Which Boiler 2

[Chris B]

Somewhat inspired by the Which Boiler thread but more what can we get thread.

My friend has a Powermax thermal store "one box" solution which is Gas burner, vented thermal store tank and domestic (mains pressure) hot water heat exchanger all in one box, situated in a large airing cupboard.

This never was very good and has a terrible reputation. (small bore heat exchanger for the DHW results in rapid furring up in the local hard water and very poor shower performance - We have descaled it 3 times in

10 years but baths have been a no-no for at least 3 years)

They were regarded as rubbish even in 2009

The upcoming summer seems an ideal time to replace. This will be a professional job but I would like to be informed on system choices prior to getting anyone in to quote.

Firstly the owner is anti combi - bad previous experience and suspicion that the hard water area will clog it up just as it has with the Powermax.

The ideal choice would be to install a conventional boiler which heats a mains pressure cylinder to provide DHW and plumbs into the existing vented heating system. Is this a possible configuration, or does the heating have to be pressurised in this sort of setup? It is feared that pressurising the heating system will result in leaks in many of the old (but still serviceable at least at low pressure) radiator valves.

Any suggestions will be welcome.

[dennis]

Did that 3 years ago, no leaks even though the rads and valves are 30 years old.

Put solar panels in to hot the water too as a twin coil cylinder isn't much more expensive than a single coil one and you get RHI payments that will just about pay for the panel.

[Dave Plowman (News)]

No reason why a pressurised system will put more of a load on seals etc than an open vented one.

[John Rumm]

Sounds like an unvented cylinder and a system boiler would be the solution. The cylinder will be mains on the DHW side, but the heating coil can be on a sealed on vented primary.

There are still a few unvented boilers about. The Vaillant 400 series spring to mind.

Having said that, converting to sealed on the primary is not normally a problem IME. If anything does spring a leak, then chances are it was due to fail in fairly short order anyway. Just make sure the existing system is well flushed before installing the new boiler.

[Dave Plowman (News)]

It also make sense. If you do have a leak, it is limited to the contents of the system. Unlike with a ball valve fed open tank. There's also generally no need to pressurise the system to the maximum allowed.

[dennis]

Really? How would that work? Only going to run at ~2PSI and remove the bit going to the header tank so no pressure from gravity?

[GB]

1 bar pressure is equivalent to a water head of 32 feet IIRC.

So, a typical vented system in a two storey house with expansion tank in the attic is working at a pressure of say 2/3rds of a bar (that's downstairs, less upstairs).

A pressurised system needs to work at up to 2-3 bar without spewing water out, but you might be kind to it and only pressurise to 1-1.5 bar most of the time.

[Dave Plowman (News)]

With a header tank the maximum pressure in the system would be in the lowest pipes, plus a bit from the pump.

[Dave Plowman (News)]

Mine - three story house - is just tickling over zero on the gauge when cold, and goes up to about 2 bar when hot. That's what it settled down to so I've left it to it. Doesn't need topping up.

[dennis]

What is it when hot as its been claimed a sealed system doesn't have to increase the pressure over a vented system. The pressure in a vented system is the same hot or cold.

[Dave Plowman (News)]

It may not change by much (if at all) if set towards the upper limit allowable.

It probably does change slightly.

[GB]

You mean because hot water is less dense?

[Vir Campestris]

Which will be at least partly compensated as the level in the header tank goes up a tad...

Andy

[GB]

But the pipework and radiators expand, so the water level may not go up or may even drop. :)

[Andrew Gabriel]

It goes up, noticably. By the way, it's called an *expansion* tank.

[GB]

LOL. I expect you are right, but it won't stop me arguing!

[dennis]

Tank is about 250mm deep so it goes up by about 150mm max, that's about 0.2 psi ignoring the fact that the column of hot water is less dense than when it was cold. Its hard to quantify as you don't actually know how much will be hot in any particular system but the density drops about 1% for a 30 degree rise. So its quite possible that the pressure could drop as it gets hot with some systems especially the ones that run hot.