What Is Thermal Expansion and Why It Matters
Water expands when heated - about 2-3% by volume. In a 50-gallon water heater, that's 1-1.5 gallons of extra volume with nowhere to go. In older "open" plumbing systems, this expansion pushed back into the city water main. But modern homes have pressure regulators, check valves, or backflow preventers that block this path.
Open vs. Closed Systems
Open System (Older Homes)
- • No check valve or PRV
- • Expansion pushes back to city main
- • No expansion tank needed
- • Becoming rare as PRVs are installed
Closed System (Most Modern Homes)
- • Has PRV, check valve, or backflow device
- • Expansion is trapped in home
- • Expansion tank required
- • Code requirement for new installations
What Happens Without an Expansion Tank
When heated water can't expand safely, pressure builds rapidly. This causes:
- T&P valve releases: The temperature/pressure relief valve opens to relieve pressure, dripping water
- Shortened tank life: Constant pressure cycling stresses the water heater tank
- Fitting leaks: High pressure causes connections to weep or fail
- Faucet/toilet wear: Internal components wear faster under high pressure
- Pipe stress: Repeated pressure spikes fatigue pipes over time
T&P Valve Dripping Is a Warning Sign
If your water heater's T&P relief valve drips periodically (especially when the heater is actively heating), it's likely responding to thermal expansion pressure. This means the valve is doing its job, but you need an expansion tank to prevent the condition.
How Expansion Tanks Work
An expansion tank is a small (typically 2-5 gallon) sealed tank with an internal rubber bladder. One side has compressed air, the other connects to your water system.
The Science
- 1. At rest: The bladder is compressed against the air side, minimal water in tank
- 2. Water heats: Expansion pushes water into tank, compressing the air bladder
- 3. Pressure absorbed: Air compresses instead of system pressure rising
- 4. Water cools: Air pressure pushes water back out of tank
- 5. Cycle repeats: Every heating cycle, the tank absorbs expansion
Proper Air Pressure Setting
The air side of the expansion tank must be pre-charged to match your water pressure. If not, the tank won't function properly:
| Water Pressure | Tank Pre-Charge | Notes |
|---|---|---|
| 40 PSI | 40 PSI | Lower pressure homes |
| 50 PSI | 50 PSI | Average Toronto home |
| 60 PSI | 60 PSI | Higher pressure areas |
| With PRV set to 55 PSI | 55 PSI | Match PRV setting |
Pro Tip: Check Air Pressure Annually
Expansion tanks lose air over time through the bladder. Check air pressure annually with a tire gauge (tank must be isolated from water pressure first). Add air through the Schrader valve if low. If pressure won't hold, the bladder has failed.
Expansion Tank Installation Costs
| Service | Cost Range | Includes |
|---|---|---|
| Tank + installation | $150-$350 | 2-5 gallon tank, fittings, labor |
| Tank only (DIY) | $50-$100 | Residential size tank |
| With water heater install | $100-$200 | Often discounted as add-on |
| Replacement only | $125-$275 | Remove old, install new |
Sizing Your Expansion Tank
Tank size depends on water heater capacity and system pressure:
- 40-50 gallon water heater: 2-gallon expansion tank
- 60-80 gallon water heater: 4-5 gallon expansion tank
- High pressure (60+ PSI): Upsize one category
- Commercial/large tank: Custom sizing required
Signs Your Expansion Tank Has Failed
Physical Signs
- • Tank feels heavy/solid when tapped (waterlogged)
- • Visible rust or corrosion
- • Leaking from fittings or seams
- • Tank feels excessively cold (full of water)
System Symptoms
- • T&P valve dripping during heating
- • Pressure gauge shows spikes when heater runs
- • Water hammer when heater cycles
- • Faucet aerators clog with debris
The Tap Test
The quickest way to check an expansion tank:
- 1. Tap the top - Should sound hollow (air space)
- 2. Tap the bottom - Should sound solid (water)
- 3. If whole tank sounds solid - Bladder has failed, tank is waterlogged
- 4. Replace immediately - A failed tank isn't protecting your system
Need an Expansion Tank Installed?
We install expansion tanks the same day - protect your water heater and plumbing from thermal expansion damage. Often combined with water heater service for best value.
Frequently Asked Questions
What is an expansion tank and why do I need one?
An expansion tank absorbs the pressure increase when water heats and expands. In closed plumbing systems (with check valves or pressure regulators), this expansion has nowhere to go. Without an expansion tank, pressure builds up, stressing the water heater and fittings.
How much does expansion tank installation cost in Toronto?
Expansion tank installation in Toronto costs $150-$350 including the tank and labor. The tank itself costs $50-$100 for residential sizes. Installation takes 30-60 minutes. When done with water heater installation, it's often discounted.
Is an expansion tank required by Toronto building code?
Yes, the Ontario Building Code requires expansion tanks for closed plumbing systems. If you have a pressure reducing valve, check valve, or backflow preventer, your system is closed and an expansion tank is required.
How do I know if my expansion tank is working?
A working expansion tank feels light when tapped at top (air) and heavier at bottom (water). If the whole tank sounds solid or waterlogged, the bladder has failed. Also check that the air pressure matches your water pressure.
How long do expansion tanks last?
Expansion tanks typically last 5-10 years. The internal bladder degrades over time, especially in hard water areas. Signs of failure include T&P valve dripping and a waterlogged-sounding tank. Check annually and replace when failed.
Mike Thompson
Master Plumber, 25+ Years Experience
Mike has installed hundreds of expansion tanks across the GTA and knows which homes need them most. He's seen the damage that thermal expansion causes and recommends proactive installation to protect water heaters.