Low hot water pressure from some taps

My in-laws have just had some work done to fix a leak in their en-suite bathroom.

To isolate the water supply, it was necessary for the builders to turn off the mains cold supply and then open all the hot and cold water taps in the house until they ran dry. The work was then done and the cold water supply turned back on. Since then, the hot water pressure from the bathroom taps has been very poor.

Their system comprises:
- Large cold water tank in the loft
- Large vented hot water cylinder in the loft, fed from the cold water tank
- Electric pump feeding hot and cold water to two showers (house bathroom and en-suite)

The showers seems to be working okay.

The pressure in the downstairs hot water taps also doesn't seem too bad. The pressure in the upstairs hot water taps is initially okay when they are first turned on, but eventually it gets worse and then turns into a trickle after about 5 minutes. If both a bath hot tap and a sink hot tap are turned on at the the same time they both run very slowly. All taps in the house are mixer taps.

Something appears to have affected the hot water pressure, especially upstairs.

Any thoughts on how to troubleshoot and resolve?

I suspect a bit of an airlock of others seem ok. This is usually cleared by backfilling a hot supply that’s connected to the cylinder and a cold main point, an easy way is usually at the washing machine valves although if there’s any flexi hoses on the system, it can be done that way too.

CBW said:

I suspect a bit of an airlock of others seem ok. This is usually cleared by backfilling a hot supply that’s connected to the cylinder and a cold main point, an easy way is usually at the washing machine valves although if there’s any flexi hoses on the system, it can be done that way too.

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Thanks Chris.

This was my first thought and I have tried to fix at one of the taps, which are probably around 15 years old (at least) by using the method where I block the end of the tap and turn the cold water on, in the hope of forcing cold water along the hot water pipes.

I have also disconnected a flexible hose from a bathroom tap and let it run for some time, filling up a few buckets with hot water. The flow seemed to be okay, but subsequently reduced when the tap was replaced.

Is the replaced two suitable for low pressure? Also check to see if the cold water storage cistern is filling back up properly.

Milleniumaire said:

Since then...

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perhaps, to avoid such inconvenience in future, the builders fitted a valve to turn off the bathroom water.

Builders very often use the ultra-cheap type

which (apart from being prone to leak for no apparent reason) constrict the water flow.

You can get a full-bore valve by a reputable maker. It costs several times as much (but insignificant in the price of a bathroom job) and is much higher quality.

Guess what I have in my house.

CBW said:

Is the replaced two suitable for low pressure? Also check to see if the cold water storage cistern is filling back up properly.

Click to expand...

Hi Chris, it is the same tap that was refitted, so no taps have been changed. It is one of the things I'm considering trying - fitting a new tap in case it is the tap that is blocked with sediment, but want to try other options before spending money on something that may not make a difference.

The cold water tank is definitely full of water as is the hot water cylinder, from what I can tell (sound of filling).

I did wonder if the issue might be caused by sediment that may have accumulated in the bottom of the hot water cylinder and which is now blocking the taps as a result of the cylinder being emptied, then refilled. However, given that I emptied a few buckets of hot water from one of the hot water flexible hoses and didn't see any signs of sediment, I guess this seems unlikely.

The hot water taps do tend to "gurgle" a little when they are in use, unlike the cold taps, which give a powerful constant flow.

If the issue was an air lock, wouldn't this completely prevent any water coming out of the hot taps upstairs?

Sounds very typical of an airlock in a dead leg (static water pressure compresses the air bubble so it is only in the dead leg. Dynamic pressure (when you open the tap) is initially maintained by the bubble expanding but subsequent flow is then blocked by the bubble extending to the live leg. Look for a section of pipework going up and over..

oldbutnotdead said:

Sounds very typical of an airlock in a dead leg (static water pressure compresses the air bubble so it is only in the dead leg. Dynamic pressure (when you open the tap) is initially maintained by the bubble expanding but subsequent flow is then blocked by the bubble extending to the live leg. Look for a section of pipework going up and over..

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I assume the only way to remove this air bubble is to pressurise water through the hot water pipe running from the cylinder?

Try pushing the cold mains water back up the hot at the kitchen sink mixer....opening hot taps upstairs will help clear the air.
With many mixer taps the hot and cold have separate waterways all the way to the spout outlet so holding your hand over the end won't work.
Tape a bag (doubled up etc) over the spout so the water can flow from cold to hot...with a towel over the lot in case it blows.

If you do have a dead leg with a big bubble in it (very common if a cylinder is moved or removed and replaced with a combi) you ideally need to get rid of it, if that's impractical then put a bleed valve at the high spot