Those of us “of a certain age” will recognize the title reference. For those who don’t, a quick internet search will introduce you to a comedy classic. Simpler times, perhaps—but in our industry, that old skit feels more relevant than ever.

As with Abbott and Costello, misunderstandings, misinformation and plain old stubbornness can send a job down the wrong path. The results may be amusing at best, but expensive—or even dangerous—at worst.

What follows is a list of items often missed by installers and frequently encountered by tech support teams across the industry. Left unchecked, these oversights can lead to recurring, costly and time-consuming problems that could be avoided with a thorough site assessment before the installation ever begins.

Begin at the end: When replacing a hydronic system, a professional, competent and conscientious contractor should begin with a clear end goal: design a system that’s safe, reliable, efficient and installed using best practices. Proper planning in the beginning helps today’s HVAC contractors achieve these targets while meeting project budgets. That gives the equipment its best chance at a long, trouble-free service life, and gives the contractor a better chance of sleeping at night without being haunted by no-heat calls. Start every job knowing what success looks like.

Moving day: Start with the most basic (but also one of the most important) questions: Can you get the new boiler into the mechanical room? Cast-iron boilers can be extremely heavy, and basement access is often an afterthought after years of remodeling. Some older homes offer limited access, meaning the boiler must travel through the house. Walking an 800-pound boiler across a tiled kitchen floor and down a flight of stairs deserves more than a passing thought.

Now’s a good time to start filling out your site visit assessment sheet. Is the new boiler available as a knockdown assembly? For example, the U.S. Boiler Co. SteamMax is offered in sectional, semi-pak or fully assembled models, depending on size, while some other models are only offered as an assembled unit.

If the unit you’re considering isn’t available as a knockdown, make sure you can either: a) get the boiler into place as delivered, or b) confirm the manufacturer permits block separation, if necessary, without voiding the warranty.

The initial site visit: When first visiting a prospective client, perform a comprehensive assessment. I recommend using a checklist so important details don’t get missed. Ask and document the homeowner’s responses to questions such as home size and conditioned area, number of zones and types of heat emitters. What has their experience been with the old system? This is the time to discuss existing issues, not after the installation when you hear the dreaded, “It never did that until you installed the new boiler!”

Selecting a replacement boiler: There are many programs available to help with a proper heat-loss calculation. It’s an often neglected but critical part of sound system design. Simply replacing the existing boiler with one of similar size can sometimes lead to a painful lesson that could’ve been avoided. This is especially true when replacing older cast-iron gas boilers with newer cast-iron, chimney-vented units. Newer boilers often contain less water and transfer heat more quickly than the units they replace. Meanwhile, large piping and cast-iron radiators can hold a substantial volume of system water compared with copper fin-tube baseboards.

High system volume, low boiler water volume and direct piping can create extended periods of low return-water temperature, which can allow flue-passage condensation to form in non-condensing boilers. That condition can shorten boiler and vent life. Fortunately, there are solutions: a simple boiler or system bypass, primary-secondary piping, thermostatic return-temperature protection, or an active bypass that responds to return temperature and helps the boiler move quickly through the range where condensation forms. It’s far better to identify these conditions up front, especially when replacing a boiler that may’ve failed early for exactly this reason. If the root cause is still present, the new boiler may not be long for this world either.

The existing system can also offer useful shortcuts when developing a game plan and putting together a budget quote. For example, if a home has a 140 MBH (thousand BTUs per hour) output boiler that doesn’t provide domestic hot water (DHW) and only heats two 3⁄4-inch fin-tube baseboard loops, one might reasonably suspect the replacement boiler can be reduced to roughly 80 to 100 MBH output. As a rough budget screen, assuming typical fin-tube water temperatures and flow rates, 3⁄4-inch tubing will only carry so much heat. That doesn’t replace a proper heat-loss calculation or emitter review, but it can help produce a reasonable preliminary estimate. It also allows homeowners to receive a realistic proposal sooner and helps contractors avoid doing full load calculations on unsigned projects.

Oversizing boilers remains a chronic issue in the industry and can produce cycle times measured in seconds rather than minutes. Short cycling is hard on components, particularly items such as combustion fan motors. Installing the right-size boiler can also help close more bids and improve your margins, because equipment cost is based on the actual needs of the home, not a historical good-faith guess that may point you toward a larger, more expensive unit.

Regarding DHW, will the new boiler be responsible for DHW production? If so, more questions are in order. Perform a fixture count. Ask about household habits: when laundry and dishwashers are run, whether that overlaps with shower schedules, what the fixture flow rates are, and what the homeowners expect for performance. What type of DHW system have they used in the past? The answers will help steer the design.

For modest homes with fixture loads within their capabilities, combi units are a popular option. However, homeowners accustomed to the quick response of an oil boiler with a tankless coil may find that a combi behaves differently than expected. DHW requirements deserve careful review. A bucket can be one of your most valuable tools here because it lets you measure fixture flow accurately.

A combi may provide enough hot water for an entire home, or it may struggle to satisfy a single shower with five rain heads. Older houses and piping can add another layer of challenge. A contractor can get into trouble with a home that has half-inch piping, very long or “creative” piping runs, low water pressure, low-flow aerators, or all the above. Combi boilers require a minimum flow rate to fire. If the boiler needs a minimum DHW flow rate of 0.5 GPM to start and a distant fixture only delivers 0.3 GPM, either the flow must be improved or an alternate DHW strategy—such as an indirect-fired storage tank—may be the better choice.

Venting considerations: Another critical item to check before replacing a boiler is where the flue gases will go. The venting layout for the new boiler should be settled before the job begins. For atmospheric chimney-vented units, consider the condition and suitability of the existing chimney.

Many current cast-iron gas boilers venting into masonry chimneys require careful review of flue sizing and, in many cases, a properly sized liner. Confirm that an appropriately sized liner will fit down the chimney if one is required and that no major offsets in the flue will prevent installation. Can the vent connector from boiler to chimney maintain the required clearance to combustibles? For power-vented or direct-vent units, is there a location for vent termination that meets local and national code requirements as well as the boiler manufacturer’s instructions?

The same question applies to condensing boilers. Make sure the vent path and combustion air path are selected and code compliant before installation. I’ve received frantic calls from contractors asking whether they can run the vent underground below the boiler—true story. As the old saying goes, an ounce of prevention is worth a pound of cure. Make sure the vent path is considered before the installation starts.

Breathe easy: There aren’t many absolute truths in our world, but here is one: fuel-fired boilers require adequate, clean combustion air, and this critical factor is often overlooked. There are clear standards covering combustion air, including available free area, grille size and location, and what to do when indoor combustion air volume is insufficient.

The old boiler may have operated without complaint, but newer equipment with additional safety devices may not be so forgiving. In tightly built homes, combustion air and draft can become especially tricky for chimney-vented or other non-sealed-combustion equipment. A substantially sealed house may not provide adequate air in real-world conditions, even when the free-area calculation appears acceptable on paper.

For direct-vent units using outdoor air for combustion, consider the relationship between the air intake and the exhaust termination, and review the manufacturer’s required clearances.

Ask the hard questions now. Could recirculation be an issue? Are there pools, plantings, chemicals, dryer vents or other contaminant sources nearby that could affect the boiler? Is the vent or intake termination located in an area exposed to frequent high winds? Adequate combustion air matters not only for burning fuel, but also for proper vent operation where draft is involved. My team receives many calls each year regarding new boilers going into intermittent lockout because the flue spill switch faults. The most common cause is insufficient combustion air.

Power considerations: Prior to a boiler replacement, always check the incoming power and grounding. Most modern boilers contain one or more processors, and digital equipment needs solid, reliable grounding for dependable operation. Older BX cable often relies on the metal sheathing as the grounding path. Loose termination screws, oxidation and other issues can all contribute to poor grounding. Bad grounds can be a bear to identify and resolve.

Make sure the proper power supply is available and that a sufficient grounding path is present, preferably by way of a dedicated and appropriately sized conductor. Check voltage, cable type, polarity and overall circuit quality before the new boiler is installed.

Fuel considerations: Finally, the boiler needs an adequate fuel supply. For electric boilers, confirm that suitable power supply and cabling are available. For oil installations, inspect the oil lines from the tank to the boiler. If the lines are bare copper and buried in concrete, now is a fine time to replace them. If oil line replacement is necessary, review the available path for the new line. If an overhead line is required, a device such as an oil deaerator may be needed to avoid fuel delivery issues, or a two-line system may be appropriate. Also consider the condition and age of the tank. Tank failures can be financially and environmentally devastating. A thorough assessment can help the homeowner avoid a very expensive problem.

For gas boiler installations, confirm that the gas supply line is sized in accordance with the applicable fuel gas code tables and the load of all connected equipment. Gas piping may be sized by approved methods, such as longest length or branch length, whichever is permitted and appropriate for the job. And please remember: tapping size does not dictate gas piping size. The new boiler may have a half-inch inlet, but based on connected input, developed length and allowable pressure drop, the installation may still require a 1-inch or larger gas main. Always use the applicable code tables and manufacturer guidance when sizing gas piping.

Heating equipment is changing rapidly, requiring an informed workforce to install and service it. Spending a little more time during the initial sales call may save a great deal of time (and aggravation) in the basement later.