timer. This will up the efficiency numbers a few more points

 

[LS6 Tommy]

I love ecobees. They beat the snot out of the Nest system...

Tommy

 

[Denwood]

Tommy, I came to the same conclusion. The next step up IMO would be dedicated building control The thermostats themselves are quite powerful, but I'm finding the data logging across our nine zones is really helping to reduce consumption. The last few days I've been tweaking schedules and times to bias the system run time to off peak hydro periods.

 

[Denwood]

I figured I'd post the results of having one full month with the 9 Ecobee stats. We cut consumption over 25%. Hydro costs dropped about 8%. So with an average outside temp of -11C (12F), the well insulated 9000 sq/ft building took $156 to heat . A few changes were made from the previous month:

1. More aggressive setbacks across the board.
2. A 3C degree setback on the main radiant zone.
3. Lowered temps in the loading bay, now safely done with remote sensors.

So if anyone tells you setbacks don't work, show them this thread. Keep in mind that the old stats were programmable and had setbacks. Now though, the setbacks are larger and the Ecobee stats can manage zones automatically where activity is not detected.

Smart recovery is working very well and my team are happy to arrive with the building at 70F for the workday.

We've gone another step and added a Vera Edge automation box ($130) and an interface card to the alarm system. For our tenants, disarming their partition also immediately instructs the Ecobee in their zone to enter " "Home" mode. Conversely, arming one of the alarm partitions now sends an "AWAY" command to Ecobee stats in that zone. This should tighten things up. In summer for example, opening a window will turn off AC in that zone based on the alarm system "talking" to the Ecobee stats via the Vera Controller. We'll also be able to intelligently manage summer night cooling.

 

[Mr onetwo]

Good work, as always, Dennis!

 

[Denwood]

Thanks Mr. One It will be interesting to see how the use curve manifests itself as things warm up. I expect the transition heat/cool months will see higher savings yet.

 

[LS6 Tommy]

I'm a big ecobee fan. Blows away the Nest. I just wish I could use it with 3 zone valves without having to buy 3 of them. Kind of defeats the purpose.

Tommy

 

[theoldwizard1]

I must have missed this the first time around. Very impressive numbers !

I did not "plowed" through the whole thread, but have you tried to calculate your "break even" timing ?

 

[Denwood]

Tommy, I hear you on the zone valves. I ended up needing nine stats to cover everything. That said, because we have 2 air handlers in the same zone as the main radiant, I've been able to data log the radiant, set it back a few degrees and use the air handlers to bump air temps during the work day. This has totally addressed the afternoon temp bump when staff activity combined with stored heat in the slab was always raising temps a few degrees. Essentially we use the slightly cooler slab to moderate the temps now in the main work area. In the end, we're taking better advantage of BTUs from staff, computers etc.

I've also adding CO2 sensor control of our HRV which will almost certainly reduce hydro and heat loss.

Wizard, it's hard to predict the exact payback as we needed nine stats. When you look at the gas savings combined with hydro costs during cooling season (hydro is the larger cost!) the payback will likely be under 2 yrs. I'm doing an internal energy audit and finding plenty of room to save power. Arming the alarm system partitions for example is now cutting power to laser printers etc. I've already knocked 300 watts from 24/7 standby power to equipment doing nothing in off hours!

 

[Westbank]

I have the Ecobee Smart Thermostat and much prefer it over the Ecobee3. I have a central humidifier, central dehumidifier and an ERV. With the Ecobee Smart Thermostat I can control all these devices via the thermostat. With the Ecobee3 you can only control ONE device (Ecobee calls them "accessories"). So you can't have a humidifier and an HRV. You have to pick one. Really sad.

Thanks for the detailed post Denwood. Much appreciated. I'm also looking for a smart hub and I'm still in shopping mode. Home automation is quite captivating !

 

[Denwood]

West, you're welcome. The older stats were almost building controllers unto themselves with the EMS version having automation if/then options onboard. That said, based on my tests/research in this thread: http://www.garagejournal.com/forum/showthread.php?t=319342 my preference would be to control an HRV using C02 levels, and/or building automation. The reason is that the Ecobee HRV control strategy is pretty basic..as are their fan control options.

If you're looking for an automation controller that works with Ecobee look at Smarthings, HomeSeer (must be the windows version for the plugin to work) or the Vera Edge, both around $100. Neither solution btw is slick out of the box...although Smarthings does have an approved plugin for your stat. I'm using the Vera Edge right now and successfully toggling HOME/AWAY/SLEEP etc. based on the security system inputs. I am testing out both Smarthings and the Vera Edge.

I let the Ecobee's handle all scheduling, set points etc. and only use the automation to toggle modes. That way if the web is down, automation has issues etc, the stats continue to do their thing.

 

[Westbank]

I'm currently looking into the Wink Hub and Smart Things V2 hub.

I prefer the clean user interface of the Wink. Sadly it doesn't support the Ecobee Smart Thermostat. I would need to migrate to the Ecobee3 thermostat and find a way to control all my equipment units (humidifier, dehumidifier and ERV). I could wire the humidifier/dehumidifier to the Ecobee3 and use the Wink hub to control the ERV based on a schedule and/or CO2 sensor. Quickly, I was not able to find a CO2 sensor compatible with the Wink Hub. That being said, the Wink hub is not the most stable hub. It's still cloud controlled. Wink said it would slowly start to shift some processing at a local level. I'm still not convinced that Wink will survive for a long time after the take over by Flex.

The Smart Things V2 is a great option but it's more developer-oriented. At this point, the Ecobee3 is not officially integrating in ST. The integration of the Smart Thermostat is really basic and provide almost no control. I like the fact that a lot of processing is done locally and the hub has a built-in backup battery. I'm not sure if I could easily control my ERV via Smart Things. I didn't check if they have CO2 sensors available.

I find it a little discouraging since most hubs have big flaws. Not easy to make a choice since we don't know what will happen in the future. The competition will be more serious when Apple's Homekit will pickup some speed.

 

[Denwood]

Based on your level of knowledge, I'd actually look at the Vera Edge. Smarthings V2 is only local for very select automations. I do like that ST has Zigby onboard as I've found the GE Smart link LED bulbs work flawlessly with it. Because the Wink app is free, you actually don't need a controller at all to automate your Ecobee stat. It's not clean, but I started with the Wink app and IFFTT to manage the stats...no hub required!

If you need any assistance with Vera, I can send you my configuration notes. Vera is talking nicely to the Ecobee stats right now...and works with the older units as well. The Ecobee web API is the same regardless. Because any Ecobee control will be via the web API, by definition your stat control has to be "in the cloud", so basically no automation controller can control an Ecobee stat without a working web connection.

You might be better off controlling your ERV like I did with our commercial HRV. The CO2 sensor I used is "dumb" but you can monitor the dry contacts with something like the MimoLite ( http://www.amazon.com/dp/B00B6RZ7MM/?tag=atomicindus08-20 ) if you need to know when the ERV is running. There was no good reason to automate this at my office as we already run the air handler ECM fans at low speed during work hours. Again, if your furnace and ERV is in the same room, you could just direct wire this to your furnace G wire so the ERV always runs with the furnace fan. My guess is that it's already connected that way? KISS is always good

Controlling HRV/ERV with a CO2 sensor: http://www.garagejournal.com/forum/showthread.php?t=319342

 

[Westbank]

Thanks for the information.

I’m currently testing the SmartThings V2.0 hub and I’m disappointed that the Ecobee integration doesn’t even handle values in Celsius. I have reported the problem to SmartThings but they don’t feel it’s a priority so it may take few months before they fix it.

My humidifier/dehumidifier and my ERV are currently connected to my Ecobee Smart Thermostat. If I switch to the Ecobee3 I could connect the humidifier/dehumidifier to the accessory terminals on the Ecobee3 and interlock the ERV to the furnace. I would need to wire the ERV so the furnace fan would turn on when there's a call for ventilation.

Right now, I simply want to run the ERV 20 minutes per hour if the temperature is above -15deg Celsius and below 30deg Celsius. Do you think this is possible with the Vera hub ? I was not able to figure out if I can do it with the SmartThings hub. I know I can set a schedule but not sure if I can dictate the temperature delta.

Do you like the Vera UI ? I have tried the iOS app and didn’t like the UI and the lack of features in there.

Thanks again

 

[Denwood]

I'm actually OK with the Vera UI, however I'm doing the majority of the work via the full web interface (which you can also of course access from a smartphone). To do what you want with the Ecobee and Vera would be pretty easy. A summary of the code bits etc. that are working for me are here. You do have to get under the hood a bit, but it's not too scary: http://forum.micasaverde.com/index.php/topic,13836.msg272778.html#msg272778

Here's a simple schedule using one of our smart switches to toggle on/off. I'm using the Wunderground Weather Plugin to get outside temps. It also gives you the predicted high and low temps for the day, so you can use them as well.

If you were to go with a C02 control, like the one I used: you may find that your ERV run time (and therefore hydro use and heat loss) are a lot less. On windy days for example your ERV likely won't run at all. If you run that in series with a zwave dry contact module like the LFM-20, you could impose your -15 to 30C temp range, but have the C02 relay/control manage the run times. Hope that makes sense.

Btw, our HRV when installed was set up with one of these: http://www.industrialcontrolsonline.com/johnson-controls-a19-series

If your ERV is close to an outside wall, you can use this simple (no power required!) device (s)to limit your ERV operation based on temp and leave it as connected currently. They are normally open or closed depending on how you connect your wires to the 3 terminals inside. I used the same device to control heat trace for my garage drain. They can switch up to 120V between -30 and 100. This would be my vote for you in terms of the KISS rule Our local Ecco supply had them on the shelf.

 

[Denwood]

West, just to update, a better way to control your ERV would be using the free Combination Switch plugin. You can add multiple AND/OR triggers using the free "Combination Switch" plugin to control your ERV based on outside weather.

I just sorted automating the 2nd smaller HRV in our building associated with our Studio air handler. This system is separate from the building system to effect acoustic isolation. If the outside temps are between -15 and 23, AND the Ecobee stat is reading 18-22C (meaning the studio is occupied) it will turn on the HRV. It those conditions are not met (so the combination switch is OFF) it turns off the HRV.

To summarize, you would add the "WunderGround Weather Plugin" and "Combination Switch" plugin. Then you would configure the Combination Switch with as many temp ranges and devices as you want. The Combination Switch is a simple way to create AND (can also create OR) statements and specify how many conditions must be met before the switch is "On". This switch in turn can be used in any of your scenes. So the net effect is pretty powerful No coding required.