What type of plumbing situation causes the shower to change in pressure and/or temperature when others taps in the household are used? And how do you make sure to avoid it?

May just be the way the pipe runs have been laid. If the shower is on the end of the hot run, for example, then pretty much any tap that's opened on the hot run ahead of the shower will drop the shower hot pressure.
Often the hot and cold runs will be different, adding to the likelihood of imbalance.

One way to reduce the problem is to run separate hot and cold feeds to the shower from the point of origin in the house for each supply (tanks, combi boiler, mains feed etc). Provided the supply pressure is OK this will usually minimise the impact of other taps being turned on. If the supply pressure is low, then it may be that this is something you just need to live with, although the effect on shower temperature can possibly be alleviated a bit by using a thermostatic mixer. This won't be able to stop the shower temperature dropping if the hot supply pressure drops to far, but should stop the shower getting too hot when the cold supply pressure drops.

This is very much a normal characteristic of having a combi boiler so I assume this is what you have. There are ways to minimise this effect, pipe size being one of them but even then it will still happen to a degree. A combi boiler is fed with cold water straight off the mains water supply however, the water flow rate through the boiler is restricted down inside the appliance so that the hot water supplied to the taps can be sufficiently heated. Although the boiler produces hot water at mains pressure the actual pressure on the hot will always be less than the cold because of resistance and pressure drop of the water having to pass through the boiler hydraulics and heat exchanger. So, when another cold tap, toilet or hot tap is used when the shower is on will cause pressure and flow changes at the shower valve. Also water will always take the least line of resistance so easier for water to flow from a downstairs kitchen tap than a first floor tap. System can be balanced by installing ball-o-fix type valves in the supply pipe work at every outlet to equalise the resistance or the method I prefer is to put a pressure reducing valve on the cold mains but take the cold supply to the boiler BEFORE the reducing valve. This essentially gives you higher pressure on the hot than you have on the cold so the hot water flow is less affected when multiple outlets are being used.

I have a pumped shower coming off the same header tank as the cylinder and never have this problem.
Old house had a gas combi and one tiny drip from another taps used to turn the shower either too hot or too cold, hence I have a vented system now.

<blockquote><cite>Posted By: joe90</cite>I have no problem with my shower because I designed it differently as I suffered from this problem in my last house.

I have a DHW tank not combi. The shower hot connection is right next to the DHW tank outlet and the 22mm from the tank is split into two 15mm pipes, one for the shower and one for everything else.

The cold feed to the shower comes from the cold tank in the loft, not the incoming main, and is seperate and below the other feed coming out of the cold tank which feeds the DHW tank (the reason for this is if the cold tank runs out of water for any reason the hot supply fails before the cold so you cant be scalded in the event of a failure)

Because both hot and cold for the shower are fed from the cold water tank in the loft the pressure is balanced. You can flush any toilet, turn on any tap and the shower temprature stays completely stable. I am not sure this design would comply with regulations but it works and has a fail safe to protect you from scalding.</blockquote>

Mine is now like this, although with a combi rather than hot tank and with mains water cold feeds, as I changed the pipework when we had the combi installed, which is why I suggested it as a possible fix earlier, although it did need some tweaking. Works like a dream now though, as we get virtually no effect on the shower from opening taps elsewhere. I did initially have a problem with the kitchen sink hot tap. If this was turned on full then it would cool the shower, because despite being on a separate pipe run it is much closer to the boiler. The fix (perhaps bodge) was to partially close the ball valve isolator on the kitchen tap, which reduces the maximum flow rate. The positive side effect of that has been less splashing and wasted hot water.

^Essex flange and dedecated pipe work will sort it out!

Same as qeipl - except our supplier did call it a balancing valve

"Poor topology (example: tap in the pipe run to a shower. Tap can act like a "short circuit". Star topology is better than point to point)."

Which is why it tends to be more of a problem in older properties where a shower is a retrofit and its plumbing taken from the nearest convenient point, usually just tee'd in.

They should sort your problem

Dynamic pressure vs static pressure - a lot of megaflo systems in larger houses perform miserably because the fitter didn't check the capabilities of the incoming water supply first - with everything off static pressure can be 3 bar, but when 4 taps are full on, dynamic pressure can drop to virtually nothing.

<blockquote><cite>Posted By: SteamyTea</cite>What's the problem, it is all contained in this simple formula, it does not matter that it is for gas as no one is going to use it:<div id="Attachments_130037" class="Attachments"><ul><div><img alt="Plumbing.jpg" src="/forum114/extensions/InlineImages/image.php?AttachmentID=2501"></img></div></ul></div></blockquote>

Perhaps worth noting that the change in static pressure at any point in a pipe network due to flow rate variation isn't connected to dynamic pressure.

Dynamic pressure is the pressure associated with the velocity of the fluid flow, and as such has no real connection with the static pressure drop in pipe systems, The formula is Pd = 1/2 rho V², where Pd is in Pascals, rho is fluid density in kg/cu m and V is velocity in m/S, but it's only really of use in water systems if you want to do work with the moving water (Pelton wheel, water jet, rotating garden sprinkler, hydrofoil etc).

affected.

(shevek, you can edit the title by going back and editing the first post. )