Hot water pressure dropping

[Kostas Kavoussanakis]

Hi,

I have an old HW tank-feeder tank set-up and two bathrooms.

I am having one bathroom redone, so all its plumbing is isolated. Friendly plumber went up in the roof and shut the water to isolate the cold (and hot?) supplies; removed the taps and loo and plugged the pipes (fitted some valves too as I tend to like this); then opened it up again at the roof before he left. All that last week.

The subsequent effect, noticed the same night as he came and went, is that when I turn on the HW tap to fill a bath in the other bathroom, I get "normal" pressure to start with, but it quickly (within seconds) dwindles. If I shut and re-open the tap I can replicate the effect, the initial pressure increasing with the period of rest. I am not sure these things work this way, but to my ignorant self it feels like the tank/circuit is choking.

Any ideas? The plumber has no idea and he is back tomorrow to re-fit the now re-tiled bathroom. It would be nice to give him a couple of hints if the problem does not auto-fix itself.

Many thanks in advance!

Kostas

[Andrew Gabriel]

I would guess some restriction between the header tank and the hot water cylinder. Either a valve isn't fully open, or some crud in the header pipe has got in the pipework and partially blocked it.

The hot water cylinder is acting as the short-term pressure reserve for the initial "normal" pressure, but the resistance in the pipework from the header tank prevents the header tank replenishing it at full flow rate.

[Kostas Kavoussanakis]

Thanks. It does not sound good if it's a blockage. What is the solution to this problem?

Kostas

[Charles]

Kosta,

How is the cold water, full flow as before?

If your bathroom is gravity fed (which it sounds like from what you say), i.e. you have a cold water tank which feeds the hot water ... dirt does collect in the cold water tank which then will go into hot etc. To test for that you have to push water up the hot pipe BUT you can only push water up the hot pipe if there is no one way valve on the hot water. Sometimes (not too sure about this) the tap may also have a one way valve.

To push water up the hot pipe ... if your bath has two taps you need some garden hose (and any possible trickery) connect cold to hot. Similar trick if you have a mixer bath tap with a single spout.

===================

I suspect the valves (I take it you mean isolating valves). I can't tell the difference but some valves are full bore and some aren't. If you have a non full-bore valve it could be what it restricts the flow, see:

am trying to figure out the valves for myself as I am about to work on my bathroom.

If it helps a little Charles (AKA Karolos)

[Kostas Kavoussanakis]

Yes. I ran it side-by-side with the hot and it does not suffer.

It sounds intriguing. I don't know if there are one-way valves, though.

As I am not testing taps that go through them, it can't be them, can it?

Thanks Charles.

Kostas

[Kostas Kavoussanakis]

Plumber thinks it would have cleared after the number of baths we have had in the period. Optimistic?

Thanks!

Kostas

[Kostas Kavoussanakis]

The are metal, but as I am not testing taps that go through them (they are not yet installed), it can't be them, can it? The plumber suspects the valves at the header tank.

Where is that vent pipe? (I presume it btreathes air?). What can obstruct it?

Thanks,

Kostas

[Dave Liquorice]

will

It should run vertically from the top of the dome of the HW cylinder with a T to take the HW supply away. Quite often the connection goes into an elbow at the top of the cylinder to a T at the side where the vent and HW connections are made. That short bit of horizontal pipe collects the air released when the water is heated, the flow of HW can draw that trapped air into the HW pipework.

Hopefully nothing, it's there to stop pressure build up in the cylinder by allowing the water in the cylinder somewhere to expand to as it is heated or in the case of fault making the cylinder boil somewhere for the steam to go...

[The Natural Philosopher]

it never clears as such..its like a partial blockage sitting at a high point. It needs to be blown away!

[Kostas Kavoussanakis]

Problem solved: a valve near the feeder tank had broken; the knob turned, but the valve did not open. A new valve later and pressure is restored.

Thanks all for the advice!

Kostas

[The Natural Philosopher]

More of the 'it worked fine till I fixed it' problems :-)