You are shopping for a new boiler and you have hit the fork every Front Range homeowner hits: a combi that heats the house and makes hot water from one wall-hung unit, or a system boiler paired with a storage cylinder. Here is the honest answer the brochures will not give you. The right choice is not about the brand or the badge. It is decided by three local realities most sizing charts ignore: the BTU you actually get at 5,280 feet, how many fixtures run hot water at the same time in your house, and whether you have room for a storage tank. Get those three right and the boiler picks itself. This post walks each one, then tells you the case where a combi is the wrong call so you do not learn it the hard way in January.
The fastest way to skip the theory is to tell us what your house does. Call (720) 880-8064 or read how we handle boiler installation and replacement across the Denver metro, flat-rate quote before anything gets touched.
Combi boiler vs system boiler: the real difference
A combi boiler is a single unit that heats your home and produces hot water on demand, with no storage tank. You open a hot tap, the burner fires, and it heats water as it flows through. A system boiler heats your home too, but it sends hot water to a separate insulated cylinder that stores a reservoir, so the hot water is drawn from the tank rather than made the instant you need it.
That one structural difference drives everything else:
Neither one is better in the abstract. A combi in a two-bath home that never runs both showers together is a clean, space-saving fit. That same combi in a busy four-bath house is the unit everyone complains about at 7 a.m. The decision is about your house, not the technology, and the next three sections are the factors that actually settle it here.
Which boiler for a Denver home: matching the unit to your house
Start with the question that decides it fastest: how often does your house draw hot water from more than one place at the same time? That is the single most useful thing to know before you spend a dollar on a boiler.
"We size the recommendation to what your house actually does at peak, not to the square footage on the listing. A 3,000-square-foot home with one couple in it has different demand than a 1,800-square-foot home with four teenagers."
Here is the framework we use on-site, tuned for typical Denver Metro homes:
The mistake we get called to fix is a combi sized for the house's square footage instead of its peak simultaneous demand. Square footage does not open the taps. People do. A house is not too big for a combi because of its floor plan; it is too big for a combi because of how many hot draws land at once during the morning rush.
There is a middle path worth knowing about. A system boiler with a properly sized indirect cylinder often gives a larger Front Range home the most stable hot water for the money, and it plays well with the way our winters load a heating system. More on the cylinder below. If you only need hot water and are not tied to a hydronic heating system at all, it is worth comparing a boiler to a water heater if hot water is all you need before you commit to either boiler type.
Not sure which side of the line your house falls on? Talk it through with a licensed tech: (720) 880-8064.
Combi boiler altitude BTU derate: the Denver factor that shrinks a combi's margin
This is the section the national sizing guides skip, and it is the one that trips up combi installs on the Front Range more than any other. Gas appliances do not deliver their full rated output at altitude. There is less oxygen per combustion cycle at 5,280 feet, so a gas boiler operates at roughly 85 to 92 percent of its sea-level BTU rating up here.
Why does this hit a combi harder than a system boiler? Because a combi's hot-water output is a real-time race. It has to heat water fast enough, right now, to satisfy whatever taps are open. When you derate the BTU it can bring to that race, you directly cut the flow rate it can deliver at a usable temperature. A system boiler feeding a cylinder is not in that race. It heats the stored water over time, so a derate slows the recovery a little but does not throttle the hot water at your fixture the way it throttles a combi.
The practical result: a combi that looks perfectly sized on a sea-level spec sheet can come up short in a Denver home once the altitude derate is applied, especially in winter when the incoming water is coldest and the unit has to raise it further to reach temperature. That is why we spec boilers for Colorado conditions instead of national-average ratings.
"The combi that disappoints in February is almost always the one that was sized off the box, not off the altitude. We would rather size it right the first time than get the no-hot-water call during a cold snap."
One more altitude-linked point on the combustion side: our hard water compounds it. Denver Metro water runs 12 to 17 grains per gallon, and that mineral load scales a combi's compact heat exchanger, the very part doing the on-demand heating, faster than it scales a cylinder-fed system. A scaled exchanger loses output, which stacks on top of the altitude derate. On any tankless-style hot-water path here, a softener is worth it to protect that exchanger and hold the flow rate you paid for.
Want the derate and your gas supply checked before you buy anything? Get a flat-rate boiler assessment and we verify the sizing for your elevation and your meter first.
Hot-water demand and the indirect tank: when storage wins
If your house has real simultaneous demand, a system boiler with an indirect cylinder is usually the stronger Front Range answer, and it is worth understanding why the storage matters so much here.
An indirect tank is a storage cylinder heated by the boiler through a coil, rather than by its own burner or element. The boiler keeps a reservoir of hot water ready, and because that reservoir is already hot, every fixture pulls full flow at temperature the instant it opens. Two showers and the kitchen sink at once is a non-event for a properly sized cylinder, exactly the scenario that stretches a combi thin.
The trade-offs, stated plainly so you can weigh them:
There is a real cost trade here, and we are not going to pretend otherwise. A combi is typically simpler and cheaper to install because there is no cylinder to house, plumb, and heat. A system boiler with an indirect tank costs more up front and takes more space, and it earns that back in a high-demand house through hot water that does not fold when the house wakes up. We hand you both numbers, flat-rate, and you decide, because we do not earn more from one answer than the other. Tell us what is going on and we will come back with a real number.
When a combi is the wrong call
Since every article online is happy to sell you on the space-saving combi, here is the part they leave out: the case where a combi is the wrong choice, even though it is cheaper and smaller.
Do not put a combi in a house with three or more bathrooms where morning routines overlap, or in a home with a large soaking tub you actually fill, or anywhere two showers running at once is a normal event. In those homes the on-demand output, already trimmed by the altitude derate up here, gets split across fixtures and everyone gets lukewarm, low-flow water at the exact moment they want a hot shower. No brand of combi fixes a demand mismatch. It is a sizing and storage problem, not a quality problem.
And do not let anyone talk you into ripping out a sound system-boiler-and-cylinder setup to "modernize" to a combi if your household has the demand the cylinder is there to serve. That is a downgrade dressed up as an upgrade. We would tell you not to spend the money.
Frequently asked questions
Does a combi boiler need a hot water tank?
No, and that is the whole point of a combi. It heats water on demand as it flows through the unit, so there is no storage cylinder and no tank taking up space. That is exactly why a combi is a clean fit for a smaller home, a condo, or any place where utility space is tight. The trade-off is that a combi's hot water is limited by its flow rate rather than by a stored reservoir, so it can be stretched thin when several fixtures draw at once. If your house has that kind of simultaneous demand, the answer flips: you want a system boiler with a separate hot-water cylinder precisely so there is stored hot water to pull from. So a combi does not need a tank, but a high-demand house often needs the tank a combi does not have.
How does Denver's altitude change boiler sizing?
At roughly 5,280 feet, a gas boiler delivers about 85 to 92 percent of its sea-level BTU rating because there is less oxygen per combustion cycle. That derate matters most for a combi, whose hot-water output is a real-time race to heat water fast enough for the open taps. Trim the available BTU and you trim the flow rate it can deliver at temperature, especially in winter when incoming water is coldest. A system boiler feeding a cylinder is less sensitive because it heats stored water over time rather than on the spot. The takeaway is to size any boiler for Colorado conditions, not the national-average number on the box, which is a common reason a "correctly sized" combi disappoints in a Denver winter.
Combi or system boiler for a larger Front Range home with multiple bathrooms?
For most larger metro homes with three or more bathrooms or overlapping morning hot-water use, a system boiler with a properly sized indirect cylinder is the more reliable choice. The stored reservoir delivers full flow to several fixtures at once, which is the exact scenario that stretches a combi thin, and it sidesteps the altitude-derate ceiling on a combi's on-demand flow. A combi still makes sense for a smaller home, a condo, or a house with tight utility space and modest simultaneous demand. We size the recommendation to your peak demand and your available space, verify it for your elevation and gas supply, and quote it flat-rate before any work starts.
Get the right boiler for your house
Deft Plumbing is licensed, bonded, and insured in Colorado, and we serve the full Denver Metro along the Front Range, from Lakewood to Arvada and Brighton, with dedicated boiler repair and installation in Aurora for the elevation and permit specifics on the east side of the metro. Whether you are leaning combi or system, the next step is the same: a real recommendation sized for your house, your water, and your altitude instead of a spec sheet from sea level.
Call (720) 880-8064 or get a flat-rate boiler quote for your home. We diagnose first, size it for Colorado, and quote the total before we pick up a wrench. You know the cost before we start. No hourly surprises.