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Author Topic: Pressurised versus vented central heating systems  (Read 13378 times)
dan_aka_jack
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« on: June 02, 2011, 04:46:25 PM »

I've been reading many of the truly excellent posts on this forum about the various different configurations possible for solar hot water systems.  We're about to undertake quite a substantial bit of surgery to our heating system.  We're replacing our ancient combi boiler with a new systems boiler, an evacuated tube panel and a heat store / heat bank.  There's (at least) one question I'm still uncomfortable answering:

Our central heating is currently a pressurised circuit but, because we're about to change our boiler and add a tank, we could change to a vented central heating system. After reading about the advantages of directly connecting the boiler to the heat store (instead of indirectly heating the heat store via a boiler coil), this got me thinking that we could change our heating system to a vented system with a fill & expansion tank, hence allowing us to directly connect our vented heat store to our boiler.  My question is: ignoring solar thermal for the moment, are there any disadvantages of having a vented central heating system rather than a pressured heating circuit?  My gut instinct is that a vented system feels inherently "less stressful" for the system as a whole because of the lower pressures involved.  IIRC, the bottom of the F&E tank needs to be installed at least 300mm above the top of the highest radiator, which means we could install our new F&E tank immediately above the heat store in our (very well insulated) airing cupboard, hence mitigating the only disadvantage I know of for F&E tanks (i.e. that they leak heat away from the heat store).

Also, can I just check that the main advantages of directly connecting the boiler to the heat store are:

  • maximise the time the boiler spends in condensing mode because the boiler will always receive the coldest water available from the tank
  • maximise the heat transfer rate from the gas boiler into the hot water tank
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dan_aka_jack
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« Reply #1 on: June 02, 2011, 05:23:16 PM »

Here's one opinion: http://www.miketheboilerman.com/openorsealed.htm  (he comes down firmly on the side of vented CH over pressurised)
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martin
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« Reply #2 on: June 02, 2011, 05:34:08 PM »

sounds right to me - http://www.navitron.org.uk/faqdetail.php?id=12 Wink
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dan_aka_jack
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« Reply #3 on: June 02, 2011, 05:38:41 PM »

And another opinion here: http://www.ukplumbersforums.co.uk/central-heating-forum/6794-sealed-system-open-vent.html (saying that vented CH systems don't last as long as pressurised system CH systems because air can easily get into vented systems... which sounds a little dubious to me because the system should have inhibitor in it, surely?)
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dan_aka_jack
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« Reply #4 on: June 02, 2011, 06:26:21 PM »

I've read a number of forum posts on plumbing forums saying that open vented systems are mechanically simpler than sealed systems but that open vented systems suffer from "sludge" because the F&E tank allows oxygen to enter the water
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billt
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« Reply #5 on: June 02, 2011, 07:46:20 PM »

I've seen similar posts, and I think they are wrong.

In a properly functioning open system the only path for the dissolved oxygen to get from the header tank into the system is the feed pipe. Diffusion of oxygen through that must be very slow indeed, if it takes place at all. Of course, if the system has a leak or it pumps over then oxygen can get into the system, but those aren't properly functioning systems.

I installed an oil fired, open CH system here in 1991. I converted it to a log boiler/ thermal store system last year. There was a bit of sludge in the boiler and adjacent pipes after 20 years of use, but not enough to affect the functioning of the system. None of the radiators has sprung a leak, or stopped working through blockage, and they are still going strong with the new heat source. Likewise the aged parents system was installed in the 70s and recently converted to oil. Likewise, that has shown no signs of these terrible problems that open systems are accused of.
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dan_aka_jack
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« Reply #6 on: June 03, 2011, 10:57:57 AM »

Many thanks for the reply...

I fear I may have stumbled upon a deal-breaker for having a vented CH circuit... our boiler will be installed only a meter or so below the bottom of our proposed F&E tank location.  From one of Wookey's old posts, I understand from that some open vented boilers required 0.2 bar to function, and hence want to be installed downstairs.  Do all open vented boilers require at least 0.2 bar to function?
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JohnS
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« Reply #7 on: June 03, 2011, 01:07:31 PM »

Make sure that you size the F&E tank properly.  Most are designed to cope with just the heating circuit.  If you add a 200 or so litre thermal store, the potential for expansion increases substantially. 

If you are going thermal solar as well, the F&E tank needs to cope with higher temperatures than usual.  E.g. 95 dC instead of 65.  Check that it can cope.  Perhaps there will be a market for decent size galvanised tanks again.

Remember to install a couple of MagnaClean or equivalent filters.  One on the boiler circuit and one on the radiator return to the store so as to maximise protection.

John
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dan_aka_jack
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« Reply #8 on: June 03, 2011, 01:51:28 PM »

excellent advice, many thanks
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Baz
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« Reply #9 on: June 03, 2011, 05:25:51 PM »

The boiler doesn't need any over pressure as such. The only thing to be aware of as with all pumped systems is that the pump can produce a 'head' which may suck water out of the tank and blow it out of the expansion vent pipe.
For galvanised tanks look at horse troughs!
Only remaining problem is finding a nice copper ball for the valve.
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Brandon
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« Reply #10 on: June 03, 2011, 09:25:57 PM »

Baz, if you need a copper ball, I can get them locally, from 4" to 12"!!

Just give me a shout and I will gladly purchase one this end for you if you are in need.
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changing the world, one roof at a time.

Quality is never an accident; It is always the result of
high intention, sincere effort, intelligent direction, and
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dimengineer
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« Reply #11 on: June 03, 2011, 10:12:21 PM »

Oh god not this old chestnut again!
Pretty well the rest of the world uses pressurised systems. There are no good arguments for vented systems. They are primitive, complex, space consuming and backwards.
Go for something modern, rather than pre-victorian.

Just my opinion

Tim (FIChemE)
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Brandon
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« Reply #12 on: June 03, 2011, 10:34:16 PM »

In their favour, they do not have perishable membranes, so how they can be construed as complex is rather beyond me. 

Yes they take skill to install well, as does a sealed system, but in a  vented system with faults, they are patently obvious, yet in a sealed system the incompetency of the installer can be masked.
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changing the world, one roof at a time.

Quality is never an accident; It is always the result of
high intention, sincere effort, intelligent direction, and
skilful execution; It represents the wise choice of many
alternatives.
martin
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« Reply #13 on: June 03, 2011, 10:34:43 PM »

Vented are cheaper, simpler, intrinsically safer, don't need regular safety inspections, and if you're subjected to "water cuts*" mean you have a reserve of water that'll
"tide you over" (we even leave the Rayburn running.... whistlie)

*which will doubtless become far more prevalent

Martin (Cycling proficiency test, 2 widths of the pool, Honours English Speaking Board) hysteria
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Unpaid volunteer administrator and moderator (not employed by Navitron) - Views expressed are my own - curmudgeonly babyboomer! - http://www.farmco.co.uk
Baz
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« Reply #14 on: June 04, 2011, 09:05:39 AM »

Pretty well the rest of the world uses pressurised systems.

"The old ways are the best". "They're foreigners - what do they know". "What would a chemical engineer know about plumbing" Grin fight

Baz (Gold Star for being me.  Smiley )

PS thanks for offer Brandon - need to remember you for all special plumbing bits.
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