Conversion to unvented system - thoughts?

zilog_jones · 03-03-2026 04:03PM #1

I've recently bought a 1960s bungalow with a pretty typical vented water system and a gas condenser boiler. The water pressure on the electric shower is mediocre, there's an asbestos cement water tank in the attic, and I would like to get solar PV in the near future so I don't want to get a combi boiler (I'd like to keep an immersion heater of some sort).

I was thinking switching to an unvented system and replacing the electric shower with a mixer bar could solve some of these problems. I suggested this to the people who were servicing the boiler, and they suggested the hot water cylinder for this could go in the attic as the current hot press in the kitchen would be too small. They also suggested some of the pipework could be a problem when put under mains pressure - there is a random selection of copper and modern plastic (PEX?) pipe around the place, and some goes under the concrete floor.

Any thoughts on this, if anyone has done similar here?

Lenar3556 · 03-03-2026 06:02PM

https://www.boards.ie/discussion/2058434931/conversion-to-unvented-system-thoughts

If the current cylinder has cylinder has sufficient capacity, you could simply install a dual impeller pump to run your proposed bar mixer. Any new works which increase pressure on existing older pipework could be an issue. But pressure testing should provide clarity in advance of making any final decision.

It might be worth doing your sums on the value of using a hot water cylinder as a dump for excess PV;

Are you going to leave the cylinder unheated ordinarily to leave ‘space’ for it to accept this excess PV? If so you might not always have hot water when you need it.

Does it make sense economically? The current feed in tariffs are in the region of 18c per unit of excess PV exported to the grid. You could heat water using gas for half this rate with a combi boiler.

zilog_jones · 04-03-2026 04:29PM

Some things to think about, thanks. Just getting a pump for the shower is something I may also consider.

My general plan would be to keep the hot water heated daily on a timer, either around peak sunlight times or cheap night tariffs depending on how battery charging works out and other things. I have heard things like the Eddi diverters may not be worth the installation costs.

I want to keep my options open for getting away from gas in the longer term, including potentially going for air-to-air heat pump, so would rather invest in hot water solutions that can work independently from GFCH in the mean time.

Conversion to unvented system - thoughts?

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