Programmable Thermostats for a Heat Pump
14 October 2006
A. I have but it’s starting to get cold now so it’s time again. Most heating and cooling contractors understand the operational nuances of heat pump operation and for that reason they know that one of the most counter-productive things you can do with on e is to monkey around with the thermostat. The term ”programmable” is an invitation to play with controls so that would most likely be the basis of their discouragement. Let’s see where they’re coming from and what you can do about it.
Simply put, heat pumps are little more than beefed-up central air-conditioning systems that can run in reverse. In summer, they remove heat from inside of the house and exchange it for cool with the outside air. When you change the setting on the therm ostat to "Heat", the technology reverses itself and pulls heat from the outside air to warm the house. Sounds simple enough but that's where everyone gets into trouble. Most people don’t think the word “heat” is really in heat pumps. In order for the s ystem to work satisfactorily a few basic conditions have to be met beginning with the contractor who installs the system.
Heat pumps are comprised of two separate parts. The inside section, which is commonly misnamed the furnace, is the air handler. Its job is to move the heated or cooled air through the ducts to the rooms of the house. The outside section is the compress or/condenser and is the heart and soul of the system where the exchange of outside heating and cooling takes place. The heated or cooled refrigerant is piped to the air handler where it passes through a set of coils. As it does, air blown over the coil s by the fan transfers the warmth or cold to the circulated air, sending it through the house.
This system works very well for cooling and pretty well for heating, until the outside temperatures get into the 30's. Heat pumps are sized for their cooling loads because at very low temperatures they don't work well and require an auxiliary technology to help or even take over. That's purely electric heat. And costly.
When you set the thermostat to 72°F on a cool morning when its thermometer reads 65°, a seven degree difference, the little green "Aux Heat" light will go on. Inside the air handler are electric heating coils, similar to the heat coils you can see in a toaster, which heat up "boosting" the system until the heat pump can catch up. If the air handler didn't have that feature, you would swear that the system wasn't working and you'll freeze to death.
Wanting a programmable thermostat, you already know the setting lever is not a gas pedal. Just setting it up to a much higher setting in a cool room will not make the room heat any faster.
The efficiency of operation of a heat pump is in maintaining the temperature that it is set to when it gets there, which is why it seems to run a lot. It's designed to.
The red "EM Heat" light tells you that the outside unit isn't working at all and stands for "Emergency Heat" and you are now on the electric coils solely-- and expensively.
The warm air temperature of an operating heat pump tends to range between 85° and 105° as it exits the room register. If the room is cool, the blown air will drag some the cool room air with it, and when it hits your skin will feel like a cold draft. O il and gas furnaces deliver air in the 140° range and have the reputation of instant warmth.
Fancy programmable electronic thermostats automatically tweak the thermostat settings and do fine but must be specifically designed for heat pumps or they will gain you nothing by using them. Honeywell makes a programmable thermostat designed for heat pu mp use called the Chromotherm III T8611 series. When it’s time to change the temperature up, this thermostat does it a fraction at a time, avoiding the electric resistance heaters from kicking on. They cost about $150.00 and I wouldn’t begin to try to co nstruct a payback formula with electricity prices heading north. But every little bit helps.