Typically, you need to control air-sourced and ground-sourced (geothermal) heat pump systems that are doing heating and cooling duty with a thermostat that is “heat pump compatible”. That’s because an additional terminal on the thermostat is required to signal to the heat pump whether it should be in heating or cooling mode, i.e. activate the heat pump’s reversing valve to switch the direction of heat flow from in (heating) to out (cooling). That terminal will be labeled “O” or “B”… and is generally not present in conventional thermostats. Also, a heat pump compatible thermostat will have at least three stages of heating – partial, maximum, and emergency back-up (typically embedded electric resistance) – labeled Y1, Y2, and W (or W1 and W2), respectively, on the thermostat wiring block. (When the “O” terminal is closed to signal cooling, Y1 and Y2 will control 2-stage cooling.)
I recently installed a geothermal heat pump for which “typically” did not apply. The client already had a geothermal system but the Econar heat pump had prematurely died; the compressor became very noisy – a sign that it’s laboring – and, within weeks, stopped working altogether. (By the way, Econar is longer manufactured; maybe its lack of durability was a contributing factor.) It was what’s known in the industry as a “combo” unit, i.e. it could provide DX forced-air heating and cooling, and hot water for radiant heated floors all in one enclosure. This type is rare; my supplier indicated that they sell only one or two a year. I replaced it with a Geostar Cypress – the only combo model in the Geostar product line.
The client wanted nothing to do with forced-air heating. And why not! All three floors had radiant flooring which is far superior to forced-air for comfort, noise, and efficiency. Unfortunately, the Cypress has an annoying “feature”: when set for radiant heating mode, a request for heat from one (or more) of the zone thermostats will cause the heat pump to heat the water in the buffer tank that supplies the radiant zones (to the exclusion of forced-air). However, if the thermostats are not satisfied after 60 minutes, the heat pump will automatically switch from heating the buffer tank to forced-air heating (and remain in that mode until all thermostats are satisfied) – not the behavior the client desired. I needed to find a way to defeat this “feature”.
After consultation with my supplier (who is also a great technical resource), we came up with a novel solution. We would use the thermostat that I’d purchase – the existing one was limited to single-stage heating/cooling and could not take full advantage of this multi-stage heat pump – in conventional (not heat pump) mode. We would separate the R and Rc terminals on the thermostat (which are normally jumpered together) to signal heating versus cooling rather than using the “O” terminal on the thermostat which communicates directly to the heat pump control panel. Rc along with the Y1 and Y2 (and other) thermostat terminals would be wired directly to the heat pump to activate and control 2-stage forced-air cooling. R and W would bypass the heat pump and be wired to the left and right T-T terminals for zone #2 in the Taco SR-206 control box (which controls the circulating pumps for the radiant zones).
When the thermostat on the main floor (designated zone #2) called for heating, the signal sent to the zone #2 T-T terminals would energize the circulator serving that zone. In addition, the end switch in the Taco box would close (whenever at least one radiant zone circulator was energized). The end switch was wired in series with the aquastat – buffer tank thermostat — and the low-voltage signal to the heat pump and its circulating pump to the buffer tank. If the buffer tank temperature was below its user-specified range (thus closing the aquastat switch) and the Taco box end switch was closed, the heat pump would be energized to heat the water in the buffer tank. In this way, a direct signal for heating to heat pump control panel was avoided and the Cypress’ “feature” that automatically switched to forced-air heating if radiant heating could not satisfy the thermostat requests within an hour was defeated. Happy client!