Having no hot water in your bathtub can be pretty frustrating, especially when the other plumbing in your bathroom seems to be functioning properly. Although the issue may appear baffling at first, it’s easy to solve in many cases. So, why isn’t your bath water heating properly?

Your bath may not be heating well due to a clogged pipe or hard water mineral buildup, which prevents efficient heat transfer to the water. It also may simply be that your hot water heater isn’t big enough to fill your tub. You can increase the heater’s temperature, but watch out for scalding.

You should be able to address some of these issues on your own, while others may require a visit from a plumber. If you’re like me, you’ll want your bath to be too hot if anything. So let’s take a closer look at the most likely causes behind bath water that won’t heat up, as well as the solutions that’ll get your water nice and steamy once again.

the science of bath water heating: what you need to know

Bath water heating issues can result from a number of problems. Remember: your hot water at your tub is coming from a hot water heater, so, it may be that the problem is at the heater.

In fact, it’s almost certain that the problem is there. A hot water heater that’s old can fail for some specific mechanical reasons like hard water or leaks, but we’ll also address a case of a perfectly working hot water heater.

Check out another article I wrote on when your boiler might need replacing.

Hard Water Mineral Buildup

Accumulation of hard water minerals is a common cause of bathwater heating issues, and it usually occurs when the water contains calcium and magnesium in excessive amounts. When heated, these minerals form a white, chalky film that covers the heating elements in the water heater.

This coating prevents efficient heat transfer to the water, and the outcome is often lukewarm water during a bath. In some cases, the buildup is so bad that the water can’t heat at all — and you’ll be taking an unintended ice bath as a result.

When the chalky film forms inside the actual tub faucet, the hot water side is likely to become obstructed because the higher temperatures boost the reactions that lead to the formation and accumulation of the deposits. Another telltale sign of mineral buildup is where your faucet leaks when your shower is on.

Interestingly, these deposits don’t necessarily have to prevent the full water flow to cause heating problems. In some cases, they’ll just cause a blockage that restricts you from turning the faucet all the way to the hot side.

A Leak

Leaks are technically a reason you may be having bathtub water heating issues. A leak in your hot water pipe can result in the hot water flowing poorly or not at all. If there’s a leak in the plumbing or a broken pipe, the hot water may be lost somewhere else.

Water damage and stains on the ceiling or crawl space below the tub are clear signs of a leak. However, leaks can be harder to spot if your tub is on the first floor of a slab foundation.

I grew up in an old Victorian house with an outdated and complicated plumbing system that never seemed to work properly. So in my case, a leak in the kitchen could’ve actually been coming from a leaking hot water pipe for the tub.

You should have a plumber inspect the system if you suspect this is the source of the heating issue. This is probably not the cause of a routine heating issue, but that’s not to say you couldn’t find a leak in the hot water system this way. The probability is low, but because the severity is high, it’s worth considering.

A Clogged Pipe

A clogged pipe is another possible reason your tub isn’t getting enough hot water. This is especially plausible if you live in an old house like mine where the pipes haven’t been replaced for many years. Old pipes sometimes accumulate mineral deposits due to chemical imbalances in the water.

These mineral deposits can clog the pipes, blocking hot water from reaching the bathtub faucet. To test if a clogged pipe is the culprit, try measuring the temperature of the heated water both at the hot water heater and at the tub and make sure you can even get that hot water there in the first place.

The Heater is Too Small

If your tub is big, and your water heater is on the smaller side, you may just not have enough heat in a “full” heated hot water tank to get your tub up to temperature after that hot water mixes with cold water.

This isn’t as bad as it sounds at first, but the fix is a bit more complicated than you’d like. But let’s turn our attention to all the fixes now.

How to Fix the Problem: 4 common solutions

You can fix bath water heating issues in the following ways:

1. Adjust the Thermostat on the Heater

If your heater is too small to fill your tub with hot water, then the heater needs to be bigger, or the water in it needs to be hotter. It’s hard to replace your water heater, but it’s not hard to adjust the thermostat.

So, a simple experiment you can run is to adjust the thermostat on your hot water heater (which may require using a screwdriver to remove a panel, although most are visible and accessible).

However, you have to use a lot of caution with this approach. If you make the water much hotter, it’s going to be hot enough to burn you, and it’s going to come out all of your faucets. So, everyone in the house needs to know about the hot water, how to manage it, how to not reach in and wash their hands really quickly after turning the hot tap full-on, etc.

So, practice some caution with this solution.

2. Fix the Hot Water Pipe Leak

To fix a leaking hot water pipe, you’ll need to find and shut off the water supply to the pipe. Use a wrench to loosen the pipe’s compression nut and remove the damaged part of the pipe. Replace that section with a new one of a similar size and diameter, then secure it with the compression nut.

Confirm that you’ve tightly secured the pipe, then turn on the water supply and test for leaks. If there are none, you’ve put an end to the issue.

3. Inspect the Faucet Valve

To inspect the condition of the faucet valve, start by turning off the water going into the bathtub. Then remove the handle and proceed to either unscrew the retaining nut or pull out the pin to free the valve. 

You can use pliers to slide the valve out of the housing or reattach the handle and pull it out. If it won’t come out, it likely means mineral deposits have built up, in which case you will need a valve puller to get it out.

4. Clean the Valve and Seat

If minerals appear to have clogged the valve cartridge, you may have to replace it. However, if the valve is otherwise in good condition, you can clean off the deposits by soaking the cartridge overnight in vinegar. Even if you don’t notice any minerals, it’s still a good idea to do this, as they could be hiding inside the cartridge.

You should also extend the cleaning to the valve seat, which you can remove using a seat wrench. To remove stubborn deposits, use a screwdriver to scrape off any visible deposits before immersing the seat in vinegar overnight.