Why does the shower/sink water get hot when someone flushes the toilet?

Because the toilet is then “using up” some of the cold water pressure, leaving less cold water pressure to the sink or shower, right? But why does this matter, given that both the hot and cold water lines have the same ultimate source of pressure–the water line coming into the house? When you flush the toilet, the pressure should drop all the way back through the cold water line to that main line coming into the house, and the reduced pressure there should affect the pressure in both the hot and cold lines running to the shower equally, right? What am I missing?

In my house there’s a tank full of hot water that keeps the pressure up during those cold water drops, although obviously the pressure would even out over a long enough period of time. I have no idea whether it works in homes with tankless hot water systems.

The problem is that the toilet is so close to the shower.

If you have a bigger house and you’re taking a shower and someone turns on the washing machine or flushes a toilet at the other end of the house it will be less noticeable.

Also, a few other things going on here:
1)They have the same source of pressure, but when you’re taking a shower the ‘system’ is open, so when someone flushes the toilet the cold water can ‘pull’ a little from the shower since that’s the path of least resistance…sort of.
2)Toilets, dishwasher, washing machines have automatic valves and since they snap open it’s much more noticeable than if someone turns on a sink. That’s slow enough that the pressure can recover as the person is doing it.
Also, most showers have a pressure balancer specifically designed to prevent this. When it notices the pressure fall on one side, it’ll reduce it on the other side. It’s actually a pretty elegant design (very simple and straight forward…until the diaphragm tears, then it causes some really bizarre symptoms like warm water coming out of your sink when you want cold water) .

The tank does not provide any pressure unless there is an air bubble in it, and water heaters do not have an air bubble.

Flow will cause the pressure drop over the length of the line. The further from the source the higher the pressure drop. The hot water line flows from the main to the heater to shower. The cold water line flows from the main to the shower and the toilet.

When taking a shower the water flow is stabilized and cold and hot water are balanced. Flush the toilet and the pressure drops all along the cold water line greatest at the end, that is the toilet and shower. The supply pressure to the house drops very little, the hot water flow at the shower drops just a little. But the pressure drop at the end of the cold water line is more therefore the shower becomes hotter.

Another thing is pipe size. Maybe a 1 inch supply to the house and as it travels through the pipe size can drop down to 1/2 inch. The smaller pipe will yield a greater pressure drop adding to the pressure drop at the end.

In my house do not flush a toilet upstairs when someone is taking a shower. Down stairs toiler will not have much effect to a shower up stairs.

It wasn’t always this way. Is this a code requirement now?

I just dealt with this exact failure a few weeks ago. The tear in the diaphragm let hot water move across the valve to the cold water side when the shower wasn’t being used. You’d think pressures are equal on both sides when nobody’s drawing any water, but they’re not: hot water is less dense, so with the water heater in the basement, and our master-bath shower on first floor, the hot water pushes up on the valve harder than the cold water does; it routinely filled the cold-water pipe with warm water, moving upstream until the branch in the cold-water supply where it would head back toward the inlet at the bottom of the water heater, forming a convectin-driven circulation loop. Apart from being a waste of heat, it led to some freaky temperature behavior:

-turn on the shower, and you immediately can find a comfortable temperature, so you jump in.
-20 seconds later, the cold-water pipe would be purged of its lukewarm water, and the shower temperature would plummet, forcing you to adjust the knob.
-20 seconds after that, the hot-water pipe would be purged of its lukewarm water, and the shower temperature would skyrocket, forcing you to adjust again.

You could get similar behavior from the faucets in that bathroom, since they were on the same hot/cold branches as the shower. I didn’t check, but presumably even the toilet was filling itself with hot water.

Thankfully, the pressure balancer was a replaceable cartridge inside the main valve body, so I didn’t have to do any soldering or anything like that: just shut off the water supply, remove the front of the valve (screws accessible from inside the shower), swap the cartridge, replace a couple of O-rings with new ones (they came with the new PB cartridge), and put it all back together.

I don’t know if it’s a requirement, but it’s nice. I can be taking a shower and someone can flush the toilet in the same room and I won’t even notice it.

My parents has theirs break. Took me a long time to track it down since I had never heard of that happening before, but some internet research and some poking around and I tracked it down. What I ended up doing (after I suspected it) was going over to their house hours after anyone had used any water, turning on the cold water at their kitchen sink (nowhere near the shower, different floor, other end of a large house) and letting it run for a few minutes. Then I put my hand on the cold water pipes behind the shower. They were hot. The only situation in which those cold water pipes should ever be hot is if someone was running only hot water through the shower. This told me that when I turned on the cold water at the sink some of that cold water was coming through the shower valve upstairs.

Any time you have hot or cold coming from the opposite side, it’s usually a cracked mixing valve/cartridge somewhere in your house (or the pressure balancer). When that happens you’ve connected your hot and cold side so when you call for either one of them you get ‘warm’.

It is because there is only one cold water pipe run to the bathroom, and the pressure in that pipe drops due to the increased flow.

If the toilet has it’s own cold water supply line, then this does not happen. If the toilet flow causes the pressure in the house to drop, then both hot and cold pressure are reduced equally.

But it does not the pressure drop the same through out the house.

Why not? Can someone explain this?

There is finite resistance to the flow of water in all the pipes. So, the pipes with the most water flowing in them will have the greatest pressure drop. If all the pipes (both hot and cold) were the same length, and all connected directly back to the same point, then you wouldn’t see this effect, but since the pipes are “T’d” off of a feeder, the pressure drop makes a difference in the water balance.

If all the valves are closed and there is no water flow then the pressure through out the whole house would be the same. But if you open one faucet and water begins to flow and a pressure drop occurs. The pressure drop is caused by friction. The friction will be greater in smaller pipes. If you want to see an example of this get a large straw and a extremely small straw. Now try sucking up a liquid, to get the same volume of drink will take more work.

Like Snnipe has been explaining the issue is caused by friction loss. the smaller the pipe the more friction loss occurs.

From a math perspective assuming my stated variables:

You have a 1/2 inch pipe running to the bathroom for cold water and a 1/2 pipe for hot water running from the water source. Both have straight lengths and fittings creating a friction loss equivalent of 100 feet. Your water source provides 60 pounds pressure. Running the shower draws 4 gallons of water 2 gallons hot 2 gallons cold.

The friction loss for 1/2 copper at 2 gallons a minute is .034psi per foot. If you are running no water the flow through that pipe is 0 gallons per minute with a friction loss of 0psi per foot.

When the shower is off the pressure throughout the 1/2inch line will be 60psi throughout the line. When the shower is on running 2 gallons per minute the pressure at the shower head is now 56.56psi.

Running both mixed hot and cold each at 2 gallons per minute both lines are experiencing the same pressure loss due to friction.

Now the have a toilet at the end of that same line. The toilet draws 2 gallons per minute of only cold water.

This means if you are running the shower and the toilet is filling at the same time you are drawing 4 gallons per minute through 100feet equivalent of 1/2 copper pipe. The friction loss for 1/2 copper at 4gpm is .1241psi per foot. So the pressure provided to the shower and toilet at the same time is 47.59psi.

So when you have just the shower running you are being sprayed by 2gpm of hot water at 56.56psi and 2gpm of cold water at 56.56psi. If during this shower someone flushes the toilet that changes the pressure to the shower head. While the toilet refills you are being sprayed with 2gpm hot water at 56.56psi and 2gpm of cold water at 47.59psi. Making the mixture a lot hotter than it normally would be.

Bigger pipes reduce friction loss considerably. If the hot and cold water had been run as 1inch pipes. the pressure loss would be .15psi at 2gpm and .056psi at 4gpm. The mixture would not change as considerably when someone flushes the toilet so you’d be unlikely to even notice a difference in the shower.

Hope that makes sense to you all.

Thanks for putting in the numbers. The last time I had to do flow rates was in 1970.