Boiler and Radiant Heat in Centennial, CO — Hydronics Only

Aug 26

7 min read

Centennial is a young city built out of old houses. It incorporated in 2001, but the neighborhoods inside it — Southglenn, Willow Creek, Foxridge and the streets around them — went up from the mid-sixties through the late seventies. The city is new; the mechanical room usually isn't.

That matters for one specific reason. Baseboard hot water from that era was sized on the assumption of hot water, around 180°F at design. A condensing boiler only condenses when return water comes back below about 130°F. Above that it runs at roughly 85%, which is what a mid-efficiency boiler does. Bolt a mod-con onto emitters that need 180°F and you've paid for condensing equipment that will never condense.

That's not an argument against a mod-con here. It's an argument for doing the arithmetic first.

Why won't my new boiler condense in a Southglenn ranch?

Because it's making water hot enough for baseboard sized for a different job.

Here's the part most people are never told: the baseboard is usually oversized too. It was laid out fast, by the foot, in a house with the insulation and windows of 1968. If that house has since had windows replaced, attic insulation added, or the rim joist sealed, the real heat loss dropped and the baseboard didn't. Front Range heat load runs around 25 BTU/hr/ft² in newer tight construction and around 40 in older brick with some weatherization. A weatherized seventies frame ranch lands between those, not above them.

So the honest question isn't "will 180°F baseboard condense." It's "what water temperature does this house actually need on a design day, and how many hours a year is it near design?" Often the answer is that 150°F covers most of the winter and 130°F a good part of it — meaning a mod-con with a real outdoor reset curve condenses most of the season and gives it up only on the coldest mornings. That's a fine outcome. What isn't fine is finding out afterward.

We settle it with a room-by-room heat loss, compared against the baseboard already on the wall, foot by foot. Two numbers come out. The first is how big the boiler needs to be — usually far smaller than what's down there now. Most boilers replaced in the Denver metro are two to three times larger than the building needs, because each replacement was sized off the last one and the padding compounded. In a ranch that's been through two or three boilers since it was built, that has had time to compound. Why oversizing is the default failure mode is worth ten minutes.

The second number is the supply temperature the existing emitters need at design. That one decides whether a condensing boiler makes sense, whether adding baseboard in two rooms buys you low enough water, or whether the right call is a well-controlled non-condensing boiler and no pretending.

Call (303) 908-5658 or book a site review and we'll start there.

Half my house is forced air and half is baseboard — is that normal here?

In Centennial, yes — it's one of the more common things we walk into. Plenty of these houses were built forced-air and had hydronic added later: a finished basement, a bump-out, an addition over a slab the trunk line could never reach. Others were built with baseboard and got forced air added for cooling, and now the two argue over the same thermostat.

Mixed systems aren't a problem in themselves. They become a problem when nobody ever decided which system owns which load. Sometimes the fix is a control strategy and a pump curve, not equipment.

Can I put radiant in a finished basement without tearing up the floor?

Usually, yes — from underneath, if the joist bays are open or can be. Staple-up radiant into existing bays is the standard ask in these houses. Tubing goes against the subfloor with plates, insulation goes below it so the heat goes up instead of into the ceiling below, and the loop runs at a water temperature the rest of the system can live with. It works. It's also slower and less forgiving than tube in concrete, and the finish floor matters — thick carpet and pad over staple-up will eat much of what you're trying to deliver.

The other honest caveat: a radiant floor is not a warm floor. It's a floor that heats a room. If the room's heat loss is small — and a below-grade finished basement here usually has a small one — the floor spends most of the winter at a temperature your feet can't pick out from room temperature, while the room stays exactly where you set it. That's the system working. What it actually feels like, and the radiant floor heating page for scope.

Pouring new slab instead — a garage floor, an addition — and tube depth is permanent. Tube sitting at the bottom of a 4" pour instead of tied at mid-height raises the required water temperature by roughly 7°F for the life of the building. Chairs at 18 to 24 inches, checked before the truck shows up.

What happens to my water heater when the boiler comes off the chimney?

This is the item that catches people in these houses, because so many of them have a boiler and a gas water heater sharing one masonry chimney.

Take the boiler off that chimney, put it on a sidewall vent, and the water heater is alone in a flue sized for two appliances. It may no longer establish draft, and combustion gas — carbon monoxide included — can spill into the basement instead of going up.

The check is the Seven Times Rule: the largest common stack must not exceed seven times the area of the smallest draft-hood outlet. A 12×12 chimney is 144 in². A 4" water heater flue outlet is 12.56 in², and seven times that is 87.9 in². 144 is not 88, and it isn't close. The water heater needs a plan — a liner, a power vent, or a change of appliance — and that plan belongs in the boiler quote, not a phone call three weeks later. Full write-up here.

Does 5,800 feet change how the boiler is sized?

Yes, once.

Centennial sits around 5,800 feet, a few hundred above Denver. Combustion equipment loses capacity in thin air, and manufacturers publish altitude derate tables for exactly this. The derate gets applied one time, to the input rating, at the sizing stage.

What we see instead is one of two errors: it's ignored, or it's applied twice — once by the sizing and again by a rule-of-thumb safety factor stacked on top — leaving a boiler even further oversized than it already was. Once. Not zero times, not twice.

Do I need a heated driveway in Centennial?

Probably not, and we'd rather say that than sell you one.

Centennial is flat to gently rolling. Most driveways here are short, level, and face a plowable street. Snowmelt earns its keep on a steep pitch, a north-facing slab that never sees sun, a shaded walk that ices over and stays that way, or an entry someone has to keep clear as a liability matter. If none of that describes your property, a shovel and a decent snowblower will outperform the economics.

Where it is justified, understand the scale: the reference snowmelt load is about 126 BTU/hr/ft² at 5°F air, 10 mph wind and a 38°F surface, per the Uponor Snow & Ice Melting Manual. A modest driveway can carry a bigger load than the house it's attached to. Snowmelt is a heat plant with a driveway attached. The snowmelt page covers scope — tubing goes down before the concrete, on 100 psi air held through the pour with the gauge visible.

Why is the water from my boiler drain black?

Because iron and oxygen have been meeting in your loop for years. The product is magnetite — fine black iron oxide, particles down to about 5 microns. It's abrasive, and modern high-efficiency circulators have permanent-magnet rotors, so the pump collects it onto the one part that has to spin.

Check it in five minutes. Draw a pint from a boiler drain into a clear glass jar. Slightly gray with a faint metallic smell is a healthy closed system. Black, gritty or oily is a circulator failure forming. On a baseboard system topped off every fall for a decade because of a slow leak nobody chased, black is what we usually find — every gallon of make-up water is a fresh delivery of oxygen.

Who inspects the work in Centennial?

Arapahoe County handles permitting and inspection here. We pull the permit and we're there for the inspection — gas, venting and combustion air included. Colorado license LIC00254926. If a bid you're comparing doesn't mention a permit, ask why.

What drives the cost of a boiler job here?

Not the badge on the boiler. Four things, roughly in order:

What the existing venting forces. A shared masonry chimney with an orphaned water heater is a different job than a boiler already venting out a sidewall.

How much near-boiler piping has to be rebuilt. Primary-secondary or a properly sized hydraulic separator, real air and dirt separation, isolation you can service around. On a system that's been added to two or three times over the decades, that's often the bulk of the labor.

Zoning. How many zones, valves or pumps, and whether the existing wiring can be reused.

Whether the emitters need help to get water low enough to condense. Sometimes that's two lengths of baseboard. Sometimes it's nothing.

We don't publish prices — the same boiler in two Centennial houses two streets apart can be two different jobs. And if the finding is that the system needs a repair rather than a replacement, you'll hear that. We'll tell you what we find and what it would take to make it right, without a running commentary on whoever was there before.

Where do you work?

Littleton, Denver, Highlands Ranch, Centennial, Englewood, Greenwood Village, Columbine, Ken Caryl, Lakewood, Golden, Wheat Ridge, Arvada, Morrison, Castle Rock and Parker. Regularly into Evergreen, Conifer and Idaho Springs. Statewide for larger boiler-plant work.

Colorado license LIC00254926. Service-area contractor — no storefront and no office in Centennial. We come to the building. Boilers, radiant floors and snowmelt are all we do.

Tell us what the system is doing and what's already been done to it. Call (303) 908-5658 or book a site review.