Twin/single pipe CH

I saw in an earlier thread that it is not possible/advisable to fit a combi boiler in a single pipe central heating system. My daughter has just bought a flat and the boiler is gravity fed. There is not enough pressure for a shower, so I thought if we replaced the boiler with a combi it would solve the problem and we could get rid of the hot water tank in the airing cupboard. However I do not know what a one pipe system is. The radiators are fed from theexisting boiler via a pump, in a circuit. Each radiator is looped off this circuit. Not a very technical description I'm afraid. Can anyone advise please?
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A one pipe system has the radiators' two connections taken from the same pipe:
_____________ _________________ | | | | | | | | | | | | +|_____________|+ +|_________________|+ | | | | =========================================================== or sometimes:
_____________ _________________ +| | +| | || | || | || | || | ||_____________|+ ||_________________|+ | | | | ===========================================================
instead of from two pipes:
_____________ _________________ | | | | | | | | | | | | +|_____________|+ +|_________________|+ | | | | ----------+-------------- | --------+----------- | --------------------------+-----------------------------+-----
There shouldn't be a problem driving this sort of system (at least, the performance should be no worse than the existing system). However given the probable age of the system it might be tempting providence to try to run it at the higher pressures common with modern sealed systems, so choose a boiler which is specified as beingf suitable for use with a vented primary (small 'feed & expansion' tank in the attic)
However if you just want a decent shower and there's nothing wrong with the heating system then a power shower at < 150 from Screwfix would be a cheaper solution :-)
-- John Stumbles -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
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You appear to be getting confused by the term "gravity", as it is used for several purposes in heating systems. 3 common ones are:
1. Gravity fed primary circuit. In this, a small tank in the loft (called the feed and expansion tank) tops up the primary circulating water. The pressure of the water is related only to how far below the tank that part of the circuit is. The alternative is a sealed pressurised system, which is now the normal way of doing things. It is easy to convert from one system to the other, although there are some issues to be checked first.
2. Gravity circulated primary circuit. This is when the primary circuit has no pump. It relies on gravity to suck down the dense cold primary water and displace the less dense hot water upwards. It is basically an art rather than a science to get it to work properly. Modern boilers can't be used with it, as the circulation happens too slowly and they overheat. It is generally a bad thing that needs to be sorted. Typically, gravity circulation was used on the hot water circuit, with pumps on the central heating side. However, some really old systems used gravity circulation for everything.
3. Gravity fed domestic hot water. This consists of a hot water cylinder that is fed from a large cold tank above (usually in the loft). This leads to reasonable flow rates, but poor pressure at the outlets, so is not really suitable for mixer showers.
The problem you have appears to be (3). This can be sorted with a pump, or by replacing your hot water system with a combi, a heat bank or an unvented cylinder. You must check your cold mains supply for pressure and flow rate before doing so, though.
Christian.
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Thanks to you both, John and Christian for your help. My house has warm air heating so I am not familiar with the conventional set up. It appears that it is a two pipe system after all. The flat is the ground floor of a house conversion, so there is no access to the loft. A cupboard has been built in the bedroom to house the hot water cylinder and a tiny tank which I assume is the 'feed and expansion tank'. all cold taps are direct from the mains, and the mains also feeds a header tank built in to the HW cylinder. This gives a 'head' of about 5 feet! For various reasons we don't want to fit a power shower. I did think I could fit a shower pump in the airing cupboard, but I have been advised against that. A combi boiler looks the best option , then we can take out the HW tank and cupboard. Thanks again, what a great newsgroup, I've only just discovered it. Ed

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Sounds like the ideal property for a combi. Flats almost beg you to install one. Limited space and nowhere to put gravity tanks even if there was. Upgrade the primary circuit to sealed at the same time.
Christian.
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The combi will give high pressure showers. Get a decent flowrate model: Ferroli Modena 102, Wickes Combi 102, Ariston Microgenus 27kW version. The first two just under 13 litres/min, the latter just above 12 litre/min.
In summer the bath filups are fast enough. In winter they slow down. Showers are not affected at all. Very fast CH warm ups.
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