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Community and Q&A

Truly “smart” thermostats and boiler controllers?

Daniel Griscom | Posted in Mechanicals on

Well, my new boiler is in (a Lochinvar WHN-85, with a Lochinvar SIT 40 indirect water heater). As far as I can tell this early in the season it’s working fine. But, I’ve been thinking about my thermostats and how they communicate with the boiler, and wondering if there might be a better way.

My old boiler was a standard dumb boiler. Each circulator was on if its thermostat was calling for heat, and the boiler was on (full blast) if a circulator was on and the water temp wasn’t at the limit. True bang-bang control.

Originally, the Honeywell Round thermostat in each zone was the smart part: it had hysteresis (the mercury sloshing back and forth in the switch) and a heat anticipator (a little adjustable heater, on when the thermostat was on). But it only had a contact closure to talk to the furnace, so its message to the boiler could only be “ON” or “OFF”.

A while ago I bought programmable thermostats so I could set the temp back at night. They replicate the hysteresis and the anticipation of the Honeywell Round, but still only have a contact closure to tell the boiler what to do.

Now I have a smart boiler, with temperature curves, outdoor reset, and all sorts of adjustments, delays, and other algorithms that help it Do The Right Thing. However, it’s still only being guided by a single contact closure per zone. So, there’s no way for the boiler to know just how much heat is being called for (“ON” versus “ON DAMMIT THE KID LEFT THE FRONT DOOR OPEN FOR FIVE MINUTES”). Nor does it know the difference between “ON: THE TEMPERATURE DIPPED A BIT” and “ON: THE NIGHT TEMPERATURE SETBACK JUST ENDED”.

I could get even smarter thermostats (e.g. the Nest), but with the communications link still so primitive there’s not a lot more it could do (excepting the improved programmability).

It seems to me that a smart boiler could do a lot better if it had better guidance from the thermostats. Are there (home) systems out there which have true “smart” temperature reporting thermostats, reporting to boilers which use the extra information intelligently?

Just wondering,

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  1. GBA Editor
    Martin Holladay | | #1

    I can certainly understand programming that recognizes when the night setback period ends.

    But what type of sensor do you envision to determine whether an exterior door has been left open? Do you intend to install sensors on your door latches? If you don't, you're just talking about basing boiler operation on the temperature in the room -- right?

  2. Daniel Griscom | | #2

    No, I'm not thinking to have additional sensors beyond temperature. But, having full temperature information per zone would certainly give the boiler controller more to work with than a simple "Someone thinks I should be on now" signal. Even better would be if the boiler knew that in twenty minutes the residents would be waking up and now is the time to goose the temperature with more than the usual burst of heat.

    I'm actually guessing that such a system isn't currently available for the residential market, and I seriously doubt there's one that will work with my Lochinvar. I'm just wondering about what might be available, perhaps looking a few years into the future, and if the Energy Cognoscenti were in charge of the market.

  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "Having full temperature information per zone would certainly give the boiler controller more to work with than a simple 'Someone thinks I should be on now' signal."

    In fact, a wall thermostat is acting on temperature information in that zone (although I'm not sure what you mean by "full temperature information"). Outdoor temperature sensors, which you say you have with your current boiler, certainly improve a system's ability to anticipate heating needs.

    If your family has a predictable routine, any programmable thermostat is able to "knew that in twenty minutes the residents would be waking up and now is the time to goose the temperature."

    If your family has an irregular but somewhat predictable routine, you may be a good candidate for a Nest thermostat (which is supposed to have the ability to monitor your family's movements and use that information, somewhat creepily, to anticipate your family's behavior).

  4. Daniel Griscom | | #4

    I understand programmable and Nest-type thermostats. My question is about the communications between the thermostat and the boiler. AFAIK, even the Nest just has a contact closure for the boiler, which it closes when it thinks the boiler should turn on. However, wouldn't the boiler be able to do a better job if it knew the difference between a) Zone 2 is a bit cold, and it's time to send the usual amount of heat, and b) Zone 2's setpoint just jumped by 10 degrees F, so it's time to send a bunch of heat?

    In the first case, I could see the boiler keeping the circulation temperature fairly low to keep the zone temperature from jumping and to provide better regulation. In the second case I could see the boiler significantly raising the circulation temperature in order to get the zone's air temperature up to the new level more quickly.

    I know my boiler will respond to a heat call with a low temperature, but if the call continues for more than X minutes it will bump the temperature up. In other words, it's trying to figure out what the need actually is on the end of that thermostat wire (beyond the simple "Need Heat"). But, wouldn't it be better if it didn't have to infer the true need, but actually knew what the need was?

    I'm guessing this is only possible with big commercial systems; will it ever be possible for residential systems?


  5. Charlie Sullivan | | #5

    You are right that what we do now is pretty primitive compared to what would be possible. Part of the problem is that we are standardized on one-bit signals between the thermostat and the boiler. To go beyond that, you'd need to make a system that included new versions of both, and you'd need to reassure consumers and contractors that they wouldn't be stuck with a system that would not be compatible with conventional replacement parts, if the product is discontinued.

    But then the other question is what benefit you'd should be more efficient to start 1 hour before the setback ends, using low-temperature water, than to start 10 minutes before it ends using high temperature water. And if the kid leaves the door open for 5 minutes, you might argue that having the house be cold for a bit might help teach him not to do that. So I'm not convinced that anything other than an outdoor reset is really needed for water temperature.

    But I do think circulation pump control is an opportunity for improvement. Smarter controls for that could save electricity used for pumping.

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