Hot water supply has suddenly become very slow

Naf

A couple of days ago our hot water suddenly started to slow to a trickle. There's not been anything change, or any sort of shortage or anything. When you turn the tap on it starts OK, then gets less and less until there's practically nothing.

We have a fairly common, albeit old, gravity setup as in the image. Header tank in the attic, dumb tank with an immersion heater upstairs, bathroom sink and bath with mixer tap shower and balanced cold supply, and downstairs kitchen mixer tap with cold fed from the mains.

I've tried/checked a few things already:

* Attic header tank seems fine. It's filling no problem and everything looks nice and clear.
* First up I tried an airlock technique by plugging my thumb over the kitchen mixer tap and opening hot and cold, forcing mains cold up the hot water system. First with the bathroom hot taps closed, eventually causing an overflow from the attic tank (into the garden, don't worry - I just didn't include that pipe on my diagram); then with the bathroom taps open, where it came out of those OK too. No luck, issue persisted.
* Next up I've tried draining the system to refill. Opened all the hot taps and shut off the attic inlet. Waited for the kitchen tap to stop, closed everything a bit, and restarted the supply. First time the kitchen tap seemed a bit better but was sputtering a bit. Second time I opened up the cold tap in the bathroom too, and now there's pretty much nothing coming out of any tap at all

I could really do with sorting this myself, and soon; but I'm now out of ideas.

nofoollikeold

With no taps working properly (presumably except the kitchen cold) I'd look for an obstruction to the outlet from the cold water storage cistern.  Could be dead animal / bird, loft insulation fallen in.  

Olinda99

I agree the pipe between the header tank and the hot water cylinder is probably obstructed. 

connect a garden hose to the mains cold water feed so you've got a bit of pressure. You could then open up all the taps that are fed from the hot water cylinder and then take the hose up into the attic and blow water from the attic header tank pipe through the pipe.

the disadvantage of this way is that the obstruction will end up in your hot water cylinder which may not be a problem depending on what it is 

a better way would be to undo the pipe from the hot water cylinder and blow water the other direction i.e back into the header tank. obviously you would have to drain the cylinder tank first

FreeBear

There should be a couple of gate valves near the DHW cylinder - Check that these are both fully open (and then backed off a tad).

Naf

nofoollikeold said:

With no taps working properly (presumably except the kitchen cold) I'd look for an obstruction to the outlet from the cold water storage cistern.  Could be dead animal / bird, loft insulation fallen in.  

The tank is completely clear, and has a secure lid over it too. I can't see how anything could get in.

Naf

Olinda99 said:

I agree the pipe between the header tank and the hot water cylinder is probably obstructed. 

connect a garden hose to the mains cold water feed so you've got a bit of pressure. You could then open up all the taps that are fed from the hot water cylinder and then take the hose up into the attic and blow water from the attic header tank pipe through the pipe.

the disadvantage of this way is that the obstruction will end up in your hot water cylinder which may not be a problem depending on what it is 

a better way would be to undo the pipe from the hot water cylinder and blow water the other direction i.e back into the header tank. obviously you would have to drain the cylinder tank first

May have to call a professional in at that point. I don't have a long enough hose, or attachments for that.

ThisIsWeird

Naf said:

A couple of days ago our hot water suddenly started to slow to a trickle. There's not been anything change, or any sort of shortage or anything. When you turn the tap on it starts OK, then gets less and less until there's practically nothing.

We have a fairly common, albeit old, gravity setup as in the image. Header tank in the attic, dumb tank with an immersion heater upstairs, bathroom sink and bath with mixer tap shower and balanced cold supply, and downstairs kitchen mixer tap with cold fed from the mains.

I've tried/checked a few things already:

* Attic header tank seems fine. It's filling no problem and everything looks nice and clear.
* First up I tried an airlock technique by plugging my thumb over the kitchen mixer tap and opening hot and cold, forcing mains cold up the hot water system. First with the bathroom hot taps closed, eventually causing an overflow from the attic tank (into the garden, don't worry - I just didn't include that pipe on my diagram); then with the bathroom taps open, where it came out of those OK too. No luck, issue persisted.
* Next up I've tried draining the system to refill. Opened all the hot taps and shut off the attic inlet. Waited for the kitchen tap to stop, closed everything a bit, and restarted the supply. First time the kitchen tap seemed a bit better but was sputtering a bit. Second time I opened up the cold tap in the bathroom too, and now there's pretty much nothing coming out of any tap at all

I could really do with sorting this myself, and soon; but I'm now out of ideas.

Hi Naf. Great info, and well done for trying everything you already have.

Although forcing mains cold back up a hot gravity supply is generally a tried and tested way to clear airlocks, when you look at your diagram, it's quite possible that the cold mains just went straight up that expansion/vent pipe, and dumped in the top of the CWS, so missing any potential airlocks around the DHW cylinder.

For an airlock to exist and cause problems, it really needs a length of level, or even wrongly-sloping, pipe. Where are these on your system, and how long are they? Your diagram shows a pretty much vertical hot pipe to the kitchen sink, for example, so I wouldn't expect any airlocks there, even on the horizontal bit at the very bottom, as this has the largest pushing force already on it, being at the lowest point of the system. And, the slow-running taps apply to all hots, so they must have summat in common.

So, it's looking like a block or restriction either in the hot cylinder itself - eg near its outlet at the centre-top - or in the cold supply to its bottom (missus!).

You refer to the cold (CWS) tank as a 'header' tank, but that usually refers to (I think) a smaller F&E tank for the central heating. You seem clued up, so I'm sure no mishtake, but could you confirm you looked into the larger CWS tank, and not the wee one?

And the CWS had clean water in it, and no obvious debris? And no loose insulation of lagging in the lid that could have fallen in?

Blimey.

Ok, where the CWS supplies the DHW cylinder, there should be a gate valve on that pipe before it enters the cylinder near the bottom, and there is usually also a drain valve there with a hose spigot. Is there? We need to check that the cold supply is getting in there without any restriction. I'm also trying to figure out the best approach to doing this!

Perhaps the 'simplest' way would be to shut off the CWS supply to the DHWC (cylinder), drain a few seconds of water from a hot tap until it stops, and disconnect the pipe that comes out the top of the DHWC at some point before it begins the journey down to the taps. Have a couple of buckets handy, aim that pipe into one - add a piece of hose or pipe if needed) and then reopen the CWS gate valve. If water comes out quite solidly, and continues to do so to fill a couple of buckets, then it would seem there's no blockage in the CWS-to-DHWC arrangement.

What do others suggest for this check?

ThisIsWeird

And please confirm that ALL the hot taps are affected? ALL!

Naf

ThisIsWeird said:

Naf said:

A couple of days ago our hot water suddenly started to slow to a trickle. There's not been anything change, or any sort of shortage or anything. When you turn the tap on it starts OK, then gets less and less until there's practically nothing.

We have a fairly common, albeit old, gravity setup as in the image. Header tank in the attic, dumb tank with an immersion heater upstairs, bathroom sink and bath with mixer tap shower and balanced cold supply, and downstairs kitchen mixer tap with cold fed from the mains.

I've tried/checked a few things already:

* Attic header tank seems fine. It's filling no problem and everything looks nice and clear.
* First up I tried an airlock technique by plugging my thumb over the kitchen mixer tap and opening hot and cold, forcing mains cold up the hot water system. First with the bathroom hot taps closed, eventually causing an overflow from the attic tank (into the garden, don't worry - I just didn't include that pipe on my diagram); then with the bathroom taps open, where it came out of those OK too. No luck, issue persisted.
* Next up I've tried draining the system to refill. Opened all the hot taps and shut off the attic inlet. Waited for the kitchen tap to stop, closed everything a bit, and restarted the supply. First time the kitchen tap seemed a bit better but was sputtering a bit. Second time I opened up the cold tap in the bathroom too, and now there's pretty much nothing coming out of any tap at all

I could really do with sorting this myself, and soon; but I'm now out of ideas.

Hi Naf. Great info, and well done for trying everything you already have.

Although forcing mains cold back up a hot gravity supply is generally a tried and tested way to clear airlocks, when you look at your diagram, it's quite possible that the cold mains just went straight up that expansion/vent pipe, and dumped in the top of the CWS, so missing any potential airlocks around the DHW cylinder.

For an airlock to exist and cause problems, it really needs a length of level, or even wrongly-sloping, pipe. Where are these on your system, and how long are they? Your diagram shows a pretty much vertical hot pipe to the kitchen sink, for example, so I wouldn't expect any airlocks there, even on the horizontal bit at the very bottom, as this has the largest pushing force already on it, being at the lowest point of the system. And, the slow-running taps apply to all hots, so they must have summat in common.

So, it's looking like a block or restriction either in the hot cylinder itself - eg near its outlet at the centre-top - or in the cold supply to its bottom (missus!).

You refer to the cold (CWS) tank as a 'header' tank, but that usually refers to (I think) a smaller F&E tank for the central heating. You seem clued up, so I'm sure no mishtake, but could you confirm you looked into the larger CWS tank, and not the wee one?

And the CWS had clean water in it, and no obvious debris? And no loose insulation of lagging in the lid that could have fallen in?

Blimey.

Ok, where the CWS supplies the DHW cylinder, there should be a gate valve on that pipe before it enters the cylinder near the bottom, and there is usually also a drain valve there with a hose spigot. Is there? We need to check that the cold supply is getting in there without any restriction. I'm also trying to figure out the best approach to doing this!

Perhaps the 'simplest' way would be to shut off the CWS supply to the DHWC (cylinder), drain a few seconds of water from a hot tap until it stops, and disconnect the pipe that comes out the top of the DHWC at some point before it begins the journey down to the taps. Have a couple of buckets handy, aim that pipe into one - add a piece of hose or pipe if needed) and then reopen the CWS gate valve. If water comes out quite solidly, and continues to do so to fill a couple of buckets, then it would seem there's no blockage in the CWS-to-DHWC arrangement.

What do others suggest for this check?

OK, lots to unpack. I have some bits of knowledge here and there, but there's a good chance I get terminology or whatever wrong based on my understanding.

• Yes, you're probably right that my forcing mains back up likely bypassed any of the troublesome pipes. It was something easy to have a go at.
• The picture doesn't necessarily show the full layout around the property, and I don't know exactly where some of the pipes are routed around the building. The balanced cold supply to the shower definitely has a lot of fairly horizontal pipes around the attic, so I'm fairly sure that'll just be an airlock to clear. That wasn't affected by the original issue - it's only happened since I drained and refilled the system. There could well be horizontal hot water pipes around the upstairs and kitchen where it comes from the main vertical pipe to get to those taps.
  
• Header tank may be the wrong term. It provides a head of water to give pressure to the hot system. We have storage heaters, no gas; so it's the only tank up there, or anywhere. The only water not on that diagram are the mains fed kitchen, bathroom, and outside cold taps. The tank has (well, had; my fiddling has of course churned it up some) some fine silt and a few bits of plastic swarf (still from when we installed the balanced cold supply); but nothing larger than what the water companies sometimes end up putting down the pipes anyway. It's got a well fitting lid that clips satisfyingly into place - nothing could have got in.
• There's a big red knob on the pipe down from the attic which seems to cut off the supply to the DHWC. "add a piece of hose" is easier said than done haha, especially as the only removable connector is the one in the top of the cylinder. Hose spigot is there at the bottom, though; so I guess same sort of procedure just with that. Guess I need to go buy some hose, and probably fittings to connect it to the taps too.
  

           

Naf

ThisIsWeird said:

And please confirm that ALL the hot taps are affected? ALL!

Yes indeed. Current state of affairs is absolutely no water coming from the balanced cold supply; and we reckon about half of the usual from all the hot taps after it settles.

ThisIsWeird

Naf said:

• Yes, you're probably right that my forcing mains back up likely bypassed any of the troublesome pipes. It was something easy to have a go at. Absolutely worth trying.
  
• The picture doesn't necessarily show the full layout around the property, and I don't know exactly where some of the pipes are routed around the building. The balanced cold supply to the shower definitely has a lot of fairly horizontal pipes around the attic, so I'm fairly sure that'll just be an airlock to clear. That wasn't affected by the original issue - it's only happened since I drained and refilled the system. There could well be horizontal hot water pipes around the upstairs and kitchen where it comes from the main vertical pipe to get to those taps. I guess such a simple layout would be too good to be true
  
• Header tank may be the wrong term. It provides a head of water to give pressure to the hot system. We have storage heaters, no gas; so it's the only tank up there, or anywhere. The only water not on that diagram are the mains fed kitchen, bathroom, and outside cold taps. The tank has (well, had; my fiddling has of course churned it up some) some fine silt and a few bits of plastic swarf (still from when we installed the balanced cold supply); but nothing larger than what the water companies sometimes end up putting down the pipes anyway. It's got a well fitting lid that clips satisfyingly into place - nothing could have got in. Cool - so that is a CWS. And, as you say, the level of debris sounds quite normal.
  
• There's a big red knob on the pipe down from the attic which seems to cut off the supply to the DHWC. "add a piece of hose" is easier said than done haha, especially as the only removable connector is the one in the top of the cylinder. Hose spigot is there at the bottom, though; so I guess same sort of procedure just with that. Guess I need to go buy some hose, and probably fittings to connect it to the taps too. The big red wheel is a gate valve, and that shuts off the CWS supply to the DHWC. It should be FULLY open, and then backed off a half turn closed just to help prevent it from seizing. Does it turn easily?
  

           

See bits in bold.

In addition, as you say, it would be hard to attach a hose to the top of the DHWC as all the pipes are soldered.