Some interaction between outlets is normal in every house. What is not normal is a kitchen tap dropping to a trickle, or a shower running cold, the moment someone flushes a toilet upstairs. That level of interference means the supply is being shared through pipework that cannot carry enough water for two demands at once.
The good news is that this is usually a plumbing layout issue with identifiable causes rather than a fault with any single appliance. Some causes are cheap to correct, some are a rewire of the pipework, and knowing which you have before you spend anything is the point of this guide.
What is actually happening
Water pressure at any outlet depends on how much flow the pipe feeding it can deliver. When a toilet cistern starts filling, it opens a new demand on the same supply. If the shared section of pipe is small, long, or partly restricted, the total available flow gets divided and every outlet downstream weakens noticeably.
The scale of the drop is the diagnostic. A slight, brief dip while a cistern fills is normal and not worth chasing. A tap falling to a dribble, or a shower swinging hot because the cold side lost pressure, means the shared pipework is genuinely undersized or obstructed for the way your household uses water.
Undersized or over branched pipework
Many houses have a 15mm supply teeing off repeatedly to serve bathroom, kitchen and utility. Each branch reduces what is left for the next, and the further along the run an outlet sits, the worse it fares. Extensions and refitted bathrooms often add branches to a run that was already at its limit.
The proper fix is to bring a larger supply, usually 22mm, further into the house and branch to outlets from there, so no single small pipe is trying to serve several demands. It is not a small job in a finished house, but it is the only fix that genuinely solves severe interaction rather than easing it.
Restrictions worth checking first
Before anyone talks about repiping, check the cheap causes. Isolating valves left partly closed, scaled or blocked filter washers, an old stop tap that has never fully opened since it was replaced, and a corroded section of galvanised or lead legacy pipework all reduce flow through a shared run.
The toilet fill valve matters too. Older ballvalve arrangements fill quickly and greedily, while a modern valve with a slower controlled fill draws far less at once. Fitting a better fill valve is inexpensive and sometimes reduces the symptom to the point where nothing else is needed.
- Check the internal stop tap is fully open.
- Check isolating valves under basins, baths and the toilet.
- Clean or replace filter washers on flexible tap and shower connections.
- Consider a slower filling toilet valve.
- Look for legacy small bore or corroded pipework due for replacement.
System type changes what is possible
On a combi boiler, everything is drawn straight off the mains, so outlets compete directly and this symptom is at its most obvious. On an unvented cylinder the hot side is fed from stored water at mains pressure, which usually copes better with simultaneous demand as long as the incoming supply is adequate.
On a gravity system with a loft tank, the cold taps fed from that tank behave differently again, and the interaction may only involve mains fed outlets such as the kitchen. Establishing which system you have is the first step to a realistic answer about improvement.
| Cause | Cost to address | Likely improvement |
|---|---|---|
| Partly closed stop tap or isolator | Minimal | Sometimes complete |
| Blocked filter washers | Minimal | Noticeable at that outlet |
| Fast filling toilet valve | Low | Reduces the dip |
| Undersized shared pipework | Significant | Solves it properly |
| Poor incoming mains supply | Varies | Needs measurement first |
Measuring before spending
Any honest recommendation starts with measurement: static pressure and flow rate at the incoming main, taken with nothing else running. Without those figures, advice about pumps, cylinders or repiping is guesswork, and it is entirely possible to spend money on hardware that the incoming supply was never going to support.
We take those readings before recommending anything on properties around Kelso, the Borders and East Lothian, because the right answer for a Victorian terrace with a long shared supply is different from the right answer for a modern estate house.
Frequently asked questions
- Why does my tap lose pressure when the toilet flushes?
- Because the cistern filling opens another demand on the same pipe. If that shared pipe is undersized, long or partly restricted, the available flow gets divided and the tap weakens. A slight dip is normal, a drop to a trickle is not.
- Is this a fault or just how my house is plumbed?
- Often it is the plumbing layout rather than a fault, particularly where a small shared pipe branches to several outlets. It is still worth checking for partly closed valves and blocked filters first, because those are faults and they are cheap to correct.
- Will a bigger pipe fix it?
- Bringing a larger supply further into the property and branching from there is the fix that genuinely works. It means pipework changes rather than a single part, so it is usually done alongside a bathroom refit or other planned work.
- Could my toilet be the problem rather than the pipework?
- It can contribute. Older fill valves draw a lot of water quickly, so replacing one with a modern controlled fill valve reduces the momentary demand. On a borderline supply that is sometimes enough to make the symptom acceptable.
- Can you tell me whether it is worth fixing?
- Yes, and we would measure first. Static pressure and flow rate at the incoming main tell us what your supply can actually deliver, and that decides whether the answer is a cheap fix, new pipework, or accepting the limit of the mains.
One outlet killing another?
We measure pressure and flow first, then tell you honestly what will and will not help.