Can One Appliance Heat Your House and Make Your Hot Water?

Two things in a Colorado basement tend to give out within a couple of years of each other — the boiler and the water heater. So every fall I get the same question: can one box replace both?

Sometimes, yes. The appliance that does it is a combi, and in the right house it is a clean answer. In the wrong house it is a hot shower that goes lukewarm the moment someone starts the dishwasher.

Here is how I tell those two houses apart, before anyone spends money.

What a combi actually does

A combi boiler makes space heat and domestic hot water from one burner. The heating side behaves like any other boiler. The hot-water side is the different part: there is no tank. You open a tap, a flow switch tells the appliance to fire, and the water is heated on its way past a plate heat exchanger.

That single design choice drives everything else. A combi is limited by flow, not by gallons. A tank runs out and then recovers. A combi never runs out — but it can only deliver so many gallons per minute, and the moment you ask for more than that, every fixture in the house gets cooler at once.

The number that decides it is gallons per minute

Before I look at a single piece of equipment, I work out what the house actually asks for at the same time. Rough hot-side flows:

• A shower — 1.5 to 2.5 gpm, depending on the head.

• A tub fill — about 4 gpm, and a big soaking tub wants it for a long time.

• Kitchen sink — about 1.5 gpm. Dishwasher, 1 to 2.5. Clothes washer, 1.7 to 2.

• Bathroom sink — 0.5 to 1.5 gpm.

You do not add up every fixture in the house. You add the ones that credibly run together on a weekday morning. Two showers and a kitchen sink is about 5.5 gpm. Four showers and a dishwasher is about 10.

The practical ceiling for a residential combi is roughly 5 to 6 gpm at a real winter temperature rise. Under that, a combi is a legitimate answer. Over it, it is the wrong tool, and no brand on the market changes that.

Hot and cold distribution on a whole-house rough-in. Peak demand is about how many of these run at the same time, not how many there are.

The rating on the box is not the rating in January

This is the part that catches homeowners, and it catches plenty of contractors too.

A combi's hot-water rating is always published as gallons per minute at a stated temperature rise — usually a 70°F rise in this market. Rise is simply delivery temperature minus the temperature of the water arriving at the house. Call delivery 120°F at the fixture, after mixing. The incoming water is the variable, and in January it is a good deal colder than it was in July.

As the rise grows, deliverable flow shrinks, because the burner's output is fixed and every gallon now needs more heat out of it. A unit rated 5.0 gpm at a 70°F rise gives about 4.7 gpm at a 75°F rise, and about 4.1 gpm at an 85°F rise.

Same appliance, same house — roughly a fifth less hot water on the coldest mornings of the year, which are precisely the mornings everyone wants a long shower. So I size on the winter rise rather than the brochure rise, and I put every competing spec sheet on the same rise before comparing them.

Then it still has to heat the house

On a combi, hot water sets the size and space heat has to fit inside it. So the order is: size to the hot-water peak first, then confirm that the same appliance covers the room-by-room heat loss of the house when nobody is running water.

Combis give hot water priority — a tap opens, full fire goes to hot water, and space heat pauses for a few minutes. In a radiant house, with a slab holding heat, nobody notices. In a house with small, fast emitters and a large load on a cold night, somebody might.

The gas line is often the real limit

A combi fires hard, because it has to make hot water on demand with no storage to lean on. The gas piping and the meter have to carry that firing rate, and on older houses they frequently cannot.

A combi fires hard with no storage behind it. The gas service has to carry that rate — on an older house, it often does not.

I check this early, because it can change the answer entirely. Finding out after the appliance is on the wall that the gas service needs upsizing is the kind of surprise nobody enjoys.

Two things the spec sheet will not tell you

Minimum activation flow. A combi has to sense a certain flow before it will fire at all — modern units around 0.26 to 0.4 gpm, older ones 0.4 to 0.6. A slow hand-wash at a low-flow aerator can sit below that threshold, which means the burner never lights, or lights and drops out while you stand there. It is worth checking against the smallest draw in the house, not the largest.

The cold-water sandwich. Turn a tap off and back on — to soap up, to shave — and you get hot, then a slug of cool, then hot again. The heat exchanger and the pipe downstream held water that cooled while the burner was off, and there is a second or two of relight and ramp on top of that. The fix is thermal mass: a small buffer tank downstream, or a unit with one built in. A recirculation loop helps as well.

When I put in a boiler and a separate tank instead

The alternative is a boiler plus an indirect water heater. An indirect makes no heat of its own — it is a tank with a heat exchanger inside it, and the boiler's own water circulates through that exchanger to heat the domestic water.

Boiler plus a separate tank. The heating side stays independent, and stored water absorbs the hot-water surge instead of the burner having to meet it live.

That buys storage back. A tank absorbs a surge a combi has to meet in real time, which is why I go this way whenever there is a big soaking tub, more than a couple of simultaneous showers, a hot-water recirculation loop, or demand above the combi ceiling.

Indirects have a trap of their own, and it is a quiet one. Their published gallons assume hot boiler water — catalogs rate them at a 180 to 200°F supply. Drop the boiler supply by 20°F and continuous output falls about 15 percent. Hang an indirect on a condensing boiler that runs 120 to 140°F on a mild day, and the gallons printed in the brochure do not turn up in the shower.

One more, while you are reading tank specs: a first-hour rating includes the water already stored, and a continuous draw does not. Comparing one brand's first-hour number against another brand's continuous number is the most common way these get chosen wrong.

Two questions worth asking before you buy either one

How hard is the water here? A combi's heat exchanger scales up exactly where the heat transfer happens, so descaling is a design assumption rather than optional maintenance. Above roughly 7 grains per gallon I plan on treatment, and I spec the isolation valve kit at installation so a flush is a 45-minute job instead of a teardown. Your water provider will give you the number if you ask.

What is the combined efficiency? AFUE rates space heating only. UEF rates water heating only. Neither one honestly describes a single appliance doing both jobs. For that there is a combined annual efficiency figure measured under ASHRAE Standard 124 — and a high steady-state number on the sticker does not guarantee a good seasonal one.

If you are replacing both this winter

There is a real answer here, and it comes out of your fixtures, your winter water temperature, your gas service and how your house is heated — not out of which appliance happens to be in fashion.

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

If you would rather I look at it, book a site review at https://www.bydesignhydronicsandair.com/contact-us-1. I will count the fixtures that actually run together, check what the gas service can carry, and tell you which of the two fits. Sometimes the answer is that the boiler is fine and only the water heater needs replacing — that is a perfectly good result, and I will say so.

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