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HEATING SYSTEMS ANTI SCALD VALVES BOILERS, HEATING BOILER OPERATION DETAILS BOILER CONTROLS & SWITCHES Air Bleeder Valves Aquastat Functions Aquastat control HI LO settings Aquastat control DIFF settings Single Function Limit Switches BOILER LEAKS CORROSION STAINS BOILER OPERATING PROBLEMS BOILER PARTS LIST Cad Cell Relay Switch Flame Sensors Circulator Pumps & Relays Expansion Tanks Gauges on Heating Equipment Limit Switches, Boilers Low Water Cutoff Valves, Boilers Mixing Valves Relief Valves - TP Valves Spill Switches Stack Relay Switch WATER HEATERS ZONE VALVES CARBON MONOXIDE/DIOXIDE CARBON MONOXIDE WARNING CHIMNEY INSPECTIONS CHIMNEYS & Flues - Asbestos Transite Pipe DRAFT HOODS - gas fired DRAFT REGULATORS - barometric dampers DUCT SYSTEMS DUCT INSULATION, Asbestos Paper DUCTS, Asbestos Transite Pipe DUST FROM HVAC? ELECTRICAL POWER SWITCH FOR HEAT FURNACES, HEATING DUCT SYSTEM DEFECTS FAN LIMIT SWITCH FURNACE CONTROLS & SWITCHES FURNACE OPERATION DETAILS HEAT EXCHANGER LEAKS STACK RELAY SWITCHES GAS PIPING, VALVES, CONTROLS GAUGES ON HEATING EQUIPMENT HEAT LOSS INDICATORS HEATING COST SAVINGS HEATING LOSS DIAGNOSIS-BOILERS HEATING LOSS DIAGNOSIS-FURNACES HEATING OIL CLOUD WAX GEL POINT HEATING OIL SLUDGE HEATING SYSTEM INSPECTION DIAGNOSIS REPAIR HEATING INSPECTION CONCEPTS HEATING INSPECTION PROCEDURE HOT WATER HEATERS HOT WATER IMPROVEMENT INSULATION MIXING VALVES NO HEAT - BOILER / FURNACE DIAGNOSIS OIL BURNERS OIL BURNER NOISE SMOKE ODORS OIL FUEL TYPES & CHARACTERISTICS OIL TANK LEAKS OIL TANK SLUDGE OIL TANKS, BURIED PLASTIC HEATER VENT RADIANT HEAT Floor Mistakes to Avoid RELIEF VALVES - TP Valves Safety Recalls BLUERAY Recall CHIMNEYS & Flues - Asbestos Transite Goodman HTPV RECALL Lennox WARNING Weil McLain RECALL SAFETY DURING HEATING INSPECTION SPILL SWITCHES - Flue Gas Detection STAIN DIAGNOSIS on Indoor Surfaces STEAM HEATING SYSTEMS Cad Cell Relay Switch Flame Sensors Low Water Cutoff Valves, Boilers Mixing Valves Relief Valves - TP Valves Sight Glass, Steam Boiler Pressure Switch, Steam Boiler Spill Switches Stack Relay Switch Steam Vents LOW WATER CUTOFF CONTROLS WATER FEEDER VALVES, STEAM TANKLESS COILS Tankless Coil Leaks THERMAL TRACKING THERMOSTATS Transite Pipe Chimneys & Flues WATER HEATERS ZONE VALVES More Information InspectAPedia TM Home & Site Map Air Conditioning InspectAPedia Bookstore Electrical Environment Exteriors Heating Home Inspection Insulate Ventilate Interiors Mold Inspect/Test Plumbing Water Septic Roofing Structure Accuracy & Bias Pledge Contact Us |
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Combination Control High Limit: On a typical combination control, the "Hi Limit" is the cut-off temperature for the heating boiler on a call for heat. The cut-on temperature is hard wired in this control at about 15 degF below the "Hi Limit" setting.
Combination Control Low Limit: On these controls the "Low Limit" is NOT the "cut on" point for heat but rather it is a setting which is intended to maintain heat inside the boiler in order to assure that the boiler can produce hot water when a tankless coil is installed.
The "Low Limit" is normally set at least 20 degF below the "Hi Limit" to avoid a "lockout" condition on this control which we discuss just below. During warm months when the boiler is not being called-on to heat the building itself, the "Low Limit" keeps heat in the boiler for the tankless coil. So the "Low Limit" is actually a "low range" operating upper limit on boiler temperature that applies out of the heating season.
Combination Control Differential: On this control, the "Diff" or differential control dial specifies the amount below the "Low Limit" to which boiler temperature can fall before the boiler should turn on to keep the boiler warm for making domestic hot water through the tankless coil.
Since tankless coils on heating boilers, especially modern small high-efficiency boilers, have rather limited ability to deliver much hot water to the building, most users set the "DIFF" control to its smallest number, assuring that the boiler will come on as soon as possible when it's needed to make hot water out of the heating season.
Lots of people, even some service technicians are confused about the actual function of combination controls and aquastats like the Honeywell R8182D. Some don't know how to set the "HI", "LO" and "DIFF" dials on a combination control. Or more commonly, we were taught to just use a "rule of thumb" which set the "HI" to 180 degF and the "LO" to 160 degF.
This was certainly a "Safe" setting for this control, and it's in use on millions of heating boilers. But with a little thought, we can adjust these settings and set the heating aquastat control so as to save a bit more on heating cost.
The "HI" on a combination heating control like this aquastat is usually set by the heating service technician to a spot between 180 degF and 200 degF. Or the "rule of thumb" fellows just park it at "180" degF as you can see in our photo.
Generally a heating boiler is more efficient if we operate it at a higher temperature. The thermal conductivity of heating water inside of finned copper tubing baseboards or through radiator surfaces is exponentially greater at higher temperatures. In other words, hotter water actually transfers heat into the occupied space more efficiently than cooler water. So you may actually save money on heating costs by running the boiler at a higher temperature.
Since this is not lab-grade equipment these settings are not precisely accurate. For this reason we like to set our HI limit at around 200 degF. We let the boiler run through a few heating cycles, watching the temperature and pressure gauges on the boiler to see what temperature we're actually reaching. And we watch the boiler relief valve to be sure we're not causing leakage there.
Setting the "HI" too high on the aquastat can result in over-temperature in the heating boiler and can result in dumping water and temperature and pressure at the relief valve.
The "LO" is set at least 20 deg F. below the "HI" limit on a combination
control. If we set the "LO" any closer top "HI" it is possible that the control, trying to give priority to making domestic hot water for someone
in the shower, will simply lock out the circulator pump entirely - the heating circulator will never run.
In our photo at left you'll see where the heating service tech left this "LO" limit control after cleaning and tuning the heating boiler.
What happens if you "cross the controls" and set "LO" above "HI" or if you set "LO" too close to "HI"? The circulator will not run.
We've inspected homes at which the owner, for decades, observed that the heat in the building was very slow to come up in response to the thermostat.
The problem was that the LO was set close to or even above the HI - the circulator pump never ran and hot heating water circulated but only very slowly by convection.
Where a tankless coil is installed on a heating boiler to make domestic hot water (for washing and bathing) we prefer to set the differential to its smallest
number (usually 5 degrees) so that the boiler will come on as soon as possible to make domestic hot water when we're out of the heating season.
(In the heating season the boiler is usually hotter due to calls for heat and the "LO" and "DIFF" are basically asleep.).
If a tankless coil is not installed on a boiler where this combination aquastat control is installed, this combination control may still be in use. In this case, the "LO" has almost no use whatsoever and, if you read the instructions provided by the control manufacturer you'll probably see that the manufacturer calls for the "LO" to be electrically disconnected entirely - it's simply a matter of moving a jumper wire (usually the blue wire pointed-to by our pen, next to the DIFF control.
If you do not disconnect the "LO" limit switch on a combination control on a heating boiler where no tankless coil is installed, during the summer you may wonder why once in a while you hear your heating boiler running. Now you can solve this mystery. In a few cases, certain cast iron boilers may cool off and begin to leak between their cast iron sections during the summer. This may be a reason to keep the "LO" wired and active on just these heating boilers.
Here is a photo of a more traditional single-function heating boiler limit controls. In this example the limit switch is being used on a tankless coil, and in the enlarged version of the photo you'll see that the temperature limit on the control is set to about 140 degF.
When a heating boiler uses a tankless coil to produce domestic hot water, a third single-function control may be installed for that purpose. In this photo a Honeywell limit control switch is being used to monitor hot water temperature at the tankless coil which is in turn mounted on a steam boiler of an older home in Portland, Maine. You can see the black-handled mixing valve in the lower right of this photo.
Cold water from the building is entering the tankless coil via the bottom pipe (green corrosion) and hot water, heated by the coil is leaving at the upper part of the tankless coil, where it turns downwards to enter the left side of the mixing valve.
Additional cold water is permitted to enter the bottom of the mixing valve, and tempered (non-scalding) hot water then leaves at the right side of the mixing valve in this photo.
More information about tankless coils, how they work, what goes wrong, and their controls, is at Tankless Coils.
On heating boilers where individual limit controls are installed, you will find two similar devices, but mounted at different locations on the boiler, controlling the boiler's HIGH limit, or cut-off on a call for heat, and the boiler's LOW limit or cut-on point as the boiler water temperature falls.
An older type of aquastat used to set boiler temperature limits was strapped to the hot water riser pipe close to the top of the heating boiler. This device is shown in Carson Dunlop's sketch at left.
Rather than a temperature probe inserted directly into the heating boiler's water (as used by modern aquastats) the strap-on type aquastat relies on metal-to-metal contact between the control and the boiler water riser pipe.
If your heating system is controlled by a strap-on aquastat such as shown here in Carson Dunlop's, you'll want to check that it has remained securely in place.
If the aquastat becomes loose its contact with the hot water pipe is lost and the system is unsafe.
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More expert information on this topic
While going through the detailed sequence in the operation of the heating boiler, watch for and inspect the condition of the heating boiler controls and safety devices (as required by ASHI 9.1.A.3 automatic safety controls).
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