You’ve all experienced this phenomenon, I’m sure…
If I want hot water in my bathroom sink, I have to let the water run a long time. Yet when I flush the toilet, BAM! Instantly I have all the scalding hot water I require.
So how come that is?
If you have warm water (half hot and half cold) running in the sink and you flush the toilet, the amount of cold water at the sink is reduced due to trying to supply cold water to 2 fixtures at once and BAM, scalding hot water.
When you are running the hot water for a long time, you are clearing all of the cold water out of the pipes. When you flush the toilet, it uses a lot of water, and thus clears out the pipes much faster
Flushing the toilet will not affect the hot water supply, since it is connected only to the cold water. So flushing the toilet will not clear out the cold water out of the hot water pipes.
Yes, but it will reduce the pressure of the cold water in the cold pipes, and since most faucets deliver a mix of the two, you will, for a short while, get 100% hot water.
Not to nit-pick but I said
And the rest of your post sounds a lot like my first post, except you would not get 100% hot water at the faucet. It would be more like the cold water level being reduced by half (since it is being divided among two fixtures with the same size supply pipe.) Assuming a 50-50 hot-cold split at the faucet, it would be more *like 75% hot - 25% cold after the flush, still enough to drastically affect the temperature of the water coming out of the faucet.
*Approximation only
I dont understand how this could happen at all.
If you have exclusively hot water running…
let’s say it’s a 30 foot run from your sink to the hot water tank.
the hot water is going to get to your faucet at exactly the same rate.
whether or not you also have cold water running should be irrelevant.
and, toilets do NOT take up a “lot” of water. the stream of water that refills is probably half to one-third the flow rate as a standard faucet.
your house is possessed.
Jrishaw is right, flushing will turn warm water (mixed hot and cold) at the faucet into hot, but it will not get the hot water to the faucet any faster.
1st of all you can have 100% hot water at the faucet. THey only mix if you turn both H&C on. A lot of newer faucets have a single handle instead of a hot knob and a cold knob, but if you put it all the way to one side or the other you should get all of that one.
2nd of all, some hot water tanks that can’t supply enough hot water have a mixing valve that mixes hot and cold water. This allows you to turn your hot water heater way up - much more then you would normally since you would get burned. If you have one of these systems and the cold pressure droped more then the hot, the amount of hot water would increase. If the distance from the mixing valve is close to the faucet (as the pipe’s run) Then the temp increase would be quick
Could be that your toilet is connected to the hot water. When you flush, the filling tank would clear the cooler water from the hot water pipes.
Mistakes can be made. A friend of mine bought a new house and found that they had forgotten to connect the toilet drain to the waste pipes. Embarassed the hell out of the city inspector. 
The builder did return and rectify the situation, including cleaning up the mess.
Peace,
mangeorge
Is the OP talking about when you flush the toilet while somebody’s running warm (mixed) water, and suddenly the toilet hogs all the cold water to fill up its tank, and so you get a big gush of really hot water from the faucet?
I think it has something to do with how old your house’s plumbing system is. I live in a 1922 house, and everybody in the family understands that if Person A is in the shower, and Person B needs to come in and use the potty real quick, but without peeking behind the shower curtain, Person B is not, under any circumstances, to flush. Flushing results in Person A screaming in fury at being scalded.
Also, we can’t run the washing machine and the shower at the same time, or fill up the kitchen sink and water the garden simultaneously. There’s only so much water pressure to go around. Every time you turn on another faucet, it diminishes the other faucets’ output by half. And actually, I always wondered why this is. Primitive plumbing arranged oddly? My mother lives in a brand-new 1990s house and you can flush the toilet, run the dishwasher and the washing machine, and water the garden, all while taking a shower, and nobody gets scalded or runs out of water pressure.
wag 1 - pipe diameter including inlet
wag 2 - water pressure
wag 3 = wag2 + wag1
I had a friend who accidentally had hot water in his toilet. The first time I took a dump and looked down to see steam rising from the bowl was a shocker…
When I first read this, I thought it was saying that you were getting all the hot water you needed from your toilet. Yuck!
Yeah, I was wondering whether modern plumbing involves arranging the pipes in the house in a different way somehow, so as to maximize what comes in from the water main. I already suspect that my kitchen sink drain has no whatchamacallit, venting/air outlet, because when you pull the plug, all the dishwater goes down with a huge loud 15-second sucking sound, like Godzilla slurping Jello in a most obnoxious way.
Also, I wondered whether maybe modern pipe diameters are bigger, where it comes into the house. That would make for more water pressure, wouldn’t it?
This is easliy corrected with a Posi-Temp valve. Get yourself to a plumbing supply or hardware store or your local Home Depot or Lowe’s. They will have it, and should be able to tell you how to install it. You might be able to do it yourself, depending on how Bob Vila-like you are. Otherwise a plumber will have to do it.
When we remodeled our bathrooms, we installed them for both showers. Now someone can flush without fear of scalding whoever is showering.
Bigger diameter pipes can carry more water with less of a pressure drop, so yes. In an older house, the pipes can become coated on the inside with scale, decreasing their inner diameter, and adding to the problem.
Can this happen to water mains as well? Friends live in a part of Toronto where the water just dribbles out of the tap, and the washing machine takes an hour to fill. It’s as if the water is going from the lake to their house via Tokyo or something…
[waves hand wildly]
OOH! OOH! I know this one! Sunspace, call on me!
That is, if it’s just the washing machine…
If it’s just that the washing machine takes forever to fill up, it’s because the hot water hose that connects the washer to the hot water faucet is clogged up with mineral icky bits that come loose off the scale in the pipes (yes, that’s the correct scientific term for it, “icky bits”). 
Tell your friends to unscrew the hose from the back of the washer, take out the little round screen that is holding the icky bits in place, keeping them from going into the washer, and rinse it off.
It’s the hot water hose that usually does this–the cold water hose doesn’t do it nearly as much.
Your kitchen sink faucet will do this, too, sometimes, and all you have to do is unscrew the bottom (top?) of the faucet, the part where it hangs out over the sink, the top (bottom?) 3/4 inch of it will unscrew, and there’s another little tiny round screen in there, clogged with mineral icky bits.
Duck Duck Goose’s comment about cleaning the screen is a good one. Use an old toothbrush (not the one you clean the toilet with, stupid!), and they clean up really easily.
For the washer hose, consider replacing it, rather than cleaning it, if it’s a rubber one. These are notorious for breaking, and flooding the house before anyone notices. I believe it’s recommended to replace them every two years.
And yes, the water main could get lined with scale also.