The Most Common Thing I Find in Denver Boiler Rooms

I open a lot of mechanical room doors. Littleton, Englewood, the older parts of Denver, newer builds down south. Different houses, different decades, different budgets.

And the most common thing I find is the same thing every time: a boiler two to three times bigger than the house it's heating.

Not a little oversized. Not "we rounded up." Two to three times.

Why it keeps happening

Nobody sets out to do this. It happens for three pretty human reasons.

The nameplate swap. Whatever the old boiler's tag said, the new one says the same thing. Except that old boiler was very likely oversized too — and the house has picked up new windows, insulation, and air sealing in the decades since. The nameplate is a record of an earlier sizing decision, not a measurement of your house today.

Fear of the callback. Undersize a boiler and the phone rings on the coldest night of the year. Oversize it and nothing happens — nobody calls to complain that their house is warm. The incentive runs one direction, and it runs there hard.

Square footage math. Finished square footage times a remembered number, and that's the load calc.

I don't fault the instinct. But the consequences land on the homeowner, and they last twenty years.

What oversizing actually costs you

A boiler that's too big doesn't run gently. It runs in short bursts.

The burner fires, satisfies the thermostat almost immediately, and shuts off. Then it does it again. On highly zoned systems, documented on-cycles run under a minute when one small zone calls. The target for a heat source is a minimum on-time of about ten minutes.

That's short cycling, and it costs you three ways:

• Efficiency drops. A burner spends the first part of every cycle just coming up to temperature. If it never gets past that phase, you pay for the warm-up over and over.

• Wear accelerates. Igniters, contactors, and controls have a finite number of cycles in them. ASHRAE puts a boiler's life expectancy at 24 to 35 years; CIBSE puts a condensing boiler at more like 10 to 15. Running it in bursts doesn't help that number.

• Emissions go up. Every ignition cycle is dirtier than steady-state operation.

The rule I hold myself to: a single heat source should land no more than 10% over the calculated design load. Not double. Ten percent.

The condensing boiler that never condenses

Here's the part that costs people the most money, and almost nobody hears it before they buy.

A condensing boiler earns its efficiency by pulling latent heat out of the flue gas. That only happens when the water coming back to the boiler is below the flue-gas dew point — roughly 130°F return, give or take a few degrees depending on combustion.

Above that line, a "95% AFUE" condensing boiler runs at about 85 to 88%. That's barely better than a good conventional boiler that costs less and lasts longer. To actually see 98% you'd need return water around 65°F — too cold to heat a room with.

Now put that together with oversizing. A condensing boiler on a system designed around 180°F water — the old fin-tube baseboard standard — may sit at 85 to 88% for most of the heating season even with outdoor reset. On that kind of legacy distribution, condensing sometimes doesn't start until outdoor temperatures are around 50°F and up.

So the homeowner paid extra — condensing units cost roughly 15 to 20% more and need their own sidewall flue — and got conventional-boiler efficiency. The equipment isn't the problem. The design around it is.

The Denver part: altitude

At Front Range elevations the air is thinner. Fewer molecules per cubic foot means less oxygen for combustion, and most gas appliances lose rated output because of it. That's the derate.

The trap is applying one memorized altitude number to everything. There are really two different numbers answering two different questions.

Sizing the gas line uses the serving utility's published altitude factor at the site's actual elevation — not the nominal 5,280 ft "Denver" number. Code requires the adjustment above 2,000 ft but doesn't prescribe the factor; the utility's table supplies it. And the serving utility varies by address around here, not just by town.

Derating the appliance uses the manufacturer's published figure for that exact model and altitude configuration. That one's model-specific in both directions. Some standard units publish 4% per 1,000 ft. Some high-altitude variants publish no derate to 5,400 ft, then 1.6% per 1,000 ft. And a fan-driven mod-con with electronic air/fuel control — the IBC SL G3, for example — is certified to hold its rated input to 12,000 feet, so it takes no derate at all when the altitude parameter is set at commissioning.

Swap one of those for the other and you get the wrong boiler. It's also why I pull the derate fresh for the actual address and the actual model, every job.

What right-sizing looks like

The equation isn't complicated. Required nameplate input equals design heat loss, multiplied by a pickup factor, divided by thermal efficiency, divided by the gas derate.

Every term is a real number from a real place. Design heat loss comes from room-by-room analysis of your house — envelope, windows, orientation, infiltration — not square footage. Pickup factor covers bringing the distribution mass up to temperature: about 1.25 to 1.5 for cast-iron radiators or black iron, closer to 1.0 to 1.10 for new low-mass distribution. Thermal efficiency runs about 0.95 for a mod-con, about 0.82 for cast iron.

Then I check the answer three ways: the calculated load, the home's actual fuel use, and the capacity of the radiation already installed. When all three agree you're within 5 to 10% of the real load.

There are rules of thumb for Denver — roughly 25 BTU/h per square foot for newer tight construction, roughly 40 for an older brick bungalow. I use them exactly once: as a sanity check after the math is done. If the calculated load lands far off those, I go re-check my work. They're a cross-check, never a sizing method.

When this isn't for you

If your boiler is oversized and you're not replacing it, don't let me talk you into tearing it out. An oversized boiler that runs is still heating your house. Right-sizing pays off at replacement time, not as an emergency.

And right-sizing doesn't automatically mean a condensing boiler. If you have cast-iron radiators or fin-tube baseboard that needs 160 to 180°F water, the return will rarely drop below that 130°F line — so a durable, well-sized cast-iron boiler can be the honest pick. I'd rather put in the right conventional boiler than the wrong condensing one.

The point isn't the brand on the front. It's whether the number on the nameplate came from your house.

More on how I approach this work: https://www.bydesignhydronicsandair.com/boiler-installation-replacement-denver

If a boiler replacement is on your horizon, book a site review at https://www.bydesignhydronicsandair.com/contact-us-1. I'd rather look at the system before anything gets ordered — I'll walk the mechanical room, look at what your emitters actually need, and give you a real number.

— Ryan Van Inwegen, ByDesign Hydronics & Air, Littleton, CO