I’m installing Ecobee 3 thermostats. I really like the one at our lake house (1990s heating plant) so I started installing them in our regular home (1900s heating plant).

Things did not go smoothly. I’m mixing late 20th century and early 20th century electrical technology. The simple job became complicated. This is called the mushroom factor  in some old-house maintenance communities: surprises in the infrastructure can make a normally easy task mushroom into a major project.

Our house has a multi-zone hot water heating system. The boiler is nice and new. The zone components are not (see right). A classic zone valve is an electric motor that responds to a thermostat. The motor either closes or opens the valve so hot water flows through radiators. The valve also sends electricity to a pump to circulate the hot water.

My three Honeywell V8043 zone valves yield a rat’s nest of wires, all connected to a 24 volt transformer. The lowest cord, sporting a spare green lead wrapped around it, goes to the thermostat. The green wire was a spare, so I reused it to supply 24v power (via the “C” lead) to the Ecobee. The wires in the middle feed 24v power to move the valves according to the thermostat.The wire attached to the top seems to hook to the circulating pump. I ignored it – you only need to deal with thermostat wires when replacing a thermostat.

New Meets Old

I’ve had several interactions with Ecobee’s customer service by now, and overall I’m impressed. The installation and support web site is filled with details of how to connect your particular rat’s nest to their thermostats. But there are so many ways to wire things up that it’s hard to choose the right wiring diagram.

Initially, someone at Ecobee customer service assured me that the thermostat would work fine right out of the box. All I had to do was follow the directions and perhaps use the power conversion kit. This didn’t work.

After a frustrating afternoon, I called Ecobee customer service and walked through my problems with the guy on the phone. He quickly realized that I needed to wire in isolation relays. He also suggested I find an HVAC contractor to handle it.

Why Isolation Relays?

Early 20th century electrical equipment uses real, live, physical-contact switches. Later 20th century equipment often uses “solid state” switches, especially when part of a smart device like a thermostat.

Now, I’m not exactly sure what the problem is, but here’s my guess: Zone valves contain bare-bones electric motors that generate a lot of electrical noise. Solid state switches don’t like that sort of noise. The motors might also draw more current than the solid state switches can handle. You can pump a lot of current through a plain dry-contact switch.

I last bought “smart” thermostats in about 1990. These were digital programmable thermostats operated by AA batteries. Thinking back, I remember I had to buy a special version that worked with zone valves. The difference was an isolation relay embedded in each thermostat.

Modern hot-water zone valve systems contain a “zone controller” board. Around here they’re called “TACO boards” in honor of the usual manufacturer. Our system is too old to have a TACO board. The HVAC contractor could perhaps have installed one when installing the new furnace a decade or two ago, but it didn’t happen.