Upgrading GCH to sealed system - Expansion Vessel Question

[Vortex]

Hi,

Shortly I want to upgrade my gas central heating from "vented" to "sealed" (primary reason to eliminate the loft tank to make way for a future loft conversion.....secondary benefit is that I'll be able to put a rad up there.)

On investigating the "tide marks" in my header tank I calculate system expansion to be about 3 litres in normal use. This is consistent with a system volume of about 100 litres...which seems plausible to me (4 bed house).

Good old bes.ltd.uk sell expansion vessels in 8, 12, 18, 24 litre (and larger) sizes.

Is there a rule of thumb for specifying the correct expansion vessel? At first glance an 8 litre one would seem adequate...but the price difference is so minimal I'm inclined towards an 18 litre one (there's plenty of space). Is there any penalty in using a vessel that's too large?

While I'm at it I want to put in "bleed valve" on a spur off the main boiler flow (actually this is where the fill and vent pipes will be removed) for easy introduction of Fernox. Is this normal practice? Seems to make a lot of sense to me.

David

[Andy Hall]

Both good reasons, and there are several more if you look in Ed Sirett's FAQ on it.

You can certainly go larger, and there is no reason not to do so. I have a boiler with a 10 litre vessel built in, but on calculating the size, it might have been marginal, so I added another one also of 10 litres.

Yes you can do that, although you may prefer to put it at a higher point. If you are adding a normal system dose of MB-1 it's 4 litres and depending on layout and drain arrangements, you might not have enough capacity below this point.

You will also need vents at high points anyway to let air out. These can be manual or automatic.

.andy

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[Vortex]

Yup I've read it. Very useful

18 litres it is then. £2.85 more than an 8 litre vessel!!

Actually I was thinking of the Fernox Gel so the capacity is a non issue really.

Now I have a new query. I've just been reading the boiler manual (Potterton Prima F, 10 years old), and for a sealed system the diagram says the Safety Valve, pressure gauge and axpansion vessel should be connected on the flow side of the boiler but before the pump (makes sense). It shows the filling point as being on the "return" side of the boiler. Why? I can see no good reason.

David

[Ed Sirett]

Sizing the vessel is one of the subjects I did not include in the FAQ below. Some boiler manufactuers suggest about 10% of total system volume. Set the initial dry side pressure to 1 bar. A 12 litre vessel wil then go to 1.5 bar with and expansion of 3 litres. That's about right.

3 litres is about the expansion you would get on a 'typical' house with around 100 litres total circuit contents.

[Andy Hall]

Exactly, and not really worth doing the sums.

OK. I've tended to use both the inhibitor and the system cleaner in this range. If you are going to use the gels (and they are a bit more expensive at around £30 rather than £20) then you don't need a filling point specifically for the purpose.

Fernox recommends injecting them into a radiator through the vent even under pressure. That can be messy. What I do is to turn off both valves on a radiator and drain some water off - I have lockshield valves with a drain c*ck on the radiator side on the tail. Then I inject in the gel - they give you a little tube to do it.

Alternatively, if you are just filling, then inject before adding water. It is best to fill and check for leaks for a week first though.

You may find that when you first pressurise that you get some seeping around spindles of radiator valves. Often, but not always, this can be corrected by tightening the gland nut. Otherwise, be prepared to replace.

When I did my system, I decided to replace all the radiator valves pre-emptively enyway because one or two had shown signs of seeping with teh open vented system. I used Pegler Terrier valves with drains as mentioned and Invensys TRV4s for TRVs.

Generally it doesn't make much difference. I fitted mine where the header tank used to be.

If you buy a sealed system kit, the pressure gauge, filling connection point and relief valve all fit to the same manifold.

One important thing is to make sure that you can vent the air properly.

.andy

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[John Stumbles]

Cheaper to buy a cheap pump-spray (Hozelok garden sprayer type) and bog-standard liquid Fernox and use the sprayer to squirt the liguid in through the filling loop connection point (with suitable adaptation to sprayer to get it to 15mm or 1/2" BSP)

[Andy Hall]

My patented adaptation to do that is:

- Cut aluminium tube of sprayer lance.

- This fits an 8mm compression fitting.

- Short length of 8mm copper tube for other side of fitting

- 8mm solder female x 15mm male adaptor

- 15mm compression service valve with nut and olive on one side discarded. A male to male compression coupler would do, but I had a discarded service valve.

- Connect filling loop braided hose to above

- Pump away. (as it were).

.andy

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[Christian McArdle]

It can make it easier to flush the system. Just close off the boiler connection at the boiler isolation valve and you'll find the water supply and drain are on opposite sides of the radiators, allowing you to flush out each radiator by turning on the filling loop and using judicious use of the valves.

Christian.

[MIKE THORNE]

Fernox do a filler kit, Wolseley centre catalogue number 603120. at £44 it could sound expensive but if you are using cleanser and inhib every 3 years it cuts down on the hassle. It comes with 2 rad vent adaptors (useless) but the hose adapts to a filling loop hose quite readily with a 15 mail iron and a 10mm insert.

[Christian McArdle]

Near the top of the system, I have a vertical stub of pipe, followed by a isolation valve and then a tupperware beaker with a tank connector on it. You just empty the system enough, open the valve and pour in the chemicals, not forgetting to close the valve before activating the filling loop!

I make that around 79p for the valve, 35p for the beaker, 85p for a tank connector, £1 for a reducing T and a few offcuts of copper. That's 3 quid, far less than you will save each time you fill by buying standard chemicals instead of expensive "sealed system" concentrates.

Christian.

[Andy Hall]

True.... but in reality, all it is is a garden type sprayer with some bits which with the above shopping list I can make in about 5 minutes for £15.

.andy

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[John Stumbles]

Fifteen squid?! Blimey, guv - they saw you coming didn't they?! ;-)

Reckon I paid about £7 for my sprayer, add say £1 for 15mm x 8mm s/r fitting reducer (the bulge of the s/r on the 8mm end helps hold the sprayer pipe), another quid for an all-plastic s/f tap connector to go onto a filling loop connection, and 1p for a cable tie to hold the pipe onto the reducer.

[Andy Hall]

I dunno. Maybe it was about £12 for the sprayer, but all the bits were junkbox items so I was inclusing having to buy them

A cable tie? (sharp intake of breath).

I must admit that when I made mine I was looking to do more than just deliver inhibitor into an unpressurised system.

For my workshop secondary circuit, I've used Fernox's Alphi11 inhibitor and antifreeze at an approx dilution of 30%. This stuff is about £15 a container, so it added up to £60 worth of chemical.

If I need to drain for any reason, I don't want to have to replace this for a while. So I can drain to a set of 25 litre containers. I can refill most of the system using a simple pump, up to the point that pressurisation is needed. I use the sprayer accessory to finally pressurise the system, which it will do, to over 2 bar, if you pump hard enough.

.andy

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