Is a Tankless Water Heater Worth It in an Idaho Custom Home?

TL;DR: A tankless water heater is worth it in an Idaho custom home when you have gas, you plan to own the house a long time, and your hot water demand is spread across the day instead of stacked into one hour. It is not automatically the better choice. Our groundwater comes out of the ground cold enough in winter that a tankless water heater here does roughly twice the work the same unit does in Phoenix, which means fewer gallons per minute, bigger gas lines, and a real conversation about propane. On a 4,000-square-foot house with five bathrooms, two well-placed tanks and one properly sized tankless bank will both deliver. What decides it is fuel, mechanical room space, water quality, and whether anyone is actually going to maintain the thing. Here is how I work through it on every build.

What are my water heater options, in plain English?

Four, and they are easy to tell apart once somebody stops using trade shorthand.

A storage tank heats 40 to 80 gallons and holds it hot until you use it. A tankless water heater holds nothing; it fires a burner the moment you open a tap and heats water as it flows past. A heat pump water heater is a tank that steals heat out of the surrounding air instead of making heat with an element, like a refrigerator running backward. An indirect tank is a storage tank with no burner of its own, heated by a boiler you already have for something else.

That last one gets skipped constantly, and it shouldn’t. If your plans already include radiant floor heat, you are installing a boiler anyway. Hanging an indirect tank off that boiler is frequently the cheapest, quietest, longest-lived hot water in the whole house, and almost nobody asks for it by name.

Is a tankless water heater worth it in an Idaho custom home?

Yes, when three things are true: you have gas service or a propane tank big enough to feed it, you are building the house to keep, and your family does not run every hot water fixture inside the same 40 minutes. Under those conditions a tankless water heater will outlive two storage tanks, free up a sizable chunk of mechanical room floor, and never leave anybody cold at the end of a long weekend with a full house.

No, when you are all-electric on a 200-amp rural service, when the mechanical room sits in an unheated space, when nobody is going to flush the unit once a year, or when the house is built on a budget that is already tight in places that matter more. A 50-gallon tank that gets replaced in year eleven is not a failure. It is a reasonable decision.

What I will not tell you is that a tankless water heater is an upgrade in the way a better window or thicker insulation is an upgrade. It changes how hot water arrives, not how much the house costs to run in any way you will notice on a bill. If you are ranking which custom home upgrades are worth the money, this one sits in the middle of the list, not the top.

Why does Idaho’s cold groundwater change the math?

Because a tankless water heater is rated on temperature rise, and ours is brutal.

Temperature rise is just the difference between the water coming in and the water you want coming out. In midwinter, well water in Teton Valley and across eastern Idaho comes in around 40 to 45 degrees. If you want 120-degree water at the tap, that is a 78-degree rise. In a warm climate with 70-degree incoming water, the same tap is a 50-degree rise. Same house, same shower, completely different load.

Here is the arithmetic a spec sheet hides. Gallons per minute equals usable BTU divided by rise, divided by 500. Take a big residential unit, 199,000 BTU input at roughly 95 percent efficiency, so about 189,000 usable BTU:

• At a 50-degree rise: 189,000 ÷ (50 × 500) = 7.6 gallons per minute
• At our 78-degree rise: 189,000 ÷ (78 × 500) = 4.8 gallons per minute

That is the whole story in two lines. The box on the pallet says 9 GPM. In a Driggs winter that unit is a 4.8 GPM appliance. Now put real fixtures against it: a shower head runs 1.8 to 2.5 GPM, a kitchen faucet 1.5 to 2.2, and a freestanding tub filler, the kind that shows up on every set of plans crossing my desk, runs 4 to 7 GPM by itself. One tub filler can consume an entire tankless unit while somebody upstairs is waiting on a shower.

This is why I size for January, not for the average. A tankless water heater that is perfect in September and embarrassing in February was sized by somebody reading the front of the brochure.

Do you even have gas? In Teton Valley, probably not piped

This single question kills more tankless plans than the physics does, and it comes down to geography.

Intermountain Gas lists Idaho Falls, Ammon, Iona, Rexburg, Menan, Firth, Basalt, Lewisville and Parker among the towns it serves. Teton County, Idaho is not on that list — no Driggs, no Victor, no Tetonia. So if you are building on the Teton Valley side of the pass, “gas” does not mean a meter on the wall. It means propane in a tank on your lot, delivered by truck, on a road that has to stay plowed in February.

That matters in three concrete ways. A 199,000 BTU unit wants a three-quarter to one-inch gas line and its own sealed vent, not a tee off whatever is already there. Two units stacked for a big house is close to 400,000 BTU, which routinely means a larger regulator and a step up from a 250-gallon to a 500-gallon propane tank. And propane is a commodity you buy by the truckload, so the operating cost conversation is different from a metered utility.

None of that is a dealbreaker. It is a design decision, and it belongs in the drawings before anybody digs. We coordinate the mechanical layout, the gas sizing and the propane tank location during the design agreement, alongside the well and septic work and the rest of the site plan — you do not go assemble that team yourself.

What about a heat pump water heater?

I like them, and they need to be placed deliberately in a cold climate or they will disappoint you.

A heat pump water heater pulls heat out of the air around it, so it runs two to three times more efficiently than a standard electric tank. The catch is in the install conditions. ENERGY STAR calls for roughly 1,000 cubic feet of air space around the unit and an interior space that stays between 40 and 90 degrees year-round, and notes that in colder spaces the unit may drop into backup electric-resistance mode, which is exactly the expensive mode you bought it to avoid.

Translate that to an Idaho house. A heat pump water heater does not belong in an unheated garage, and it does not belong in a vented crawl space. It belongs inside the conditioned envelope, in a mechanical room with real air volume or with ducted intake and exhaust. The other thing nobody mentions: it cools and dries the room it sits in. Put it next to a finished basement media room and somebody is going to complain in January. Put it in a mechanical room that runs warm, and it gets more efficient.

Where your mechanical room ends up is partly a foundation decision, which is one more reason the basement versus crawl space versus slab question gets settled early rather than late.

Tank vs. tankless vs. heat pump, side by side

This is the comparison I actually walk through at the table, with Idaho conditions baked in rather than national averages.

Factor	Storage tank (gas or electric)	Tankless water heater (gas)	Heat pump water heater
Hot water supply	Finite. 50 gallons, then recovery time	Endless, but capped at gallons per minute	Finite, with slower recovery than gas
Cold-climate penalty	Minor. Longer recovery in winter	Significant. Output drops about 35% at our winter rise	Significant if the space is cold or small
Floor space	Largest. Needs real footprint plus drain pan	Smallest. Wall-hung, frees the floor	Largest, plus about 1,000 cu ft of air around it
Fuel needed	Gas, propane or standard electric	Gas or propane, upsized line and dedicated vent	Electric only, but modest draw
Maintenance	Flush occasionally, replace anode rod	Annual descaling is not optional on well water	Clean the air filter, keep the condensate clear
Typical service life	8–12 years (market data, Fixr 2026)	15–20 years (market data, Fixr 2026)	Longer than standard electric; varies by install
Best fit here	Simple layouts, tighter budgets, all-electric lots	Gas or propane, long-term owner, spread-out demand	Conditioned mechanical room, all-electric house

Read the “cold-climate penalty” row twice. Everything else on that chart is the same conversation you would have in Texas. That row is the Idaho-specific one, and it is the row most quotes ignore.

How big does it need to be for my house?

Not by square footage. By simultaneous fixtures, which is a different question and the only one that matters.

The method is simple. List the fixtures that could realistically run at the same time on your worst morning, add up their flow rates, then size to that number at a 78-degree winter rise. Not the average rise. The winter one.

House profile	Realistic simultaneous demand	What I generally spec
2,500 SF, 2.5 baths, no large tub	About 4 GPM (two showers plus a sink)	One 199K tankless, or a single 50-gal gas tank
3,500 SF, 4 baths, one freestanding tub	About 7 GPM (tub filler plus a shower)	Two tankless units banked, or a 75-gal gas tank
5,000 SF and up, 5 baths, tub plus multi-head shower	9–12 GPM	Two to three banked units, or boiler plus indirect tank
All-electric lot, any size	Whatever the electrical service allows	Heat pump water heater, often two, inside the envelope

On that last row, here is why whole-house electric tankless almost never pencils in Idaho. Four gallons per minute at a 78-degree rise is 156,000 BTU per hour, which is about 46 kilowatts, which is roughly 190 amps at 240 volts. On a house with a 200-amp service, that is the entire service for one appliance. I have never specified one on a custom home and I do not expect to.

Why does hot water take two minutes to reach the far bathroom?

Because hot water has to physically travel from the heater to the tap, and in a big house that is a long pipe full of cold water that has to be pushed out first. This complaint has nothing to do with which heater you bought, and a tankless water heater can actually make it feel slightly worse, because the burner has to light before the first hot water even starts moving.

There are three fixes, and the right time to choose one is during plumbing rough-in, not after drywall.

A dedicated return line is the real answer: a small pipe that loops back from the far end of the house so hot water is always close by. A crossover valve at the farthest fixture is the retrofit version, cheaper and less effective because it uses the cold line as the return. Demand control is the part people get wrong — a button, motion sensor, or smart control that runs the pump only when someone is actually about to use water. An always-on recirculation loop on a big house burns energy around the clock and short-cycles a tankless unit for no good reason.

Roughing in a return line while the walls are open costs a fraction of what it costs to add later, when it becomes a demolition project. It is one of the cheap decisions that quietly separates a house that feels expensive from one that just cost a lot, and it is the same logic behind rough-ins for smart home features and leak detection.

What does this cost, and what is not in that number?

Market data first, so you have a reference point that is not mine. Fixr’s 2026 figures for a typical home put installed cost at roughly $600 to $2,500 for a conventional tank, $1,000 to $4,000 for tankless and $1,200 to $4,600 for a heat pump unit. Those are market numbers for an average house, not SwagerBuilds pricing.

Now the part the market data leaves out, because on a custom home it is most of the cost. Those figures do not include upsizing a gas line, a dedicated sealed vent through a roof assembly, a larger propane tank and regulator, a second unit when one will not carry the fixture count, a recirculation return line, a condensate drain and neutralizer with a pump if the unit sits below grade, isolation valves for annual descaling, or the water treatment that well water demands. A five-bath house with two banked units, recirculation and softening lands above the top of every one of those ranges, and anybody quoting you the bottom of the range is quoting a water heater, not a hot water system.

For reference on the whole project: SwagerBuilds custom homes start at $400 per square foot and go up on the Teton Valley and Idaho side, and start at $1,000 per square foot and go up in Jackson Hole. Design is sold as a fixed design fee, quoted per project. Mechanical selections like this one are made inside that design work, before a construction price is signed, which is the entire point of getting to a real number before you sign instead of discovering it later. If you want the broader picture, we keep an honest list of the hidden costs of building a custom home in Idaho and how construction allowances hide decisions like this one inside a single line item.

Will the 2029 water heater rule affect my house?

Not the unit you install in 2026. It will affect the one that replaces it.

The Department of Energy finalized amended efficiency standards for water heaters that take effect in 2029. The headline change is on the electric side: DOE expects more than half of newly manufactured electric storage water heaters to use heat pump technology, up from about 3 percent today. Under the final rule, electric storage units above 35 gallons are headed toward heat pump technology, and gas storage sees a more moderate efficiency bump. Gas tankless was largely left alone in that rulemaking.

The practical takeaway for somebody building right now is small but worth doing: if there is any chance a future owner puts an electric unit in that mechanical room, give the room the air volume, the clearance and the floor drain a heat pump water heater needs. That is free on paper today and expensive in 2031.

One more timing note, because it changes the arithmetic people did two years ago. The federal Energy Efficient Home Improvement Credit under Section 25C, which paid up to $2,000 toward a heat pump water heater, applies to property placed in service before December 31, 2025. If you are reading a 2024 blog post that leans on that credit to justify an upgrade, the credit is no longer there. Spec the equipment because it is right for the house, not because of a credit that expired. The same correction applies to a lot of the older math behind whether a green custom home is worth it.

What does a well and hard water do to a water heater?

It shortens its life, and it does it faster to a tankless water heater than to a tank.

Most of eastern Idaho and Teton Valley sits on hard water. Hard water means dissolved minerals, and when you heat it, those minerals come out of solution and plate onto the hottest surface they can find. In a tank, that means sediment on the bottom, which is annoying and slow-acting. In a tankless unit, it means scale inside a narrow heat exchanger, which is the single most common reason a tankless water heater dies early.

Three things prevent it, and all three are cheap at rough-in. Install isolation valves so the unit can be descaled in 45 minutes without cutting pipe. Put a whole-house softener or conditioner in the plan rather than treating it as an afterthought. Then actually flush the unit once a year. That last one is a homeowner commitment, and if the honest answer is that nobody is going to do it, a storage tank is the smarter specification. I would rather hear that at the design table than warranty it out in year six.

Water quality is a well decision as much as a plumbing decision, which is why it gets tested and planned alongside the rest of the build sequence rather than after the house is dried in.

What I spec, and how we decide it on your build

Twenty years in construction and I have never once treated the water heater as a default line item, because it is one of the few mechanical choices a homeowner notices every single day.

My general starting points, which get overridden by the specific house every time: if there is piped natural gas and the fixture count is moderate, a single properly sized tankless water heater wall-hung in the mechanical room. If there is a boiler for radiant floor heat, a boiler plus indirect tank, nearly always. If the lot is all-electric, a heat pump water heater inside the conditioned envelope with the air volume it needs. If the house has five baths and a tub filler somebody is going to use, two banked units and a demand-controlled return line. And if the budget is working harder somewhere else in the house, a well-placed gas tank with no apology.

This gets settled during the design agreement, on paper, with the mechanical room drawn and the gas and electrical loads sized — not negotiated in the field when the plumber is standing there. I am involved in every phase of every build, and there are 24/7 cameras on every job, so when the mechanical room gets roughed in you can watch it happen. That is the same reason the rest of the envelope decisions get made early too: insulation, windows, the heating system, the roof assembly, and the energy-efficiency features worth paying for are one coordinated set of decisions, not six separate arguments.

Built different. Built right. Built by SwagerBuilds.

Frequently asked questions about tankless water heaters

Does a tankless water heater really give you unlimited hot water?

Unlimited in duration, limited in flow. A tankless water heater will run a single shower all day, but it can only heat so many gallons per minute at once. At our winter incoming water temperature, a large 199,000 BTU residential unit delivers roughly 4.8 gallons per minute, which is two showers or one big tub filler, not both.

How long does a tankless water heater last in Idaho?

Market data puts tankless service life at 15 to 20 years versus 8 to 12 for a storage tank. On untreated hard well water with no annual descaling, I have seen that advantage disappear entirely. The maintenance is what buys the extra decade.

Can I put a heat pump water heater in my garage in Idaho?

Not in an unheated one. ENERGY STAR specifies an interior space that stays between 40 and 90 degrees year-round with about 1,000 cubic feet of air around the unit. An unheated Idaho garage fails the temperature requirement for months at a time, so the unit falls back to electric-resistance heating and the efficiency you paid for disappears.

Is electric tankless an option if my lot has no gas?

Practically, no. Delivering 4 gallons per minute at a 78-degree winter rise takes about 46 kilowatts, roughly 190 amps at 240 volts, which is the entire service on most houses. On an all-electric lot the right answer is a heat pump water heater, or two.

Should I run a hot water recirculation line in a large custom home?

Yes, rough it in. A dedicated return line with demand control costs a fraction during plumbing rough-in compared to adding it later, and it is the actual fix for waiting two minutes for hot water at the far end of the house. Skip the always-on pump; use a button or sensor.

Planning a build in Teton Valley or eastern Idaho?

Hot water is one of a few hundred decisions that are cheap on paper and expensive in the field. We settle them during design, in writing, before a construction price gets signed — drawings, engineering coordination, interior selections, permitting and construction all run through us.

If you are building in Driggs, Victor, Tetonia, Rexburg, Rigby, Idaho Falls or Jackson Hole, start a conversation with us or call (208) 520-0636. If you are still a year out and just reading, that is fine too — start with cost per square foot to build in Teton Valley, cost-plus versus fixed price, and how involved you actually have to be.