Putting isolation valve on hot water outlet pipe on vented cylinder

[Samsg30]

Hi,

I have a vented cylinder where hot water outlet pipe is going from the side (marked as blue in picture) of the cylinder and the expansion water pipe is going from the top (marked with red) which is not feeding hot water but just the expansion pipe leading to the gravity fed tank above. Cylinder is fed via gravity tank.
Now the issue is, if we have to work on the hot water line, by just turning off the cold water feed in line doesn't drain the enough water out of the cylinder since the hot water outlet is on the side (marked by blue colour in the picture).. Therefore, everytime, any hot water line is disconnected, around a bucket full of hot water is discharged. I think this is because water level drops until it reaches the same level as the side outlet pipe. So, can I install an isolation valve on the hot water outlet pipe (22mm) where it is marked in yellow. Is that going to cause any safety concerns, let say, in case it's left in close position by accident if hot water demand is on through the boiler?

 

[terryplumb]

Closing the cold feed to the cylinder is all you need to do. Any water draining out isn't from the cylinder it's from the distribution pipework. Fitting isolation valve on the hot water outlet side is simply not needed or recommended.

 

[stem]

Something a bit unusual here. The draw off to the taps is usually from the same top cylinder connection as the expansion pipe like the picture below, and not as per your photo notes & description. Otherwise the hot water at the top of the cylinder would never be available to use at the taps.

So normally, turning off the cold supply from the feed tank then stops the hot water at the taps almost immediately, and the cylinder remains full of hot water.

Drawing the water off lower down the cylinder using a connection in a similar way to the one shown in your photo is sometimes used just to supply a Power Shower (Pump) to avoid it pumping any air that can collect at the top of the hot water cylinder. In which case, it's normal to fit a manual valve to allow the pump to be isolated for maintenance. However, the vent pipe and main supply to the taps remain un-isolated.

 

[Samsg30]

Thanks Stem.
I forgot to mention that the hot water line is indeed attached to the pump hence it's on the lower side of the cylinder. Now, given that the whole house 'HOT' water is supplied via pump only, the pipe going up from the top of the cylinder is only leading to the vent only and not supplying any other taps. The cold water feed coming to pump is from the tank to only cater for 2 showers. Below is the slightly updated picture to yours.

You have answered by original question on putting a valve here to isolate the hot supply to the pump without risking anything and yes.. there's no isolation valve on the pipe going out from the top of the cylinder to the vent. When we take the hot water inlet pipe out to install / clean pump it discharge around 2 buckets full water even after turning the cold water supply to the tank off and leaving the hot tap / shower open to drain any excess water out.. but still 2 bucket full water (good 15 -20 Litres) drains out so my assumption is that it is draining water from the top to the cylinder to the same level as the outlet pipe marked by the white line on the picture. Otherwise It would be surprising to see 20L of water just hanging around in the pipe work.

But I have bit of a niggling question.. and that is.. if I have closed the cold water inlet and left the hot water tap on (which is sitting at the lower level than the cylinder outflow (as shown in the picture), should it not have drained all the water to the level from where water is actually fed in to the tap from the side of the cylinder? The Vent pipe above should be pushing enough air back in the cylinder to allow water to flow out even if cold water inlet is not creating pressure for hot water to flow out? If yes, then why would I be getting additional 15-20L hot water from?

Hope the question above makes sense?

 

[ianmcd]

Simple answer is absolutely NO and you dont need it anyway, if you fit a valve there and it is closed and you are heating the cylinder , the best that can happen is the cylinder will start leaking, the worst it could explode, DO NOT fit a valve there

 

[gas112]

the valve would be on a draw off from side of cylinder it will not cause any harm at all.
I dont think some of people have looked at pic correctly it is not interfering with THE open vent in any way

 

[Madrab]

.

 

[stem]

Provided the vent remains open, I don't see a problem either. It would be no different from the norm except the branch to the taps is missing, which would be closed anyway when the taps were all shut.

Still a bit of an odd way to do it though. At the moment you have lost a significant portion of your hot water supply, bearing in mind that heat rises and the bottom of the cylinder where the cold water enters won't be very warm. The hot water at the top is unused / unavailable to you.

It would be more usual to have a special flange such as a 'Surrey Flange' fitted at the top to provide the air free supply, that way the whole cylinder becomes usable and the isolation valve on the cold inlet would provide you with the desired isolation.

 

[ianmcd]

My apologies, after looking at the pic again yest that is fine but again it will not do anything that closing the cold water inlet wont do, I thought you meant replacing the pushfit staight with a valve

 

[stem]

But I have bit of a niggling question.. and that is.. if I have closed the cold water inlet and left the hot water tap on (which is sitting at the lower level than the cylinder outflow (as shown in the picture), should it not have drained all the water to the level from where water is actually fed in to the tap from the side of the cylinder?

Sounds reasonable, once the cylinder above the where the side pipe is connected has emptied no more water should come out of the cylinder. Provided that the isolation valve on the cold water inlet is doing its job. If it's a gate valve they can have a habit of 'letting by'.

then why would I be getting additional 15-20L hot water from?

How do you know you are getting this extra water? Does it start flowing from the cylinder outlet again once it's stopped? Or could it be the water contained in the pump and interconnecting pipework around the house?

 

[Samsg30]

Thanks you all for the replies. I agree a flange would have been the best solution. I actually need pump replacing from a positive head to a negative head as the flow rate from overhead tank to the cylinder is not enough to trigger the pump on the hot water side.. therefore only cold water is triggering the pump even though both hot and cold water is being fed to the pump from the overhead feeder tank only with a difference that one is going directly to the pump and other is going to the cylinder and output is coming back from the cylinder .. so I guess that is reducing the gravity flow on the hot water side to less than 2L per minute.
However, when I took the hot water inlet pipe out to check the filter on the pump (after isolating the cold water inlet and leaving hot water tap on until it runs dry), still 15-20L water popped out. So putting a lever ball valve on the outflow should give enough isolation without needing to drain the one quarter of cylinder using the fiddly drain valve at the bottom end of the cylinder.
This leaves the vent pipe open for any air / pressure built up in case valve goes wrong in future or accidently left closed for whatever reason

 

[Samsg30]

Sounds reasonable, once the cylinder above the where the side pipe is connected has emptied no more water should come out of the cylinder. Provided that the isolation valve on the cold water inlet is doing its job. If it's a gate valve they can have a habit of 'letting by'.


How do you know you are getting this extra water? Does it start flowing from the cylinder outlet again once it's stopped? Or could it be the water contained in the pump and interconnecting pipework around the house?

@stem Sorry just noticed your message before I posted the final comments.
So, yesterday, I took the hot water inlet pipe out of the pump after running the usual process of isolating cold water inlet and running a few hot water taps / shower dry. As soon as the flexible hot water inlet hose from the pump was taken out, water started coming with quite a good flow. Initially I thought it must be the remaining water in the pipe so only kept a small jar and towel but I was lucky enough to have a large bucket by the side which got filled up almost twice (amounting to 15+Litres) so this must not be the remaining water from the pipe and only reasonable explanation is that it was the differential water stored between the top of the cylinder to the side outlet of the cylinder.
Though I would have expected that particular area to have drained already.
Gate valve must not have given up else I would have a constant flow of water for all that time even if that would be a trickle.. but that wasn't the case.