Pressurising a CH system

[GMM]

For a lot of good reasons that are nothing to do with the operation of the system, I've been thinking about converting my gravity-fed CH system. Now Im sure this will precipitate a bunch of leaks, but where are these most likely to occur? If it were to reveal pin-holes in upstairs radiators, that's not so bad as most of them could bear replacement anyway, and this would hardly be a panic job over the summer. The pipework, however, is a different matter....

[Vortex11]

Just do it.

Obviously if leaks occur they will be in the least convenient places. Make sure you have a pressue gauge on the system so you can see what's going on.

Just prepare for leaks and all will be fine.

[johnejeg]

There is no good reason why there would leaks in the pipe work or radiators.

The most likely candidates would be compression joints, pump valves, rad valve glands and the like.

You should be aware that for safety reasons that not all boilers can be converted from open vent to pressurised system.

You are advised to consult the boiler manufacturers installation instructions to verify that the change over can be done.

[Fredxxx]

I don't see any reason why you should expect leaks.

But why change your system? For one it's much easier to add inhibitor with a standard vented system.

[Harry Bloomfield]

GMM a écrit :

So why change it?

Leaks can be very difficult to find, especially if they are small leaks.

If you must, a good way to find any leaks without soaking the place, is to fill it with air pressure rather than water. Air leaks out faster and can be easier to hear plus no damage.

[John Rumm]

+1

I have converted systems in the past and not had any problem - I think you would only normally expect problems if the rads and pipework really were well past it. Even problems often cited (like older lockshield valves weeping under pressure) seem very rare.

Its about the only advantage though...

IMHO sealed systems have many advantages:

Easier and quicker to refill and get rid of airlocks.

Greatly reduced corrosion

No need for a header tank (and hence space used, possibility of freezing etc)

No pumping over

Less sludge build up (no free oxygen introduction)

Less boiler kettling

Easy to run rads at any height in the system (loft conversion etc)

Much bigger choice of boilers

Limited scope for a leak to cause substantial damage to the property.

[Chris French]

And anyway, not necessarily easier. Adding inhibitor to ours means getting the ladder, getting up into the loft, making way way across the loft insulation to the awkwardly placed F&E tank (cough cough, where was that mask...). Realise I've forgotten the bottle, etc. etc.

I just turn off the feed from the tank (valve in airing cupboard) drain a bit of water out of the system and add the inhibitor . through the top of the bathroom towel rad (highest point other than the F&E tank, so drain first)

[John Rumm]

I add mine using my Fernox TF1 filter - you can isolate and drain just it, and then it takes 1/2" a litre to refill.

[GMM]

Well, the boiler here is maybe 5 - 7 years old, but I'm sure the rads and pipework pre-date that to a significant extent. Sod's law sort of dictates that there will be some issue, but I was thinking it would be a lot simpler to replace a rad or two (and likely a benefit for some of them) than to have to lift carpets/furniture/etc to find a drip under a floor.

It would make sense (to me at least) if upstairs rads were more likely to fail as they will presumably come under unaccustomed pressure after conversion. Similarly, I would sort of expect rads to be more prone to corrosion than copper pipework.

The driving reason here is that the F&E tank (etc) is in the way of another job I need to do. Of course, I could remove it and replace it later but that would put the system (and DHW) out of action until the other job is complete. It would seem that taking the opportunity to convert (and solve the issue once and for all, given the advantages listed) could make more sense.

[Chris French]

I doubt the difference in pressure will make a difference between up and downstairs. 3m difference in height = 0.3 bar. A pressurised system runas a t about 1.5 bar IIRC

Yep, but non of the old rads here (guess 35- 40 years old have given any evidence of leaks. Compression jionts, valaves etc. that is waht I would expect to leak. However, when I put a combi in the old house onto an old system there were no leaks.

[Andrew Gabriel]

This is true, but it mainly applies to new systems which are still dry inside, and needs some massive safety warnings about what happens if some part of the system does fail whilst pressurised with air - you really don't want to be in the house when that happens, but certainly you don't want anyone else in the house whilst the system is pressurised with air. Plumbers are not allowed to do this because of the safety issues.

[GMM]

Well, it's a 3-storey house so the rads at the top probably run at about

0.15 bar (5' of head), while those on the ground floor run at about 0.75 (25'). Pressurising to 1.5 bar all round (though I guess there is still a hydrostatic effect that adds to this) means that the lowest would now experience twice the pressure they're used to, while the highest would experience 10x their current pressure. It seems to me that this is more likely to reveal marginal defects on the upper floors.

If there weren't any leaks then happy days, but I always like to factor a bit of Sod's Law into any job. Joints in and around the rads, including valves etc, are pretty trivial to deal with, as they're all readily accessible and I would drape a bit of blue paper towel under each rad before starting to provide a tell-tale for any small leaks.

[Dave Plowman (News)]

If you have pin hole leaks in steel rads, they're only ever going to get worse. Pressurized or not.

I converted my about 30 year old system when I fitted a new boiler - it didn't give the choice of open vented and had the pressure vessel built in anyway. Didn't have any problems.

Personally, I'd wait until the boiler needs changing and do it then.

[Lobster]

On 26 Jun 2015, John Rumm grunted:

We changed ours about 15 years ago; and despite looking OK (the existing CH system system having been installed by a previous occupier), an upstairs rad blew a hole in the middle of a panel, at the back, a few days after the system was recommissioned.

First we knew of it was finding the soaked sofa in the room below :(

At least being a sealed system there was a limit to how much water could leak out!