Q: When someone is using any of our three showers and the water is turned on elsewhere in the house, the water temperature in the shower fluctuates instantly. Several months ago I saw a product at the end of, I think, “This Old House,” that is installed near the showerhead and ensures a constant temperature even when other water sources are used at the same time. I believed they called it a surge protector. No one appears to know what I am referring to. Could you please help?

A: You’ve articulated a common problem. Someone is in the shower; someone else flushes the toilet; the water pressure drops dramatically; and the warm shower turns real hot, real fast.

Toilet flushing isn’t the only cause. The washing machine could kick into its rinse cycle or the automatic sprinkler could come on. Maybe someone just gets a drink from the kitchen faucet. Whatever the source, the cause is the same — increased demand on the home’s water supply drops the pressure in the system. The problem can be exacerbated in an older house where corrosion in galvanized piping further restricts the flow of water in the pipes.

Whether there’s an easy solution depends on the type of shower control valve you’ve got. If you have a single-control valve, the fix is probably pretty easy — it’s a matter of changing a cartridge inside the valve. If you have separate hot and cold valves, the fix is tougher — you have to replace the valve.

We remember the TV program you mentioned. It wasn’t “This Old House”; it was “Ask This Old House.” Tommy Silva, Roger Cook and Rich Trethewey make house calls to help homeowners in need of a little professional assistance. In this particular episode, plumbing pro Trethewey was faced with the same problem you’ve got: fluctuating water pressure. His solution was to install a pressure balancing valve in the shower control. This is also known as a mixing valve or an anti-scald valve.

A pressure-balanced shower valve is designed to compensate for changes in water pressure. Although it looks like any other shower or tub valve from the outside, it has a special diaphragm or piston mechanism inside that moves with a change in water pressure to immediately balance the pressure of the hot- and cold-water inputs.

Any licensed plumber should be able to tackle the job. Repair for the single valve should take an hour or so once the replacement part is in hand. This is also something a willing do-it-yourselfer can tackle. The dual control is a much bigger job requiring removing the existing valve and replacing it with a new single-control valve. Count on some wall or tile repair also.

If it’s a single control, the first step is to determine the manufacturer. If it’s a name-brand valve, such as Delta or Moen, replacement parts should be readily available. If not, you’re probably looking at replacing the valve. If it’s a name brand, go to the company’s Web site. You should be able to download a schematic of the valve. See how the valve goes together and decide whether you want to take on the job.

If it’s a go, call a local plumbing-supply house and ask if they carry repair parts for your particular make and model of valve. While the average homeowner might be a little leery about contacting a business that sells mostly to the trade, our experience is that if you go in with a clear idea of what you want, these establishments are most happy to sell to you. Don’t expect a how-to course, though. A few judiciously chosen questions are OK.

Once you have the part, change it out. Because every valve is a bit different, we can give you only a general process. First, turn off the water. Next, remove the faucet handle and the escutcheon to expose the interior of the valve. Remove the cartridge and replace it with the new part. Reinstall the escutcheon and handle. Then test the job by turning on the shower and have someone flush the toilet. Hopefully, the pressure problems will be solved.

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