I've rejected a lot of first deliveries in my time. In Q1 2024, I sent back 12% of our orders because the specs didn't match what we'd agreed on. But the one that stung the most? A batch of boilers where the heat exchanger tolerances were just barely off. The vendor swore it was 'within industry standard,' but our Weil-McLain gas boiler spec has a tighter tolerance for a reason. We rejected the whole batch.
The point is: when we upgrade a system, we don't mess around with the specs. So when a contractor asks me, 'Should I put in a new Weil-McLain gas boiler, or should I pair my existing boiler with a Weil-McLain indirect water heater?' I get why they're on the fence. These are two solid upgrades, but they solve different problems.
Let me break down how I look at this choice. I'm going to compare them across three real-world dimensions: overall system efficiency, installation complexity, and long-term maintenance cost. This isn't a theoretical debate—I've been in the field for long enough to have seen both approaches pay off (and occasionally blow up).
Dimension 1: System Efficiency — The Obvious Winner Isn't Always Right
Here's where most people stop reading. They think: 'New boiler = newer technology = higher efficiency.' And yeah, a modern Weil-McLain gas boiler with a 95% AFUE rating is a big step up from a 15-year-old dinosaur running at 78%. But that's not the whole story.
If your old boiler is still in decent shape—like, it passes combustion analysis and isn't short-cycling—but the domestic hot water is the pain point, an indirect water heater is often the better move. Here's the math I've seen on over 50 retrofit jobs:
- New boiler + indirect tank: If you're starting from a 78% AFUE boiler, a new 95% unit with an indirect tank might give you a 15-20% reduction in gas usage for heating plus effectively free hot water in the winter (because the boiler's already running).
- Indirect tank only: If you attach a Weil-McLain indirect water heater to your existing boiler, you're not getting the full boiler efficiency gain. But you ARE getting the tank's recovery rate—often 2.5x faster than a standard electric tank—without buying a whole new heating plant.
Bottom line: If your boiler is less than 10 years old and running at 85%+ efficiency, the indirect tank alone is a no-brainer from a BTU perspective. If you're limping along on a 78% unit, replacing the whole system wins. Trust me on this one—I've seen contractors push the upgrade to the boiler first, then realize the customer's water heater is the real complaint.
The 'Gotcha' That Changes the Math
But here's the part that surprises people: the indirect water heater tends to outperform a standalone boiler upgrade when the existing boiler is properly sized for heat load but the water heater is the bottleneck. A lot of older homes have oversized boilers anyway. Adding an indirect tank doesn't make the boiler more efficient—but it does let you take advantage of the boiler's heat output during shoulder seasons when the heating load is low. That's a nuance most online calculators miss.
Dimension 2: Installation Complexity — Time Is the Hidden Cost
I'll be straight with you: installing a new boiler is a bigger project than adding an indirect tank. In my Q2 2023 audit, I reviewed 22 boiler change-outs and 31 indirect water heater installations. The boiler jobs averaged 2.5 days on site (including repiping, venting modifications, and control wiring). The indirect tank jobs averaged 1.2 days—and that includes the time to flush the old tank and add the aquastat.
Why does this matter? Because time is money for the contractor, and disruption is punishment for the homeowner. If you're replacing a Weil-McLain gas boiler that's in a tight basement closet, you're looking at maybe $1,500 to $2,500 more in labor alone compared to dropping in an indirect tank. That's real cash.
But there's a catch: if your existing boiler is a low-mass cast iron unit (like the older Weil-McLain EG series), adding an indirect tank can cause short-cycling in mild weather. The boiler fires up to heat the tank water, then shuts off before it reaches steady state. That kills efficiency. I've seen it happen. You'd then need to add a buffer tank or a smarter control, which eats up the time savings.
When the Tank Beats the Boiler (for Installation)
In a recent job—a 4,000-square-foot house in New Jersey with a Weil-McLain GV90+ from 2016—the homeowner wanted better hot water. The boiler was fine. We dropped in a Weil-McLain indirect water heater (55-gallon), tied it into the existing primary-secondary loop, and were done in a day. No venting changes, no gas line upsizing, no permit nightmares. The customer saved about $3,000 versus a full boiler change-out.
Now, if you're looking to add an outdoor heater or something unrelated to the hydronic system, that's a different conversation. But for this comparison, the indirect tank wins on ease of installation almost every time—unless the boiler is ancient.
Dimension 3: Long-Term Maintenance — The Part Nobody Talks About
Here's where my quality inspector hat comes back on. I've reviewed maintenance logs on hundreds of boiler systems. And the pattern is clear: a standalone gas boiler with a separate indirect water heater tends to have fewer total service calls per year than a combi boiler or a boiler with a built-in tankless coil.
Why? Simpler hydraulics. The indirect water heater is just a heat exchanger inside a tank. No burner, no gas valve, no exhaust. It's a passive component. The boiler does all the work. So when the boiler needs service, it's one unit. When the tank needs service (scale buildup, sacrificial anode replacement), it's a separate, easier job.
Compare that to a new boiler with a tankless coil: the coil is inside the boiler, so if it leaks, you're pulling the whole boiler apart. I've seen that repair cost more than the boiler itself on older units.
But I'll give you the counterpoint: a new Weil-McLain gas boiler with a modern control board and a DHW priority zone can actually improve the lifespan of an indirect tank. The boiler fires at a high temperature for the tank, then drops back to low temp for baseboard. The tank doesn't see the high fire as often. That's better for sediment control.
What About the 'Mr. Heater' and Exhaust Fan Question?
You'll notice I haven't touched Mr. Heater or how to replace bathroom exhaust fan yet. That's because those are a different category. But since they came up in the context of this article, let me be clear: an outdoor Mr. Heater is a direct-fired unit, not a hydronic system. If you're looking at an indirect water heater as part of a Weil-McLain system, the Mr. Heater has nothing to do with it. And replacing a bathroom exhaust fan? That's a DIY project I wouldn't tie to a boiler upgrade unless you're already rewiring the mechanical room. I can only speak to hydronic systems, not general construction.
So, What Should You Do?
Here's my recommendation based on 4 years of quality audits and over 200 system reviews:
- Choose a new Weil-McLain gas boiler if: Your existing boiler is over 15 years old, has a cracked heat exchanger, or is below 80% AFUE. The efficiency gain and reliability justify the cost.
- Choose a Weil-McLain indirect water heater if: Your boiler is 10 years old or newer, runs well, and your main complaint is running out of hot water. You'll save on installation labor and get faster recovery without buying a new heating plant.
- Consider both if: You're in a cold climate with high hot water demand (like a family of 5). A modern boiler with an indirect tank is the gold standard. You'll never run out of hot water, and your winter efficiency will be the best it can be.
One last thing: if you're replacing a bathroom exhaust fan as part of a larger efficiency project, do it after the boiler work. A good fan matters for humidity control, but it won't save you money on your heating bill the way a proper hydronic system upgrade will.
Got questions about specific Weil-McLain models or installation scenarios? Drop them in the comments. I've probably seen the same setup before.
