Help please - oddball DHW problem

[robkgraham]

I've been defeated by a hot water supply problem in my daughter's house. The pressure is OK when you open the tap but then drops off steadily to something just a bit more than a healthy trickle over about a minute. She can get enough to do the washing up etc, but showering is hopeless. The problem appeared suddenly about a week before Christmas. Always a bit difficult to judge what 'suddenly' means from the non-technical but her partner is reasonably switched on and he agrees to that.

The house is 1950's terrace and is two storey with a traditional vented dhw system heated by an indirect coil from a modern gas CH system. The header tank is modern plastic with a cover and it's base is some 4 feet off the attic floor - it looks clean inside. The hw tank is on the floor of the bedroom below. The kitchen and bathroom are on the ground floor next to one another and fed with a 3/4" pipe; the kitchen sink, bathroom basin and shower all show the same characteristic.

I drained the header tank, removed the 22mm hw feed gate valve and checked that for a blockage. I also took off a compression T feeding the vent pipe (it was the only thing that was easily accessible and helped to look as if I knew what I was doing !) and checked for something coming down the vent pipe; everything was clean. There are no isolator valves in the main hw supply pipe (that I could find anyway!)

Anybody got any ideas other than something to do with the hw tank itself; removing that is going to be a pig !! There's no drain point to start with.

Any help would be appreciated - thanks

Rob

[Set Square]

Sounds like either a blockage or an air-lock - although it's hard to explain why it flows ok for the first minute or so.

Have you tried back-flushing it? Assuming that the cold tap is mains fed, connect the hot and cold taps together with a short piece of hose and open both taps - hot first fully, and then cold, gently. That will force water backwards into the header tank - and is the usual cure for this type of problem. You need an assistant to watch the header tank to ensure that it doesn't overflow *and* to make sure that some water is flowing into it from the bottom connection. [What you *mustn't* do is subject the hot cylinder to full mains pressure - which might happen if there's a serious blockage between the cylinder and header tank, and you turn on the cold tap too fast].

[meow2222]

1 minute is header tank time, not hw tank time. My best guess is your header tank ballcock is only slowly dribble filling the header tank. Once you've exhausted the header, the pressure head then falls from header tank heoght to hw tank height, so you then only get low output.

If this is right, you'll find the ballcock only provides an inadequate fill rate, and the header runs dry after a minute or so of tap full on.

NT

[Chris Bacon]

The cold water tank should hold much more water than this. To the OP, what size is your header tank (resists any attempts at jokes)?

[Dave Plowman (News)]

Airlock.

[Tony Williams]

That sounds like a partial blockage, somewhere between the outlet of the c/w tank and the outlet at the top of the h/w tank, (before the vent pipe take-off).

The initial apparently healthy flow is the water stored in the vent pipe. When that empties, the draw is from the h/w tank, flow-rate limited by the blockage.

Shut the tap off, and the vent pipe slowly refills, ready for the next attempt.

No words of comfort there.

Crud from the c/w tank settling at the base of the feed pipe to the h/w tank?

Hard water area, chalk settling in the base of the h/w tank? Try tapping down the side of the tank, see if the sound changes as you reach the bottom.

Who on earth plumbed in a h/w tank without allowing a means to drain and replace it?

[robkgraham]

Header tank - big one ! (60 gallons at least I reckon) and there is no way that it's level is going to drain down to cause this problem; it took something like 20 minutes to drain it down through the bathroom tap.

I like Set Squares idea. I'll drain the c/w tank down first a bit I think but that is the sort of tip I was looking for. Many thanks.

Actually there is a possible drain pipe that disappears down the back to somewhere on the ground floor where there is now a gas fire with the CH heater, but not knowing anything about these things I would need to get a gas fitter in to get access to this pipe if indeed it does exist

- I suspect siphoning with a hose from the DHW tank top outlet would be easier and less expensive.

Thanks guys

Rob