Electronic thermostat myths
Carol Fey | Jun 07, 2012
Last time I talked about myths and misunderstandings regarding non-electronic thermostats, especially as applied to hydronics. Now I’m going to talk about electronic thermostat myths.
Typical of electronic thermostats are the Honeywell non-programmable T8400, the programmable T8600, and the White Rodgers 1F90.
Myth: You can’t use electronic thermostats with hydronics.
Fact:Electronic thermostats work very well with hydronics.
Myth:All electronic thermostats are programmable, and therefore cannot be used with hydronics.
Fact:There are two parts to this myth. First, electronic and programmable are not the same thing. Electronic means that the inside of the thermostat is made up of resistors, diodes and thermistors — elements of a printed circuit board. Programmable means that the thermostat can be set to automatically reduce or increase the temperature setting, depending upon the time of day. Another word for programmable is setback.
Second, programming, or night setback works well with hydronics if it’s done right. And doing it right is easy. People with hot water heat enjoy saving money as much as those with forced air furnaces. And many people are more comfortable sleeping in a lower temperature than they live in during the day.
The consideration with programming is recovery time. See the next myth for an explanation.
Myth: A hydronic system cannot recover from night setback.
Fact:Too often the assumption is that you have to setback at least 15 degrees, say from 70°F daytime to 55°F at night. But you can set back a smaller amount. Consider the difference most people feel between 70°F and 65°F. Reducing the temperature even a few degrees every night saves significant energy over time. You can work with a home or building owner who is motivated to save money to figure out the right amount of setback that will also allow good morning recovery.
Cast iron radiators, with their slow reaction time, do have trouble with that much setback. But most modern systems are fin tube, which has a much faster recovery time. Setback can also be used with infloor radiant heat. The key is experiment with the recovery time required for that system.
Myth:Electronic thermostats can’t be used for hydronics because they don’t have anticipators.
Fact:True, electronic thermostats don’t have anticipators. Instead they have cycle rate adjustment. The only reason that mechanical thermostats have anticipators is to set cycle rate, so electronic thermostats just cut out a step to get to the same place.
That desired place is controlling cycle rate, or how long the heat is on and how long it is off. It has been found that for most hydronic and forced air systems, the ideal is five or six cycles per hour (cph). Electronic thermostats come out of the box pre-set for five or six cycles per hour. This means that on-time plus off-time is 10 or 12 minutes. (Sixty minutes divided by six cycles per hour equals ten minutes per cycle.) As the temperature gets colder outside, heat-on time takes more minutes of the cycle and off-time takes less.
Industry-recommended cycle rate for cast iron radiators is three cycles per hour (20 minutes). That’s because it takes longer for cast iron to give off all its heat. Electric heat is usually nine cycles per hour because it gives off heat very quickly. It’s easy to go into installer setup and set cycle rate.
Myth:Setting up and programming an electronic thermostat is difficult.
Fact:Setting up and programming a thermostat is quite easy. But you do need to read the instructions.
Myth:Electronic thermostats need to be leveled.
Fact:An electronic thermostat doesn’t care what position it is in. You could even hang it upside down—it would work the same. The only thermostat that needs to be leveled is a mercury-bulb.
Myth:Electronic thermostats need to be calibrated.
Fact:Hardly any thermostat, electronic or otherwise, needs to be calibrated. Most cannot be calibrated anyway. An interesting feature in some high-end electronic thermostats, though, is that temperature can be “offset.” That means that you can go into installer setup and tell the thermostat to display a temperature a couple degrees warmer or cooler than the room actually is. This is usually when your client is trying to fool a spouse or building occupant, so you’ll be participating in a little trickery.
Myth:An electronic thermostat is always an improvement over a mechanical one.
Fact:There are lots of electronic thermostats that have horrible temperature control, allowing room temperature to swing many degrees above and below set point. We expect this of very cheap thermostats, but it can also happen with some expensive models. Best temperature control seems to come from manufacturers long committed to the business, such as White Rodgers, Honeywell and Roberts haw. Remember, the heating industry definition of comfort is that the temperature doesn’t fluctuate more than one degree above or below set point.
Myth:Wireless thermostats are unreliable.
Fact:Wireless thermostats have been perfected. With the traditional thermostat manufacturers, there is no interference with any other signals, such as garage door openers. Wireless thermostats are great for adding air conditioning or heating zones without having to pull wires. They also make it easy to move the thermostat to a location that the homeowner prefers.
In conclusion, electronic thermostats don’t necessarily control the temperature better than mercury-bulb thermostats, but there are advantages. Electronic thermostats make the whole job look more modern. After all, the only part of the system that your client normally sees is the thermostat. A programmable thermostat, even with just a few degrees of set back, can save your client some money. Residential customers may enjoy cooler sleeping temperatures.
Old thermostats don’t often fail. But your customers may enjoy the updated look of a new one. It makes your work look modern and complete. It’s your calling card.
Carol Fey is a technical trainer who has been in the HVAC industry for over 25 years. You can find her books and DVD atwww.carolfey.com. To see her adventures while a heating mechanic in Antarctica, go towww.carolfey.blogspot.com.