Shower Temperature Swings Hot and Cold? Find the Cause
You are halfway through a shower and someone flushes a toilet. The water turns scalding, or ice cold, then drifts back. A pressure balancing shower valve is built to prevent exactly this. Moen says its pressure balancing valves balance the incoming hot and cold water pressure, keeping the temperature within about 3 degrees F of where you set it despite a toilet flushing or a dishwasher starting.
So when a shower swings, there are three places to look: a balancer that is worn or stuck, a valve that never had one, or a problem upstream at the water heater or the supply pressure. This guide sorts out which one you have, starting with the checks that cost nothing.
The U.S. Consumer Product Safety Commission urges lowering water heaters to 120 degrees F, and says most adults will suffer third-degree burns from 150 degree water in two seconds, from 140 degree water in six seconds and from 130 degree water in thirty seconds. CPSC also says to always hand-test hot water before using it. Until the swing is fixed, test the water with your hand before you step in. Before you take a valve apart, shut the water off at the valve stops or the main, and after any repair Delta says to turn the valve on and make sure cold water flows first.
How a pressure balance valve holds the temperature
A flush or a dishwasher pulls water from the same pipes as the shower. This Old House explains what the balancer does about it: when pressure drops in one line, such as when a toilet is flushed, the balancer adjusts the flow from both lines to keep the set temperature. Moen adds that its pressure balancing valves need pressure on both the hot and the cold side to work.
The ASSE 1016 standard for shower valves covers three types: pressure balancing, thermostatic, and a combination of the two. Its definitions put the difference simply. A pressure balancing valve senses the incoming hot and cold pressures. A thermostatic valve senses the outlet temperature and corrects for changes in the incoming water temperature. Kohler (in the homeowners guide for one of its pressure balanced shower towers), Delta and Gerber each note that their pressure balanced valves do not compensate for a change in the supply water temperature, which is why a shower can run cooler in winter. Our guide to a shower not getting hot in winter covers that case.
Part 1: Which kind of swing is it?
- Swings only at this shower, when water runs elsewhere. This Old House says to start by checking whether the problem is confined to the shower or affects other bathroom fixtures. If only the shower is affected, the pressure balancer is likely the culprit. Go to Part 2.
- Every faucet runs lukewarm, or the hot water runs out fast. Rheem, a water heater maker, says that if your hot water runs out fast the problem may not be pressure alone, and lists an undersized water heater, sediment buildup and faulty dip tubes among the causes. Family Handyman, quoting a restoration contractor, explains that a broken dip tube lets cold water mix with the hot water near the top of the tank, and that small pieces of plastic in your faucet screens can be a sign of one. Start with the water heater.
- An older valve with no balancer. Rheem says older shower valves cannot automatically compensate for pressure changes. Not every valve has a balancer: Moen's own cartridge chart lists its two and three handle shower valves, and its standard push pull single handle valve, without marking them pressure balanced. On those, in our view, no cartridge swap adds it. This Old House and Fine Homebuilding both describe replacing an old two handle valve with a pressure balance valve, which means opening the wall and cutting and soldering pipe. In our view that is a job for a plumber. ASSE notes that in-line pressure balancing devices exist for two handle sets, but that they do not protect against changes in the incoming water temperature.
Part 2: Checks that cost nothing
Step 1: Open the stops all the way
Moen, Kohler and Delta all say to make sure any stops or shut-off valves for the shower are fully open. Delta's instructions say to install the stops but leave them full open. Moen's optional Posi-Temp stop check valves are open when the slot is horizontal.
Step 2: Look at the water pressure
For Kohler Rite-Temp valves, Kohler's first step for temperature fluctuations is to inspect the pressure of the water coming into the valve. Kohler says a difference of more than 5 psi between hot and cold will cause fluctuations, that Rite-Temp valves need at least 45 psi on both supplies, and that a plumber can measure it with a pressure gauge. Other makers set their own figures. Pfister, for one, recommends at least 25 psi for its valve, so check your valve's instructions.
Step 3: Check for a shutoff added after the valve
Delta and Kohler both warn against installing a shut-off device on the outlet of a pressure balance valve. Delta says it can defeat the valve's ability to balance, and Kohler says it can create a cross-flow that affects the water temperature. If someone added one after the valve, ask a plumber about it.
Part 3: Fix the balancer in your valve
Turn the water off first. These are what the makers say about the part that does the balancing.
Moen Posi-Temp
Posi-Temp valves use the 1222 cartridge. Moen has no page on swings that we could open, but its troubleshooting for tub and shower water that will not get hot enough lists a stuck balancing spool among the possible causes, and for Posi-Temp models says that if the water pressure drops when you go from cold to hot, or the problem continues after the simple checks, you replace the 1222 cartridge and flush the valve body. Moen recommends cleaning and flushing the valve body every time a new cartridge goes in, because debris could damage the new cartridge or obstruct the flow of water. Moen warns that during flushing the water may come out of the open valve body with considerable force, and recommends standing outside the tub or shower when the water is turned on. If the retainer clip will not go back in or looks damaged, Moen says the water cannot be turned back on until the clip is replaced. Install the new cartridge with the H and C tab pointing up and the notched side of the brass stem facing away from the H and C mark. Our Moen 1222 replacement guide has the full steps.
Delta Monitor and MultiChoice
Delta's maintenance sheet for its 13 and 14 Series valves lists the symptom by name: unable to maintain constant water temperature. The fix Delta lists is to replace the housing assembly with RP46074, or follow Delta's helpful hints. One of those hints is a quick test: turn the valve to the full mix position and shake the cartridge. If it rattles, Delta says the unit is working; if there is no rattle, replace it. Delta adds that in water with lime, rust, sand or other contaminants, the pressure balance valve needs periodic inspection. Before you pull the cartridge, note the position of the rotational limit stop on the cap, because Delta says the cartridge must always go back in the same position.
Delta's reference sheet tells the two common cartridges apart by the lower housing: the Monitor 1300 and 1400 Series uses the RP19804, with the shorter lower housing, and the MultiChoice 13 and 14 Series uses the RP46074, with the longer one. For the Monitor cartridge, see our Delta Monitor 1300 and 1400 install guide.
Kohler Rite-Temp
Kohler's advice for temperature fluctuations on Rite-Temp valves starts with the incoming pressure, as in Part 2, and Kohler explains why: a pressure balancing unit is designed to react to the pressure it has been provided. Kohler also notes that the Rite-Temp mixer cap is designed for hot water coming in from the right and cold from the left. Kohler does not name a part for swings on that page. Our own advice, not Kohler's: if a plumber finds the hot and cold pressure within 5 psi of each other and at least 45 psi, and the swings continue, the pressure balancing unit is the part that does the balancing, so it is the part we would replace next. On that page Kohler names the mixer cap and pressure balancing unit kit only as the fix for a valve leak. FourHome sells a replacement unit on its own and in a kit with the mixer cap.
Gerber Safetemp and Grohe
Gerber says its Safetemp valve meets ASSE 1016, and Grohe lists the 47080000 as the pressure balance valve cartridge for its 35 000 series rough valves. Neither maker page we could open gives troubleshooting for temperature swings, so match the part number on your old cartridge, and check the fit notes on our part page, before you order.
The temperature limit stop restricts how far the handle turns toward hot, which limits how much hot water is mixed in. Moen recommends limiting the shower to 120 degrees F or less, checked with a thermometer, and Pfister notes that the water heater controls the actual temperature of the hot water. Delta and Gerber both say the limit stop may need adjusting with the seasons. Holding the mix steady when the pressure changes is the balancer's job, so a swing is a balancer, pressure or water heater question.
What you found, and the fix
| What you notice | Where to look | The fix |
|---|---|---|
| Swings on a flush, only at this shower | The pressure balancer or cartridge | Open the stops, check pressure, then replace the cartridge or balancing unit |
| Delta cartridge does not rattle when shaken | The cartridge assembly | Replace it (RP46074 or RP19804) |
| Moen Posi-Temp still swings after the checks | The 1222 cartridge | Replace it and flush the valve body (our reading of Moen) |
| Kohler Rite-Temp swings | The incoming pressure first | Have a plumber measure the hot and cold pressure |
| Every faucet lukewarm, or hot runs out fast | The water heater | Water heater service |
| Older valve with no balancer | The valve itself | A plumber fits a pressure balance or thermostatic valve |
When to call a plumber
- The swings continue after you have tried the repair, the valve shows extensive damage or corrosion, or the shower system is old or complex. This Old House lists all three.
- Your shower has no balancer and you want one. Replacing the valve means opening the wall, as This Old House and Fine Homebuilding describe.
- Kohler's pressure check calls for a pressure gauge, and Kohler says a plumber can measure the hot and cold pressure for you.
- You are not sure about the repair. Pfister says to call a professional if you are uncertain about installing its valve.
Frequently asked questions
Why does my shower get hot when someone flushes the toilet?
A flush draws cold water, and Rheem explains that a sudden drop in cold water pressure can let hot water dominate the mix. A pressure balancing valve is built to correct for that: Moen says its pressure balancing valves keep the temperature within about 3 degrees F despite a flush or a dishwasher. If yours still swings, the balancer may be worn or stuck, the supply pressure may be off, or the valve may be an older one without a balancer.
Will a new cartridge fix temperature swings?
It can, on a valve that has one. Moen's troubleshooting for Posi-Temp water that will not get hot enough includes replacing the 1222 cartridge and flushing the valve body, and Delta's sheet for its 13 and 14 Series valves says to replace the housing assembly, RP46074, when the valve is unable to maintain constant water temperature. Kohler starts somewhere else for Rite-Temp valves: check the incoming water pressure first.
What temperature should my water heater be set to?
The U.S. Consumer Product Safety Commission urges lowering water heaters to 120 degrees F, and the Department of Energy says most households usually only need 120 degrees F. CPSC notes that most adults will suffer third-degree burns from 150 degree water in two seconds, so in our view a lower setting matters most in a home with a shower that swings.
Is a thermostatic valve different from a pressure balance valve?
Yes. The ASSE 1016 standard covers three kinds of shower valve: pressure balancing, thermostatic and a combination of both. Its definitions say a pressure balancing valve senses the incoming hot and cold pressures, while a thermostatic valve senses the outlet temperature and also corrects for changes in the incoming water temperature. Which to install is a question for a plumber or the valve maker.
Does adjusting the limit stop fix temperature swings?
Not in our reading of the makers' instructions. The limit stop restricts how far the handle turns toward hot, so it sets the hottest water the shower can deliver. Moen recommends a maximum of 120 degrees F at the shower. Holding the mix steady when pressure changes is the balancer's job, so a swing points to the balancer, the pressure or the water heater.
Sources
The steps above come from these pages, except where a step says the advice is our own. We opened each one on October 12, 2026.
- Moen, What is meant by a pressure-balancing tub and shower faucet, Water temperature is not hot enough on tub shower units, 1222 Cartridge: Single Handle Shower, PosiTemp, Valve Body Cleaning and Flushing, Faucet Cartridges: Shower, How to Adjust the PosiTemp Temperature Limit Stop and How to Operate PosiTemp Stop Check Valves.
- Delta, maintenance and installation sheet 107528 Rev I (13 and 14 Series, cartridge reference sheet).
- Kohler, Rite-Temp valve troubleshooting and K-498 homeowners guide (pressure balanced valve warnings).
- Gerber, Safetemp pressure balance tub and shower valve installation manual.
- Grohe, 47080000 pressure balance valve cartridge.
- Pfister, single control pressure balancing valve instructions.
- ASSE, Guidelines for Temperature Control Devices in Domestic Hot Water Systems (Scald Awareness Task Group white paper) and ASSE 1016, 1988 edition (definitions).
- U.S. Consumer Product Safety Commission, Avoiding Tap Water Scalds.
- U.S. Department of Energy, Energy Saver page on water heater temperature.
- Rheem, Why Does My Shower Get Cold When Someone Uses Another Faucet?.
- This Old House, How To Repair a Shower Anti-Scald Pressure Balancer and Making Your Shower Safer.
- Fine Homebuilding, Replace a Shower Mixing Valve.
- Family Handyman, What to Do If Your Water Heater Has a Defective Dip Tube.
Replacement cartridges and balancing units for Moen, Delta, Kohler, Gerber and Grohe single handle tub and shower valves, shipped from Amazon fulfillment centers with Prime delivery.
See the full FourHome assortment on AmazonFourHome is an independent brand and is not affiliated with Moen Incorporated, Delta Faucet Company, Kohler Co., Gerber Plumbing Fixtures, Grohe, Pfister, Rheem, ASSE, This Old House, Fine Homebuilding, Family Handyman or any government agency. Brand names and part numbers are used only to identify parts or to cite their published instructions. Shut off the water before you work on a valve, test the water by hand before you step in, and call a licensed plumber for valve replacement, pressure measurement or water heater work.
FourHome is not affiliated with or endorsed by any faucet manufacturer. Brand names and part numbers are used only to identify compatibility.