Aquastat Relays - What's A Differential Anyway?

An aquastat relay is a combination of three controls. As its name suggests, it's an aquastat, a relay and a transformer. All three components are mounted together in what looks like a gray painted brick, with a skinny copper cigar hanging out of the back on a very thin copper tube. The cigar is the temperature-sensing element of the aquastat.

The purpose of the aquastat relay is to provide a place for most of the controls in a boiler system to be wired together. Take the cover off and you'll see pairs of screws that are labeled, for example, T-T, C1-C2, B1-B2 and L1-L2. These are called terminal screws. The label tells you what to wire in there. For example, the T-T terminals are where you'd connect the two wires from the thermostat. The C terminals are for the circulator. The B terminals are for the burner (oil burner or gas valve). The L terminals are for line voltage (120V). What this means is that you don't have to think about the circuits at all. It's a kind of plug-and-play, invented years before we had that terminology.

The circuits that connect everything are inside the aquastat relay where you can't see them. The aquastat relay makes sure that everything goes into the right place: the aquastat, the relay, the transformer, the thermostat, the circulator, the burner or valve, and line voltage.

What the circuits do is this: You wire in line voltage electricity (about 120V) into L1 and L2. As you know, some things connected to the aquastat relay are line voltage, such as a circulator. And some things are low voltage, such as the thermostat. With the circuits inside the aquastat relay, line voltage goes directly to the line voltage devices. Line voltage also goes to the primary side of the transformer in the aquastat relay. The transformer changes 120V into about 24V. The T-T terminals and the low voltage gas valve, if there is one, are provided with 24V low voltage. As long as you wire each device to the right terminals, it will get the correct voltage without you even thinking about it.

Inside the aquastat relay, if the thermostat calls for heat, normally the burner and the circulator are brought on. Simple enough. You could just about wire that yourself. But here's something special the aquastat relay does. If there's a call for heat and if, according to the aquastat, the water's already hot enough, the relay keeps the burner off and brings on only the circulator. We don't need to heat the water if it's already hot enough. Circulating the already-hot water can satisfy the call for heat.

There are two questions I get most often about the aquastat relay. The first is, “Why aren't the parts replaceable (aquastat, transformer, relay)?” I decided to find out. I asked Chip Troost, who is the product manager for that control at Honeywell. Chip has been there, well, pretty near forever, and he knows the whole story.

Chip says, “The L8148 and L8124 are safety controls that go through rigorous testing to assure they are wired correctly and perform to specifications. Allowing parts to be changed in the field, without the test equipment to assure proper performance after the parts were changed, could put the homeowner, service company and the manufacturer at risk. This is the same reason we do not recommend the use of rebuilt controls. They have not been tested to the rigorous requirements in the Honeywell engineering and manufacturing documentation.”

A second frequent question is, “When I look at an aquastat relay, how can I tell what the differential is?” The answer is, you can't. Chip says, “Historically, the differential has not been specified on the product label. I have no idea why. The OEMs (boiler manufacturers) typically specify a differential that meets their particular appliance design specifications, and perhaps at one time that was seen as proprietary information.”

What is differential anyway? Chip says, “Well, first you need to specify what control you are talking about. For a high limit-type control where the device is carefully calibrated to switch off (open the switch) at a specified maximum temperature, differential is the 'difference' between when the control opens the switch to stop the heating of the boiler, and the temperature where the switch turns back on because the boiler water has cooled below the set-point minus the differential."

“For instance, let's say the boiler limit control is set to 180 degrees F and the differential is 20 degrees F. When there is a call for heat and the water temperature is, let's say, room temperature, the control will turn on the energy source (gas, oil or electric) and continue to allow the boiler to heat until the water reaches 180 degrees F. At that time, the switch opens, turning off the energy source. The boiler water begins to cool. When the boiler water temperature drops to 160 degrees F (180-degree F set point minus 20-degree F differential), the switch will close and the boiler will begin to heat again."

“The differential is important to reduce fast cycling, which will wear out the components and reduce energy efficiency. Too large of a differential can cause lack-of-heat complaints when the water temperature drops too low."

“Controls can be calibrated to both the Make (closes switch) temperature and the Break (opens the switch), depending upon the application and what is being controlled. For instance, cooling controls typically are calibrated to Make at a specified temperature, and the differential is subtracted from the Make temperature. Thus, if it gets too warm, the system turns on and cools until the temperature drops below the Make temperature minus the differential. Too wide of a differential results in energy loss and complaints of over-cooling.”

Finally, I asked Chip if electronic aquastat relays are coming. He says they're already here for the oil industry. “The new L7224 and L7248 series of electronic aquastats provide all sorts of diagnostics that were formerly only available on commercial-grade equipment. Communication capability provides for remote monitoring and system diagnostics before the service technician is even dispatched to the home.”

And back to the “What's the differential?” question, Chip says, “With electronic aquastats, you can read the differential setting from the display along with other important settings and operating status.”

So, I guess electronics aren't all bad, are they?

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