Conversion of primary heating circuit to unvented

[Roger Mills]

Following on from the earlier thread about "pumping under", I'm seriously considering converting my friend's system to unvented - making it much easier to get the air out, and hopefully solving the circulation problems. [And hopefully not introducing too many leaks as the result of the higher pressure!]

I am aware that I would need to disconnect and blank off the connections to the F&E tank, and install an expansion vessel, pressure relief valve and filling loop with pressure gauge. From what I can see, complete kits can be obtained for 70 or 80 quid.

The question is where in the system to install these items.The boiler and pumps are in an integral garage, surrounded by heavy workbenches, tool chests and shelves - making them very difficult to get to without moving lots of stuff. The F&E tank is in the attic above the airing cupboard, and connects into the flow and return of the HW circuit.

The airing cupboard would be by far the easiest place to install the kit. There is ready access to mains water for the filling loop. Because the secondary HW system is unvented, there is a tundish into which anything coming out of the hot cylinder's temperature and pressure relief valve would spill - so that could also accept the output from the primary circuit's PRV. There is an uninterrupted connection to the boiler - albeit in 15mm pipe, and probably 3 metres vertical and 3 metres horizontal away.

The boiler's installation manual says to put the the PRV on the flow pipe, close to the boiler and before the pump inlet. But that seems to assume a single pump on the flow side, prior to any zone valves, etc. But this system ain't like that! There are two pumps - one for CH and one for HW - both on the return side, feeding *into* the boiler.

Are there any technical - or even legal - reasons for not installing all the kit in the airing cupboard - some way from the boiler, but directly connected to it? If so, could I put the PRV near to the boiler, and the expansion vessel and filling loop somewhere else (preferably in the airing cupboard)?

[Fredxx]

I thought where the boiler is specified to work in an unvented system, it was allowable to have a single 22mm pipe (20mm internal) as a cold feed from the expansion tank to the system?

It would guarantee no pump under, or over!

Wouldn't that be simpler and allow the simple addition of Fernox etc?

[Andrew Gabriel]

No, the PRV outlet must go outside without any air break, and it should point back against the outside wall. If the system boils, it will be ejecting super heated water and steam.

It still wants to go on the flow pipe near the boiler and before any other things that might restrict flow.

Filling and pressure guage can go anywhere (but should be next to each other so you can see the pressure as you fill the system).

Expansion vessel should ideally be as near to the pump inlet as possible (heating pump), as that will give longest pump life and lowest pump noise. You can put it anywhere, but then you will lose those benfits. If the system needs more than one expansion vessel, it doesn't matter where the second one is.

PRV must be connected exactly as it says in the boiler installation manual or the system won't conform the manufacturer's safety requirements and would fail commisioning.

[Michael Chare]

Must the system have a pressure switch so that the boiler stops if the system looses pressure?

[Andrew Gabriel]

That will be stated in the installation manual.

Mine has one built in to the boiler. It operates at 1/4 bar, because the boiler can also be used in vented systems.

The boiler may have some internal settings which need changing for sealed versus vented. The OP needs to read through the installation manual thoroughly.

[Roger Mills]

But surely the same thing could happen with near boiling water coming out of the hot cylinder's temperature and pressure relief valve? That goes into a tundish with an air gap. Why the distinction?

OK, if I move a lot of stuff in the garage, it could go in the flow pipe very close to the boiler, with no intervening flow restrictions. Technically it would be *before* the pump inlet - since the water then goes right round the house to get to the pump, which is quite close to the boiler on the return side. Does that sound ok?

In that case, it could go in the garage near the heating pump inlet which, as noted previously, is in the return pipe.

OK, it makes sense to put it close to the boiler. what "commissioning" are you referring to? It would be me doing it!

[Andrew Gabriel]

There's a massive difference between nearly boiling, and super-heated water and steam. Also, an immersion heater is typically 3kW when it gets stuck on (think, a kettle boiling). A central heating boiler is going to be 25-50kW when stuck on - a completely different ball game and very dangerous for anyone caught in the steam, particularly in the confines of a house. Yes, mostly it just handles a failed pressure vessel, but it is also there to handle much more serious failures.

It sounds OK, but you really need to read the installation manual, which I haven't. You become responsible for the installation, even any related parts done wrongly initially.

The commisioning instructions will be in the installation manual. Some of them will need to be repeated since you are changing the system design. Commissioning means testing to ensure the system is operating according to the manufacturer's design. It might including things like testing that you have sufficient pressure vessel capacity for the system volume at max running temperature.