You know the drill – you’re the last person in the shower, the water turns cold, and you’re rinsing shampoo out of your hair with a trickle. That 40-gallon tank just can’t keep up. Or maybe your energy bill has been creeping up, and you’ve heard tankless heaters are the answer. So you start looking into how to switch from a tank water heater to a tankless. And the internet gives you a dozen conflicting opinions.
This article walks you through what actually matters – the pipe sizes, the venting, the gas line math, and the hidden costs most guides skip. I’ve installed a few of these, fixed a few, and learned the hard way what gets overlooked. You’ll walk away knowing exactly what to ask a contractor – or what to brace for if you’re doing it yourself.
A.A
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- On/Off switch for Suburban SW Water Heater 110 element. This switch allows you to make sure the element stays off it the water hea…
One small note before we dive in: If you own an RV or a small cabin with a Suburban SW series water heater and you’re temporarily keeping the old tank unit, there’s a handy little switch – the Water Heater Element Switch from A.A. – that lets you turn off the 110V element when the tank is empty. That prevents burnouts during winterization. It’s not a tankless part, but it’s a cheap insurance piece while you still run a tank system. Check the current price on Amazon if that sounds useful for your setup.
Myth: Tankless Heaters Always Save You Money
They can. But not always. The real savings depend on how much hot water you use, your fuel type, and your climate.
A tankless gas unit is about 82-94% efficient on paper, while a modern tank heater sits around 60-70%. That 20-point gap sounds huge. But if you live alone and use 20 gallons of hot water a day, the dollar difference might be $30 a year. Not enough to justify a $2,000 install.
On the flip side, a family of five running two showers back-to-back will see real savings – $150-250 per year in gas alone, plus the tankless doesn’t waste heat keeping 40 gallons hot 24/7. You also free up floor space and get endless hot water. But those benefits come with trade-offs I’ll cover next.
The First Thing Everyone Gets Wrong: Sizing
People think ‘just swap it out, same connections.’ No. You cannot replace a 50-gallon tank with any tankless and expect it to work the same. Tankless units are rated by flow rate – gallons per minute (GPM) at a specific temperature rise.
Here’s how to figure what you need. Start with your coldest incoming water temperature. In the northern US, that might be 40°F in winter. You want your shower at 105°F, so you need a 65°F temperature rise. A typical 0.7 GPM shower head plus a kitchen faucet running at 0.5 GPM means you need 1.2 GPM at that rise. Most tankless heaters can handle that. But if you run two showers, a dishwasher, and a washing machine at once – say 3.5 GPM – you need a larger unit or install two in parallel.
Manufacturers publish tables. Use them. If you undersize, you’ll get lukewarm water any time two taps are open. Oversizing wastes money on a unit you don’t need. I’ve seen people buy an 11 GPM unit for a one-bathroom house – it’s overkill and costs hundreds more upfront.
Gas Line and Venting: Not Optional Upgrades
This is the part that catches DIYers off guard. Most tank water heaters use a 1/2-inch gas line. A tankless heater often needs 3/4-inch or even 1-inch pipe. Why? A typical tank uses about 40,000 BTU per hour. A tankless can pull 150,000 to 200,000 BTU. That’s 3-5 times more gas. The existing line may not have the capacity. Running a new gas line from the meter can cost $500-1,500 depending on distance and obstacles.
Venting is another mess. A tank heater usually vents through a standard chimney or a 3-inch PVC for power-vent units. Tankless heaters require stainless steel venting (Category III) because exhaust temperatures are higher and more corrosive. That stainless vent pipe is expensive – around $20-40 per linear foot. And you often need a new roof termination. Total venting cost can easily hit $400-800.
Add in an electrical outlet (tankless needs a small amount of electricity for the control board and fan), a water shutoff valve kit, and a condensate drain line, and you’re looking at $1,500-3,500 for a professional install. That’s before the unit itself.
Comparing Tank vs. Tankless: The Real Numbers
| Factor | Tank Water Heater | Tankless Water Heater |
|---|---|---|
| Upfront cost (unit + install) | $800 – $1,500 | $1,500 – $3,500 |
| Lifespan | 10-12 years | 20+ years (with proper maintenance) |
| Energy factor | 0.60 – 0.70 | 0.82 – 0.94 |
| Space needed | ~16 sq ft floor area | Wall-mounted, 2-3 sq ft |
| Hot water supply | Limited to tank size | Endless (at rated flow) |
| Maintenance | Annual drain + anode check | Annual flush + descale |
| Cold water sandwich effect | None | Yes – cold burst between uses |
| Recovery time | 30-60 min | Instant |
Notice something: tankless lasts longer, but the payback period is long. If you know you’ll move in 5 years, skip it. If you’re staying put, it makes sense.
Myth: Tankless Installation Is a Weekend DIY Project
I’ve heard this one a lot. ‘Just mount it, connect the water lines, plug it in.’ The reality: you’re dealing with gas piping that must be pressure-tested, venting that must meet manufacturer specs to avoid carbon monoxide risk, and often a new gas meter if your existing one is undersized. Local codes require permits and inspections in most places. A leak at a gas fitting isn’t like a water leak – it can kill you.
I’m not saying nobody can DIY it. I’ve done it. But it took me three weekends, a borrowed pipe threader, and two trips to the supply house for fittings I didn’t know I needed. If you’ve never sweated copper or cut steel pipe, hire a licensed plumber. The $800 you save upfront isn’t worth the risk.
5 Questions People Actually Ask About Switching
Can I use my existing vent pipe?
Almost certainly no. Tank water heaters typically use 3-inch or 4-inch PVC for power-vent models or B-vent steel for atmospheric. Tankless requires stainless steel Category III vent, and the size depends on the unit and total vent length. Some manufacturers allow concentric venting, but mixing with old pipe is a no-go. You will need new venting.
Do I need a permit for converting to tankless?
In most jurisdictions, yes. Gas line work, venting, and structural support for a heavy hanging unit all require a permit. Unpermitted work can cause problems when you sell the house – inspectors flag unapproved gas lines. Plus insurance companies may deny claims if they find unpermitted modifications. Cost of a permit is usually $100-300. Just get it.
What happens to my electric bill if I switch from gas to electric tankless?
Electric tankless heaters draw massive current – 110-150 amps for a whole-house unit. Your existing panel might not support that. You could need a panel upgrade ($1,000-2,000). And electric rates in many areas make them more expensive to run than gas units. Unless you have cheap electricity (hydro or solar), stick with gas.
Will a tankless work with a recirculation pump?
Yes, but it kills the efficiency. A tankless unit fires up whenever flow is detected. A recirculation pump creates frequent small flows, causing the burner to cycle on and off constantly. That wears out the heat exchanger faster and wastes fuel. If you want instant hot water at every tap, get a tankless with a built-in bypass loop and a timer – or just accept the few seconds of cold water.
How often do I need to descale a tankless heater?
That depends on your water hardness. With hard water (8+ grains per gallon), descale every 6 months. With soft water, once a year. Use a pump and white vinegar or a specialty descaling solution. Hard water scale buildup is the #1 killer of tankless units. Skip maintenance, and you’ll buy a new heat exchanger in 5 years – which costs nearly as much as a new unit.
What to Remember When Making the Switch
- Get a firm quote that includes gas line upgrade, venting, permits, and disposal of the old tank. Don’t trust a verbal estimate that seems too low.
- Measure your current peak hot water demand – count fixtures running at once. Multiply shower GPM (showerhead rating) + kitchen + laundry. That’s your required flow.
- Check your incoming water temperature in winter. A northern house with 40°F ground water needs a bigger unit than a Florida home with 70°F water.
- Factor in maintenance cost. Vinegar and a pump cost $50 a year. Neglect kills the unit.
- Expect a ‘cold water sandwich’ – a burst of cold water between hot water uses for a few seconds – that’s normal. A recirculation loop or a small buffer tank can solve it, but adds cost.
- If your tank is in the garage or an unconditioned space, tankless units need freeze protection. Some have automatic freeze prevention, but in extreme cold you’ll need to insulate pipes and install a drip line.
- Consider a hybrid approach: keep the tank for low-demand periods and add a tankless for peak times. Not the cheapest option, but some families prefer the redundancy.
