Changing a combi valve on unvented cylinder

[armyash]

Sorry for the stupid question but i have to change a combi valve on an unvented cylinder tomorrow, i havent been to the job but the part has been ordered by another engineer. I'm not overly clued up on fault finding but have fitted a few.

Just want to confirm a few things.

1. Isolate mains and drain cylinder

2. Change valve

3. When refilling, open furthest hot tap until full.

Another thing, apparently the ex vessel needs recharging, do I do this once the cylinder is drained or does not particularly matter?

Thanks

I do have my unvented.

 

[Chalked]

You might not need to drain the cylinder if the valve is fitted high enough. Swapping should only need a spanner.
As for recharging the vessel. Firstly, before you turn the water off, depress the shrader valve and make sure there is no water ( vessel goosed if there is!) . Then while you have the water off and an open hot water tap, even better. Remove the vessel. Recharge to the manufactures setting, this will be printed on it. A bui crude but after you have pumped the vessel up, check that the Schrader valve is not passing, by covering the end in a bit of spit or gas leak detection fluid.
Reconnect and turn mains back on.

Get the bill in and get paid before the tpr valve starts passing :devil2:

 

[townfanjon]

Ash , as Chalked has pointed out , if the inlet control set is level or above the cylinder its straight forward. This is why its fitted up there, as well as helping achieve the discharge pipe dimensions.
I only wish people did it !! I go mad if they are on the floor.

 

[armyash]

Thanks. I have seen a photo of the combi valve and from what i can make out it's fitted about midway of the cylinder. So a drain down is needed.

Thanks for the replies, pretty much what i had in mind just good to get it confirmed to give me that extra bit of confidence going it.

 

[chris watkins]

You should charge the expansion vessel to slightly more than whatever the pressure reducing valve is set at, most are 3.5bar so set it at 3.6-7 that way the expansion will be accommodated as soon as the cylinder starts to heat.

If you start off at higher pressure the hot water system pressure will have to rise until it is greater than the air charge pressure, you can end up with quite a difference between the hot & cold water systems.

 

[Best]

I hear on this forum of some unvented units imploding if the mains is turned off to the unit and an outlet is opened at a lower floor. Maybe better after turning water off to first open a tap on same floor, or slackening unit outlet connection.
The combination valves are probably sometimes fitted too low because the photos showing order of connections on some leaflets, show it at bottom of unit for to illustrate order only.
Not getting at you personally Armyash, but this shows how the G3 qualifications don't really give confidence. It never taught me where the combination valve should be and had to learn that by working it out myself.

 

[armyash]

I hear on this forum of some unvented units imploding if the mains is turned off to the unit and an outlet is opened at a lower floor. Maybe better after turning water off to first open a tap on same floor, or slackening unit outlet connection.
The combination valves are probably sometimes fitted too low because the photos showing order of connections on some leaflets, show it at bottom of unit for to illustrate order only.
Not getting at you personally Armyash, but this shows how the G3 qualifications don't really give confidence. It never taught me where the combination valve should be and had to learn that by working it out myself.

I fully understand what you are saying. To be honest the course I did was ok but as with most things in the trade I have come to realise that the only way you will learn anything is from experience.

I have fitted 5 or 6 unvented cylinders and they are reasonably straight forward. For me it's fault finding as usual that I need to get to grips with.

 

[Best]

Armyash. Didn't mean that to read that you didn't know combination valve position.
It's just the G3 training I was talking about being inadequate for some. It's terrible that we often have to learn the basics AFTER doing exams, by other means. Sadly, I had to learn about unvented by reading.
You are spot on about experience, - things seem so difficult and worrying until you do a few of them.

 

[JCSPLUMBING]

I agree that the G3 ticket course is not great. How can it be in one day? I learned more before I got the ticket by watching and just being around unvented installs. However I do find that having it gives you more work as general folk are not allowed near it.

 

[Ermintrude]

Drain and Hoover out the scale via bottom immersion

 

[armyash]

Just had a look at the photo that i have been sent for the job. Handy drain off.

Got the job tomorrow afternoon.

 

[Ch4 plumbing]

The problem with how that's piped (valve at bottom) is you have to drain the cylinder to change the valve!! If possible I try and get the valve up high!!

 

[Best]

Looks a bit DIY.

 

[Best]

The problem with how that's piped (valve at bottom) is you have to drain the cylinder to change the valve!! If possible I try and get the valve up high!!

And it then means the D1 pipe from the pressure relief valve to the tundish will be going upwards & so against regs.

 

[armyash]

I think it falls down hill but only just, you can see the tundish in the picture. It might be the same level, it's hard to tell. Not sure how much time I will have on the job but will re-pipe what I can.

 

[Ch4 plumbing]

And it then means the D1 pipe from the pressure relief valve to the tundish will be going upwards & so against regs.

Agree with that, seen loads where it's not 300mm on the D2 to the 1st elbow

 

[Best]

I missed the tundish! Think I can just about see it and does look like the D1 might be ok.
Don't bother repiping anything unless it is seriously wrong & against regs, or you are getting paid for to make it all tidy. First things I check when I see an unvented unit is the D2 pipe diameter for its overall length & also it's discharge point.

 

[Best]

Agree with that, seen loads where it's not 300mm on the D2 to the 1st elbow

Yea, you would think that would be the first consideration for the installer. I mean, if anything goes wrong, or if the customer has any complaint, - things like that are just so obviously wrong to anyone who inspects it.

 

[JCSPLUMBING]

Here's one I changed a couple of weeks ago. The expansion valve was knackered on this one. Made me laugh as is was my old boss who fitted it as per the cylinder commissioning sheet and sticker

 

[armyash]

All changed, and all working fine.

Easy when you know how

Thanks for the replies