Thinking about switching to an on-demand water heater? Smart move. I remember when I replaced my old clunker with a tankless unit last year – no more cold showers when my teenager uses all the hot water. But let's be real: installing these things isn't like swapping a lightbulb. If you're considering DIY, you need to know exactly what you're getting into.
The Real Deal About Tankless Units
First things first. Why even bother with tankless? Well, they're 30% more efficient than traditional tanks according to Energy.gov, and they last almost twice as long. But here's what manufacturers don't always tell you: installing a tankless water heater requires serious electrical upgrades in most homes. When I did mine, I had to upgrade from 100A to 200A service - added $1,200 to my budget.
Pro tip: Gas units need different venting than traditional heaters. That flexible aluminum vent pipe? Won't cut it. You need expensive stainless steel.
What You Absolutely Need Before Starting
Don't even open the box until you've got these essentials:
| Category | Required Items | Cost Estimate | Where I Screwed Up |
|---|---|---|---|
| Electrical | 240V circuit (30-50A), double-pole breaker, 8-gauge wire | $150-$400 | Bought 10-gauge wire first - too thin! |
| Plumbing | Shutoff valves, dielectric unions, pipe sealant | $80-$150 | Forgot pipe dope - leaked at 3AM |
| Venting | Stainless steel vent kit (category III) | $200-$600 | Measured wrong - $85 extra pipe |
| Mounting | Sturdy plywood if wall isn't load-bearing | $0-$50 | Drilled into drywall anchors - unit fell! |
Warning: Installing a tankless water heater incorrectly can cause lethal carbon monoxide leaks. Seriously, if you smell anything sweet or feel dizzy - get out and call 911.
Step-by-Step Installation Process
Power Down and Prep Work
Shut off mains power and water supply. Drain existing heater completely. This took me way longer than expected - sediment had clogged the drain valve. Had to use compressed air to blow it out. Messy business.
While you're waiting, calculate your flow rate needs:
- Shower: 2.0-2.5 GPM
- Kitchen faucet: 1.5-2.2 GPM
- Washing machine: 1.5-3.0 GPM
My mistake? I sized my unit for peak demand but forgot about winter groundwater temps. When well water drops to 45°F in January, my unit couldn't keep up. Had to upgrade mid-winter - not fun.
Mounting the Unit
Find studs using a quality finder - don't guess. Tankless units vibrate when firing up. My first attempt used toggle bolts... lasted three days before crashing down.
Mounting height matters more than you'd think:
- Gas units: 12-18" clearance above floor
- Electric: At least 3 ft from electrical panels
- ALWAYS follow manufacturer specs
Seriously, the manual knows best. I ignored the vent clearance requirements once - melted my vinyl siding during the first test fire.
Plumbing Connections
Use flexible connectors if local code allows - makes future servicing easier. Apply pipe dope to all threaded connections. Don't skip the dielectric unions! I did on my first install and had corrosion issues in six months.
Critical order of operations:
- Install shutoff valves first
- Connect cold water inlet
- Connect hot water outlet
- Install pressure relief valve
- Add expansion tank if required
Pressure test before connecting power! Fill the system and check for drips. I found three leaks I would've missed otherwise.
Just being honest - this took me two full weekends as a moderately skilled DIYer. The YouTube videos make it look like a 3-hour job. It's not. Budget twice the time you think you'll need.
Electrical Nightmares (and How to Avoid Them)
Here's where most DIYers get shocked - literally. Installing a tankless water heater often needs heavier wiring than your existing setup. My unit required:
| Wire Gauge | Amperage | Breaker Size | Circuit Length Max |
|---|---|---|---|
| 8 AWG | 40A | 40A dual-pole | 60 ft |
| 6 AWG | 55A | 60A dual-pole | 85 ft |
| 4 AWG | 70A | 70A dual-pole | 110 ft |
Biggest gotcha? Voltage drop over distance. My panel was 50 ft from installation point - needed 6 AWG instead of 8. Electrician friend caught it during inspection.
Grounding is non-negotiable. Use green grounding screws and proper clamps. I've seen units fry because someone used a sheetrock screw for grounding. Not kidding.
Venting: Where DIY Goes Wrong
This is the make-or-break step. Get venting wrong and you risk CO poisoning. Gas units require category III stainless steel venting - that cheap stuff from the big box store won't cut it.
Critical venting rules:
- Maximum equivalent length: Check manual (usually 10-20 ft)
- 1/4" slope per foot toward outdoors
- No more than two 90° elbows
- Termination must be 12" from windows/doors
I learned the hard way: Using PVC for venting on a gas unit is illegal and dangerous. Inspector red-tagged my first attempt.
Life-saving tip: Install CO detectors in every bedroom and the hallway near your tankless water heater. Test monthly. This isn't optional.
First Startup: Don't Panic
When you first fire up your new tankless water heater, it'll make scary noises. Rattling? Probably air in the lines. Clicking? Normal ignition sequence.
Follow this startup checklist:
- Open farthest hot water faucet
- Turn power on at breaker
- Set temperature to 120°F max initially
- Check for error codes
- Verify water heating at faucet
- Check for leaks at all connections
Error E05 haunted me for days. Turns out I had the flow sensor installed backward. Felt like an idiot.
Maintenance: Keep It Running Smooth
Think you're done after installing a tankless water heater? Not quite. These units need annual maintenance or they'll die early. Here's my maintenance ritual:
| Frequency | Task | Tools Needed | Cost If Skipped |
|---|---|---|---|
| Monthly | Check error codes | None | Replacement parts ($80+) |
| Quarterly | Flush with vinegar | Submersible pump, bucket | Heat exchanger failure ($400+) |
| Annually | Professional inspection | Combustion analyzer | Voided warranty |
Flushing takes me 45 minutes now that I've got the kit. First time? Three hours of frustration. Pro tip: Buy the manufacturer's flush kit - generic ones leak.
Cost Breakdown: What You'll Actually Pay
Let's talk real numbers. Installing a tankless water heater isn't cheap, even DIY. Here's my actual cost breakdown:
| Item | Big Box Store Price | Actual Cost | DIY Savings |
|---|---|---|---|
| Electric Unit (8kW) | $550 | $550 | N/A |
| Breaker & Wiring | $120 | $185 (distance issues) | -$65 |
| Vent Kit (gas) | $350 | $420 (custom bends) | -$70 |
| Plumbing Parts | $90 | $140 (forgot unions) | -$50 |
| Permits | $75 | $75 | N/A |
| TOTAL | $1,185 | $1,370 | Still saved $1,800 vs pro install |
Permits matter. My city charged $75 for plumbing/electrical permits. Skipping this could void your homeowners insurance if something goes wrong.
FAQ: Your Burning Questions Answered
Can I install a tankless water heater myself legally?
In most areas, homeowners can DIY plumbing/electrical in their primary residence. But you STILL need permits and inspections. My inspector made me redo three things - worth the hassle for safety.
How difficult is installing a tankless water heater?
On a scale of 1-10? Solid 7 for electric, 8.5 for gas. Venting is tricky and electrical requires precision. If you've never wired a 240V circuit, hire an electrician for just that part.
Why does my new unit keep shutting off?
Probably mineral buildup or flow sensor issues. Mine did this after six months - vinegar flush fixed it. If problem persists, check for kinked supply lines.
Can I replace my 50-gallon tank with same-size tankless?
Size differently! Tankless is rated by flow rate (GPM) not capacity. Calculate your peak demand: If two showers run simultaneously at 2.5 GPM each - you need 5 GPM capacity at your groundwater temperature.
Do tankless water heaters really save money?
Yes, but not immediately. My gas bill dropped $18/month but with $1,370 installation cost? Payback takes 5+ years. Do it for endless hot water, not quick savings.
When to Call a Pro
After installing three of these units (my house and two friends'), here's when I'd hire out:
- Gas line work: Messing with gas is Russian roulette
- Service panel upgrades: 200A service requires utility coordination
- Complex venting runs: More than two elbows? Get help
- If local codes confuse you: My inspector failed me for using Teflon tape on flare fittings - turns out it's prohibited
Good plumbers charge $1,500-$3,500 for full installation. Get three quotes. Watch for "gotcha" fees like drywall repair or upgrade charges.
Final thought: Installing my tankless water heater was satisfying but stressful. Would I DIY again? For electric - absolutely. For gas? Only if I had a licensed buddy helping.
The endless hot showers are glorious though. Totally worth the skinned knuckles and extra trips to Home Depot.
The endless hot showers are glorious though. Totally worth the skinned knuckles and extra trips to Home Depot.