Why Is My Shower Hot Water Not Working But Works Everywhere Else?

If your shower has no hot water but the kitchen sink, bathroom sink, and washing machine all get hot just fine, you’re not alone. This is one of the most common water heater complaints we see in Toronto homes - and it’s rarely about the heater itself. The problem is almost always somewhere between the heater and the shower. Here’s what’s likely going on, and how to fix it without calling a plumber right away.

Check the Mixing Valve First

Most modern showers use a thermostatic mixing valve to blend hot and cold water for a safe, steady temperature. If that valve gets clogged, worn out, or misadjusted, it can block hot water from reaching the showerhead - even if the water heater is working perfectly.

These valves are usually behind the shower wall or inside the handle assembly. If you’ve noticed your shower suddenly going cold after a few minutes, or if the temperature feels off even when you turn the knob all the way to hot, the valve is suspect. Older brass valves can build up mineral deposits from Toronto’s hard water. Newer plastic ones can crack or warp over time.

To test it: Turn off the water supply to the shower. Remove the handle and faceplate. Look for a small cylindrical valve with a stem. If it’s stiff, discolored, or covered in white crust, it’s clogged. You can try soaking it in white vinegar for an hour, then scrubbing gently with an old toothbrush. If it’s cracked or doesn’t move smoothly, replace it. A new thermostatic valve costs around $60-$120 and takes under an hour to swap if you’re handy.

Look at the Shower Cartridge

If your shower has a single-handle control, it likely uses a cartridge instead of a mixing valve. These cartridges control both flow and temperature. Over time, rubber seals inside them harden, and sediment gets trapped. The result? Hot water gets blocked or mixed incorrectly.

Here’s how to check: Turn off the water. Remove the handle. Use a wrench to pull out the cartridge. Compare it to the replacement part in the box - they’re usually labeled by brand (Moen, Delta, Kohler). If the old one looks warped, cracked, or covered in grit, that’s your culprit. Replacing it costs less than $40 and fixes the problem in 20 minutes. You don’t need to turn off the whole house’s water - just the shower line.

Is the Hot Water Line to the Shower Blocked?

Hot water travels through pipes from your water heater to each fixture. If only the shower is affected, the pipe feeding it might be partially blocked. This often happens in older homes with galvanized steel pipes, which rust from the inside. The rust flakes build up and narrow the pipe - enough to choke flow to a high-demand fixture like a shower, but not enough to stop a low-flow sink.

Look for signs: If your shower had good pressure and hot water last year, but it’s been getting worse over time, that’s a red flag. If you’ve had other fixtures (like a bathtub or laundry tub) lose hot water over the past few years too, it’s probably pipe corrosion.

There’s no easy fix for this without replacing the pipe. But you don’t need to repipe the whole house. A plumber can isolate the section feeding the shower and replace just that run with PEX tubing - a flexible, rust-proof plastic pipe. It’s cheaper and faster than copper, and works well in tight walls. Expect to pay $400-$800 for this repair, depending on wall access.

Check the Water Heater’s Temperature Setting

It sounds simple, but many people don’t realize their water heater’s thermostat can drift. If it’s set too low - say, below 120°F (49°C) - the hot water might reach the sink just fine, but not make it to the shower, especially if the pipe run is long or the showerhead is low-flow.

Find your water heater. It’s usually in the basement, utility room, or garage. Look for a dial or digital display. Turn it up to 130°F (54°C) and wait an hour. Test the shower again. If it improves, you’ve found the issue. But don’t go above 130°F - that’s the safety limit to prevent scalding, especially for kids and elderly people.

Also check if your water heater is electric. If it has two heating elements, the top one might be working (heating water near the top of the tank for sinks), but the bottom one is dead. That means hot water lasts only a few minutes before it runs out - and showers, which use more water, are the first to suffer.

Is the Showerhead or Aerator Clogged?

Sometimes the problem isn’t the plumbing - it’s the showerhead. Mineral buildup from hard water can clog the tiny holes inside the showerhead, reducing flow so much that the hot water gets diluted by cold water before it even reaches you.

Try this: Remove the showerhead. Soak it overnight in a bowl of white vinegar. Rinse it, then reattach. Turn the water on. If the flow improves and the water feels hotter, that was the issue. You can buy a new showerhead for under $25. Look for ones labeled “hard water friendly” - they have self-cleaning nozzles.

Don’t forget the aerator on the bathroom sink. If that’s clogged too, it might explain why the sink still gets hot water - because it’s using less volume, so even a partial blockage doesn’t stop it.

Could the Water Heater Be Too Small?

If your water heater is older than 10 years, it might be undersized for your household. A 40-gallon tank is fine for one or two people, but if you’ve added a teenager or started taking longer showers, you’re running out of hot water faster than it can recover.

Check the label on your heater. It’ll say the capacity in gallons. If you have three or more people in the house and a 40-gallon tank, you’re likely pushing its limits. The kitchen sink might still get hot because it’s used briefly. The shower, which runs for 10-15 minutes, drains the tank completely.

Consider upgrading to a 50- or 60-gallon tank - or even a tankless water heater. Tankless units heat water on demand, so you never run out. They cost more upfront ($1,500-$3,000), but they last twice as long and use 30% less energy. Many Toronto homes qualify for rebates through Enbridge’s Home Efficiency program.

What About the Dip Tube?

The dip tube is a plastic pipe inside your water heater that sends cold water to the bottom of the tank, where it’s heated. If it breaks, cold water gets mixed in with the hot water right at the top - so you get lukewarm water even when the heater is working.

This usually happens in heaters made between 1993 and 1998, when a bad batch of dip tubes was used. If your heater is from that era and you’re getting inconsistent hot water, this could be the cause. You can’t see the dip tube without draining the tank, but if you notice white plastic flakes in your faucet filters or showerhead, that’s a telltale sign.

Replacing the dip tube is a job for a pro - it requires draining the tank and removing the cold water inlet. But if your heater is older than 12 years, it’s probably cheaper to replace the whole unit than to repair it.

Quick Diagnostic Checklist

• ✅ Does the kitchen sink get hot water? If yes, the heater is working.
• ✅ Is the shower’s temperature knob turned all the way to hot? Try turning it past the “H” mark.
• ✅ Did you recently replace the showerhead or cartridge? New parts can be faulty.
• ✅ Are other fixtures (like the bathtub) also losing hot water? If yes, it’s likely the heater or pipes.
• ✅ Do you see white flakes in your faucet filters? That points to a broken dip tube.
• ✅ Has your water heater been serviced in the last 2 years? Sediment buildup reduces efficiency.

When to Call a Professional

You can fix most of these issues yourself - but call a plumber if:

• You’ve tried replacing the cartridge and mixing valve and it didn’t help.
• You suspect pipe corrosion and need to replace copper or galvanized lines.
• Your water heater is over 12 years old and you’re running out of hot water quickly.
• You smell gas or hear hissing near the heater - that’s a gas leak, and you need to shut off the gas and call immediately.

Most Toronto plumbers offer same-day service for this issue. The average diagnostic fee is $99, but many waive it if you hire them for the repair.

Prevent It From Happening Again

Hard water is the #1 enemy of hot water systems in Ontario. Install a water softener if you don’t have one - it cuts mineral buildup by 90%. Even a simple inline filter on the water heater inlet helps.

Flush your water heater every year. Drain a few gallons from the bottom valve to remove sediment. It takes 20 minutes and costs nothing. It extends the life of your heater by 5-10 years.

Replace shower cartridges and mixing valves every 8-10 years, even if they seem fine. They’re cheap insurance against sudden cold showers in winter.