Unfortunately, many homeowners, business owners, and technicians are willing to “let the boiler slide” for months or even years without a proper checkup.

In many cases, this happens because the equipment was designed to go a considerable time without maintenance.

Today, the National Boiler Codes specify all commercial boilers must be checked by a licensed boiler contractor at least once per year.

The truth is, as the market pushes for higher efficiency equipment — such as condensing, fully-modulating systems (mod-cons) — boilers and burners may require two or more maintenance checks each year.


Many boiler problems stem from mistakes made during installation, so the first order of business in establishing a preventive maintenance schedule is to assess the overall picture.

Record what you see in regard to the system piping, venting, gas or oil supply, and all other key facets of the mechanical room.

The boiler start-up sheet can and should be used to evaluate system performance and to spot trends that affect the operating pressure (for a steam boiler), operating temperature (for hot water systems), stack temperature, and water-level controls.

Completing a check-sheet not only records the basic situation as you see it but also sets a baseline for later maintenance and service calls whether they’re routine or generated by a no-heat call. If, for instance, there’s a progression of ignition problems — which could tie to a score of possible causes — a skilled technician will begin a series of diagnostic procedures to learn the source of the problem.

Think of it as hydronic forensics, and you’re the investigator looking for clues. Granted, there’s (hopefully) no blood but plenty of fingerprints and a bountiful supply of clues.

One problem could have many possible causes. Keeping a detailed record of the forensics makes sense, right? Yet, the trail of information is often neglected, lengthening the time it may take you to do the diagnostics.

Key categories for routine maintenance include venting, combustion air, gas or oil supply piping and/or filtration, water quality and piping, electrical wiring and diagnostics, and controls.

Let’s take a quick look at each of these.

Venting — Check all venting under category I venting (non-condensing, negative pressure) for a white chalky substance on the outside of the vent. This often indicates condensation inside the flue. If signs of condensation are present, verify the boiler is piped properly and operating at the prescribed temperature.

“One common remedy is to increase boiler operating temperatures,” said Nate Warren, hydronic sales manager, Bradford White. “Also ... it may be necessary to check the flue or the boiler for damage.”

Oftentimes, when large boilers are replaced with smaller, more efficient ones, the chimney is too big and requires a liner. Perform a draft test. If it’s an oil system, use your smoke tester first because smoke could damage your analyzer.

Category III and IV appliances require sealed venting with stainless steel or specific plastic vent material to withstand the corrosive effects of condensation. Installers and service professionals need to ensure the correct material was used. Positive pressure vent systems (Category III and IV) must be sealed.

Combustion — For atmospheric systems, check for any blockage of combustion air openings.

Personally, I like to use fan-in-the-can units; when the equipment starts, so does the fresh air in the mechanical room — now ready to be used for combustion.

“Check the free flow of air into atmospherically-fired boiler rooms at least every six months,” said Joan Mishou, customer service manager for Laars Heating Systems Co. “For ducted combustion units, check for blockages routinely.”

Also, clean and check filters every six months, or at least once a year.

Check gas pressures while at peak load or on the coldest day of the year if possible to determine if there’s a measurable reduction during peak operation. This would help to answer any questions about a boiler’s ability to meet heat demand and quality of combustion.

Warren pointed out gas line pressure can be a nuisance in certain parts of the country. Areas that rely on older gas lines may not be able to supply sufficient pressure during peak demand.

Inadequate gas pressure can cause rough light-off and lock-outs, which are often misdiagnosed as faulty igniters, ignition controls, or gas valves. Proper sizing of the gas piping between the boiler and the gas meter should always be confirmed.

Typical gas pressure required by older commercial boilers is 4 inches and up water column (4+ inches W.C.).

If there’s any doubt about the pressure available to the system, you may need to put in a call to the gas company to see if line pressure can be increased.

Gas or oil supply piping and/or filtration — Clean or replace fuel-line filters on oil boilers once a year or every six months if the tanks are older. If this is the case, take the boiler offline for a few hours during and after fuel deliveries.  

Water quality and piping — Evidence of corrosive activity on piping should be addressed immediately and could result from air infiltration. All visible piping should be checked for signs of deterioration during each visit. Check the boiler’s relief valve to ensure it’s not leaking.

Many newer boiler designs use stainless steel or aluminum in their heat exchangers. The manufacturer may have requirements for water quality. Know the requirements and test for them when the boiler is inspected.

Electrical wiring, diagnostics, and controls — Check all wiring in the system for overheating. Hardening or melting of insulation will cause problems. Among other things, this can incapacitate or otherwise influence diagnostic systems or disable safety (boiler-off) checks.

Check that the boiler shuts down on high limit and at low water cutoff. Check operation of the aquastat to be sure the boiler shuts down at the set-point temperature.

According to Warren, a service tech should also check the flow switch to verify the boiler shuts off under no-flow conditions. Be sure to test the igniter for ohm resistance or micro amp signal (depending on type) and verify readings are within acceptable guidelines.


If an igniter fails, the many possible causes include condensation, venting, or lack of proper combustion air.

If condensation was dripping from the heat exchanger onto the igniter, it could be caused by return water temperatures being too low. This could be a piping issue or a system issue where a bypass loop is needed to mix supply and return water.

Typically, the recommended minimum return temperatures are 140°F for cast iron boilers, 110° for copper atmospheric, and 120°-130° for sealed combustion copper.

Other contributors to premature igniter failure would be improper venting or poor combustion air — either of which could lead to fouling of the heat exchanger, which would overheat the combustion chamber. This is most common with atmospheric units.

If these are ruled out, there’s the possibility of low gas pressure.

Another key facet to identifying and solving problems in the field is having the right tools on hand. A combustion analyzer, gas manometer, and volt meter are essential.

A combustion analyzer delivers information quickly and can lead you to check venting or combustion air. As you move progressively through the diagnostics, this may lead to use of a gas manometer to check the gas pressure.


Mod-cons must be inspected routinely. For commercial systems, every few months isn’t overkill.

Some mod-con designs have very small combustion chambers that really take a beating. The refractory material may need to be replaced often. When deterioration is apparent through the sight glass, it’s time to replace it.

If refractory deterioration isn’t addressed promptly, it’s possible to damage the heat exchanger permanently.

Igniters — Mod-con boilers can operate with wet combustion chambers, so they’re susceptible to moisture damage. Oxidation on the flame rod/igniter can cause shutdowns. Over time, the flame rod can fatigue due to heat/cool cycles and can also change the gap between the igniter and the flame rod. If this gap gets too wide, lockouts or rough lightoffs can result.

Routine maintenance of commercial systems should entail changing the igniter at least once a year.

Fireside cleaning — With mod-cons, water vapor mixes with impurities in the air and fuel to form “mouse turds” or “coffee grounds” inside the combustion chamber, which must be removed not to impede drainage. If the combustion chamber doesn’t drain properly, the refractory tends to get soggy and deteriorates prematurely.

The heating surfaces need to be cleaned annually to keep the efficiency as high as possible. Scale the thickness of an eggshell can reduce efficiency by 10 percent or more. 

Condensate can have low, acidic levels and should be drained and neutralized. Often in my service area, I see condensing boilers installed by others that drain condensate without neutralizers — this dumps the acidic fluid right into a cast iron floor drain.

Even something as mundane as drainage can’t be ignored with mod-cons.

The systems produce heat efficiently, but they must be maintained. Follow the manufacturer’s service recommendations at a minimum.

The result will be a long and harmonious life for the boilers you service, as well as harmony between you and your customers.

Think of it as money in the bank. After all, a happy customer is a staying and paying customer.

Publication date: 1/15/2017

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