Maintaining A Hot-Water Heat System - part 2

Testing and fixing an aquastat

An aquastat is simply a thermostat (not to be confused with the thermostat upstairs) that measures water temperature in the boiler and activates a switch when the temperature reaches a certain point. A hot-water heat system has at least two aquastats. One turns the burner off to keep the boiler temperature from getting too hot. The other keeps the circulator from pumping water through the radiator unless it’s hot enough to do some good. On some furnaces the aquastats are individual units. On others the aquastats are housed in the same box and share a sensor that passes through the furnace wall and into the boiler.

What an aquastat looks like inside the box depends on the make and model. If there are two dials, one controls the boiler temperature while the other controls the circulator temperature. Both temperatures are allowed to vary below the setting by a preset amount. If there are three dials, one controls the boiler temperature, the second controls the circulator, and the third sets the amount the two are allowed to vary. Aquastats are usually mounted on the furnace but are sometimes found on a pipe leading from the circulator.

Testing An Aquastat

1 REMOVE THE COVER ON THE AQUASTAT. Testing an aquastat is a matter of resetting the dials and watching what happens. On this furnace, there are two aquastats (one for the burner and one for the circulator) mounted in different locations. On other furnaces, both aquastats may be housed in the same box. The larger aquastat dial controls the set point—the temperature at which it will turn off the burner or pump. The smaller dial controls the differential, or number of degrees below the set point at which the burner or circulator will come on.

2 TURN THE WALL THERMOSTAT UPSTAIRS TO ITS HIGHEST SETTING. If the furnace is functioning correctly it should turn on. If not, test the burner (set point) aquastat on the furnace—the one set to the highest temperature—by lowering the setting to less than 100°F (38°C). If the aquastat is working, the burner should gooff. Return the burner aquastat to its original setting. If the aquastat is working, the burner should go back on within 10 minutes. If the aquastat fails at either setting, replace it. Make a note of the original settings before you adjust them.

3 RUN A SIMILAR TEST ON THE CIRCULATOR AQUASTAT. First raise the temperature setting to higher than 100°F (38°C). If the aquastat and circulator are both working, the circulator will go on. If not, find the source of the problem by putting a multimeter set to 120 V AC across the aquastat terminals labeled C1 and C2, which lead to the circulator. If you get no reading, the aquastat is defective. If you get a reading, check the circulator (pump) motor. Once you've tested the setting, reset the aquastat to lower than 100°F (38°C). The circulator should go off. If not, replace it.

Replacing An Aquastat

1 TURN OFF THE POWER AT THE CIRCUIT BREAKER BOX AND AT THE EMERGENCY CUTOFF SWITCH. Make sure the power is off by putting a meter or voltage tester across the two terminals labeled Line. Double-check by putting one lead on a piece of bare metal inside the box and the other lead on first one line terminal and then the other. If the light comes on or the meter gives you a reading other than zero, you've turned off the wrong breaker. Do not work on the furnace until your meter or voltage tester tells you that you've turned off the correct breaker.

2 DISCONNECT THE WIRES LEADING TO THE AQUASTAT ONE AT ATIME. Mark each wire with a piece of tape showing which terminal the wire came from. When replacing parts, bring the old ones with you to the store. It will save you time and frustration.

3 REMOVE THE AQUASTAT. in most cases, including this one, the aquastat fits on the surface of the furnace and has a sensor that fits into a well inside the boiler. Remove the screws holding the aquastat in place and remove it from the furnace. On rare occasions the aquastat is mounted on a pipe leading to a baseboard or radiator. If so, remove the screws holding it in place. Take the aquastat to a dealer and buy a replacement that matches,

4 REMOUNT AND REWIRE THE AQUASTAT. Follow the manufacturer's directions when reinstalling an immersion aquastat. Among other things they will tell you how to properly position the sensor in the well. Reattach a pipe aquastat following the directions that come with it.

5 IN SOME CASES THE EXISTING WELL MAY NOT MATCH THE REPLACEMENT AQUASTAT. Make sure you've got the correct aquastat and replace the well. Drain the boiler using the pressure-relief valve, then follow the manufacturer's directions for installing a new well. Put the sensor in the well as directed and reattach the aquastat.

6 REATTACH THE WIRES USING THE LABELS TO MAKE SURE each one is attached to the correct terminal. Set the dials on the new aquastat to match the pretesting settings on the old one.

Testing and replacing the pump motor

MATERIALS: Wire nuts, masking tape (for marking electrical leads], coupler spring assembly

TOOLS: Multimeter, alien wrench, open-ended or adjustable wrench, gloves

If you’re reading these pages, you’re either extremely curious or extremely cold. If curiosity brought you here, you’re just interested in the way things work. If a failed motor brought you here, it means that no matter how hard the furnace is working, the heat isn’t reaching the rest of the house. You should have already tested the aquastat, which controls when the pump motor turns on. If you have, the problem is probably in your motor or very close to it. Traditionally the pump (known technically as a circulator) is a three-part assembly: motor, bearing housing, and pump. Inside the bearing housing a safety device called the coupler acts as a once-and-done clutch. If the pump jams, the coupler breaks the connection between the motor and pump so that the motor won’t burn itself out. Once the coupler goes into action, though, it’s broken. You’ll have to replace it.

Have someone turn the heat way up while you’re standing next to the pump. If the motor comes on but the pump doesn’t run, the problem is most likely the coupler. Replacing the coupler is a simple matter, but usually only a temporary cure. Whatever caused the coupler to break in the first place is likely to act up again. Unless you fix the core problem, sooner or later the new coupler will break. If the coupler’s intact and further tests indicate the motor or pump is broken, talk to your HVAC supplier about a combination pump, motor, and coupler sold as a single unit. Service people love them, and they’re considerably less expensive than the traditional assemblies. Installing one is a matter of loosening a few bolts, putting the assembly in place, then retightening the bolts.

1 TURN OFF THE POWER AT THE BREAKER BOX AND EMERGENCY CUTOFF SWITCH. Make sure the power is off by putting a meter across the line terminals of the aquastat. Remove the cover from the junction box on the pump motor. Gently remove the wire caps on the wires and double-check to make sure no power is going to them. Label the wires with pieces of tape so that you'll know how to put them back together.

2 TEST THE MOTOR BY SETTING A MULTIMETER TO RX1 and putting a lead on each of the motor wires. The meter needle should swing orthe readout change to indicate continuity. If you get no reading, replace or repair the motor. Talk to your supplier about replacing the motor and bearing assembly with a less expensive (but reliable] unit that combines the two.

3 IF THERE IS CONTINUITY THE MOTOR IS PROBABLY USABLE. The problem may be the coupler, the spring-loaded device between the motor and pump that protects the motor if the pump jams. The coupler is located inside the bearing housing, the funnel-shaped piece between the motor and pump. Look inside for the setscrew on the pump end of the assembly and loosen it.

4  REMOVE THE BOLTS HOLDING THE MOTOR TO THE BEARING ASSEMBLY. Slide the motor and coupler off the pump shaft. Motors can be heavy and awkward; have a helper handy m case you need assistance.

5 LOOK AT THE COUPLER AND SPRINGS TO CHECK FOR BROKEN PARTS. If there are any, remove the setscrew holding the coupler to the motor and replace the coupler. If the coupler appears to be intact and you are still not getting heat, call a service technician.

6 SLIDE THE MOTOR AND COUPLER BACK ONTO THE PUMP SHAFT. The motor is heavy and it can be difficult to get the spring assembly back over the shaft. Enlist a helper to support the motor while you wrestle the assembly into place.

7 REATTACH THE MOTOR TO THE BEARING HOUSING. Tighten the setscrew attaching the coupler to the pump shaft and rewire the motor. Turn on the furnace. If the motor doesn't come on by the time the thermometer on the side of the furnace reaches the temperature on the aquastat, call a service technician.

A SHORT-TERM SOLUTION IF THE PUMP FAILS - If the pump motor fails at 2:30 a.m. and the baby's crying and the pipes are freezing as you try to figure out what to do—you're having a bad night. But as long as the boiler is firing, there may be a short-term solution. You still must repair the pump but, in the meantime, try turning up the thermostat all the way. Hot water will still move slowly through the system of pipes and radiators without the pump by natural convection and cold water will return to the boiler. The thermostat won't control the temperature until you've repaired the pump. But the house will be warmer and the pipes less likely to freeze.

Log in to comment