Pressurised Primary Circuit - Why?

I am fully familiar with gravity feed hot water systems and vented primary heating circuits run from my trivial-to-maintain ancient gas boiler. I keep reading here about expansion vessels, leaking pressure relief valves etc and can't fathom why modern systems seem to have pressurised primaries (ignoring what appear to be devil-spawn Combi boilers).

I know that cars have pressurised circuits to elevate the boiling point of the coolant but central heating flow temperatures are much lower than this especially with condensing temperatures.

Surely all the apparent problems with pressurised systems cant solely be justified by the convenience of not having a little header tank somewhere.

So are there other good reasons for pressurising the primary circuit?

One of these days I expect to have to change my boiler so I'd like to make the leap in understanding that I appear to need to choose, fit and maintain a modern device.

Are there any good text books on domestic heating out there?

TIA

Bob

Reply to
Bob Minchin
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Indeed - and the pressure we are talking about is not that high - typically 1 bar when cold, so not dissimilar to that of a vented system with a header tank 30' up.

Yup, quite a few....

Sealed system plus side:

No header tank - so quicker and easier to install, also better suited to properties with no loft space, or converted loft space etc. [1] System leaks less likely to cause damage (i.e. only limited volume of water in the system) Easy and quicker filling - no air locks etc Lower corrosion (system not open to the atmosphere) so no source for air to enter the system or to re-oxygenate the water No problems with "pump over" etc.

[1] Made even simpler with "system" boilers - that basically have everything you need in one box - just add rads and a hot water cylinder, and no need for separate pumps, zone valves etc.

On the down side, you have the problems that can arise from a failed expansion vessel, or dripping over pressure relief valve etc that you mention.

You can still elect for a vented system if you want. They are less common these days, but many boiler makers still offer a vented option. See a Vaillient EcoTec 415+ for example.

You can't really elect for the basic simple boiler any more - other than by keeping the one you have.

My personal preference is for a sealed system, but each have their plus points.

The articles on the wiki are probably your best bet.

Reply to
John Rumm

A pretty good summary! One additional downside of sealed systems is that it's a bit more difficult to add inhibitor - but I still think that sealed systems are better, on balance.

The system in my main home is vented, and works perfectly well - but I have had the occasional air-lock problem after drain-downs. The system in my holiday flat (all on one level, no loft-space, etc.) is sealed - and that, too, works perfectly well.

If I were doing anything major to my main system, I think I'd convert it to sealed - and get rid of the F&E tank.

Reply to
Roger Mills

Treloar on plumbing:

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Reply to
dom

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Yep, agree with all that.

The other very attractive feature is that there is no loft tank to freeze, split and flood your house. The Europeans don't go for tanks in lofts since their winters are usually more severe. The sealed system became a lot more common with the EU conformity thing, with the same boilers being sold throughout the EU. I suspect that, if you were to look at a few manufacturers' European websites, you wouldn't find any open vented boilers on offer at all; they're probably modified for UK Luddites.

I've had both and the added complications are reliable components (PRV, overheat thermostat, expansion vessel, filling loop) and well worth the benefits. I'd only ever use an open vent on a solid fuel system.

Reply to
Onetap

A pressure vessel? What other additional cunning bits and pieces?

FWIW, I had problems with the float valve on my open system too. It would sometimes stick - and it has failed causing the overflow to run too. And was far less accessible than the pressure vessel in my system boiler.

Reply to
Dave Plowman (News)

Changing a pressure vessel is hardly rocket science either. Although I've not needed to.

The thing I most like about the change to pressurized is the ease of filling after draining down for whatever reason. My open system was a nightmare of air locks. Partially offset by the greater difficulty in using an inhibitor. But on balance, I prefer it. And I do believe in keeping things simple unless their are real benefits on offer.

Reply to
Dave Plowman (News)

It might, but it probably doesn't. It's one the same floor, and at the same temperature, as the bedrooms.

Yes, I believe it is (rare).

Reply to
Martin Bonner

Total tripe. Buy a quality boiler and they last and last. Look at an Intergas, ATAG or Broag

They are for flats. Condensing boilers are no more complex than non-condensing boilers.

Then get a thermal store heated by a condensing boiler with two anti-cycle stats. They just don't go wrong.

Reply to
Doctorr Drivel

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