Changing Honeywell T87 to smart thermostat

Adie

Hi! We have two T87 thermostats, old ones. They're connected to baseboard heating, I the kind with the layers of thin metal. We would like to switch to Google Nest or Amazon Thermostat. Is this possible and doable? It doesn't say high voltage anywhere on the unit. 

Jamie Hall

What provides the heat to those baseboards? If it's electric, there must be a contactor or relay somewhere controlled by the thermostats. If it's a boiler, the thermostats will control valves or pumps directly.

OK. That said. It is possible to install a "smart" thermostat on such a system, such as a Nest. However, it isn't straightforward, as the "smart" thermostat needs its own power supply, which the T87 doesn't. By far the simplest way to do that is to buy and install an appropriate wall wart power supply for the Nest. Which sounds simple, but various Nest models, as it turns out, have various requirements. Other "smart" thermostats have yet other requirements.

If you want the wi-fi capability, though, you will have to figure out how to do it -- and every installation is different. If all you need is programming, I'd suggest any one of a wide array of battery powered thermostats by Honeywell and others which work very well and are fully compatible. If you don't need programming... the T87, particularly older ones, are among the best and most reliable thermostats ever made.

Ironman

Are you doing this to save energy?

Hydronic systems are slow response and using a smart stat with setback doesn’t really save anything while making the system less comfortable.

A Nest is the worst possible choice for a hot water system. If you must have one, look at a Honeywell that was designed by heating engineers, not computer geeks.

I have a 25 year old T87f controlling mine and a NIB Nest that someone gave me when they first came out. After seeing all the problems with the Nest, I have no intention of using it on my house or anyone else’s.

Ironman

You will also need to have a “common” wire going to any smart stat to power its logic. If that additional wire isn’t available in your present thermostat cable, then you’ll need to pull a new cable.

SteamBoiler

Adie said:

Hi! We have two T87 thermostats, old ones. They're connected to baseboard heating, I the kind with the layers of thin metal. We would like to switch to Google Nest or Amazon Thermostat. Is this possible and doable? It doesn't say high voltage anywhere on the unit. 

Whatever thermostat replacement you are thinking about doing, suggest doing the investigation now but actual implementation, if any, in late spring when you are less likely to need heat as you transition. Expect some hiccups. It helps to be handy. If you are handy, pull the T87 off the wall and measure the voltage between the 2 wires, should be 24V. Alternatively, if you can access the thermostat wires at the baseboard unit furnace, measure it there. Any of the "smart" thermostats are only compatible with 24V. If your T87 only has 2 wires in, any smart thermostat will additionally require a C wire (basically a ground wire from the same transformer board that is giving you the 2 regular heat control wires). Battery powered non-smart thermostats are likely plug and play replace.

I had a Honeywell T8302 mercury thermostat and replaced it with, and am happy with, Nest on my steam radiator system, but considering moving to Ecobee because it provides more control over start and stop than does Nest.

At the very basic, a heat-only thermostat compares a user set temperature with its own measured ambient temperature and if the measured temperature is lower (undershoot threshold varies by unit and may be programmable, your T87 doesn't appear to have it) than the set temperature, will call for heat. The duration of heat call (or overshoot threshold) varies, and in some units, is programmable. My T8302 had a mechanical anticipator that one could move to set duration of heat call (but I don't think it worked properly). Your T87 doesn't appear to have it. My Nest doesn't have a programmable lower threshold but it does learn that the steam system takes a while to put out heat and most heat calls are 15-25 minutes duration. The smart thermostats log all heat calls - timing and duration - and really make you think about what you want from your system in terms of economy and comfort. For comfort you want the temperature to be as close as possible to your setpoint, all the time. That may result in frequent short calls. For economy you may want infrequent long calls but the ambient temperature will under- and over-shoot the setpoint. My steam system takes 10 minutes to get steam to the radiators, so I prefer calls that are relatively long - a 10 minute call is just a waste. My old Honeywell T8302 seemed to make frequent short calls though I never really measured them - no way to, really. The basement utility room was always warm but the home wasn't! But with Nest I can see each heat call, and the home is finally warm - not the basement.

The battery-powered next step up thermostats (eg Honeywell T3) will likely work fine and be robust and provide programmable temperature schedule, they won't log your heat calls or provide remote access or programmable heat call start-stop points. The "smart" thermostats will provide schedules, logs and remote access, you can program the thermostat 10 degrees below normal setpoint while you are on vacation and recover on the way back. I have my Nest set to 68F starting 7am, 64F at 10pm, and recover a degree an hour starting 4am. Ecobee additionally allows you to set the lower threshold (can program setpoint to 68F and start heat call at 67F or 67.5F, eg) and minimum heat call duration. For Nest, the programmers appear to have hardcoded a zone of approx 0.5F below and 0.5F above setpoint.

But the smart thermostats all require constant robust power and react in various ways to system disruptions that the designers may not have anticipated. I am still trying to figure out how Nest reacts to power disruptions in my system relating to low water boiler alarm and high pressure cutoff. I had some issues with shorter calls that I believe are related to power disruptions from low-water cut off unit chatter.

Jamie Hall

T87s come in two basic types. The older ones -- very reliable, particularly the ones with a mercury switch, do have an anticipator. If it's set properly, both undershoot and overshoot are minimal. The newer ones are digital, and have a cycles per hour setting. Not as good, but no worse than most other digital thermostats.

SteamBoiler

Jamie Hall said:

T87s come in two basic types. The older ones -- very reliable, particularly the ones with a mercury switch, do have an anticipator. If it's set properly, both undershoot and overshoot are minimal. The newer ones are digital, and have a cycles per hour setting. Not as good, but no worse than most other digital thermostats.

That is good input. I did a quick search for T87 and the first link appeared relevant, is still sold, and does show an anticipator that I missed in my quick look. The good thing about the old-style thermostats is they don't get thrown for a loop and go unstable when external factors - power loss, pressure limits, low water etc - mess up regular functionality. For my Nest I am planning a couple experiments in late spring with intentional low water (it is connected in series with the LWCO and I believe I saw some weird behavior related to LWCO chatter as water ran low) and intentional pressure cut off (should only result in an extra long heat call but I don't know) on my steam system.

For a power user I would absolutely recommend a smart thermostat but keep a known good battery powered one on hand.

HVACNUT

Do you have 3 wires available at the thermostats? The third could be behind the sub base. If so, that's a good start. 

tmeyer2000

I have the same problem as the OP. An old Honeywell thermostat that controls 240V baseboard heating via a relay. There are three wires, two are connected to the old thermostat, and the third is free on both ends. If I use that as the C wire for the Nest, where do I connect the other end?

109A_5

Hello @tmeyer2000,
One possible example.