Troubleshooting Condensing Boilers in Hydronic Systems – What is the System Doing?

This a guest post from Roy Collver. Roy is a condensing boiler expert. Here’s what John Siegenthaler, author of “Modern Hydronic Heating,” says about Roy’s work: “When I have a detailed question about the inner operation of a modulating / condensing boiler, Roy Collver is the first person I contact. The investment in Roy’s HeatSpring course is a fraction of the cost of a single mod/con boiler, but it will teach you concepts, procedures, and details that will return that investment many times over.”

Learn from Roy

• Free. Roy is teaching a two-part free course on how to sell mod-con boilers. The second live lecture is happening on Wednesday, July 30th. Sign up for the free mod-con course here.
• Paid: Roy Collver teaches an advanced 5-week course on mastering condensing boiler design in hydronic systems with the folks at HeatSpring. If you need to increase your skills and confidence around selling, quoting, designing, setting up controls, or troubleshooting condensing boilers in new construction or retrofit applications, this course is for you. Each session is capped at 50 students, but there are 30 discounted seats. Get your discount and sign up for Condensing Boilers in Hydronic Systems.

Enter Roy…

Understanding the Simple Basics

Cold weather is never too far away in most parts of North America. Be ready when it hits, and review the basics of hydronic system operation so you can quickly locate the problems that always come up. When you approach an operational hydronic system it will exhibit one of the following six states. Quickly understanding what you are dealing with will greatly reduce head-scratching time and point you in the right direction. Standing slack-jawed in front of a boiler with no clear path to determining what is wrong is very uncomfortable and a waste of time. Confidence is a key factor in successful troubleshooting, and to be able to indicate to a customer what the BASIC problem is right away buys you time to be able to work the problem, find out the SPECIFIC cause, and fix it. Using this guide as a quick reference should help speed the troubleshooting process along.

Hydronic systems are all about Delta T (the difference in temperature between the heating fluid, the system components and the surrounding air and objects). Heat always travels to cold, and if heat is not added to the heat transfer fluid (usually water), the fluid and all of the components in the system will eventually cool down to the temperature of the surroundings.

THINGS TO LOOK FOR:  NO POWER – NO FUEL – PILOT OUT – MANUAL RESET SAFETY OFF – NO CALL FOR HEAT – FLOW SWITCH OR LOW WATER CUT-OFF TRIPPED – DEFECTIVE BOILER COMPONENT (GAS VALVE, VENT DAMPER, IGNITOR, ETC.)

The boiler is on and the hot combustion gases create a large Delta T between the combustion chamber and the water in the surrounding heat exchanger. Because heat travels to cold, the water heats up. The circulation pump moves the hot water through the distribution piping to the terminal units. The terminal units heat up and a Delta T develops between the hot terminal units and the colder air. The air will get warmer at the expense of the water, which cools slightly. The cooler water circulates back through the system back to the boiler where it is heated up again. If the heat going into the boiler is more than the system can use, the water will continue to get hotter until the boiler cycles off on its operating control. The temperature difference between the water leaving the boiler and the water returning to the boiler will be “normal” for the system (usually 15°F to 40°F depending on the load and system design).

The boiler is on, adding heat to the water, but for some reason the hot water is not circulating through the distribution piping to the terminal units. The terminal units will cool down to the temperature of their surroundings and a “no heat” condition will result. The water in the boiler will continue to get hotter until the boiler cycles off on its operating control or internal high limit control. The supply and return piping near the boiler will be close to the same temperature.

THINGS TO LOOK FOR:  –  NO POWER TO PUMP – DEFECTIVE PUMP (BURNED OUT MOTOR, WORN IMPELLER, ETC.) – BLOCKAGE IN PIPING (TOTALLY PLUGGED “Y” STRAINER, CLOSED VALVE, CRUD BUILDUP BLOCKAGE, ETC.)

The boiler is on, adding heat to the water, but the hot water is not circulating fast enough through the system. The first terminal unit may become warm, but because the water is moving so slowly, all of the usable heat is transferred out of it before it gets very far. The last terminal units do not become warm enough to heat the space and a “not enough heat” condition will result. The water in the boiler will continue to get hotter until the boiler cycles off on its operating control or internal high limit control. There will be a large Delta T between the water leaving the boiler and the water returning to the boiler. (The supply will be a bit hotter than normal, but the return will be much colder than normal.)

THINGS TO LOOK FOR:  –  DEFECTIVE PUMP (WORN IMPELLER, ETC.) – PARTIAL BLOCKAGE IN PIPING (PLUGGED “Y” STRAINER, CRUD BUILDUP, PARTIALLY CLOSED VALVE, ETC.)

The boiler is on, adding heat to the water. The water is being circulated through the distribution piping to the terminal units, but the terminal units are not properly transferring the heat to their surroundings. The water remains hot and circulates back to the boiler without adding heat to the space. A “not enough heat” condition will result. The water in the boiler will continue to get hotter until the boiler cycles off on its operating control or internal high limit control. There will be little temperature difference between the water leaving the boiler and the water returning to the boiler. (Both supply and return will be hot.)

THINGS TO LOOK FOR:  –  DIRTY FIN TUBE ELEMENTS, FAN COIL BLOWER NOT WORKING, CURTAINS OR FURNITURE COVERING ELEMENTS, HEAVY CARPET OVER RADIANT FLOOR.

The boiler is on, adding heat to the water. The water is being circulated through the distribution piping to the terminal units, but the terminal units are too big for the heat output of the boiler. The water cools off too much and circulates back to the boiler without adding enough heat to the space, and a “not enough heat” condition results. The water in the boiler never gets hot enough, and the boiler never shuts off. Both supply and return will be cooler than they should. This condition can exist temporarily when starting up a cold system.

THINGS TO LOOK FOR:  – BOILER UNDERSIZED – BOILER UNDERFIRED – HEAT EXCHANGER PARTIALLY PLUGGED. –  UNUSUALLY HEAVY LOAD (COLD START-UP OF A SYSTEM).